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
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- 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.pdfIt 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.
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- 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_layerhttps://en.wikipedia.org/wiki/Ekman_spiralhttps://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 Modelhttp://goo.gl/IMgwBMThis 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-14-00090.1The 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 w
ater 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
