Seasonal cycle of the overturning circulation in the subpolar North Atlantic

Understanding the mechanisms driving variability in the Atlantic Meridional Overturning Circulation (AMOC) on different timescales is essential for better predictions of our evolving climate. The newly updated time series (August 2014 to June 2020) from OSNAP (Overturning in the Subpolar North Atlantic Program) continues to reveal strong intra-annual and interannual variability. However, this six-year record allows us, for the first time, to examine the observation-based seasonal variability of the subpolar overturning circulation. We find that the overturning peaks in late spring from April through June and reaches the minimum in winter for both OSNAP West (a section from the coast of Labrador to West Greenland) and OSNAP East (a section from East Greenland to the Scottish shelf). An analysis of seasonality in the Labrador Sea (OSNAP West) suggests that the delay between wintertime transformation and the observed overturning peak in late spring is consistent with the advection and export of dense Labrador Sea Water along the western boundary. Further analysis is required to understand the mechanism driving seasonal overturning across OSNAP East.