A Lagrangian view of seasonal overturning variability in the eastern North Atlantic subpolar gyre.

The Atlantic Meridional Overturning Circulation (AMOC) plays a critical role in the global climate system through the uptake and redistribution of heat, freshwater and carbon. At subpolar latitudes, recent observations show that the strength of the AMOC is dominated by water mass transformation in the eastern North Atlantic Subpolar Gyre (SPG). Both observations and ocean reanalyses show a pronounced seasonality of the AMOC within this region. However, the distribution of the strength and seasonality of overturning across the individual circulation pathways of the eastern SPG remains poorly understood. To investigate the nature of this seasonal overturning variability, we use Lagrangian water parcel trajectories evaluated within an eddy-permitting ocean sea-ice hindcast simulation.

By introducing a novel Lagrangian measure of the density-space overturning, we show that water mass transformation along the circulation pathways of the eastern SPG accounts for 8.9 ± 2.2 Sv (55%) of the mean strength of AMOC in the eastern subpolar North Atlantic. Our analysis highlights the crucial role of water parcel recirculation times in determining the magnitude of the strength and seasonality of overturning. We find that upper limb water parcels flowing northwards into the eastern SPG participate in a recirculation race against time to avoid wintertime diapycnal transformation into the lower limb of the AMOC. Upper limb water parcels sourced from the central and southern branches of the North Atlantic Current typically recirculate on interannual timescales (1-5 years) and thus determine the mean strength of overturning within this region. The seasonality of Lagrangian overturning is explained by a small collection of water parcels, recirculating rapidly (≤ 8.5 months) in the upper Central Iceland and Irminger Basins, whose along-stream transformation is dependent on their month of arrival into the eastern SPG.