Decadal changes in Atlantic overturning due to the excessive 1990s Labrador Sea convection.

Changes in the Atlantic Meridional Overturning Circulation (AMOC) represent a crucial component of Northern Hemisphere climate variability. In modelling studies decadal overturning variability has been attributed to the intensity of deep winter convection in the Labrador Sea. This linkage is challenged by transport observations at sections across the subpolar gyre. Here we report simulations with an eddy-rich ocean model which captures the observed concentration of downwelling in the northeastern Atlantic and the negligible impact of interannual variations in Labrador Sea convection during the last decade. However, the exceptionally cold winters in the Labrador Sea during the first half of the 1990s induced a positive AMOC anomaly of more than 20%, mainly by augmenting the downwelling in the northeastern North Atlantic. The remote effect of excessive Labrador Sea buoyancy forcing is related to rapid spreading of mid-depth density anomalies into the Irminger Sea and their entrainment into the deep boundary current off Greenland.