Support the Arctic Sea Ice Forum and Blog

Author Topic: Arctic ice divergence/compaction and Ekman Transport  (Read 11854 times)

seaicesailor

  • Guest
Arctic ice divergence/compaction and Ekman Transport
« on: July 31, 2015, 05:02:40 PM »
The idea I had of this thread (if possible) is that, rather than discussing, we collected material on Arctic ice divergence and compaction under low/high pressure systems, and associated Ekman upwelling/downwelling.
Please feel encouraged to add your input/link/reference. If possible, but not necessarily, accessible to a wide audience, written in layman terms. If we use this thread as a reference, we can stop recurrent discussions on this matter that are off-topic in other threads.

I will be collecting suggested links or references below.

- Check out the videos related to Ekman layer from A. Muncheow webpage:
http://muenchow.cms.udel.edu/html/classes/gfd/notes.html
------

- Video: Watch a 5-min. explanation of Ekman Spiral starting at 3:00-ish

------

- Chapter on the Ekman layer from a Geophysical Fluid Dynamics textbook (from the web of Andreas Muenchow department, U. Delaware):
http://muenchow.cms.udel.edu/html/classes/gfd/book/IntroGFDChapt5.pdf
It deals with both the bottom atmospheric layer, the top ocean layer, and the problem of both combined. Requires medium/advanced knowledge on Fluid Mechanics. The whole book on GFD is available from that web.
------

- This interesting (but advanced) grad course text deals with the coupling of atmospheric and ocean Ekman layers from page 26 thereon:
http://www.rsmas.miami.edu/users/isavelyev/GFD-2/Ekman
------

- Abstract on Ekman transport and Upwelling in the Arctic Ocean
http://www.whoi.edu/fileserver.do?id=92424&pt=2&p=44107
------

- Wikipedia entries
https://en.wikipedia.org/wiki/Ekman_layer
https://en.wikipedia.org/wiki/Ekman_spiral
https://en.wikipedia.org/wiki/Ekman_transport

----------------------------

- Paper: On the Link Between Arctic Sea Ice Decline and the Freshwater Content of the Beaufort Gyre: Insights From a Simple Process Model
http://goo.gl/IMgwBM

This paper deals with the dynamics of the Beaufort Gyre and associated downwelling and eddy (turbulent?) stresses; the authors develop a simple model to understand how its strength has been changing over the recent years and seasonally, caused by ice loss; reciprocally, how this change affects the ice.

A-Team has made a great summary of the paper for all of us (thank you!):

Here is the first round of notes I took on this first not-great but recent paper, just cutting out redundant verbiage and too technical parts. Then I highlighted a few core concepts that will most likely be in all Ekman pumping Arctic papers and looked for web site data displays.

Note first that no one is talking about Ekman pumping except in the Beauford Gyre. Second, this is about large scale wind patterns and seasonal drag on water and ice. Ekman pumping is balanced by something called eddy diffusion. Ekman pumping here has nothing to do with warming lower waters melting more ice but rather potentially affects North Atlantic processes.

So far, there are just a handful of ideas. With 3-4 more papers, we would have these better explained and possibly a few more additional concepts. So it is not a mammoth project but a few hours of work from where we are now.

On the link between Arctic sea ice decline and the freshwater content of the Beaufort Gyre
PED Davis, C Lique, HL Johnson - Journal of Climate, 2014
http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-14-00090.1

The rate of freshwater accumulation in the Beaufort Gyre of the Arctic Ocean has accelerated over the past decade (coinciding with the dramatic decline observed in Arctic sea ice cover) which will modify momentum transfer into the upper ocean. We find a linear relationship between the annual mean momentum flux and the amount of freshwater accumulated in the Beaufort Gyre.

A balance between Ekman pumping and eddy-induced volume flux determine the response time scale and total quantity of freshwater accumulated. Eddies are an important adjustment of the Arctic Ocean to a change in forcing. The decline in Arctic sea ice cover impacts the magnitude and seasonality of the freshwater export into North Atlantic Deep Water -- that's the potential significance.

The Arctic Ocean receives inputs of freshwater from Eurasian and North American river runoff, net precipitation over evaporation (positive P2E), and sea ice melt. The Atlantic contributes relatively salty water via Fram Strait and the Barents Sea, and the Pacific contributes relatively freshwater via the Bering Strait.

Freshwater is exported both as liquid and ice into the Nordic and Labrador Seas through Fram Strait and CAA. A moderate freshening of the surface waters in these exit regions may impact the formation of North Atlantic Deep Water and thus affect both the global thermohaline circulation and the global climate.

Within the Arctic Ocean itself, more than 70 000 km^3 of freshwater is stored within a very fresh surface layer separated by a strong halocline from relatively warm and saline Atlantic-derived layer beneath. The largest freshening occurs in the Beaufort Gyre, a permanent anticyclonic circulation driven by the winds associated with the Beaufort high. These winds cause water to converge in the center of the gyre, and the resulting downwelling (Ekman pumping) leads to an accumulation of freshwater through the mechanical deformation of the salinity field.

During an anticyclonic regime, freshwater is accumulated within the Beaufort Gyre over several years because of a strengthened atmospheric Beaufort high. In contrast, during the cyclonic regime the atmospheric Beaufort high weakens, and freshwater is released to the shelves where it may be exported into the North Atlantic. As a result, there is a strong linear relationship between the freshwater content of the Beaufort Gyre and the wind stress curl on interannual time scales.

Since 1997, the Arctic has been in the longest anticyclonic regime on record, leading to a large excess accumulation of freshwater within the Beaufort Gyre, some 8400 km^3 of freshwater between 1995 and 2012.

The positive trend in the sea surface height associated with the Beaufort Gyre and the negative trend in the curl of the wind field toward more anticyclonic have a clear spatial but not seasonal correlation, the correlation is less clear. Winds drive more effective freshwater accumulation via more efficient momentum transfer into the upper ocean.

Another theory proposes Eurasian river water have shifted their input pathways because of a positive phase of the Arctic Oscillation during 2005–08 with no role played by the Beaufort Gyre wind-driven circulation.

In a partly ice-covered Arctic, momentum transfer is determined by surface wind and icewater stress components, in proportion to sea ice concentration.

In the past, the thick and extensive Arctic sea ice cover reduced momentum transfer as large internal ice stresses reduced the ice-water stress component, shielding the ocean from direct wind forcing. However, as the sea ice cover has begun to break up and retreat farther and longer each year and the number of leads, melt ponds, and ice floe edges has increased, changing the shape of the ice pack.

A thinner and weaker sea ice cover is more easily forced by winds. The changing shape of the ice pack provides more near-vertical faces for the wind to push against. As a result, not only is the annual mean ocean surface stress increasing (net forcing), but its seasonality is also changing.

The form drag coefficient measures the efficiency of momentum transfer into the upper ocean; it exhibits a small positive trend over the Beaufort Gyre in summer between 1990 and 2012.
 
The dynamical response of the Beaufort Gyre to the thinning, weakening, and changing shape of the Arctic sea ice cover will depend upon exactly how much more stress is transferred from the surface of the ice pack to the ocean below and how the seasonal distribution changes. Processes such as stratification, atmospheric boundary layer stability, ocean circulation, and sea ice conditions also affect momentum transfer through sea ice.

The model is forced with an anticyclonic ocean surface stress centered over the domain. The magnitude of the curl of the stress field (which is proportional to the strength of the Ekman pumping) is at a maximum in the center of the domain, and is zero at the boundaries and in the outflow.

To balance the input of vorticity from the winds, the effect of eddies and diapycnal [perpendicular to isopycnal direction: surfaces of constant density not always horizontal because of wind] mixing have been incorporated.

Ekman transport driven by the anticyclonic ocean surface stress will cause water to accumulate in the center of the domain, steepening the pressure gradient and driving an anticyclonic geostrophic current. Baroclinic instability associated with this current will result in an eddy-induced bolus transport toward the boundary of the domain.

While eddies are the most important process responsible for balancing Ekman pumping , lateral friction against the Chukchi Cap [?] may play a role.

Away from the boundaries, however, Kelvin waves have little effect.

The efficiency of momentum transfer into the upper ocean is at an optimum when the sea ice concentration is approximately 80% (i.e., in fall and spring). Above this point, a thick and extensive sea ice cover damps the transfer of momentum due to the large internal ice stresses and shielding of the ocean from direct wind forcing. Below this point, the momentum transfer into the upper ocean decreases, as the drag associated with drifting sea ice is greater than that of open water.

At some point, the accelerated accumulation of freshwater in the Beaufort Gyre will stop and no longer be supported by Ekman pumping.

Fig. 8 shows the annual cycle in liquid freshwater content between both the isohaline from upward-looking sonar deployed as part of the Beaufort Gyre Exploration Project and the magnitude of the annual cycle in Ekman pumping from the ERA-Interim reanalysis.

Freshwater peaks in December–January due to stronger Ekman pumping during winter and is at a minimum during August–September due to weaker Ekman pumping and a relaxation of the salinity field.

Given the far-reaching consequences that changes in the export of freshwater to either side of Greenland may have on the circulation of the Atlantic Ocean, future studies should aim to quantify the contribution that the seasonal release of freshwater from the Beaufort Gyre has on the total freshwater export from the Arctic Ocean.

In Great Salinity Anomaly events the subpolar North Atlantic and Nordic seas underwent decadal periods freshening affecting formation of North Atlantic Deep Water. Consequently, the accelerated accumulation of freshwater in the Beaufort Gyre may exacerbate the effects of a switch to a cyclonic wind regime, by making the Arctic even more anomalously fresh beforehand, and thus increasing the quantity of freshwater that may be exported into the North Atlantic.

The total quantity of freshwater accumulated for a given change in ocean surface stress depends on the eddy diffusivity (i.e., on the length of time it takes for the eddy field to balance the change in Ekman pumping).

As Arctic sea ice is becoming weaker, thinner, and more broken up, the annually averaged momentum flux into the upper ocean is increasing for the same wind speed, resulting in an accelerated linear accumulation of freshwater through an enhanced mechanical deformation of the salinity field.

The upward-looking sonar data were collected and made available by the Beaufort Gyre Exploration Project based at the Woods Hole Oceano- graphic Institution http://www.whoi.edu/beaufortgyre

---------------------------------------------------------------------------------------------

The Seasonal Variability of the Arctic Ocean Ekman Transport and Its Role in the Mixed Layer Heat and Salt Fluxes
Jiayan Yang
http://journals.ametsoc.org/doi/full/10.1175/JCLI3892.1 free full




« Last Edit: August 08, 2015, 09:45:08 PM by seaicesailor »

6roucho

  • Frazil ice
  • Posts: 296
  • Finance geek
    • View Profile
  • Liked: 0
  • Likes Given: 1
Re: Arctic ice divergence/compaction and Ekman Transport
« Reply #1 on: July 31, 2015, 05:30:57 PM »
seaicesailor, isn't it somewhat restrictive to restrict input to people with a very solid knowledge of the matter? One of the great benefits of these forums is the opportunity for scientists and science enthusiasts from many disciplines to argue and learn. As long as the discussion remains on-topic and scientific then I'm sure that'd be fine with the admins.

seaicesailor

  • Guest
Re: Arctic ice divergence/compaction and Ekman Transport
« Reply #2 on: July 31, 2015, 06:07:18 PM »
seaicesailor, isn't it somewhat restrictive to restrict input to people with a very solid knowledge of the matter?

Yeah I edited it and removed the restriction, makes sense. Thanks

Neven

  • Administrator
  • First-year ice
  • Posts: 7899
    • View Profile
    • Arctic Sea Ice Blog
  • Liked: 1143
  • Likes Given: 566
Re: Arctic ice divergence/compaction and Ekman Transport
« Reply #3 on: July 31, 2015, 09:03:55 PM »
Thanks for opening a separate thread for this, seaicesailor (there was another one somewhere, I believe, but I couldn't find it).
Il faut comparer, comparer, comparer, et cultiver notre jardin

TerryM

  • First-year ice
  • Posts: 6002
    • View Profile
  • Liked: 892
  • Likes Given: 5
Re: Arctic ice divergence/compaction and Ekman Transport
« Reply #4 on: July 31, 2015, 09:26:07 PM »
Neven
Could there have been an article with comments at the ASI blog in 2012/13?


I recall a fairly long discussion with lots of diagrams delving into Ekman spirals, pumping etc. possibly during or after one of the major cyclonic events.


I'll try to dig out any bookmarks I may have saved.


Terry

TerryM

  • First-year ice
  • Posts: 6002
    • View Profile
  • Liked: 892
  • Likes Given: 5
Re: Arctic ice divergence/compaction and Ekman Transport
« Reply #5 on: July 31, 2015, 11:10:34 PM »

Ps. I hate the wikipedia entry for some reason, and it is not specialized to ocean ice


Is it the Wiki for Ekman Transportation or Ekman Spirals that you're referencing? Personally I've found the Ekman Spiral Wiki to be less dependant on having a strong math background.


The 2nd figure at:


http://www.whoi.edu/fileserver.do?id=92424&pt=2&p=44107


demonstrates an Ekman pump that had uplifted the halocline layer by 30 m between 1991 and 1998 in the Amundsen Basin.


This video, from about 3 min. in offers a quick (5 min.) explanation of an Ekman Spiral.





An Ekman Spiral occurs whenever wind blows across the water, however, when combined with our knowledge that the wind in a low pressure system moves in an anti-clockwise direction this 90 degree diversion explains why warm, deeper waters are drawn upward in the center as surface ice is dispersed.


While all the details of Ekman Transportation and all the effects of Coriolis forces on Arctic ice & water are far to complex for me. Familiarity with Ekman Spirals and the resulting pumping/suction have helped me to understand some of what I've been witnessing as the Arctic continues it's warming.


Terry

A-Team

  • Young ice
  • Posts: 2788
    • View Profile
  • Liked: 757
  • Likes Given: 35
Re: Arctic ice divergence/compaction and Ekman Transport
« Reply #6 on: July 31, 2015, 11:12:55 PM »
Really good idea. There are slack times when we might be developing long-range educational resources, not just following breaking news events. Wikipedia can be crazy-making in its formal treatment of the general case that never gets around to explaining special aspects of the Arctic.

This falls under physical oceanography in the academic pantheon, so I wonder if Andreas Muenchow or Rebecaa Woodgate have posted lecture notes or explanatory blogs. However they are not operating so much in the open sea as in straits. Andreas would be too busy to help in the next two months or more.

Ekman pumping is well-established physical theory so in theory does not experimental demonstration. Yet that precept has turned sour in many another situation. What, if anything, is there for us to see in satellite photos, underwater moorings, algal growth, water temperature and salinity profiles, ice movement, ice melt, wind charts, and drifting buoy data that evidences, or serves as good proxy, for actual Ekman pumping? 

On the quantitative side, is Ekman pumping really that significant in the Arctic Ocean compared to other seasonal processes that we are more familiar with, even restricting to vertical heat transport or disruption of density stratification?

How would a chart display this quantitatively, why isn't there one out there now, is it subject to re-analysis, can we make a time series, at what level does it become significant to Arctic sea ice processes?

The mice can all agree that it would be good to bell the cat. Hopefully a mouse or two can step forward. A lot could be done just with strong synthesizing skills and good taste in internet search, ie beyond just copy/paste/blather.

These forums let someone go back and edit later. So we could develop a quality resource at a fixed position on this first page with someone assimilating later stream-of-conscious musings (if warranted).
« Last Edit: August 02, 2015, 06:33:36 AM by A-Team »

seaicesailor

  • Guest
Re: Arctic ice divergence/compaction and Ekman Transport
« Reply #7 on: August 01, 2015, 04:02:26 PM »

Ps. I hate the wikipedia entry for some reason, and it is not specialized to ocean ice

Is it the Wiki for Ekman Transportation or Ekman Spirals that you're referencing? Personally I've found the Ekman Spiral Wiki to be less dependant on having a strong math background.

The 2nd figure at:

http://www.whoi.edu/fileserver.do?id=92424&pt=2&p=44107

demonstrates an Ekman pump that had uplifted the halocline layer by 30 m between 1991 and 1998 in the Amundsen Basin.
This video, from about 3 min. in offers a quick (5 min.) explanation of an Ekman Spiral.



An Ekman Spiral occurs whenever wind blows across the water, however, when combined with our knowledge that the wind in a low pressure system moves in an anti-clockwise direction this 90 degree diversion explains why warm, deeper waters are drawn upward in the center as surface ice is dispersed.

While all the details of Ekman Transportation and all the effects of Coriolis forces on Arctic ice & water are far to complex for me. Familiarity with Ekman Spirals and the resulting pumping/suction have helped me to understand some of what I've been witnessing as the Arctic continues it's warming.

Terry

Thank you Terry! I will be adding these links in the first post following the suggestion of A-team. I also have been finding useful texts, mostly from university lectures.

Really good idea [...] These forums let someone go back and edit later. So we could develop a quality resource at a fixed position on this first page with someone assimilating later stream-of-conscious musings (if warranted).

Indeed. Thank you

A-Team

  • Young ice
  • Posts: 2788
    • View Profile
  • Liked: 757
  • Likes Given: 35
Re: Arctic ice divergence/compaction and Ekman Transport
« Reply #8 on: August 02, 2015, 06:59:38 AM »
Searching Google Scholar with 'Ekman Arctic' for the most recent on-topic open source journal article, from which explanatory snippets can be compiled from the introductory paragraphs and discussion, is the best way forward. While the real beef of the article will be way too technical for us at first, we do need to properly understand wind stress curl, the very heart of the matter.

This 'gateway drug' article will also provide a very good review of previous publications,skipping over misguided earlier efforts and any conceptural errors that might have prevailed for a few years in the past, ie hit the 2014 or later button.
The first one I looked at is quite promising.

I'll let someone else pull the best snippets ...

On the link between Arctic sea ice decline and the freshwater content of the Beaufort Gyre
PED Davis, C Lique, HL Johnson - Journal of Climate, 2014
http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-14-00090.1

Andreas T

  • Nilas ice
  • Posts: 1148
    • View Profile
  • Liked: 18
  • Likes Given: 4
Re: Arctic ice divergence/compaction and Ekman Transport
« Reply #9 on: August 02, 2015, 04:46:05 PM »
Is that what you have in mind,
 the article is on research gate http://www.researchgate.net/publication/265139890_On_the_Link_Between_Arctic_Sea_Ice_Decline_and_the_Freshwater_Content_of_the_Beaufort_Gyre_Insights_From_a_Simple_Process_Model
Quote
[/On interannual time scales, Proshutinsky and Johnson (1997)
and Proshutinsky et al. (2002) have suggested
that two different wind regimes exist within the Arctic:
cyclonic and anticyclonic. During the anticyclonic re-
gime, freshwater is accumulated within the Beaufort
Gyre over several years because of a strengthened at-
mospheric Beaufort high. In contrast, during the cy-
clonic regime the atmospheric Beaufort high weakens,
and freshwater is released to the shelves where it may be
exported into the North Atlantic. As a result, there is
a strong linear relationship between the freshwater
content of the Beaufort Gyre and the wind stress curl on
interannual time scales (Proshutinsky et al. 2002, 2009).
Since 1997, the Arctic has been in the longest anticy-
clonic regime on record, leading to a large excess accu-
mulation of freshwater within the Beaufort Gyre.
quote]

seaicesailor

  • Guest
Re: Arctic ice divergence/compaction and Ekman Transport
« Reply #10 on: August 02, 2015, 08:47:16 PM »
Thank you Andreas and A-Team.
I did not find a clear reference from the google scholar search. I'll try later again

A-Team

  • Young ice
  • Posts: 2788
    • View Profile
  • Liked: 757
  • Likes Given: 35
Re: Arctic ice divergence/compaction and Ekman Transport
« Reply #11 on: August 02, 2015, 09:33:05 PM »
Here is the first round of notes I took on this first not-great but recent paper, just cutting out redundant verbiage and too technical parts. Then I highlighted a few core concepts that will most likely be in all Ekman pumping Arctic papers and looked for web site data displays.

Note first that no one is talking about Ekman pumping except in the Beauford Gyre. Second, this is about large scale wind patterns and seasonal drag on water and ice. Ekman pumping is balanced by something called eddy diffusion. Ekman pumping here has nothing to do with warming lower waters melting more ice but rather potentially affects North Atlantic processes.

So far, there are just a handful of ideas. With 3-4 more papers, we would have these better explained and possibly a few more additional concepts. So it is not a mammoth project but a few hours of work from where we are now.

On the link between Arctic sea ice decline and the freshwater content of the Beaufort Gyre
PED Davis, C Lique, HL Johnson - Journal of Climate, 2014
http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-14-00090.1

The rate of freshwater accumulation in the Beaufort Gyre of the Arctic Ocean has accelerated over the past decade (coinciding with the dramatic decline observed in Arctic sea ice cover) which will modify momentum transfer into the upper ocean. We find a linear relationship between the annual mean momentum flux and the amount of freshwater accumulated in the Beaufort Gyre.

A balance between Ekman pumping and eddy-induced volume flux determine the response time scale and total quantity of freshwater accumulated. Eddies are an important adjustment of the Arctic Ocean to a change in forcing. The decline in Arctic sea ice cover impacts the magnitude and seasonality of the freshwater export into North Atlantic Deep Water -- that's the potential significance.

The Arctic Ocean receives inputs of freshwater from Eurasian and North American river runoff, net precipitation over evaporation (positive P2E), and sea ice melt. The Atlantic contributes relatively salty water via Fram Strait and the Barents Sea, and the Pacific contributes relatively freshwater via the Bering Strait.

Freshwater is exported both as liquid and ice into the Nordic and Labrador Seas through Fram Strait and CAA. A moderate freshening of the surface waters in these exit regions may impact the formation of North Atlantic Deep Water and thus affect both the global thermohaline circulation and the global climate.

Within the Arctic Ocean itself, more than 70 000 km^3 of freshwater is stored within a very fresh surface layer separated by a strong halocline from relatively warm and saline Atlantic-derived layer beneath. The largest freshening occurs in the Beaufort Gyre, a permanent anticyclonic circulation driven by the winds associated with the Beaufort high. These winds cause water to converge in the center of the gyre, and the resulting downwelling (Ekman pumping) leads to an accumulation of freshwater through the mechanical deformation of the salinity field.

During an anticyclonic regime, freshwater is accumulated within the Beaufort Gyre over several years because of a strengthened atmospheric Beaufort high. In contrast, during the cyclonic regime the atmospheric Beaufort high weakens, and freshwater is released to the shelves where it may be exported into the North Atlantic. As a result, there is a strong linear relationship between the freshwater content of the Beaufort Gyre and the wind stress curl on interannual time scales.

Since 1997, the Arctic has been in the longest anticyclonic regime on record, leading to a large excess accumulation of freshwater within the Beaufort Gyre, some 8400 km^3 of freshwater between 1995 and 2012.

The positive trend in the sea surface height associated with the Beaufort Gyre and the negative trend in the curl of the wind field toward more anticyclonic have a clear spatial but not seasonal correlation, the correlation is less clear. Winds drive more effective freshwater accumulation via more efficient momentum transfer into the upper ocean.

Another theory proposes Eurasian river water have shifted their input pathways because of a positive phase of the Arctic Oscillation during 2005–08 with no role played by the Beaufort Gyre wind-driven circulation.

In a partly ice-covered Arctic, momentum transfer is determined by surface wind and icewater stress components, in proportion to sea ice concentration.

In the past, the thick and extensive Arctic sea ice cover reduced momentum transfer as large internal ice stresses reduced the ice-water stress component, shielding the ocean from direct wind forcing. However, as the sea ice cover has begun to break up and retreat farther and longer each year and the number of leads, melt ponds, and ice floe edges has increased, changing the shape of the ice pack.

A thinner and weaker sea ice cover is more easily forced by winds. The changing shape of the ice pack provides more near-vertical faces for the wind to push against. As a result, not only is the annual mean ocean surface stress increasing (net forcing), but its seasonality is also changing.

The form drag coefficient measures the efficiency of momentum transfer into the upper ocean; it exhibits a small positive trend over the Beaufort Gyre in summer between 1990 and 2012.
 
The dynamical response of the Beaufort Gyre to the thinning, weakening, and changing shape of the Arctic sea ice cover will depend upon exactly how much more stress is transferred from the surface of the ice pack to the ocean below and how the seasonal distribution changes. Processes such as stratification, atmospheric boundary layer stability, ocean circulation, and sea ice conditions also affect momentum transfer through sea ice.

The model is forced with an anticyclonic ocean surface stress centered over the domain. The magnitude of the curl of the stress field (which is proportional to the strength of the Ekman pumping) is at a maximum in the center of the domain, and is zero at the boundaries and in the outflow.

To balance the input of vorticity from the winds, the effect of eddies and diapycnal [perpendicular to isopycnal direction: surfaces of constant density not always horizontal because of wind] mixing have been incorporated.

Ekman transport driven by the anticyclonic ocean surface stress will cause water to accumulate in the center of the domain, steepening the pressure gradient and driving an anticyclonic geostrophic current. Baroclinic instability associated with this current will result in an eddy-induced bolus transport toward the boundary of the domain.

While eddies are the most important process responsible for balancing Ekman pumping , lateral friction against the Chukchi Cap [?] may play a role.

Away from the boundaries, however, Kelvin waves have little effect.

The efficiency of momentum transfer into the upper ocean is at an optimum when the sea ice concentration is approximately 80% (i.e., in fall and spring). Above this point, a thick and extensive sea ice cover damps the transfer of momentum due to the large internal ice stresses and shielding of the ocean from direct wind forcing. Below this point, the momentum transfer into the upper ocean decreases, as the drag associated with drifting sea ice is greater than that of open water.

At some point, the accelerated accumulation of freshwater in the Beaufort Gyre will stop and no longer be supported by Ekman pumping.

Fig. 8 shows the annual cycle in liquid freshwater content between both the isohaline from upward-looking sonar deployed as part of the Beaufort Gyre Exploration Project and the magnitude of the annual cycle in Ekman pumping from the ERA-Interim reanalysis.

Freshwater peaks in December–January due to stronger Ekman pumping during winter and is at a minimum during August–September due to weaker Ekman pumping and a relaxation of the salinity field.

Given the far-reaching consequences that changes in the export of freshwater to either side of Greenland may have on the circulation of the Atlantic Ocean, future studies should aim to quantify the contribution that the seasonal release of freshwater from the Beaufort Gyre has on the total freshwater export from the Arctic Ocean.

In Great Salinity Anomaly events the subpolar North Atlantic and Nordic seas underwent decadal periods freshening affecting formation of North Atlantic Deep Water. Consequently, the accelerated accumulation of freshwater in the Beaufort Gyre may exacerbate the effects of a switch to a cyclonic wind regime, by making the Arctic even more anomalously fresh beforehand, and thus increasing the quantity of freshwater that may be exported into the North Atlantic.

The total quantity of freshwater accumulated for a given change in ocean surface stress depends on the eddy diffusivity (i.e., on the length of time it takes for the eddy field to balance the change in Ekman pumping).

As Arctic sea ice is becoming weaker, thinner, and more broken up, the annually averaged momentum flux into the upper ocean is increasing for the same wind speed, resulting in an accelerated linear accumulation of freshwater through an enhanced mechanical deformation of the salinity field.

The upward-looking sonar data were collected and made available by the Beaufort Gyre Exploration Project based at the Woods Hole Oceano- graphic Institution http://www.whoi.edu/beaufortgyre

A-Team

  • Young ice
  • Posts: 2788
    • View Profile
  • Liked: 757
  • Likes Given: 35
Re: Arctic ice divergence/compaction and Ekman Transport
« Reply #12 on: August 04, 2015, 10:56:39 PM »
I'm looking right now at a 2006 paper from a physical oceanographer at Woods Hole. It is apparently the first paper that used actual data (wind, sea ice concentration, ice motion) for Arctic Ocean Ekman transport. The data situation is obviously much better in 2015.

This is still not the ideal paper for us but does have some useful commentary in places. Basically the Arctic Ocean is a big place, very few measurements are made in situ, and conditions affecting Ekman transport are rapidly changing. The vast majority of forums here discuss the superficial layer of ice and water whereas the three-dimensional structure of heat, salinity, influxes, currents are critical to understand.

The Seasonal Variability of the Arctic Ocean Ekman Transport and Its Role in the Mixed Layer Heat and Salt Fluxes
Jiayan Yang
http://journals.ametsoc.org/doi/full/10.1175/JCLI3892.1 free full

In the ice-covered Arctic Ocean, the surface momentum flux comes from both air–water and ice–water stresses. The data required to compute these stresses are now available from satellite and buoy observations  but no basin-scale calculation of the Ekman transport in the Arctic Ocean has been done to date.

In this study, a suite of satellite and buoy observations of ice motion, ice concentration, surface wind are used to calculate  daily Ekman transport over the whole Arctic Ocean from 1978 to 2003 at 25-km resolution, examining the relationship of seasonal variability to  surface forcing fields and discussing Ekman transport contributions to the seasonal fluxes of heat and salt to the Arctic Ocean mixed layer.

The greatest seasonal variations of Ekman transports of heat and salt occur in the southern Beaufort Sea in the fall and early winter when a strong anticyclonic wind and ice motion are present. The Ekman pumping velocity in the interior Beaufort Sea reaches as high as 10 cm day−1 in November while coastal upwelling is even stronger. The contributions of the Ekman transport to the heat and salt flux in the mixed layer are also considerable in the region.

The heat content in the oceanic mixed layer directly influences the water–ice heat flux and thus is considered to be a leading factor that determines the state of the Arctic sea ice. The oceanic heat flux to the ice in the central Arctic, estimated to be around about 2 W m−2, has been shown to be required in order to simulate the observed thickness of perennial ice.

Because of the insulation of a very stable Arctic halocline, which is replenished by water formed in the shelf regions, a major source of heat flux is likely the solar radiation to the oceanic mixed layer through open leads and thin ice. How this heat flux is redistributed by oceanic currents is unknown.

The water temperature in the mixed layer in areas  of open water or thin-ice covering is usually considerably higher than the freezing point because of both an enhanced solar radiation associated with a lower albedo and the influence of warmer Pacific and Atlantic waters that tend to flow along the boundary after entering the Arctic basin.

How can the large amount of heat stored in those areas be released or redistributed? Will intense cooling from the atmosphere in the following fall and winter remove the heat locally from the ocean or will advection transport it elsewhere to remotely influence the ice condition there?

From the perspective of physical oceanography, the Ekman-layer transport is perhaps the most fundamental field from where a more complicated three-dimensional circulation structure can be examined.

The stress that acts on the surface of the ice-covered Arctic Ocean is due to both wind and ice motion. Before the satellite remote sensing era, the ice motion data in the Arctic had been inferred from geostrophic wind  or from positioning a number of sparsely distributed drifting buoys. A lack of high-resolution and good-quality data of ice motion is perhaps a leading reason for why previous calculations of Ekman velocity and pumping rate were limited in the ice-free oceans.

This study attempts to examine the role of Ekman transport in the seasonal variability of the mixed-layer heat and salt fluxes but not higher-order dynamic oceanic processes, such as the geostrophic transport. Without comprehensive oceanographic data, the three-dimensional circulation field can only be dealt with by using ocean general circulation models.

Sea level pressure, geostrophic wind, sea ice concentration, sea ice motion, and the temperature and salinity in the upper Arctic Ocean, are used to determine seasonal time scale. Interannual variations of the Ekman transport and pumping rate aren't considered.

The ice motion, like the wind field, changes rapidly in short time scales. The whole basin-scale motion pattern can be reversed within a month. Strong cyclonic patterns of ice motion were observed in several winters although climatology indicates that the motion should be strongly anticyclonic.

A rare set of direct measurements of the Ekman spiral indicates that the depth of the frictional influence is about 20 m (Hunkins 1966). This depth should vary regionally depending on the latitude and vertical mixing processes. The salinity and potential temperature averaged in the upper 20 m can be used to represent their distributions in the Ekman layer.

The monthly salinity field reveals a basin distribution that can be characterized by the high salinity in regions to the north of the Nordic Seas where the influence from the inflow of the high-salinity Atlantic water is evident. Away from Fram Strait and Barents Sea, the influence of the Atlantic water wanes and the salinity gradually decreases. The salinity is particularly low along the Canadian/Alaskan and Siberian coasts where river runoffs are a major source of freshwater.

In the interior Arctic Ocean, there is a low salinity center in the Beaufort Sea associated with the anticyclonic Beaufort Gyre driven by the Ekman pumping. The seasonal change in the interior is generally within 1 psu but the variability in the coastal areas is much larger.

The Arctic Ocean is known to have very shallow halocline (∼50 m) and thermocline (∼100 m) depths. Thus the vertical gradients of salinity and temperature are typically large between the mixed layer and at 30 m depth. The salinity is always higher at 30 m than in the mixed layer. The greatest difference occurs in the coastal areas and especially so in the summer months. The difference is greater than 2.5 psu in those areas. Such a large gradient is mainly due to the freshening associated with runoff and sea ice melting as reflected in the mixed layer salinity distribution

The strong upwelling occurs also along the coasts and thus this strong salinity gradient does contribute to a significant salt flux to the mixed layer in those regions. The vertical temperature gradient is nearly opposite to that of salinity with subsurface water at 30 m being generally cooler than that in the surface layer. Like in the salinity case, the largest temperature gradient occurs also along the coast. This is due to the warming of the mixed layer in the summer  when sea ice cover retreats and the solar radiation increases.

The Arctic Ocean Ekman layer is forced by wind stress in the open-water areas and by ice–water stress in the ice-covered areas. The sea level pressure data are used for surface geostrophic wind.  The ice–water stress is computed by using the daily sea ice motion vectors. Neglecting the geostrophic velocity can induce considerable errors. Although geostrophic velocity is usually considered to be much smaller than ice drifting speed, it can be large along the coast, in fronts, and in Fram Strait.

With all those data, we can now calculate the Ekman layer velocity by using the textbook Ekman layer equation  The Ekman velocity is the vertically averaged velocity within the Ekman layer. The stress and the Ekman velocity are dependent on each other.

The seasonal climatology of the Ekman transport velocity eflects the surface forcing fields, the wind and ice motion. The wind  and ice motion start their anticyclonic phases in September and this transition leads to the development of the offshore Ekman transport in the fall and winter.

The Ekman transport is particularly strong along the southern boundary in the Beaufort and Chukchi Seas. The offshore transport intensifies rapidly into the winter months, reaching the maximum in November and December. But even in these two months, the transport velocity is generally weaker than 1 m2 s−1 and so the depth-averaged Ekman velocity is less than 5 cm−1. The Ekman transport in the Beaufort and Chukchi Seas weakens gradually from January to March.

Because of the migration of the high SLP center from Siberia to the Beaufort Sea  the pattern of the anticyclonic ice motion reintensified in April and May. This resulted in a second peak of offshore Ekman transport in May not only along the southern Beaufort Sea boundary but also along Russian coast. During the summer, both the surface wind and ice motion are weak. So the Ekman transport is also weak over the whole Arctic basin.

Another area with a strong seasonal variation is within and to the north of Fram Strait. The Ekman transport in this region is typically westward. It becomes the strongest in the fall and winter just like in the Beaufort Sea. This westward Ekman flow is due to the strong southward ice transport directed toward the Nordic Seas and through Fram Strait. Upwelling and downwelling field are induced by the divergence and convergence of the Ekman transport. 

The most prominent change of the Ekman velocity occurs in the Beaufort Sea, especially along the southern boundary. Starting in September, the Ekman transport starts to intensify in the southern Beaufort Sea. It is directed away from the Alaskan and Canadian coast toward the central Beaufort Sea. This offshore transport persists through the winter and spring, and peaks in a 4-month period from October to January.

This leads to a strong Ekman convergence and thus downwelling in the Beaufort Sea and coastal upwelling along the boundary in these four months. The reintensification of the Ekman transport in April and May, as discussed in the previous section, results in the reappearance of strong downwelling in May.

The maximum downwelling rate in the interior Beaufort is about 15 cm day−1 in November and December. The coastal upwelling is restricted within a narrow zone along the boundary and the upwelling velocity is much larger than that of the interior downwelling. During the summer months, the interior Beaufort Sea is still dominated by downwelling although w is very weak, less than 5 cm day−1. The seasonal variability of w in the eastern Arctic Ocean is not as well organized and also is considerably weaker except in the Laptev Sea.

Another area that shows a great seasonal variability is along Fram Strait. There is a strong westward Ekman transport from Svalbard toward Greenland between October and April. This transport is forced mainly by the strong southward sea ice motion associated with the Arctic sea ice export to the Nordic Seas. Consequently, upwelling dominates the eastern Fram Strait while downwelling persists off the Greenland’s coast (Fig. 10). This contrast becomes the most striking in the winter months when the southward ice transport is the largest.

The Ekman transport and its divergence, that is, upwelling rate, vary profoundly on the seasonal time scales. How this variability affects the heat and salt contents in the upper Arctic Ocean has seldom been examined, at least to the basin scale.

The most pronounced contribution from Ekman advection occurs in the southern Beaufort and Chukchi Seas in the fall season. The anticyclonic wind and ice motion starts in September which drives a strong offshore Ekman transport. This strong Ekman transport coincides with the period when the temperature gradient is large in the area.

In the coastal upwelling case, the water being upwelled usually originates from a bottom boundary layer that is separated from the surface Ekman layer. It is drawn from the shelf breaks, which is typically much deeper than that of Ekman-layer depth, to the coastline upward along the slope of shelves. The water tends to be considerably warmer than used in this study. In such cases, upwelling will result in a positive heat flux to the mixed layer along the coast. Similar to the heat flux calculation, we have also computed the salt flux to the surface mixed layer:

The maximum upwelling rate in this 26-yr climatology reaches as high as 15 cm day−1 in November and December in the southern Beaufort Sea. The heat advection associated with the Ekman transport is high in the Beaufort and Chukchi Seas. The maximum heat transport occurs in the fall when the offshore Ekman transport velocity is large and the mixed layer temperature gradient is also large.

Into the winter months, however, the heat flux decreases rapidly even though the Ekman transport remains strong but the mixed layer has been cooled to be nearly uniformly at the freezing point and thus the temperature gradient is small. The Ekman velocity also transports low-salinity coastal water and high-salinity Pacific Ocean water toward the Beaufort Sea in the fall and winter seasons.

The freshwater flux associated with this advection is significant and comparable with other leading freshwater fluxes such as the summer melting of sea ice. The annual precipitation–evaporation rate in the Arctic is about 15 cm yr−1.

The ice motion in the Beaufort Sea, for instance, can be opposite in the same month in two different years. So the Ekman velocity and pumping rate can be much greater in some years than the 25-yr averaged fields presented here. One would wonder whether the interannual and decadal variability of Ekman heat transport affects the sea ice condition in the regions.
« Last Edit: August 05, 2015, 03:18:14 AM by A-Team »

seaicesailor

  • Guest
Re: Arctic ice divergence/compaction and Ekman Transport
« Reply #13 on: August 08, 2015, 09:55:38 PM »
There is this book

https://books.google.es/books?id=cwP0ZAusFusC

Matti Lepparanta
"The drift of sea ice", nothing more nothing less.

It costs 200 euro, electronic version.

But in the preview it shows chapter 6 :"free drift". Interesting for what it is going on in the Beaufort sea right now. Very mathematical (anyway the preview only lets see a few pages)

I will see if I find texts or papers similar to this.

seaicesailor

  • Guest
Re: Arctic ice divergence/compaction and Ekman Transport
« Reply #14 on: August 09, 2015, 03:11:36 PM »
A-Team, time permitting I must make a summary of your summaries to place them in the opening post (there is a limitation of characters).

In the meantime, thank you! The second  paper was even more 'illuminating' for me.