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AbruptSLR

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Trends for the Southern Ocean
« on: June 27, 2013, 05:01:59 PM »

Following-up on one of Sidd's suggestion, I am opening this thread in order to break-out future discussions of trends in the Southern Ocean with regard to such topics as: Ocean Heat Content, OHC;  overturning, upwelling, CO₂ absorption, salinity, circulation patterns, eddies, etc.

I begin with the following paper (and link); which, indicates that the Southern Ocean continues to act as a sink for absorbing CO₂, but that this absorption could stabilize in the near future, with continuing CO₂ emissions:

Sea–air CO2 fluxes in the Southern Ocean for the period 1990–2009
by: A. Lenton et al, 2013;
Biogeosciences, 10, 4037–4054, 2013
www.biogeosciences.net/10/4037/2013/; doi:10.5194/bg-10-4037-2013

http://www.biogeosciences.net/10/4037/2013/bg-10-4037-2013.pdf

“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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AbruptSLR

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Re: Trends for the Southern Ocean
« Reply #1 on: June 28, 2013, 03:46:44 PM »
For those in the UK who want the latest information about the Southern Ocean you might want to attend the following up-coming session of The Royal Society:

New models and observations of the Southern Ocean, its role in global climate and the carbon cycle
9:00 am on Tuesday 16 July 2013 – 5:00 pm on Wednesday 17 July 2013
at The Royal Society at Chicheley Hall, home of the Kavli Royal Society International Centre, Buckinghamshire

http://royalsociety.org/events/2013/southern-ocean-models/
 
Event details
The Southern Ocean is the most remote and the least understood of the world’s oceans, but plays a crucial role in past and present climate change. Currently it is the focus of intense physical and biogeochemical research. This meeting will bring together observationalists and modellers to exchange their latest insights, and will reach across the disciplines to bring together physical oceanographers, climatologists and carbon cycle scientists.

Session 1: The Southern Ocean: large-scale circulation and climate
 Dr Stephen Rintoul, CSIRO and Antarctic Climate and Ecosystems Cooperative Research Centre, AustraliaSouthern Ocean circulation, variability and links to climate
Dr Stephen Rintoul, CSIRO and Antarctic Climate and Ecosystems Cooperative Research Centre, Australia

Dr Sarah Gille, University of California San Diego, USAPoleward heat transport in the Southern Ocean: identifying roles of winds and fronts
Dr Sarah Gille, University of California San Diego, USA

Professor Mike Meredith, British Antarctic Survey, UKDense water export in the Atlantic sector of the Southern Ocean: mechanisms, changes and consequences
Professor Mike Meredith, British Antarctic Survey, UK

Dr Kevin Speer, FSU, USAFloat observations of the Southern Ocean: insights and implications
Dr Kevin Speer, FSU, USA


Professor Karen J Heywood, University of East Anglia, UKProcesses at the Antarctic continental slope important for climate and the carbon cycle
Professor Karen J Heywood, University of East Anglia, UK

AbstractAcceleration of Antarctic ice sheet loss is mainly driven by basal ice shelf melt, in turn determined by ocean-ice interaction and related to the heat transport onto the Antarctic continental shelf. Processes of water mass transformation through sea-ice formation/melting and ocean-atmosphere interaction on the Antarctic continental shelf are key to the formation of deep and bottom waters as well as determining the heat flux beneath ice shelves. Climate models however cannot include such small-scale processes and struggle to reproduce the water mass properties of the region.
Changes in temperature and salinity of Southern Ocean water masses have been identified regionally. Here we discuss recent changes in water mass properties on the Antarctic continental shelf. Some of the mechanisms through which the warm waters offshore in the Southern Ocean may penetrate onshore are discussed, including eddies and along-slope waves.
In early 2012 the GENTOO project deployed three Seagliders for up to two months to sample the water to the east of the Antarctic Peninsula in unprecedented temporal and spatial detail. We discuss evidence in the Seaglider data of exchanges across the shelf-break front (the Antarctic Slope Front), including observations of dense water spilling off the continental shelf, and of a subsurface lens of Warm Deep Water on the shelf emanating from offshore. GENTOO demonstrated the capability of ocean gliders to play a key role in a future Southern Ocean Observing System.

 Session 2: Southern Ocean mixing and controls on circulation
 Dr James R Ledwell, Woods Hole Oceanographic Institution, USADiapycnal mixing from coordinated tracer and turbulence measurements
Dr James R Ledwell, Woods Hole Oceanographic Institution, USA


Professor Raffaele Ferrari, MIT, USARecent observations of Southern Ocean mixing and their implications
Professor Raffaele Ferrari, MIT, USA

AbstractThe Meridional Overturning Circulation (MOC) of the ocean is a critical regulator of the Earth's climate processes. Climate models have been shown to be highly sensitive to the representation of lateral eddy mixing in the southern limb of the MOC, within the Antarctic Circumpolar Current latitudes, although the lack of extensive in situ observations of Southern Ocean mixing processes has made evaluation of mixing somewhat difficult. We present the first direct estimate of the rate of lateral eddy mixing across the Antarctic Circumpolar Current is presented. The estimate is computed from the spreading of a tracer released upstream of Drake Passage as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). The meridional eddy diffusivity, a measure of the rate at which the area of the tracer spreads along an isopycnal across the Antarctic Circumpolar Current, is approximately 700 m^2/s at 1500 m depth. The estimate is based on an extrapolation of the tracer based diffusivity using output from numerical tracers released in a 1/20th of a degree model simulation of the circulation and turbulence in the Drake Passage region. The model is shown to reproduce the observed spreading rate of the DIMES tracer and suggests that the meridional eddy diffusivity is weak in the upper kilometer of the water column with values below 500 m^2/s and peaks at the steering level, near 2 km, where the eddy phase speed is equal to the mean flow speed. The implications of these results for the ventilation of deep water masses and for the representation of oceanic turbulence in ocean models used for climate studies will be discussed.


Dr Emily Shuckburgh, British Antarctic Survey, UKSubmesoscale processes and mixing in the Southern Ocean
Dr Emily Shuckburgh, British Antarctic Survey, UK

 Session 3: Southern Ocean overturning and ventilation
 Professor John Marshall FRS, MIT, USAResponse of the southern ocean and sea-ice to changing winds
Professor John Marshall FRS, MIT, USA
Biography not yet available


Dr Andy Hogg, ANU, AustraliaCirculation in the Southern Ocean: a conspiracy between wind, buoyancy, eddies and geometry
Dr Andy Hogg, ANU, Australia

AbstractDisentangling the individual contributions of surface wind stress and surface buoyancy forcing to the Southern Ocean circulation is complicated by the dynamical role played by eddies, as well as interactions between flow and topography in this region. Here we show a suite of recent results from idealised (but high resolution) ocean models, which are helping to unravel the governing dynamics of the Southern Ocean. It is now clear that eddies may partially moderate the Southern Ocean response to future changes in wind stress, but that the sensitivity of the overturning circulation and the circumpolar transport differ considerably. Surface buoyancy forcing (both local and remote) plays a strong role in controlling the system response, and is likely to dominate Southern Ocean change on long timescales. Idealised model have the twin advantages of complete equilibration and model efficiency; however, an important caveat on the application of idealised model results is that details of the model topography can dominate the behaviour of the system.


Professor Darryn Waugh, John Hopkins University, USAChanges in the ventilation of the southern oceans
Professor Darryn Waugh, John Hopkins University, USA

AbstractSurface westerly winds in the Southern Hemisphere have intensified over the past few decades, primarily in response to the formation of the Antarctic ozone hole.  I will discuss the impact of this intensification on the transport of surface waters into the interior (“ventilation”) of the southern oceans. Measurements of CFC-12 made in the southern oceans in the early 1990s and mid- to late-2000s will be used to show  large-scale coherent changes in the ventilation, with a decrease in the age of subtropical subantarctic mode waters and an increase in the age of circumpolar deep waters. Model simulations will be used to examine the possible mechanisms involved with these changes in ventilation, and the possible impact on the oceanic uptake of heat.


Professor Jorge Sarmiento, Princeton University, USASOBOM: new observations of the Southern Ocean system
Professor Jorge Sarmiento, Princeton University, USA


Professor Andrew Watson FRS, University of East Anglia, UKGlacial atmospheric CO2 and role of the Southern Ocean
Professor Andrew Watson FRS, University of East Anglia, UK
Biography not yet available

 Session 4: Carbon cycle and biogeochemical processes
 Dr Mario Hoppema, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, GermanyPenetration of anthropogenic carbon into the deep Southern Ocean with special emphasis on the Weddell Sea
Dr Mario Hoppema, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany

AbstractUsing 10 cruises spanning 1984 to 2011, we investigate the time rate of change of TCO2 in the Weddell Gyre (i) along the Prime Meridian, (ii) on the continental slope near the tip of the Antarctic Peninsula, and (iii) at the bottom of the Weddell Sea interior. In the Weddell Sea Bottom Water at the Prime Meridian, the spatial distribution of the increase in DIC bears a high resemblance to that of CFCs, suggesting that the changes in Cant are propagated from the surface. However, other variables like dissolved oxygen and silicate also show trends through time, pointing to non-steady state conditions which might also affect the derived CO2 increase. Near the tip of the Peninsula, the coldest and most recently ventilated waters, hugging the continental slope, exhibit increasing DIC over time in clear dependence of temperature. In the bottom layer of the Weddell Sea interior, no relationship is found between DIC and potential temperature. The mean values of DIC in these waters are observed to have remained essentially constant, suggesting that no significant ventilation of these waters has taken place over the time scale of observations. This finding is in line with the low levels of CFCs at this location.
Co-authors:
Steven van Heuven, Centre for Isotope Research, University of Groningen, The Netherlands
Elizabeth Jones, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Germany
Hein J W de Baar, Royal Netherlands Institute for Sea Research, The Netherlands


Professor Corinne Le Quéré, Tyndall Centre for Climate Change Research, University of East Anglia, UKRecent trends in the Southern Ocean CO2 sink
Professor Corinne Le Quéré, Tyndall Centre for Climate Change Research, University of East Anglia, UK


Dr Dorothee Bakker, University of East Anglia, UKCarbon uptake in the Southern Ocean, where ‘old’ (deep water) and ‘new’ (carbon dioxide from fossil fuels) meet
Dr Dorothee Bakker, University of East Anglia, UK


Dr Robert Anderson, LDEO, USABiological response to variable dust supply in the South Atlantic sediment record
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

AbruptSLR

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Re: Trends for the Southern Ocean
« Reply #2 on: June 28, 2013, 05:05:09 PM »
The following extract and images come from the following website about recent efforts to document a cross-section of oceanographic properties from the Cape of Good Hope to the coast of Dronning Maud Land:

http://www.coriolis.eu.org/Science/Research-Activities2/Atlantic-Ocean/GOODHOPE-SAMOC

"With the relatively important number of GoodHope full-depth hydrographic cruises, high resolution XBT sampling, deployed profiling floats and satellite altimetry in complement with numerical simulation analyses we have been able to already improve quantitatively the knowledge on regional dynamics and water properties exchanged south of Africa. In particular the increased number of vertical profiles obtained by the repeat deployment of Argo floats along the GH line allowed us to make important progresses on the understanding and quantifying particular aspects of the regional dynamics. They include the estimate of the ACC variability for the upper 2500 m  (Swart et al. 2010; Swart and Speich 2010; the regional mixing layer heat budget (Faure et al. 2010); aspects of the regional mesoscale dynamics (Gladyshev et al. 2008; Dencausse et al. 2011; Arhan et al. 2011); the Indo-Atlantic Antarctic Intermediate Water (AAIW) exchanges (Rusciano et al. 2012); global estimates of halothermosteric variability in connection with sea-level changes (von Schukman et al. 2012).
Hereafter we describe some of the results we obtained that are based, at least partially, on analyses of Argo data within the GoodHope project.
While the oceanic region located south of South Africa has been studied extensively for its dynamical processes contributing to the transfer of Indian Ocean Central Water to the South Atlantic, other issues related to air-sea fluxes and water mass conversion, though also influencing the inter-oceanic exchanges, have been comparatively less examined in this area than at other longitudes of the Southern Ocean. A reason for this certainly resides in the fact that no Subantarctic Mode Water (SAMW) is formed in the SAZ south of Africa, unlike in the Indian Ocean and Pacific Ocean.
In this study we used ARGO hydrographic profiles, two hydrographic GH transects, and satellite measurements of air-sea exchange parameters to characterize the properties and seasonal heat budget variations of the Surface Mixed Layer (SML) south of Africa (Faure et al. 2011). Two recent studies using ARGO floats, though not focused on the region south of Africa, provide information on the SML heat balance in this sector of the Southern Ocean. Sallée et al. (2006), while studying the formation of SAMW in the southeastern Indian Ocean, found that upstream (west) of this formation area heating by eddy diffusion related to the nearby South Indian western boundary current system (the Agulhas Current and Agulhas Return Current) counterbalances the cooling due to air-sea fluxes and Ekman transport. Dong et al. (2007), in a circumpolar study of the Southern Ocean SML heat budget, underlined the role of air-sea fluxes at the seasonal time scale, and the relative weakness of the geostrophic advection term, in contrast with western boundary current regions.
The analysis distinguishes the Subtropical domain (STZ), and the SAZ, Polar Frontal Zone (PFZ) and Antarctic Zone (AZ) of the ACC. While no Subantarctic Mode Water forms in that region, occurrences of deep SML (up to ~450 m) are observed in the SAZ in anticyclones detached from the Agulhas Current retroflection or Agulhas Return Current. These are present latitudinally throughout the SAZ, but preferentially at longitudes 10°E-20°E where, according to Dencausse et al. 2011, the S-STF is interrupted. Likely owing to this exchange window and to transfers at the SAF also enhanced by the anticyclones, the SAZ shows a wide range of properties largely encroaching upon those of the neighbouring domains (Fig. 4).
Heat budget computations in each zone reveal significant meridional changes of regime. While air-sea heat fluxes dictate the heat budget seasonal variability everywhere, heat is mostly brought through lateral geostrophic advection by the Agulhas Current in the STZ, through lateral diffusion in the SAZ, and through air-sea fluxes in the PFZ and AZ. The cooling contributions are by Ekman advection everywhere, lateral diffusion in the STZ (also favoured by the ~10-degree breach in the Subtropical Front), and by geostrophic advection in the SAZ. The latter likely reflects eastward draining of water warmed through mixing of the subtropical eddies.
Here again, a combination of Argo hydrographic profiles collected between 2004-2009 in the South Atlantic south of Africa and observations from the GoodHope hydrographic transect is achieved to describe the characteristic and the flow of Antarctic Intermediate Water (AAIW). The Argo raw data are reorganized in a 1°x1° grid in an area extending from 10°W to 40°E and from 20°S to 60°S. The AAIW characteristics are compared in nine different regions defined on the base of the regional Southern Ocean front that are relevant to the WWIW dynamics : the S-STF and the SAF. Following the method developed by  Faure and al., 2011 for the determination of the fronts calculated from the Argo profiles, the different regional varieties of AAIW and their origins are determined : the A-AAIW (south-west Atlantic AAIW) with salinities lower than 34.2 ; I-AAIW (Indian AAIW) with salinities exceeding 34.3 ; and a new intermediate water - defined as IA-AAIW (Indo-Atlantic AAIW)- found north of the S-STF between 10°W and 12°E and south of the S-STF between 12°E and 40°E with salinities comprised between 34.2 and 34.3.
The collected Argo profiles show (fig.5) a quasi-zonal distribution of the salinity minimum values computed within AAIW on the isoneutral surfaces (37.3) on a grid 1°x1°. The zonal AAIW matches fairly well the SO fronts location. The Indian and Atlantic varieties of AAIW are separated by the S-STF in the western part of the domain ; the area of the north of the S-STF is largely dominated by I-AAIW with area-normalized volume values of about 5.4*102 m3/m2, west of 23°E. The A-AAIW volume, abundant between the S-STF and the SAF, decrease very importantly eastward of 12°E certainly due to the strong mixing between Indian and Atlantic varieties induced by the spawning of eddies in the Cape Basin.

Making use of the recent developed ANDRO velocity dataset developed by Ollitrault and Rannou, 2011, the regional AAIW absolute geostrophic velocity and transport is estimated within the isoneutral layer. The AAIW has speed between 0.1-0.3 m/s in the Agulhas Current and 0.1-0.23 m/s in the Agulhas Return Current. AAIW flows in the subtropical region have a speed approximatively of 0.03 m/s. A net increase of the eastward transport is evident from 40°S to 60°S, in particular at the S-STF and PF locations. The transport accross the latitudinal lines shows an evident variability between 12°E-23)E, which represents the « frontal » window characterized by high mesoscale and submesoscale activity due to eddies and rings detected from in-situ observations and satellite altimetry. "

The captions are for the attached image in order:

Figure 2 : Locations of the Goodhope CTD and XBT sections

Figure 3 :  Schematic of the proposed trans-basin array along 34.5°S and the oblique Goodhope transect. Note the x-axis scale is stretched over western and eastern boundaries. Stars indicate the different components of the array that have been (or will be) submitted to respective funding agencies : eastern boundary PIES/CPIES by France-ANR (black stars), and western boundary bottom pressure gauges, CPIES and ADCP by Brazil/FAPESP/FACEPE (green stars) dynamic height moorings to USA/NSF (red stars), western boundary PIES/CPIES and interior PIES-DP to USA/NOAA (blue stars). Colour contours are of 27-year mean OGCM from the Earth-simulator (OFES) meridional velocity at 200m-depth. Jason ground-tracks are overlaid as light gray lines.

Figure 4 : Seasonal variations of the SML heat budget terms averaged over the period 2004-2008 (Faure et al., 2011)

Figure 5 : Salinity vertical minimum maps from Argo floats data. The subplots show Smin (top-left panel), S<=34.2 (top-right panel) ; 34.2<S<34.3 (bottom-left panel) and S>=34.3 (bottom-right panel) (source : Rusciano and al, 2012).
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

AbruptSLR

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Re: Trends for the Southern Ocean
« Reply #3 on: June 28, 2013, 05:33:46 PM »
The following information comes from an article that can be downloaded from:

http://www.erik.vansebille.com/science/vansebille2013a.pdf

The citation for the paper is:
van Sebille, E., P. Spence, M. R. Mazloff, M. H. England, S. R. Rintoul, and O. A. Saenko (2013), Abyssal connections of Antarctic Bottom Water in a Southern Ocean State Estimate, Geophys. Res. Lett., 40, doi:10.1002/grl.50483.

The abstract for the paper is:

"Antarctic Bottom Water (AABW) is formed in a few locations around the Antarctic continent, each source with distinct temperature and salinity. After formation, the different AABW varieties cross the Southern Ocean and flow into the subtropical abyssal basins. It is shown here, using the analysis of Lagrangian trajectories within the Southern Ocean State Estimate (SOSE) model, that the pathways of the different sources of AABW have to a large extent amalgamated into one pathway by the time it reaches 31oS in the deep subtropical basins. The Antarctic Circumpolar Current appears to play an important role in the amalgamation, as 70% of the AABW completes at least one circumpolar loop before reaching the subtropical basins. This amalgamation of AABW pathways suggests that on decadal to centennial time scales, changes to properties and formation rates in any of the AABW source regions will be conveyed to all three subtropical abyssal basins."
The following are the captions for the two attached figure (in order):

Figure 3: a) The connectivity between the four formation regions and the three subtropical ocean basins as diagnosed from the Lagrangian particles on top of the bathymetry of the SOSE model in blue shading. The legends on the bottom show the percentage of particles that forms (right legend) and ends (left legend) on each of the sections, color-coded on the map. The pie charts (the surface of which is scaled to the proportion of particles in that region) show for each of the four formation regions in which subtropical basins (green) the particles end; and for each of the three end sections in which of the four formation regions (red) the particles form. This analysis suggests that the outflow distributions on the different formation and end sections are relatively similar, indicating a merging of bottom water types within the Southern Ocean. (b) The time it takes the particles to reach the abyssal ocean at 31°S from their formation region, color-coded for the source regions. Most particles reach 31S within 100 years. (c) The number of circumpolar loops made by the particles before they reach 31S, also color-coded for the source regions. Most particles perform at least one circumpolar loop.

Figure 4. The pathway of AABW in the SOSE model, as a function of the three major source regions. The map is very similar to that of Figure 2 and shows the percentage of particles that cross through each 1degree X 1degree grid cell at some time in the 500 year integration for the particles formed in (a) eastern Antarctica, (b) the Weddell Sea, and (c) the Ross Sea. Equatorward of 60degrees S, the pathways are very similar for the three source regions (with pointwise correlations ranging from 0.74 between the Ross Sea and eastern Antarctica sources to 0.95 between the Weddell Sea and eastern Antarctica sources), indicating a high degree of amalgamation of AABW pathways in the Southern Ocean.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

AbruptSLR

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Re: Trends for the Southern Ocean
« Reply #4 on: June 29, 2013, 01:45:24 AM »
At the following website, there is a considerable amount of metadata about Antarctic investigations such as the following about the investigation of CDW on the Ross Sea continental shelf in the austal summer of 2013:

http://gcmd.nasa.gov/KeywordSearch/Titles.do?Portal=GCMD&KeywordPath=Locations%7CCONTINENT%7CANTARCTICA&MetadataType=0&lbnode=mdlb5#1

Summary
Abstract: In order to understand the role of CDW (Circumpolar Deep Water) in controlling the hydrodynamics and related biochemical processes on the continental shelf of the Ross Sea, the oceanographic research was conducted from 2013 January 19 to March 02. The vertical temperature, salinity and depth were obtained at 41 stations using CTD and Rosette water sampler.

Purpose: In order to identify the temporal and spatial distribution of CDW on the Ross shelf and estimate the heat transport and its effect on the melting of ice shelves by CDW intrusion. A total of the oceanographic investigation was conducted using ship (ARAON) in 2013.


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Data Center URL: http://www.kopri.re.kr/

Data Center Personnel
Name: JAE LIM KIM
Phone: +82-260-6083
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“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Trends for the Southern Ocean
« Reply #5 on: June 29, 2013, 05:31:12 PM »
The following summary notes that the ocean bottom pressure (as measured by GRACE) for the Southern Ocean has been measured to be increasing recently.  Increasing bottom pressure contributes to the retreat of thinning Antarctic ice sheet grounding lines.

Satellite-derived interannual ocean bottom pressure variability and its relation to sea level
by: Christopher G. Piecuch; Katherine J. Quinn and Rui M. Ponte
19 JUNE 2013, Geophysical Research Letters; DOI: 10.1002/grl.50549

Summary:
"Knowledge of the relationship between bottom pressure pb and sea level ζ is important for understanding ocean circulation and climate. We use recent Gravity Recovery and Climate Experiment (GRACE) Release-05 data along with altimetry to investigate the relationship between ζ and pb over long periods (>1 year) and large scales (>750 km). Elevated pb signals are observed over deep extratropical regions (e.g., Southern Ocean basins) and shallow or semi-enclosed areas (e.g., Indonesian and Nordic seas). In these places, considerable ζ variance is explained by pb variance. Correlation between ζ and pb is significant in many regions, including instances of significant negative correlation suggestive of active baroclinic processes. Results exemplify the good quality of GRACE Release-05 data and demonstrate that contemporary regional ζ variability cannot always be interpreted in terms of steric changes alone."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Trends for the Southern Ocean
« Reply #6 on: June 30, 2013, 03:56:38 PM »
Regarding the findings of the following referred paper; I would like to point out that while the sea surface temperature of the Southern Ocean is currently not trending upwards (resulting in more sea ice extent); the deep ocean water temperatures are trending upward and the low-level atmospheric circulation is becoming stormier; both of which contribute directly to the loss of ice mass from AIS:

Southern Ocean Sector Centennial Climate Variability and Recent Decadal Trends
By: Mojib Latif, Torge Martin, and Wonsun Park; Journal of Climate 2013; doi: http://dx.doi.org/10.1175/JCLI-D-12-00281.1

"Abstract
Evidence is presented for the notion that some contribution to the recent decadal trends observed in the Southern Hemisphere, including the lack of a strong Southern Ocean surface warming, may have originated from longer-term internal centennial variability originating in the Southern Ocean. The existence of such centennial variability is supported by the instrumental sea surface temperatures (SSTs), a multi-millennial reconstruction of Tasmanian summer temperatures from tree rings, and a millennial control integration of the Kiel Climate Model (KCM). The model variability was previously shown to be linked to changes in Weddell Sea deep convection. During phases of deep convection the surface Southern Ocean warms, the abyssal Southern Ocean cools, Antarctic sea ice extent retreats, and the low-level atmospheric circulation over the Southern Ocean weakens. After the halt of deep convection the surface Southern Ocean cools, the abyssal Southern Ocean warms, Antarctic sea ice expands, and the low-level atmospheric circulation over the Southern Ocean intensifies, consistent with what has been observed during the recent decades. A strong sensitivity of the timescale to model formulation is noted.
In the KCM, the centennial variability is associated with global average surface air temperature (SAT) changes of the order of a few tenths of a degree per century. The model results thus suggest that internal centennial variability originating in the Southern Ocean should be considered in addition to other internal variability and external forcing when discussing the climate of the 20th century and projecting that of the 21st century."

I am also including the attached figure in this post indicating that the frontal zones of the circulation patterns in the Southern Ocean are trending southward; which will also increase ice mass loss from the AIS.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

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Re: Trends for the Southern Ocean
« Reply #7 on: June 30, 2013, 04:21:24 PM »
The source of attached images by Fyfe can be downloaded at:

http://www.clivar.org/sites/default/files/Fyfe.pdf

The first and second attached image shows how with increasing atmospheric CO₂ concentration; the zonal wind stress and the residual overturning, respectively; both increase and migrate southward.  Both of these trends would result in an increase in ice mass loss from AIS.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

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Re: Trends for the Southern Ocean
« Reply #8 on: June 30, 2013, 04:55:51 PM »
I have mentioned the DIMES (Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean) program in previous posts.  If you want to review an overview of that study you can go to the following link:

http://dimes.ucsd.edu/

One of the participants in this study is John Marshall (MIT) who states: “The parcel of tracer dropped in the Southern Ocean will take about 20 to 30 years to circulate around the Antarctic–this is a multi-decadal endeavor, …”, furthermore, the attached image shows how far tracers have migrated in one year.  This indicates that the large amount of Ocean Heat Content, OHC, now entering the ocean in the tropics of the Pacific, Indian and Atlantic Oceans, will take several decades to fully impact the AIS ice mass loss (in other words the impact of the current El Nino hiatus period will be felt for decades to come).

For more details see the story at the following link:

http://oceans.mit.edu/featured-stories/dimes-title
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Trends for the Southern Ocean
« Reply #9 on: June 30, 2013, 06:08:59 PM »
I have previously posted about, but I am providing an additional review the Marshall & Speer 2012 paper that can be found at the following site:

http://www-pord.ucsd.edu/~ltalley/sio219/marshall_speer_natgeo2012.pdf

Closure of the meridional overturning circulation through Southern Ocean upwelling
By: John Marshall and Kevin Speer; NATURE GEOSCIENCE j VOL 5 j MARCH 2012 j www.nature.com/naturegeoscience

The first attached image from this paper shows an overview of the global MOC

The second attached image show a north-south cross section through the MOC with air-sea flux patterns.

The third attached images shows an idealized simulation of the ACC.

The fourth attached image elaborates on the wind – sea energy flux relationship in the Southern Ocean.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Trends for the Southern Ocean
« Reply #10 on: June 30, 2013, 06:10:53 PM »
The following two attached images are a continuation of the immediately prior post:

The first attached image illustrates the dynamic relationships contributing to residual circulation around the Southern Ocean per Marshall & Speer 2012

The second attached image shows hydrography for selected sections of the Southern Ocean per Marshall & Speer 2012
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

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Re: Trends for the Southern Ocean
« Reply #11 on: July 01, 2013, 05:29:41 AM »
The following two references and associated abstracts (from the website below) indicate: (a) the CDW entering the troughs leading to PIG have increased in volume between 2000 and 2010; and (b) the rate of increase of CO2 being absorbed by the AABW from the Weddell Sea area is not keeping pace with the rate anthropogenic increase of CO2; respectively.
 
http://www.journals.elsevier.com/deep-sea-research-part-i-oceanographic-research-papers/recent-articles/

From circumpolar deep water to the glacial meltwater plume on the eastern Amundsen Shelf
Y. Nakayama | M. Schröder | H.H. Hellmer
Deep Sea Research Part I: Oceanographic Research Papers; Volume 77, July 2013, Pages 50–62

"Abstract: The melting of Pine Island Ice Shelf (PIIS) has increased since the 1990s, which may have a large impact on ice sheet dynamics, sea-level rise, and changes in water mass properties of surrounding oceans. The reason for the PIIS melting is the relatively warm (∼1.2°C) Circumpolar Deep Water (CDW) that penetrates into the PIIS cavity through two submarine glacial troughs located on the Amundsen Sea continental shelf. In this study, we mainly analyze the hydrographic data obtained during ANTXXVI/3 in 2010 with the focus on pathways of the intruding CDW, PIIS melt rates, and the fate of glacial meltwater. We analyze the data by dividing CTD profiles into 6 groups according to intruding CDW properties and meltwater content. From this analysis, it is seen that CDW warmer than 1.23°C (colder than 1.23°C) intrudes via the eastern (central) trough. The temperature is controlled by the thickness of the intruding CDW layer. The eastern trough supports a denser CDW layer than the water mass in Pine Island Trough (PIT). The eastern intrusion is modified on the way into PIT through mixing with the lighter and colder CDW from the central trough. Using ocean transport and tracer transport calculations from the ice shelf front CTD section, the estimated melt rate in 2010 is ∼30myr−1, which is comparable to published values. From spatial distributions of meltwater content, meltwater flows along the bathymetry towards the west. When compared with earlier (2000) observations, a warmer and thicker CDW layer is observed in Pine Island Trough for the period 2007–2010, indicating a recent thickening of the CDW intrusion."

Decline of deep and bottom water ventilation and slowing down of anthropogenic carbon storage in the Weddell Sea, 1984–2011
Oliver Huhn | Monika Rhein | Mario Hoppema | Steven van Heuven
Deep Sea Research Part I: Oceanographic Research Papers; Volume 76, June 2013, Pages 66–84

"Abstract: We use a 27 year long time series of repeated transient tracer observations to investigate the evolution of the ventilation time scales and the related content of anthropogenic carbon (Cant) in deep and bottom water in the Weddell Sea. This time series consists of chlorofluorocarbon (CFC) observations from 1984 to 2008 together with first combined CFC and sulphur hexafluoride (SF6) measurements from 2010/2011 along the Prime Meridian in the Antarctic Ocean and across the Weddell Sea. Applying the Transit Time Distribution (TTD) method we find that all deep water masses in the Weddell Sea have been continually growing older and getting less ventilated during the last 27 years. The decline of the ventilation rate of Weddell Sea Bottom Water (WSBW) and Weddell Sea Deep Water (WSDW) along the Prime Meridian is in the order of 15–21%; the Warm Deep Water (WDW) ventilation rate declined much faster by 33%. About 88–94% of the age increase in WSBW near its source regions (1.8–2.4 years per year) is explained by the age increase of WDW (4.5 years per year). As a consequence of the aging, the Cant increase in the deep and bottom water formed in the Weddell Sea slowed down by 14–21% over the period of observations."
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Re: Trends for the Southern Ocean
« Reply #12 on: July 01, 2013, 04:47:03 PM »
The CHIP3 & 5 Ocean Heat Uptake (delta Ocean Heat Content, OHC), OHU projections presented in the following paper are probably non-conservative from the point of view of public safety; they clearly indicate the trend of increasing OHU into the CDW through 2090 (for SRES A1B).

Ocean heat uptake and its consequences for the magnitude of sea level rise and climate change
by: T. Kuhlbrodt and J. M. Gregory; GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L18608, doi:10.1029/2012GL052952, 2012

Abstract
"Under increasing greenhouse gas concentrations, ocean heat uptake moderates the rate of climate change, and thermal expansion makes a substantial contribution to sea level rise. In this paper we quantify the differences in projections among atmosphere-ocean general circulation models of the Coupled Model Intercomparison Project in terms of transient climate response, ocean heat uptake efficiency and expansion efficiency of heat. The CMIP3 and CMIP5 ensembles have statistically indistinguishable distributions in these parameters. The ocean heat uptake efficiency varies by a factor of two across the models, explaining about 50% of the spread in ocean heat uptake in CMIP5 models with CO₂ increasing at 1%/year. It correlates with the ocean globalmean vertical profiles both of temperature and of temperature change, and comparison with observations suggests the models may overestimate ocean heat uptake and underestimate surface warming, because their stratification is too weak. The models agree on the location of maxima of shallow ocean heat uptake (above 700 m) in the Southern Ocean and the North Atlantic, and on deep ocean heat uptake (below 2000 m) in areas of the Southern Ocean, in some places amounting to 40% of the top-to-bottom integral in the CMIP3 SRES A1B scenario. The Southern Ocean dominates global ocean heat uptake; consequently the eddyinduced thickness diffusivity parameter, which is particularlyinfluential in the Southern Ocean, correlates with the ocean heat uptake efficiency. The thermal expansion produced by ocean heat uptake is 0.12 m YJ_1, with an uncertainty of about 10% (1 YJ = 1024 J)."

Figure 4a & b is presented in the first attachment while Figure 4c and the figure caption is presented in the second attachment; and the following text indicates how these figures for delta OHC (or OHU) were determined:

"The ensemble-mean top-to-bottom integrated OHU is shown in Figure 4a. It was calculated as the difference between the 20-year averages 2080–2099 and 1980–1999. It is largest in the Southern Ocean, in a band around 40_S, with maxima in the Argentine Basin and south of Africa. This leads to a clear signal in steric sea level rise [cf. Pardaens et al., 2011, Figure 2], which is predominantly thermosteric in the Southern Ocean. The models agree on these features
(R > 1, thin black contours), and they are also visible in the top 700 m alone (Figure 4b), which accounts for up to 50% of the heat uptake in the full depth.  OHU below 2000 m is substantial in several large areas of the Southern Ocean (Figure 4c), including the Argentine basin and the area west of the Drake Passage, where there are maxima of top-to-bottom OHU. The pattern bears resemblance to observations [Purkey and Johnson, 2010].  In these areas, the deep OHU can amount to up to 40% of the total. In the deep-water formation areas in the Southern Ocean and in the North Atlantic the ensemble mean OHU displays minima above 700 m. The models show a large spread in these areas (R < 1)."
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Re: Trends for the Southern Ocean
« Reply #13 on: July 01, 2013, 05:14:32 PM »
I believe that the pdf for the following reference is small enough to be attached to this post:

Recent Changes in the Ventilation of the Southern Oceans
Darryn W. Waugh et al.; Science 339, 568 (2013); DOI: 10.1126/science.1225411

While the conclusions to this paper indicate some scientific reticience; nevertheless, the documented trends in Southern Ocean ventilation are clearly of concern from a hazard assessment point of view.
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Re: Trends for the Southern Ocean
« Reply #14 on: July 01, 2013, 11:22:31 PM »
The following links provide information on the June 2013 insurance industry report entitled:

Warming of the Oceans and Implications for the (Re)insurance Industry - A Geneva Association Report
http://www.earthtechling.com/2013/06/climate-change-not-something-insurance-is-likely-to-cover/

https://www.genevaassociation.org/media/616661/GA2013-Warming_of_the_Oceans.pdf

This report states: "Ocean warming from climate change could make some parts of the world “uninsurable,” …"
While this report does not specifically address the risk of contributions from the AIS to sea level rise, SLR, it does indicate that flood indundation associated with both storms and SLR will be a major contributor to making areas at risk of flood inundation "uninsurable".  Of particular concern from this report is the attached figure that indicates that the Indian Ocean (in addition to the Pacific & Atlantic Oceans) is a major contributor the the increase of ocean heat content in the Southern Ocean between 1955 and 2010.
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Re: Trends for the Southern Ocean
« Reply #15 on: July 04, 2013, 12:27:01 AM »
I thought that I would post the following CDW related abstracts without comment, other than none of them disagree with any of the points that I have made in this folder; and they all elaborate on this complex matter:


Weddell SeaWarm DeepWater and the Southern Annular Mode
Sunke Schmidtko, Karen Heywood, Andrew Thompson, and Adrian Matthews
Geophysical Research Abstracts; Vol. 15, EGU2013-11919, 2013

Abract:
"Weddell Sea Warm Deep Water (WDW) circulates anti-cyclonically in the Weddell gyre, cooling as it does so.  WDW is fed by Circumpolar Deep Water (CDW) that bifurcates between 40oE and 80oE from the Antarctic Circumpolar Current (ACC). WDW is remarkably stable in its theta and salinity properties with no statistically significant trend over the last three decades. Small decadal variability is superimposed. A different state of the Weddell Sea gyre is found in the late-70ies and early-80ies, with significant colder temperatures downstream of the CDW bifurcation. The decadal variations are correlated with the Southern Annular Mode (SAM), in particular the meridional atmospheric pressure gradient in the Atlantic sector.  We propose this occurs due to a reduction in the supply of new WDW into the Weddell gyre during times of negative SAM. Impact of the WDW-SAM connection on AABW and polynya formation is discussed."


Sensitivity of Circumpolar Deep Water Transport and Ice Shelf Basal Melt along the West Antarctic Peninsula to Changes in the Winds
Dinniman, Michael S., John M. Klinck, Eileen E. Hofmann, 2012:. J. Climate, 25, 4799–4816. doi: http://dx.doi.org/10.1175/JCLI-D-11-00307.1

Abstract :
"Circumpolar Deep Water (CDW) can be found near the continental shelf break around most of Antarctica. Advection of this relatively warm water (up to 2°C) across the continental shelf to the base of floating ice shelves is thought to be a critical source of heat for basal melting in some locations. A high-resolution (4 km) regional ocean–sea ice–ice shelf model of the west Antarctic Peninsula (WAP) coastal ocean was used to examine the effects of changes in the winds on across-shelf CDW transport and ice shelf basal melt. Increases and decreases in the strength of the wind fields were simulated by scaling the present-day winds by a constant factor. Additional simulations considered effects of increased Antarctic Circumpolar Current (ACC) transport. Increased wind strength and ACC transport increased the amount of CDW transported onto the WAP continental shelf but did not necessarily increase CDW flux underneath the nearby ice shelves. The basal melt underneath some of the deeper ice shelves actually decreased with increased wind strength. Increased mixing over the WAP shelf due to stronger winds removed more heat from the deeper shelf waters than the additional heat gained from increased CDW volume transport. The simulation results suggest that the effect on the WAP ice shelves of the projected strengthening of the polar westerlies is not a simple matter of increased winds causing increased (or decreased) basal melt. A simple budget calculation indicated that iron associated with increased vertical mixing of CDW could significantly affect biological productivity of this region."



Seasonal inflow of warm water onto the southern Weddell Sea continental shelf, AntarcticaÅrthun, Marius; Nicholls, Keith W.; Makinson, Keith; Fedak, Michael A.; Boehme, Lars
Geophysical Research Letters, Volume 39, Issue 17, CiteID L17601; DOI: 10.1029/2012GL052856
http://onlinelibrary.wiley.com/doi/10.1029/2012GL052856/abstract

Abstract:
"To capture the austral summer to winter transition in water mass properties over the southern Weddell Sea continental shelf and slope region, 19 Weddell seals were tagged with miniaturized conductivity-temperature-depth sensors in February 2011. During the following 8 months the instruments yielded about 9000 temperature-salinity profiles from a previously undersampled area. This allows, for the first time, a description of the seasonality of warm water intrusions onto the shelf, as well as its southward extent towards the Filchner Ice Shelf. A temperature section across the Filchner Depression and eastern shelf shows a pronounced decrease in warm water inflow from summer to winter, further supported by an almost 3-year long time series from a shelf-break mooring. The seasonal variability is related to the surface wind stress and an associated deepening of the off-shelf core of Warm Deep Water."


Modelling the impact of ocean warming on melting and water masses of ice shelves in the Eastern Weddell Sea
Malte Thoma, Klaus Grosfeld, Keith Makinson, Manfred A. Lange

Ocean Dynamics, June 2010, Volume 60, Issue 3, pp 479-489

Abstract:
"The Eastern Weddell Ice Shelves (EWIS) are believed to modify the water masses of the coastal current and thus preconditions the water mass formation in the southern and western Weddell Sea. We apply various ocean warming scenarios to investigate the impact on the temperature-salinity distribution and the sub-ice shelf melting in the Eastern Weddell Sea. In our numerical experiments, the warming is imposed homogeneously along the open inflow boundaries of the model domain, leading to a warming of the warm deep water (WDW) further downstream. Our modelling results indicate a weak quadratic dependence of the melt rate at the ice shelf base on the imposed amount of warming, which is consistent with earlier studies. The total melt rate has a strong dependence on the applied ocean warming depth. If the warming is restricted to the upper ocean (above 1,000 m), the water column (aside from the mixed surface layer) in the vicinity of the ice shelves stabilises. Hence, reduced vertical mixing will reduce the potential of Antarctic Bottom Water formation further downstream with consequences on the global thermohaline circulation. If the warming extends to the abyss, the WDW core moves significantly closer to the continental shelf break. This sharpens the Antarctic Slope Front and leads to a reduced density stratification. In contrast to the narrow shelf bathymetry in the EWIS region, a wider continental shelf (like in the southern Weddell Sea) partly protects ice shelves from remote ocean warming. Hence, the freshwater production rate of, e.g., the Filchner-Ronne Ice Shelf increases much less compared with the EWIS for identical warming scenarios. Our study therefore indicates that the ice-ocean interaction has a significant impact on the temperature-salinity distribution and the water column stability in the vicinity of ice shelves located along a narrow continental shelf. The effects of ocean warming and the impact of increased freshwater fluxes on the circulation are of the same order of magnitude and superimposed. Therefore, a consideration of this interaction in large-scale climate studies is essential."
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Re: Trends for the Southern Ocean
« Reply #16 on: July 07, 2013, 05:27:40 PM »
The attached image from the following reference show both the extent of the recent southward migration of the Southern Hemisphere westerlies over the indicated 30-yr period, and the relative strength of the Agulhas Current and its interaction with the Southern Ocean.  While the Subtropical Front, STF, may or may not be migrating southward, eddies from the southern branch of Southern Hemisphere gyres (including the Agulhas Current) feed heat into the Southern Ocean across the STF, and with increasing global warming this heat transfer can be expected to accelerate:

The Location and Variability of Southern Ocean Fronts
By Robert M. Graham, 2013, Thesis, Stockholm

The caption for the accompanying figure is:
"Agulhas leakage affected by westerly winds and position of subtropical front. Schematic of the greater Agulhas system embedded in the Southern Hemisphere supergyre. Background colours show the mean subtropical gyre circulation, depicted by climatological dynamic height integrated between the surface and 2,000 dbar, from the CARS database [Ridgway and Dunn, 2007]. Black arrows and labels illustrate significant features of the flow. An outline of the Southern Hemisphere supergyre is given by the grey dashed line. The plot on the right shows the southward expansion of the Southern Hemisphere westerlies over a 30-yr period, from the CORE2 wind stress [Large and Yeager, 2004] averaged between longitudes 20uE and 110uE (Indian Ocean sector). The expected corresponding southward shift of the subtropical front is illustrated by red dashed arrows and would affect Agulhas leakage (shown as eddies) and the pathway between leakage and the AMOC, which is highlighted with a red box. (from Beal et al. [2011])"
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Re: Trends for the Southern Ocean
« Reply #17 on: July 10, 2013, 06:48:32 PM »
I thought that I would post the first attached image to show how the Peru (Humboldt) Current draws cold water from the Southern Ocean and into the South Pacific Gyre; while the second attached image shows how eddies introduce a large amount of heat into the Southern Ocean particularly from the Indian Gyre.
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Re: Trends for the Southern Ocean
« Reply #18 on: July 15, 2013, 03:29:09 AM »
A few points of interest for those who do not yet appreciate the rapid warming of the Southern Ocean and some of the consequences:

- Antarctica have warmed at a rate twice the global average over the last few decades.

- The ocean surrounding Antarctica soaks up about 40 percent of the oceans’ anthropogenic carbon and transports much of it to the deep sea, where the element can remain for centuries. In some parts of the Southern Ocean, increase wind speeds are bringing deep water to the surface faster than they once did, raising concerns about the release of sequestered carbon; and it is possible that stronger winds could also disrupt the ocean currents in the Antarctic that circulate heat to other parts of the world.

From an article entitled: Weddell Sea Warm Deep Water and the Southern Annular Mode; by Schmidtko, Sunke; Heywood, Karen; Thompson, Andrew; Matthews, Adrian

"Abstract: Weddell Sea Warm Deep Water (WDW) circulates anti-cyclonically in the Weddell gyre, cooling as it does so. WDW is fed by Circumpolar Deep Water (CDW) that bifurcates between 40°E and 80°E from the Antarctic Circumpolar Current (ACC). WDW is remarkably stable in its theta and salinity properties with no statistically significant trend over the last three decades. Small decadal variability is superimposed. A different state of the Weddell Sea gyre is found in the late-70ies and early-80ies, with significant colder temperatures downstream of the CDW bifurcation. The decadal variations are correlated with the Southern Annular Mode (SAM), in particular the meridional atmospheric pressure gradient in the Atlantic sector. We propose this occurs due to a reduction in the supply of new WDW into the Weddell gyre during times of negative SAM. Impact of the WDW-SAM connection on AABW and polynya formation is discussed."
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Re: Trends for the Southern Ocean
« Reply #19 on: July 16, 2013, 01:50:48 AM »
Note that according to the following article the changes in the Antarctic Ozone has resulting in major changes in the age of the Circumpolar Deep Water, CDW:

Recent Changes in the Ventilation of the Southern Oceans;
by: Darryn W. Waugh, Francois Primeau, Tim DeVries, Mark Holzer; Science 1 February 2013: Vol. 339 no. 6119 pp. 568-570; DOI: 10.1126/science.1225411

Abstract:
"Surface westerly winds in the Southern Hemisphere have intensified over the past few decades, primarily in response to the formation of the Antarctic ozone hole, and there is intense debate on the impact of this on the ocean's circulation and uptake and redistribution of atmospheric gases. We used measurements of chlorofluorocarbon-12 (CFC-12) made in the southern oceans in the early 1990s and mid- to late 2000s to examine changes in ocean ventilation. Our analysis of the CFC-12 data reveals a decrease in the age of subtropical subantarctic mode waters and an increase in the age of circumpolar deep waters, suggesting that the formation of the Antarctic ozone hole has caused large-scale coherent changes in the ventilation of the southern oceans."
Furthermore, the attached image indicates that while the Antarctic Ozone hole may be healing, it is doing so slowly; so the recent changes (that have contributing the recent increase in ice melting) in the CDW are not likely to go away any time soon, and they may get worse if the atmospheric methane concentration continues to increase:

From:

http://ozonewatch.gsfc.nasa.gov/

Note that per NASA: "The Dobson Unit (DU) is the unit of measure for total ozone. If you were to take all the ozone in a column of air stretching from the surface of the earth to space, and bring all that ozone to standard temperature (0 °Celsius) and pressure (1013.25 millibars, or one atmosphere, or “atm”), the column would be about 0.3 centimeters thick. Thus, the total ozone would be 0.3 atm-cm. To make the units easier to work with, the “Dobson Unit” is defined to be 0.001 atm-cm. Our 0.3 atm-cm would be 300 DU."
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Re: Trends for the Southern Ocean
« Reply #20 on: July 17, 2013, 05:56:24 PM »
Meehl et al (2013) (see abstract below and attached image) is an update to their previous work, and the authors show that accelerated warming decades are associated with the positive phase of the IPO. This is a result of a weaker wind-driven ocean circulation, when a large decrease in heat transported to the deep ocean allows the surface ocean to warm quickly, and this in turn raises global surface temperatures.

Journal of Climate 2013 ; e-View doi: http://dx.doi.org/10.1175/JCLI-D-12-00548.1
Externally forced and internally generated decadal climate variability associated with the Interdecadal Pacific Oscillation
by: Gerald A. Meehl, Aixue Hu, Julie Arblaster, John Fasullo, and Kevin E. Trenberth

"Abstract
Globally averaged surface air temperatures in some decades show rapid increases (accelerated warming decades) and in other decades there is no warming trend (hiatus decades). A previous study showed that the net energy imbalance at the top of atmosphere of about 1 Wm-2 is associated with greater increases of deep ocean heat content below 750m during the hiatus decades while there is little globally averaged surface temperature increase or warming in the upper ocean layers. Here we examine processes involved with accelerated warming decades, and address the relative roles of external forcing from increasing greenhouse gases and internally generated decadal climate variability associated with Interdecadal Pacific Oscillation (IPO). Model results from CCSM4 show that accelerated warming decades are characterized by rapid warming of globally averaged surface air temperature and greater increases of heat content in the upper ocean layers and less heat content increase in the deep ocean, opposite to the hiatus decades. In addition to contributions from processes potentially linked to Antarctic Bottom Water (AABW) formation and the Atlantic Meridional Overturning Circulation (AMOC), the positive phase of the IPO, adding to the response to external forcing, is usually associated with accelerated warming decades. Conversely, hiatus decades typically occur with the negative phase of the IPO, when warming from the external forcing is overwhelmed by internally generated cooling in the tropical Pacific. Internally generated hiatus periods of up to 15 years with zero global warming trend are present in the future climate simulations. This suggests that there is a chance the current observed hiatus could extend for several more years."

The attached image shows the average sea surface temperature trends from the climate model simulations for a) 'hiatus' decades, i.e. decades with no warming of global mean surface temperatures, and b) 'accelerated' decades, i.e. decades with greater-than-average rises in global surface temperatures. The subtropical ocean gyres (green ellipses) are key players in the downward transport of heat. The stippling indicates areas where this trend is statistically significant (note particularly the ASE area). From Meehl et al (2013).
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Re: Trends for the Southern Ocean
« Reply #21 on: July 17, 2013, 07:36:19 PM »
As a follow-up to the immediately preceding post (about the IPO and the El Nino Hiatus period and their influence on the ocean temperatures in the Amundsen Sea Embayment, ASE), and at the risk of slightly over simplifying matters:

The first attached image of ASE ice velocities in the pre-hiatus period from 1994-1996 shows that the Thwaites Glacier ice velocities got up to about 4km/year.

The second attached image of ASE ice velocities during the current hiatus period in 2008, shows a marked reduction in Thwaites Glacier ice velocities.

This supports my position that when the current El Nino hiatus period ends, ASE ice velocities (and particularly for the Thwaites Glacier) will likely accelerate, and thus increasing SLR.

This consideration must be evaluated when projecting any SLR trends based on GRACE data, which have only been gathered during the current El Nino hiatus peroid (note that the GRACE mission is scheduled to wind-down soon and its replacement is not scheduled until 2017, so we may be basing SLR projections on non-conservative [from a public safety point of view] GRACE data for at least the next decade [a situation which many denialists are very happy about]).
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Re: Trends for the Southern Ocean
« Reply #22 on: July 17, 2013, 11:08:41 PM »
Hello Abruptslr,
Do you mean they will destroy the GRACE  satellites soon ? When ?

Laurent

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Re: Trends for the Southern Ocean
« Reply #23 on: July 18, 2013, 01:17:37 AM »
Laurent,

I mean that the attached image from the following website indicates that the GRACE obit may decay sufficient so that it fails sometime in 2016 (while its replacement will not be available until 2017):

http://www.csr.utexas.edu/grace/operations/lifetime_plots/
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Re: Trends for the Southern Ocean
« Reply #24 on: July 18, 2013, 01:20:38 AM »

The article at the following website (and the attached graph) show that El Nino events are becoming more active (almost certainly due to anthropogenic warming), and as indicated in many of my prior posts, once the current El Nino hiatus period ends, this increasing El Nino activity could be a serious problem (ie will accelerate ice mass loss from the WAIS), particularly for the ASE glaciers.

http://phys.org/news/2013-06-el-nino-unusually-late-20th.html

The caption for the attached image is:
"This graph shows El Niño variability derived from tree rings (blue) and instrumental measurements (red). The dashed lines indicate boundary for natural variability. Recent El Niño behavior is largely beyond natural variability. Credit: International Pacific Research Center"
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Re: Trends for the Southern Ocean
« Reply #25 on: July 25, 2013, 08:51:18 AM »
I am not qualified to comment on the "non-linear" implications of changes in the Antarctic Intermediate Water (AAIW, see the link below for a discussion of this zone of ocean current that I have not discussed previously), particularly from cooler, fresher AAIW (presumably from possible future ice melting from West Antarctica and the Antarctic Peninsula) projected to result in "… changes in meridional ocean heat transport, along with surfacing anomalies, cause basin-wide changes in the surface ocean and overlying atmosphere on multi-decadal time-scales."  Nevertheless, these non-linear changes on multi-decadal time-scales sound like they have serious implications that will be non-reversible for a long time to come.

http://en.wikipedia.org/wiki/Antarctic_Intermediate_Water

Non-linear climate responses to changes in Antarctic Intermediate Water
vy: Jennifer A. Graham;  David P. Stevens & Karen J. Heywood; Journal of Climate, 2013; doi: http://dx.doi.org/10.1175/JCLI-D-12-00767.1 

Abstract
"The global impact of changes in Antarctic Intermediate Water (AAIW) properties is demonstrated using idealized perturbation experiments in a coupled climate model. Properties of AAIW were altered between 10 and 20°S in the Atlantic, Pacific and Indian oceans separately. Potential temperature was changed by ±1°C, along with density-compensating changes in salinity. For each of the experiments, sea surface temperature responds to changes in AAIW, when anomalies surface at higher latitudes (> 30°). Anomalous sea-to-air heat fluxes leave density anomalies in the ocean, resulting in non-linear responses to opposite sign perturbations. In the Southern Ocean, these affect the meridional density gradient, leading to changes in Antarctic Circumpolar Current transport. The response to cooler, fresher AAIW is both greater in magnitude and significant over a larger area than that for warmer, saltier AAIW. The North Atlantic is particularly sensitive to cool, fresh perturbations, with density anomalies causing reductions in the meridional overturning circulation of up to 1 Sv. Resultant changes in meridional ocean heat transport, along with surfacing anomalies, cause basin-wide changes in the surface ocean and overlying atmosphere on multi-decadal time-scales."
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Re: Trends for the Southern Ocean
« Reply #26 on: July 25, 2013, 09:01:03 AM »
The following reference provides further evidence of the various comments that I have made about AABW:

Decadal freshening of the Antarctic Bottom Water exported from the Weddell Sea
by: Loïc Jullion,  Alberto C. Naveira Garabato, Michael P. Meredith, Paul R. Holland, Peggy Courtois, &  Brian A. King; Journal of Climate 2013;  doi: http://dx.doi.org/10.1175/JCLI-D-12-00765.1

Abstract
"Recent decadal changes in Southern Hemisphere climate have driven strong responses from the cryosphere. Concurrently, there has been a marked freshening of the shelf and bottom waters across a wide sector of the Southern Ocean, hypothesised to be caused by accelerated glacial melt in response to a greater flux of warm waters from the Antarctic Circumpolar Current onto the shelves of West Antarctica. However, the circumpolar pattern of changes has been incomplete: no decadal freshening in the deep layers of the Atlantic sector had been observed. In this study, we document a significant freshening of the Antarctic Bottom Water exported from the Weddell Sea, which is the source for the abyssal layer of the Atlantic overturning circulation, and we trace its possible origin to atmospheric-forced changes in the ice shelves and sea ice on the eastern flank of the Antarctic Peninsula that include an anthropogenic component. These findings suggest that the expansive and relatively cool Weddell gyre does not insulate the bottom water formation regions in the Atlantic sector from the ongoing changes in climatic forcing over the Antarctic region."
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Re: Trends for the Southern Ocean
« Reply #27 on: August 02, 2013, 01:19:23 AM »
While the following research is over six years old it nevertheless supports idea that increasing upwelling in the Southern Ocean will contribute to some increase in CO₂ atmospheric content.

 
Wind-Driven Upwelling in the Southern Ocean and the Deglacial Rise in Atmospheric CO2
by:R. F. Anderson, S. Ali, L. I. Bradtmiller, S. H. H. Nielsen, M. Q. Fleisher, B. E. Anderson, L. H. Burckle; Science 13 March 2009: Vol. 323 no. 5920 pp. 1443-1448; DOI: 10.1126/science.1167441

"Abstract
Wind-driven upwelling in the ocean around Antarctica helps regulate the exchange of carbon dioxide (CO2) between the deep sea and the atmosphere, as well as the supply of dissolved silicon to the euphotic zone of the Southern Ocean. Diatom productivity south of the Antarctic Polar Front and the subsequent burial of biogenic opal in underlying sediments are limited by this silicon supply. We show that opal burial rates, and thus upwelling, were enhanced during the termination of the last ice age in each sector of the Southern Ocean. In the record with the greatest temporal resolution, we find evidence for two intervals of enhanced upwelling concurrent with the two intervals of rising atmospheric CO2 during deglaciation. These results directly link increased ventilation of deep water to the deglacial rise in atmospheric CO2."

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Re: Trends for the Southern Ocean
« Reply #28 on: August 05, 2013, 04:47:03 PM »
The reference article and accompanying abstract indicate that both ice mass loss and increased precipitation is freshening the ocean of the northwestern Weddell Sea continental shelf, despite the increase of salinity from increased sea ice formation in the areas of the former Larsen Ice Shelves A & B:

On the freshening of the northwestern Weddell Sea continental shelf
by: H. H. Hellmer, O. Huhn, D. Gomis, and R. Timmermann; Ocean Sci. Discuss., 7, 2013–2042, 2010; www.ocean-sci-discuss.net/7/2013/2010/; doi:10.5194/osd-7-2013-2010

"Abstract
We analysed hydrographic data from the northwestern Weddell Sea continental shelf of three austral winters (1989, 1997 and 2006) and two summers following the last winter cruise. During summer a thermal front exists at ~64o S separating cold southern waters from warm northern waters that have similar characteristics as the deep waters of the central basin of the Bransfield Strait. In winter, the whole continental shelf exhibits southern characteristics with high Neon (Ne) concentrations, indicating a significant input of glacial melt water. The comparison of the winter data at the tip of the Antarctic Peninsula, spanning a period of 17 years, shows a salinity decrease of 0.09 for the whole water column. We interpret this freshening as a reduction in salt input to the water masses being advected northward on the western Weddell Sea continental shelf.  Possible causes for the reduced winter salinification are a southward retreat of the summer sea ice edge together with more precipitation in this sector. However, the latter might have happened in conjunction with an increase in ice shelf mass loss, counteracting an enhanced salt input due to sea ice formation in coastal areas formerly occupied by Larsen A and B ice shelves."
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Re: Trends for the Southern Ocean
« Reply #29 on: August 05, 2013, 07:03:27 PM »
The image at the following link shows how much the SST has increased all around the Antarctic coastal (continental shelf) waters; which has significant implications not only for increased rates of basal ice melting from ice shelves but also probabily for the decomposition of coastal methane hydrates in these areas of increasing SST.

http://www.scientificamerican.com/article.cfm?id=graphic-science-map-shows-vast-regions-ocean-warmer
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Re: Trends for the Southern Ocean
« Reply #30 on: August 06, 2013, 01:54:56 AM »
The article at the following link indicates just how rapidly the warming of the ocean water in the Southern Hemisphere inducing the fish species to migrate further south.

http://www.heraldsun.com.au/news/national/australia8217s-warmer-waters-chasing-more-than-220-fish-species-further-south/story-fnii5v70-1226690968547
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Re: Trends for the Southern Ocean
« Reply #31 on: August 06, 2013, 03:47:24 PM »
To provide further support for the trend of increasing El Nino temperatures (from 1936 to 2010) discussed in my Reply #24; I post the accompanying image from NOAA's website showing an alternate presentation of this long-term trend (driven by anthropogenic global warming, AGW):
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Re: Trends for the Southern Ocean
« Reply #32 on: August 06, 2013, 04:36:57 PM »
According to the information at the following link, not only are the fish distribution changing in the Southern Hemisphere; but the bottom life in the coastal waters around Antartica are projected to become more dominated by algae as ice shelves retreat:

http://www.nbcnews.com/science/antarctic-sea-ice-melts-seaweed-could-smother-seafloor-6C10848682
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Re: Trends for the Southern Ocean
« Reply #33 on: August 09, 2013, 01:53:09 AM »
The first attached figure shows an Antarctic Circumpolar Wave, ACW, pattern of alternating warm and cold sea surface temperatures, SSTs.  As discussed in the linked reference from the Los Alamos National Laboratory; the ACW in the Southern Ocean appears to have a dominate influence on the ENSO.

http://www.ees.lanl.gov/staff/cal/acen.html

Antarctic Circumpolar Wave and El Nino
Chung-Chieng "Aaron" Lai and Zhen Huang
"Abstract
El Nino in the tropical eastern Pacific has profound consequences for weather around the globe. It occurs aperiodically (usually in the 2- to 9-year time frame). Prediction of El Nino events is now the focus of a major scientific initiative. The societal impacts of accurately forecasting El Nino up to a year in advance are huge, allowing economic and agricultural policy makers to adapt to short-term climate fluctuation in a beneficial way.
The El Nino cycle is the largest source of interannual climate variability on a global scale. At present, researchers know the sequence of phenomena once an El Nino event begins. But, if we want to predict El Nino events, we must know what the trigger is and where it comes from.
The cause of the El Nino cycle has been investigated extensively. So far, however, there is no overall theory that can explain all aspects of the event. An understanding of the complex processes at work to produce El Nino requires information about phenomena occurring all across the Pacific, not just its eastern boundary, the west coast of South America. Present theory says that the weakening of Walker Circulation leads to an El Nino. This happens once some water mass with warmer (than normal) sea-surface temperature (SST) comes into the eastern Pacific. But where does that water mass come from?
We hypothesize that the source of that water mass is the Southeast Pacific as part of the Southern Ocean. The Southern Ocean contains the strong eastward flow of the Antarctic Circumpolar Current (ACC). Recent investigations have found an Antarctic Circumpolar Wave (ACW). The SST anomalies associated with an ACW propagate eastward with the circumpolar flow, with a period of from 4 to 5 years and taking 8 to 10 years to encircle the South Pole. The water mass with warmer (than normal) SST is a spinoff from the ACW on the ACC. Northward-flowing Humboldt (Peru) Currents transmit the water mass towards the equator.
The main objective of this research is to expand our knowledge in the interannual climate variations that might be attributed to El Nino and ACW cycles. This will help us understand not only the complex processes at work to produce El Nino but also the role of the ACW in the global climate system. The goals of this research are (1) to answer key questions related to the occurrence, triggering mechanism, and aperiodicity of El Nino, and (2) to understand the origination of and the atmospheric and oceanic processes in the ACW. "
The caption for the second attached image is: "Simplified schematic summary of interannual variation in sea-surface temperature (Warm and Cold), atmospheric sea-level pressure (bold H and L), meridional wind stress (denoted by MWS), and sea-ice extent (grey lines), together with the mean course of the Antarctic Circumpolar Current. Heavy black arrows depict the general eastward motion of anomalies, and other arrows indicate communications between the circumpolar current and the more northerly subtropical gyres."
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Re: Trends for the Southern Ocean
« Reply #34 on: August 09, 2013, 04:39:31 PM »
In the way of additional background on the Antarctic Circumpolar Wave that I presented in my pervious post:

According to Wikipedia, some researchers now consider the ACW to be part of the ENSO.

http://en.wikipedia.org/wiki/Antarctic_Circumpolar_Wave

And from Springer publishing:

http://link.springer.com/article/10.1007%2Fs00376-011-1143-z#

Interdecadal change in the Antarctic Circumpolar Wave during 1951–2010
By: Lingen Bian and Xiang Lin, Advances in Atmospheric Sciences, May 2012, Volume 29, Issue 3, pp 464-470, Springer.

"Abstract
In this study, we defined an index of the Antarctic Circumploar Wave (ACW) and analyzed its variability for the period 1951–2010. A regime shift of the circumpolar westerly in the Southern Ocean and an interdecadal change of the ACW, which occurred around the mid-1970s, were identified. Associated with these changes, the variations of the ACW show three distinct sub-periods: 1951–1973, 1974–1980, and 1981–2010. They are characterized by different speeds, amplitudes, and wave structures. We briefly investigated possible mechanisms responsible for the different behaviors of the ACW during the three periods."
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Re: Trends for the Southern Ocean
« Reply #35 on: August 09, 2013, 05:55:48 PM »
The following linked reference makes it clear that the ACW and the ENSO reinforce each other by positive feedback mechanisms:

White, W. B., S.-C. Chen, R. J. Allan, and R. C. Stone, Positive feedbacks between the Antarctic Circumpolar Wave and the global El Niño–Southern Oscillation Wave, J. Geophys. Res., 107(C10), 3165, doi:10.1029/2000JC000581, 2002.

http://onlinelibrary.wiley.com/doi/10.1029/2000JC000581/abstract

Abstract
"Atmospheric and oceanic teleconnections link the Antarctic Circumpolar Wave (ACW) in the Southern Ocean [White and Peterson, 1996] and the global El Niño-Southern Oscillation (ENSO) wave (GEW) in the tropical Indo-Pacific Ocean [White and Cayan, 2000], both signals characterized by eastward phase propagation and 3- to 5-year- period variability. We extend the tropical standing mode of ENSO into the extratropics by regressing the Niño-3 sea surface temperature (SST) index against sea level pressure (SLP) anomalies over the globe, finding the Pacific-South America (PSA) pattern in SLP anomaly [Cai and Baines, 2001] straddling Drake Passage in the Southern Ocean. The amplitude of this PSA pattern is ∼1/3 that of the ACW in this domain and thus cannot be considered its principal driver. On the other hand, suppressing the tropical standing mode of ENSO in interannual ST (surface temperature) and SLP anomalies over the globe allows the GEW to be observed much more readily, whereupon its eastward phase propagation across the Warm Pool is found to remotely force the ACW in the eastern Pacific and western Atlantic sectors of the Southern Ocean through atmospheric teleconnections [Sardeshmukh and Hoskins, 1988] which propagate along with it. Subsequently, the ACW propagates this imposed GEW signal throughout the remainder of the Southern Ocean as a coupled wave in covarying ST and SLP anomalies, whereupon entering the Indian sector 1.5 to 2.5 years later it spawns a northern branch which takes another 1.5 to 2.5 years to propagate the ACW signal equatorward into the Warm Pool south of Indonesia. There it interferes constructively with the GEW. Thus the two forms of teleconnection, one fast and directed from the tropics to the high southern latitudes via the atmosphere and the other slow and directed from the high southern latitudes to the tropics via the ocean, complete a global circuit of 3- to 5-year duration that reinforces both the ACW and GEW and influences the tropical standing mode of ENSO."
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Re: Trends for the Southern Ocean
« Reply #36 on: August 09, 2013, 06:53:12 PM »
I thought that my immediately prior post touched on the complexities of the global atmospheric-oceanic interactions (including discussion of such concepts as the Global ENSO Wave, GEW), therefore, I thought that it might be valuable to provide the following link to a pdf which provides a good global overview focused on: "The South Atlantic and the Atlantic Meridional Overturning Circulation", which include discussions of the primary circulations in all oceans of the world including the Southern Ocean:

http://www.aoml.noaa.gov/phod/docs/2011_DSRII_Garzoli_Matano.pdf
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Re: Trends for the Southern Ocean
« Reply #37 on: August 19, 2013, 11:42:34 PM »
The following reference/abstract conclude that transient mesoscale ocean structures can significantly affect much larger atmospheric low-pressure systems that swiftly pass through the Southern Ocean; therefore, the influence of future ice melt on the freshening of the Southern Ocean will have a significant influence both on the Southern Ocean and on the local atmosphere:

http://www.nature.com/ngeo/journal/v6/n8/full/ngeo1863.html

Imprint of Southern Ocean eddies on winds, clouds and rainfall;
by: I. Frenger, N. Gruber, R. Knutti & M. Münnich; Nature Geoscience; 6, 608–612(2013)doi:10.1038/ngeo1863

Abstract:
"Owing to the turbulent nature of the ocean, mesoscale eddies are omnipresent. The impact of these transitory and approximately circular sea surface temperature fronts on the overlying atmosphere is not well known. Stationary fronts such as the Gulf Stream have been reported to lead to pronounced atmospheric changes. However, the impact of transient ocean eddies on the atmosphere has not been determined systematically, except on winds and to some extent clouds Here, we examine the atmospheric conditions associated with over 600,000 individual eddies in the Southern Ocean, using satellite data. We show that ocean eddies locally affect near-surface wind, cloud properties and rainfall. The observed pattern of atmospheric change is consistent with a mechanism in which sea surface temperature anomalies associated with the oceanic eddies modify turbulence in the atmospheric boundary layer. In the case of cyclonic eddies, this modification triggers a slackening of near-surface winds, a decline in cloud fraction and water content, and a reduction in rainfall. We conclude that transient mesoscale ocean structures can significantly affect much larger atmospheric low-pressure systems that swiftly pass by at the latitudes investigated."
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Re: Trends for the Southern Ocean
« Reply #38 on: August 21, 2013, 05:37:23 PM »
The following reference indicates that the ENSO dynamics since the 1970's cannot be explained as simple "Frequentist" extrapolation of past frequencies and amplitudes.  This supports the idea that changes in the Southern Ocean/Atmospheric system caused first by the formation of an ozone hole, and increasingly now by concentrations of atmospheric methane over Antarctica, are contributing directly to this trend change in ENSO dynamics (see replies: #24, and #33-36, in this thread):

http://onlinelibrary.wiley.com/doi/10.1002/grl.50264/abstract

The 1970's shift in ENSO dynamics: A linear inverse model perspective;
by: Christopher M. Aiken, Agus Santoso, Shayne McGregor, & Matthew H. England; Geophysical Research Letters; Volume 40, Issue 8, pages 1612–1617, 28 April 2013; DOI: 10.1002/grl.50264

Abstract:
"Inverse methods are used to investigate whether the observed changes in El Niño–Southern Oscillation (ENSO) character since the 1970's climate shift are consistent with a change in the linear ENSO dynamics. Linear Inverse Models (LIMs) are constructed from tropical sea surface temperature (SST), thermocline depth, and zonal wind stress anomalies from the periods 1958–1977 and 1978–1997. Each LIM possesses a single eigenmode that strongly resembles the observed ENSO in frequency and phase propagation character over the respective periods. Extended stochastically forced simulations using these and the LIM from the combined period are then used to test the hypothesis that differences in observed ENSO character can be reproduced without changes in the linear ENSO dynamics. The frequency and amplitude variations of ENSO seen in each period can be reproduced by any of the three LIMs. However, changes in the direction of zonal SST anomaly propagation in the equatorial Pacific cannot be explained within the paradigm of a single autonomous stochastically forced linear system. This result is suggestive of a possible fundamental change in the dynamical operator governing ENSO and supports the utility of zonal phase propagation, rather than ENSO frequency or amplitude, for diagnosing changes in ENSO dynamics."
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Re: Trends for the Southern Ocean
« Reply #39 on: August 25, 2013, 03:22:40 AM »
The following linked reference supports the concern that the Southern Ocean is losing its ability to sequester carbon such as the atmospheric CO₂being introduced by anthropogenic sources into the deep ocean; which would result in more CO2 accumulating in the atmosphere:


http://onlinelibrary.wiley.com/doi/10.1002/grl.50219/abstract


Maiti K., M. A. Charette, K. O. Buesseler, and M. Kahru (2013), An inverse relationship between production and export efficiency in the Southern Ocean, Geophys. Res. Lett., 40, 1557–1561, doi:10.1002/grl.50219

Abstract:
"In the past two decades, a number of studies have been carried out in the Southern Ocean to look at export production using drifting sediment traps and thorium-234 based measurements, which allows us to reexamine the validity of using the existing relationships between production, export efficiency, and temperature to derive satellite-based carbon export estimates in this region. Comparisons of in situ export rates with modeled rates indicate a two to fourfold overestimation of export production by existing models. Comprehensive analysis of in situ data indicates two major reasons for this difference: (i) in situ data indicate a trend of decreasing export efficiency with increasing production which is contrary to existing export models and (ii) the export efficiencies appear to be less sensitive to temperature in this region compared to the global estimates used in the existing models. The most important implication of these observations is that the simplest models of export, which predict increase in carbon flux with increasing surface productivity, may require additional parameters, different weighing of existing parameters, or separate algorithms for different oceanic regimes."
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Re: Trends for the Southern Ocean
« Reply #40 on: August 31, 2013, 01:21:03 AM »
The following reference discusses sea ice and the ocean mixed layer over the Antarctic shelf seas:


Sea ice and the ocean mixed layer over the Antarctic shelf seas;
by: A. A. Petty, P. R. Holland, and D. L. Feltham; The Cryosphere Discuss., 7, 4321-4377, 2013; www.the-cryosphere-discuss.net/7/4321/2013/; doi:10.5194/tcd-7-4321-2013


http://www.the-cryosphere-discuss.net/7/4321/2013/tcd-7-4321-2013.html

"Abstract. An ocean mixed layer model has been incorporated into the Los Alamos sea ice model CICE to investigate regional variations in the surface-driven formation of Antarctic shelf waters. This model captures well the expected sea ice thickness distribution, and produces deep (> 500 m) mixed layers in the Weddell and Ross shelf seas each winter. This results in the complete destratification of the water column in deep southern coastal regions (leading to HSSW formation) and also in some shallower regions (no HSSW formation) of these seas. Shallower mixed layers are produced in the Amundsen and Bellingshausen seas. By deconstructing the surface power input to the mixed layer, we show that the freshwater flux from sea ice growth/melt dominates the evolution of the mixed layer in all seas, with a smaller contribution from the surface heat flux. The Weddell and Ross shelf seas receive an annual surplus of energy at the surface, the Amundsen shelf sea energy input in autumn/winter is balanced by energy extraction in spring/summer, and the Bellingshausen shelf sea experiences an annual surface energy deficit, through both a low energy input in autumn/winter and the highest energy loss in spring/summer. An analysis of the sea ice mass balance demonstrates the contrasting mean ice growth, melt and export in each region. The Weddell and Ross shelf seas have the highest annual ice growth, with a large fraction exported northwards each year, whereas the Bellingshausen shelf sea experiences the highest annual ice melt, driven by the advection of ice from the northeast. A linear regression analysis is performed to determine the temporal and spatial correlations between the autumn/winter mixed layer power input and several atmospheric variables. The temporal mean Weddell and Ross autumn/winter power input shows stronger spatial correlation to several atmospheric variables compared to the Amundsen and Bellingshausen. In contrast the spatial mean autumn/winter power input shows stronger temporal correlation to several atmospheric variables, in the Amundsen and Bellingshausen. All regions show strong temporal correlation between the autumn/winter surface power input and the meridional wind speed except the Ross, which instead shows moderate correlation to the zonal wind speed. Further regressions demonstrate that this is probably due to the Ross shelf-sea geometry and impact of the ocean turning angle on ice motion, with a more zonal (eastward) wind preventing ice build up along the Cape Adare coast in the eastern Ross shelf sea, increasing ice export."


The following reference discusses the HSSW:


A model of the formation of high-salinity shelf water on polar continental shelves;
by: Robert W. Grumbine; Jounal of Geophysical Research: Oceans; Vol. 96, Issue C12, pp 22049-22062; 1991.

http://onlinelibrary.wiley.com/doi/10.1029/91JC00531/abstract;jsessionid=62CCC029B6596E17EF08B9CB0224A4C9.d01t01?systemMessage=Wiley+Online+Library+will+be+disrupted+on+31+August+from+10%3A00-12%3A00+BST+%2805%3A00-07%3A00+EDT%29+for+essential+maintenance&userIsAuthenticated=false&deniedAccessCustomisedMessage=


Abstract:

"A model of the flow and salinity fields forced by sea-surface salinity flux and wind stress curl is developed and used to examine the processes that create High-Salinity Shelf Water (HSSW). The flow field is the sum of the baroclinic geostrophic flow driven by salinity variations and a barotropic geostrophic flow driven by wind stress curl. The salinity field is controlled by advection, convection, and sea surface salinity flux associated with sea ice formation. The model domain represents the Weddell Sea or Ross Sea continental shelf without topography. To examine the relative effects of wind stress and buoyancy forcing in HSSW production, the peak polynya freezing rate in the model is varied from 0.0 to 0.30 m d−1, and the Ekman pumping derived from the wind stress curl is varied independently from 0.0 to 1.8×10−6 m s−1. The Ekman pumping was seen to control the magnitude of the circulation, while the polynya freezing rate controlled the extent of salinization in the shelf water. The flux of HSSW increases linearly with increasing Ekman pumping above 0.3×10−6 m s−1. The flux of HSSW is linear with respect to the polynya freezing rate. The modelled flux of HSSW and the flux of derived Bottom Water for present estimates of the forcings (a peak freezing rate of 0.10 m d−1 and Ekman pumping of 0.2×10−6 m s−1) agree with with the fluxes inferred from physical and chemical observations in the deep Weddell Sea by oceanographic field programs. The modelled flux of Bottom Water for the Ross Sea also agrees with observations."
« Last Edit: September 02, 2013, 01:20:01 AM by AbruptSLR »
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Re: Trends for the Southern Ocean
« Reply #41 on: September 01, 2013, 09:30:48 PM »
Robert Grumbine has a blog at moregrumbinescience.blogspot.com

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Re: Trends for the Southern Ocean
« Reply #42 on: September 02, 2013, 01:37:53 AM »
Sidd,

Safe travels, thanks for the lead to Grumbine blog; also, the following linked reference provides support for the concept (cited earlier in this thread) that the southward migration of the ACC (induced by the effects of the Antarctic ozone hole) is in particularly introducing warm CDW into the eastern branch of the Weddell Gyre; and is likely introducing more warm CDW southward wherever the ACC flows close to the coast:

http://onlinelibrary.wiley.com/doi/10.1002/grl.50526/abstract


Couldrey, M. P., L. Jullion, A. C. Naveira Garabato, C. Rye, L. Herráiz-Borreguero, P. J. Brown, M. P. Meredith, and K. L. Speer (2013), Remotely induced warming of Antarctic Bottom Water in the eastern Weddell gyre, Geophys. Res. Lett., 40, 2755–2760, doi:10.1002/grl.50526.


Abstract:
"Four repeat hydrographic sections across the eastern Weddell gyre at 30°E reveal a warming (by ~0.1°C) and lightening (by ~0.02–0.03 kg m−3) of the Antarctic Bottom Water (AABW) entering the gyre from the Indian sector of the Southern Ocean between the mid-1990s and late 2000s. Historical hydrographic and altimetric measurements in the region suggest that the most likely explanation for the change is increased entrainment of warmer mid-depth Circumpolar Deep Water by cascading shelf water plumes close to Cape Darnley, where the Indian-sourced AABW entering the Weddell gyre from the east is ventilated. This change in entrainment is associated with a concurrent southward shift of the Antarctic Circumpolar Current's (ACC) southern boundary in the region. This mechanism of AABW warming may affect wherever the ACC flows close to Antarctica."
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Re: Trends for the Southern Ocean
« Reply #43 on: September 02, 2013, 02:32:14 AM »
The linked reference provides additional data documenting the well discussed trend for the increasing warming and freshening of AABW:

http://journals.ametsoc.org/doi/abs/10.1175/JPO-D-12-0182.1

 
Deep Ocean Changes near the western boundary of the South Pacific Ocean
by: Bernadette M. Sloyan, Susan E. Wijffels, and Bronte Tilbrook, Katsuro Katsumata and Akihiko Murata, Alison Macdonald; Journal of Physical Oceanography 2013; doi: http://dx.doi.org/10.1175/JPO-D-12-0182.1

Abstract:
"Repeated occupations of two hydrographic sections in the Southwest Pacific Basin from the 1990’s to 2000’s track property changes of Antarctic Bottom Water (AABW). The largest property changes - warming, freshening, increase in total carbon, and decrease in oxygen - are found near the basins deep western boundary between 50°S and 20°S. The magnitude of the property changes decreases with increasing distance from the western boundary. At the deep western boundary, analysis of the relative importance of AABW (γn > 28.1 kg m −3) freshening, heating or isopycnal heave suggests that the deep ocean stratification change is the result of both warming and freshening processes. The consistent deep ocean changes near the western boundary of the Southwest Pacific Basin dispels the notion that the deep ocean is quiescent. High latitude climate variability is being directly transmitted into the deep Southwest Pacific Basin and the global deep ocean through dynamic deep western boundary currents."
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Re: Trends for the Southern Ocean
« Reply #44 on: September 08, 2013, 04:34:49 AM »
The linked reference indicates that the GRACE satellite is a valuable tool for observing short-term transport variability of the ACC:


http://onlinelibrary.wiley.com/doi/10.1029/2012JC007872/abstract;jsessionid=0BD807A4AF5649740DD46F6FAA56696F.d04t03


Bergmann, I., and H. Dobslaw (2012), Short-term transport variability of the Antarctic Circumpolar Current from satellite gravity observations, J. Geophys. Res., 117, C05044, doi:10.1029/2012JC007872.


Abstract:
"Ocean bottom pressure gradients deduced from the satellite gravity mission Gravity Recovery and Climate Experiment (GRACE) were previously shown to provide barotropic transport variations of the Antarctic Circumpolar Current (ACC) with up to monthly resolution. Here, bottom pressure distributions from GRACE with monthly (GFZ RL04) and higher temporal resolution (CNES/GRGS with 10 days, ITG-GRACE2010 with daily resolution) are evaluated over the ACC area. Even on time scales shorter than 10 days, correlations with in situ bottom pressure records frequently exceed 0.6 with positive explained variances, giving evidence that high-frequency nontidal ocean mass variability is captured by the daily ITG-GRACE2010 solutions not already included in the applied background models. Bottom pressure is subsequently taken to calculate the barotropic component of the ACC transport variability across Drake Passage. For periods longer than 30 days, transport shows high correlations between 0.4 and 0.5 with several tide gauge records along the coast of Antarctica. Still significant correlations around 0.25 are obtained even for variability with periods shorter than 10 days. Since transport variations are predominantly affected by time-variable surface winds, GRACE-based transports are contrasted against an atmospheric index of the Southern Annular Mode (SAM), which represents the Southern Hemispheric wind variability. Correlations between the SAM and GRACE-based transports are consistently higher than correlations between any of the available sea level records in all frequency bands considered, indicating that GRACE is indeed able to accurately observe a hemispherically consistent pattern of bottom pressure (and hence ACC transport) variability that is otherwise at least partially masked in tide gauge records due to local weather effects, sea ice presence and steric signals."
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Re: Trends for the Southern Ocean
« Reply #45 on: September 08, 2013, 05:21:38 PM »
The linked reference provides valuable insight about implications of Ekman layer dynamics for cross shelf transport of warm CDW into the Amundsen Sea Embayment, ASE:

http://journals.ametsoc.org/doi/abs/10.1175/JPO-D-11-041.1?journalCode=phoc

Wåhlin, A. K., R. D. Muench, L. Arneborg, G. Björk, H. K. Ha, S. H. Lee, H. Alsén, 2012: Some Implications of Ekman Layer Dynamics for Cross-Shelf Exchange in the Amundsen Sea. J. Phys. Oceanogr., 42, 1461–1474. doi: http://dx.doi.org/10.1175/JPO-D-11-041.1


Abstract:
"The exchange of warm, salty seawater across the continental shelves off West Antarctica leads to subsurface glacial melting at the interface between the ocean and the West Antarctic Ice Sheet. One mechanism that contributes to the cross-shelf transport is Ekman transport induced by along-slope currents over the slope and shelf break. An investigation of this process is applied to the Amundsen Sea shelfbreak region, using recently acquired and historical field data to guide the analyses. Along-slope currents were observed at transects across the eastern and western reaches of the Amundsen slope. Currents in the east flowed eastward, and currents farther west flowed westward. Under the eastward-flowing currents, hydrographic isolines sloped upward paralleling the seabed. In this layer, declining buoyancy forces rather than friction were bringing the velocity to zero at the seabed. The basin water in the eastern part of the shelf was dominated by water originating from 800–1000-m depth off shelf, suggesting that transport of such water across the shelf frequently occurs. The authors show that arrested Ekman layers mechanism can supply deep water to the shelf break in the eastern section, where it has access to the shelf. Because no unmodified off-shelf water was found on the shelf in the western part, bottom layer Ekman transport does not appear a likely mechanism for delivery of warm deep water to the western shelf area. Warming of the warm bottom water was most pronounced on the western shelf, where the deep-water temperature increased by 0.6°C during the past decade."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Trends for the Southern Ocean
« Reply #46 on: September 08, 2013, 05:34:40 PM »

The linked reference (with a free pdf) provides valuable insight about how CDW gets transformed into modified CDW, MCDW, at the shelf break.  Achieving a better understanding of this process is important for achieving a better understanding of the future stability of the Ross Ice Shelf, RIS:


http://www.esr.org/~padman/papers_for_peter/Kohut_etal(RossSeaMCDW).pdf

Small scale variability of the cross shelf flow over the outer shelf of the Ross Sea;
by:Josh Kohut; Elias Hunter, and Bruce Huber; 2013 American Geophysical Union; doi: 10.1002/JGRC.20090

Abstract:
"The importance of cross-shelf transport across the Ross Sea on local and remote processes has been well documented. In the Ross Sea, mid-water intrusions of Circumpolar Deep Water (CDW) are modified by shelf water near the shelf break to form Modified Circumpolar Deep Water (MCDW). In 2010-2011 we deployed multiplatform technologies focused on this MCDW intrusion in the vicinity of Mawson and Pennell Banks to better understand its role in ecosystem processes across the shelf. The high-resolution time and space sampling provided by an underwater glider, a short-term mooring, and a ship based survey highlight the scales over which these critical cross-shelf transport processes occur.  MCDW cores were observed as small-scale well-defined features over the western slopes of Pennell and Mawson Banks. The mean transport along Pennell Bank was estimated to be about 0.24 Sv, but was highly variable in time (hours to days).  The observations suggest that the core of MCDW is transported by a predominately barotropic flow that follows topography around the banks toward the south until the slope of the bank flattens and the warmer water moves up and over the bank. This pathway is shown to link the source MCDW with an area of high productivity over the shallows of Pennell Bank."

“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

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Re: Trends for the Southern Ocean
« Reply #47 on: September 08, 2013, 05:51:57 PM »
The following link provides a free pdf of the results of primarily field measurements (together with numerical model results) of water transport on the continental shelf and slope in the southeastern Weddell Sea (which will help to calibrate model projections related to the future stability of the FRIS):

http://ismer.uqar.ca/IMG/pdf/Graham_2013_JGR.pdf


Seasonal variability of water masses and transport on the Antarctic continental shelf and slope in the southeastern Weddell Sea;
by: Jennifer A. Graham, Karen J. Heywood, Cédric P. Chavanne, and Paul R. Holland; JOURNAL OF GEOPHYSICAL RESEARCH: OCEANS, VOL. 118, 1–14, doi:10.1002/jgrc.20174, 2013



Abstract:
"An array of five moorings was deployed from February 2009 to February 2010 across the Antarctic shelf and slope in the southeastern Weddell Sea (~18_W). Observations demonstrate the key processes responsible for variability in water masses and transport in the region. Rapid fluctuations in temperature and salinity throughout the year are linked with variability in wind stress over the array. This causes the deepening or shoaling of the pycnocline, past the depth of the moorings. In the upper 500 m, the seasonal cycle in salinity shows freshening in autumn, with the strongest freshening at the shallowest mooring (~250 m), furthest on-shelf. The sea ice concentration over the array exceeds 90% during this period and contributes a positive salt flux into the ocean during autumn.  Freshening begins during strong along-shore (easterly) winds in late April 2009. This demonstrates that variations in Ekman transport and wind-driven mixing play a key role in determining the salinity of shelf waters around Antarctica. Transport of the Antarctic Slope Current also shows a seasonal cycle with a maximum during late April. Model simulations show the importance of along-shore advection, as the arrival of a fresh anomaly from upstream determines the timing of the salinity minimum at the array. These processes are likely to be important for other regions around the Antarctic continent."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Trends for the Southern Ocean
« Reply #48 on: September 18, 2013, 05:33:10 PM »
Regarding the following linked reference about CMIP5 projections about Southern Ocean Circulation and Eddy Compensation:

First, it is telling that modelers are using a climate pathway where the equivalent atmospheric CO₂ reaches 1370 ppm by 2100; which acknowledges that we are currently following a BAU pathway.

Second, it should be understood that the coarse grid CMIP5 models are not good at modeling the risk of warm CDW impinging on the grounding lines of the various ice streams and glaciers in Antarctica.

Third, while the study finds that by 2100 (for the BAU pathway) that the ACC current velocities will be within 15 percent of their historical mean values; again this does not imply that more warm CDM will not be increasingly impinging on the grounding lines of the various ice streams and glaciers of the AIS as this matter is strongly influenced by such factors as: (a) the east wind drift current (or the Antarctic Coastal Current); (b) the coastal wind velocities and direction; (c) the production rates of AABW; (d) the integrity of the sea ice; (e) the southward drift of the ACC and (f) the Amundsen-Bellingshausen Seas Low [which the CMIP5 models do not capture well]:

http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-12-00504.1


Downes, Stephanie M., Andrew McC. Hogg, 2013: Southern Ocean Circulation and Eddy Compensation in CMIP5 Models. J. Climate, 26, 7198–7220. doi: http://dx.doi.org/10.1175/JCLI-D-12-00504.1


Abstract
"Thirteen state-of-the-art climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are used to evaluate the response of the Antarctic Circumpolar Current (ACC) transport and Southern Ocean meridional overturning circulation to surface wind stress and buoyancy changes. Understanding how these flows—fundamental players in the global distribution of heat, gases, and nutrients—respond to climate change is currently a widely debated issue among oceanographers. Here, the authors analyze the circulation responses of these coarse-resolution coupled models to surface fluxes. Under a future CMIP5 climate pathway where the equivalent atmospheric CO2 reaches 1370 ppm by 2100, the models robustly project reduced Southern Ocean density in the upper 2000 m accompanied by strengthened stratification. Despite an overall increase in overlying wind stress (~20%), the projected ACC transports lie within ±15% of their historical state, and no significant relationship with changes in the magnitude or position of the wind stress is identified. The models indicate that a weakening of ACC transport at the end of the twenty-first century is correlated with a strong increase in the surface heat and freshwater fluxes in the ACC region. In contrast, the surface heat gain across the ACC region and the wind-driven surface transports are significantly correlated with an increased upper and decreased lower Eulerian-mean meridional overturning circulation. The change in the eddy-induced overturning in both the depth and density spaces is quantified, and it is found that the CMIP5 models project partial eddy compensation of the upper and lower overturning cells."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Trends for the Southern Ocean
« Reply #49 on: September 18, 2013, 05:42:46 PM »
For those who want to know more about Antarctic coastal and regional currents and their interaction with warm Circumpolar Deep Water, CDW, I attach the accompanying pdf.



“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson