Shelf-basin exchange in the Western Arctic Ocean

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Presentation transcript:

Shelf-basin exchange in the Western Arctic Ocean (R. Pickart and M. Spall) Objective: To observe and understand the processes that contribute to the exchange of water (heat, salt, carbon, etc) between the shelf and basin interior. General approach: Combine in-situ observations, idealized and more realistic models, remotely sensed data products, simple theory For today: Oceanic response to upwelling wind events Wrangel Island (Photo by D. Torres)

Shelf-break jet response to storms mooring array Chukchi Sea Beaufort Sea 0.8 Sv Beaufort Slope Mooring Array 2002-4 horizontal spacing of O(5km) (resolves the internal deformation radius)

High spatial and temporal resolution identifies several water masses Present focus is on upwelled Atlantic water First year time series at center of boundary current Winter-transformed Pacific water Alaskan Coastal water Upwelled Atlantic water potential temperature (oC)

Western Arctic Boundary Current Spring average - shelfbreak jet of Pacific water

Western Arctic Boundary Current Summer average – surface intensified warm outflow

Western Arctic Boundary Current Note deep velocity core of Atlantic Water Winter average – focus today

Storms impacting the Beaufort Sea Pacific-born storm (upwelling) Arctic-born storm (downwelling) L L Focus is on upwelling storms (predominant, 28 in 2002)

WRF model simulation of storm (3 km resolution) Composite wind speed during a storm event in Nov. 2002 Maximum winds in the Chukchi Sea, over open water Located to the west of the SBI array

Observations from SBI array Eastward flowing modified Pacific water at shelfbreak (deep core is renmant from previous storm)

Observations from SBI array Offshore Ekman transport, Atlantic Water pulled onto, shelf, strong westward flow at surface

Observations from SBI array Wind ceases, isopycnals remain steep but current reverses, driving deep eastward flow of Atlantic Water

Idealized primitive equation model Initial Condition Salinity MITgcm : 30 levels grid spacing 2-10 km Initialize at rest, blow wind for 4 days, turn off No Wind Wind No Wind 2725 km Y Shelf and slope X 3400 km

Model fields (east of wind forcing) Salinity Alongstream velocity (cm/s) 1 day after winds are turned off Ekman transport lifts isopycnals, depresses SSH near the coast, and drives a westward flow

Model fields (east of wind forcing) Salinity Alongstream velocity (cm/s) 1 day after winds are turned off Salinity is slow to adjust to change in wind but velocity adjusts rapidly – due to fast propagation of SSH Results in deep eastward flow similar to observations 3 days after winds are turned off

Storm drives anticyclonic circulation over northern Chukchi Results from a more realistic model forced by NCEP winds – 3 phases Broad scale response prior during after SSH Storm drives anticyclonic circulation over northern Chukchi Sea, eastward flow along shelf break, slow to decay

Similar reversal in Barrow Canyon, enhancement of Observed currents at various sites – 3 phases of storm prior during after Similar reversal in Barrow Canyon, enhancement of eastward flow along Chukchi shelfbreak

Summary: upwelling storms are common along north slope of Alaska in winter Ekman transport upwells Atlantic Water, drives westward flow Rapid propagation of barotropic waves results in deep eastward flow of Atlantic Water Wind stress curl (due primarily to ice) drives basin-scale anticyclonic circulation over the Chukchi Sea which is slow to decay, loss of upwelled Atlantic Water Next: momentum and tracer budgets