Subducted Convection
Tom Rossby
graduate School of Oceanography, University of Rhode Island
(Abstract received 08/14/2009 for session A)
ABSTRACT
A recently concluded study of the Nordic Seas hydrography from an isopycnal perspective reported that the waters in the Lofoten Basin exhibited significantly greater spiciness than the inflowing Atlantic waters from the south of the same density. It was shown that the increase in spiciness stemmed from downward mixing of overlying saltier water in wintertime. Clearly open ocean convection plays a major role in providing the mixing, especially where isopycnal surfaces outcrop in winter. But other mechanisms exist by which water can be mixed downward. These include the subduction of surface eddies that form at the Northwest Corner. Similar processes have been documented in the Sea of Japan. In the Lofoten Basin large anti-cyclonic eddies form where the Norwegian Atlantic Current passes the steep escarpment off the Lofoten Islands. These drift towards the center of the basin where they build up a huge reservoir of heat. In winter this pool loses a significant fraction of the previous summer’s build-up, a loss that leads to increased spiciness on the oncropping surfaces. Inspection of individual CTD profiles reveals what look like subducted eddies reminiscent of the lenses that originate at the Northwest Corner. In this talk I will suggest the possibility that these features result from the cooling of Lofoten eddies without their destruction, unlike what happens to eddies that form due to intense heat loss at the surface. The somewhat more general question might be whether preexisting anticyclonic eddies at the surface can serve as nuclei for coherent subsurface eddies. Next year a 2-year field program will take place in the Lofoten Basin to study the circulation and the hydrographic changes that take place over the summer-winter heating-cooling cycle. Each year 25 isopycnal RAFOS floats will be deployed to map the circulation and probe diapycnal processes if and when float experiences an outcropping process. The question is how best to deploy these and what measurement strategies will best recognize and distinguish between these diverse processes.
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