Atlantic Deep Water Formation Occurs Primarily in the Iceland Basin and Irminger Sea by Local Buoyancy Forcing

The Atlantic Meridional Overturning Circulation (AMOC), a key mechanism in the climate system, delivers warm and salty waters from the subtropical gyre to the subpolar gyre and Nordic Seas, where they are transformed into denser waters flowing southward in the lower AMOC limb. The prevailing hypothesis is that dense waters formed in the Labrador and Nordic Seas are the sources for the AMOC lower limb. However, recent observations reveal that convection in the Labrador Sea contributes minimally to the total overturning of the subpolar gyre. In this study, we show that the AMOC is instead primarily composed of waters formed in the Nordic Seas and Irminger and Iceland basins. A first direct estimate of heat and freshwater fluxes over these basins demonstrates that buoyancy forcing during the winter months can almost wholly account for the dense waters of the subpolar North Atlantic that are exported as part of the AMOC. Plain Language Summary The Atlantic Meridional Overturning Circulation (AMOC) is a key mechanism in the climate system, as it contributes to the uptake and storage of anthropogenic CO2 in the deep ocean. As global warming continues apace, climate scientists are concerned with the possibility of a slowing AMOC due to changes in deep water formation at high latitudes in the North Atlantic. Although wintertime conditions over the Labrador Sea have long been considered a strong predictor of downstream AMOC change, recent results in the subpolar North Atlantic have revealed that the volume of deep waters formed in the Labrador Sea is small compared with the waters carried in the AMOC, creating an unresolved question as to the source of deep waters that compose the lower AMOC limb. Here, we combine a collection of observations from the subpolar gyre and Nordic Seas to show that the Iceland Basin and Irminger Sea are the main sources for the lower limb. Additionally, we reveal that the production of deep waters can almost entirely be accounted for by wintertime air‐sea fluxes over those basins and that some of the dense waters formed via convection are stored in the Irminger and Iceland basins in subsequent months. Key Points New observations reveal Atlantic Subpolar Gyre deep water is formed primarily in the Irminger and Iceland basins by local buoyancy forcing The deep water formed by buoyancy forcing in winter is not entirely exported the following months; a portion is stored in those basins The transformation and subsequent export of deep water s