Circulation and overturning in the eastern North Atlantic subpolar gyre

This study describes new transport estimates of the North Atlantic Current in the Iceland Basin, and uses these results along with other contemporaneous measurements to determine mass and overturning budgets for the eastern North Atlantic subpolar gyre. As part of the Overturning in the Subpolar North Atlantic Program (OSNAP), estimates of the North Atlantic Current are determined using three full-depth dynamic height moorings spanning the Iceland Basin and are supplemented by Argo and satellite altimetry data. Along with historical estimates of the exchanges over the Iceland-Scotland Ridge, additional OSNAP results from the Rockall Trough and Rockall-Hatton Bank regions are used to calculate transport budgets in different density layers over a broad portion of the eastern subpolar gyre. Results show that 13–14 Sv of the North Atlantic Current (σθ < 27.8 kg m−3) flow northward into the middle of the Iceland Basin through a primary baroclinic flow near 23.5°W and a secondary quasi-barotropic flow near 26°W. Together with the observed northward flow in the Rockall-Hatton area, we conclude that 19–20 Sv of the upper limb of the Atlantic Meridional Overturning Circulation (σθ < 27.56 kg m−3) flows into the region where nearly 40 % of it (7.3 Sv) is converted into the lower limb primarily through progressive water mass modification from atmospheric cooling. This accounts for nearly half of the strength of Atlantic Meridional Overturning Circulation defined by the full OSNAP array extending across the basin from Greenland to Scotland.