Variable northward throughput between the North Atlantic gyres and implications for overturning

The northward transport of warm, salty waters within the extended Gulf Stream system along the borders of the subtropical and subpolar gyres is climatically important. However, the role of variable gyre circulation in modulating the northward ‘throughput’ of water between subtropical and subpolar latitudes remains unknown. Here, we use the Lagrangian analysis tool TRACMASS with the 1/12° ocean reanalysis GLORYS12 to quantify variability in the northward throughput between the gyres. Lagrangian particles are seeded in the Gulf Stream at 30°N between 1993 and 2017, and tracked forward in time to quantify the volume of water recirculating within the subtropical gyre versus the throughput to the subpolar gyre. On average, 64% of the Gulf Stream water recirculates within the subtropical gyre while 36% is transported north of 45°N into the subpolar gyre within the four years of tracking. The subtropical recirculation is strongly correlated to the net volume transport at the Gulf Stream seeding section on interannual time scales, indicative of an overall stronger/weaker subtropical gyre with a strong/weak western boundary current. The subtropical-subpolar throughput is not significantly correlated to the net volume transport at the seeding section. This indicates that regional wind patterns and/or stratification exist that favours enhanced subtropical-subpolar throughput. Variable subtropical-subpolar throughput is potentially a mechanism contributing to reducing meridional coherence in the AMOC strength between subtropical and subpolar latitudes.