Multidecadal Warming and Density Loss in the Deep Weddell Sea, Antarctica

The World Ocean is estimated to store more than 90% of the excess energy resulting from man-made greenhouse gas-driven radiative forcing as heat. Uncertainties of this estimate are related to undersampling of the subpolar and polar regions and of the depths below 2000 m. Here we present measurements from the Weddell Sea that cover the whole water column down to the sea floor, taken by the same accurate method at locations revisited every few years since 1989. Our results show widespread warming with similar long-term temperature trends below 700-m depth at all sampling sites. The mean heating rate below 2000m exceeds that of the global ocean by a factor of about 5. Salinity tends to increase-in contrast to other Southern Ocean regions-at most sites and depths below 700 m, but nowhere strongly enough to fully compensate for the warming effect on seawater density, which hence shows a general decrease. In the top 700m neither temperature nor salinity shows clear trends. A closer look at the vertical distribution of changes along an approximately zonal and a meridional section across the Weddell Gyre reveals that the strongest vertically coherent warming is observed at the flanks of the gyre over the deep continental slopes and at its northern edge where the gyre connects to the Antarctic Circumpolar Current (ACC). Most likely, the warming of the interior Weddell Sea is driven by changes of the Weddell Gyre strength and its interaction with the ACC.