Freshwater-driven subpolar gyre cooling and atmospheric regime responses using a large ensemble climate model.

The North Atlantic subpolar gyre is a highly dynamic region where ocean–atmosphere interactions are shaped by variations in freshwater export from the Arctic, import of subtropical waters via the Gulf Stream, mixing, in addition to large-scale atmospheric circulation patterns such as the NAO and the strength and position of the North Atlantic jet. Building on our earlier analysis using the CANARI Large Ensemble (HadGEM3-GC3.1), we previously showed that winters with anomalously fresh surface waters systematically exhibit shallower mixed layers, cooler SSTs, and weaker surface heat loss. These conditions imply enhanced freshwater-driven stratification, reduced deep convection, and a tendency for heat to be trapped below the surface, features consistent with the structure and persistence of the North Atlantic Warming Hole (NAWH).

In this study, we extend that framework to examine how this wintertime surface cooling and associated changes in the surface heat fluxes interact with the overlying atmosphere across a range of background circulation states. Using ensemble member–specific sea level pressure anomaly patterns and a regime-classification approach, we identify multiple atmospheric response modes that differ in the strength and latitude of the North Atlantic pressure gradient. These regimes reveal that the atmospheric response to subpolar cooling is not uniform. The background fields play a decisive role in determining how the surface cooling interacts with the large-scale atmospheric circulation.

Together, our results highlight a dynamically consistent pathway linking freshwater import, ocean stratification changes, regional winter SST cooling, heat flux responses, and large-scale atmospheric circulation shifts. This work provides new insight into the range of possible atmosphere–ocean climate feedbacks associated with ongoing and future freshening of the subpolar North Atlantic.