Evolution of the deep and bottom waters of the Scotia Sea, Southern Ocean, during 1995-2005

The Southern Ocean hosts the formation of the densest layers of the oceanic overturning circulation, and provides a climatically sensitive element of deep ocean ventilation. An oceanographic section across the eastern Scotia Sea occupied in 1995, 1999 and 2005 reveals significant variability in the deep and bottom waters of Southern Ocean origin. Warming (~0.1ºC) of the warm mid-layer waters in the Scotia Sea between 1995 and 1999 reversed through to 2005, reflecting changes seen earlier upstream in the Weddell Sea. The volume of deep waters with potential temperature less than 0ºC decreased during 1995-2005, though such a reduction was only clear between 1995 and 1999 at the southern end of the section. The abyssal waters of the eastern Scotia Sea apparently changed circulation between 1995 and 1999, with the dominant point of their entry to the basin shifting from the south to the northeast; by 2005, the former route had regained dominance. These changes are best explained by interannual variations in the deep waters exiting the Weddell Sea, superimposed on a longer-term (decadal) warming trend. The interannual variations are related to changes in the strength of the Weddell Gyre, reflecting large-scale atmospheric variability that may include the El Niño / Southern Oscillation phenomenon. The Scotia Sea is the most direct pathway for dense waters of the overturning circulation emanating from the Weddell Sea to fill much of the world ocean abyss. The regional changes reported here have the potential to affect the climatically significant ventilation of the global ocean abyss.

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