Michael Spall

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Circulation and Water Mass Transformation in a model of the Chukchi Sea

Ventilation of the Arctic Halocline by Shelfbreak Eddies

The circulation and water mass transformation in a regional ocean-ice model of the Chukchi Sea are discussed. The model has horizontal resolution of O( 4 km), is forced by fluxes derived from daily NCEP reanalysis fields, and has seasonally varying transport, temperature, and salinity imposed at Bering Strait. Many of the observed characteristics of the mean circulation and seasonal cycle in the Chukchi Sea are reproduced. The discussion focusses on: the branching of the inflow transport into pathways following Herald Canyon, Central Channel, and the Alaskan coast; the pattern of ice melt; and the water mass transformation and formation of winter water and hypersaline water. The ice melt pattern and timing is strongly influenced by advection through Bering Strait. High frequency forcing results in a larger region of ice melt, particularly over the shoals and in the northern Chukchi Sea, compared to monthly mean forcing. In the model, the seasonal cycle of salinity in the southern and central Chukchi Sea is dominated by advection through Bering Strait, while local atmospheric forcing and brine rejection are more important north of Herald and Hanna Shoals and in Barrow Canyon. However, since the residence time in the Chukchi Sea is generally less than 1 year, interannual variability in the Bering Strait salinity will be reflected in the salinity across the Chukchi Sea and at Barrow Canyon.

The Arctic halocline is a layer of cold, fresh water in the upper 200 m that overlies the deeper warm, salty waters of Atlantic Ocean origin. The maintenance of this halocline is not well understood, but is crucial for the ice cover in the Arctic, and for the basin-scale balances of heat and salt. Observations indicate that the halocline in the interior of the Beaufort Sea is populated with very small anticyclonic eddies composed of cold, fresh waters that originated on the shelf. It is speculated that these eddies are important for the maintenance of the halocline and also transport tracers and nutrients from the shelf region into the basin interior. In this project, an idealized, high resolution ocean general circulation model is used to study this eddy formation process and its possible role in the maintenance of the Arctic halocline. The model results are being compared to and evaluated against observations collected by Dr. R. Pickart (WHOI) along a high resolution mooring array just east of Barrow Canyon during 2002-2004 as part of the Shelf Basin Interaction Program. The results indicate that cold fresh waters flow from the Chukchi Sea along the shelf-break in a narrow boundary current and that this boundary current is strongly unstable and able to form numerous small-scale anticyclonic eddies, in general agreement with observations.

This work has been generously supported through grants from the National Science Foundation Office of Polar Programs.

Recent manuscripts on these subjects :

Spall, M. A., 2007. Circulation and Water Mass Transformation in a Model of the Chukchi Sea. J. Geophys. Res. ., 112, C05025,doi:10.1029/2005JC002264

Spall, M. A., R. S. Pickart, P. Fratantoni, A. Plueddemann, 2007. Western Arctic Shelfbreak Eddies: Formation and Transport. submitted: Journal of Physical Oceanography .


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