[A-CG32] Extratropical oceans and atmosphere

[ACG32-12] Importance of frontal-scale and mesoscale ocean-atmosphere coupling to subtropical mode water formation

*Jingjie Yu¹, Bolan Gan¹, Lixin Wu¹ (1.Ocean University of China)

Keywords:Subtropical mode water, Frontal-scale and mesoscale ocean-atmosphere coupling, Gulf stream and Kuroshio Extension, Eddy-resolving coupled global climate model

Subtropical mode water (STMW) is a thick layer of water mass characterized by homogeneous properties within the main pycnocline, important for oceanic oxygen utilization, carbon sequestration, and climate regulation. STMWs are formed in western boundary current regions where thermal fronts and vigorous mesoscale eddies are pervasive with strong feedbacks to atmosphere. However, their roles in the STMW formation have been overlooked. By conducting twin simulations with an eddy-resolving global climate model, we find that suppressing local frontal-scale ocean-to-atmosphere (FOA) coupling leads to STMW formation being reduced almost by half in the North Atlantic Ocean. This is because FOA coupling enlarges STMW outcropping, attributable to the mixed layer deepening associated with cumulative excessive latent heat loss due to higher wind speeds and greater air-sea humidity contrast driven by the Gulf Stream fronts. Regarding to the role of the role of mesoscale ocean-atmosphere (MOA) coupling in STMW production, it is found that approximately 25% more STMW is formed in the North Pacific Ocean with the MOA coupling than without it. This is attributable to a significant increase in ocean latent heat release primarily driven by higher wind speed over the Kuroshio Extension region, which is associated with the southward deflection of storm tracks in response to oceanic mesoscale imprints. Furthermore, the enhanced heat loss in the presence of FOA and MOA coupling overshadows the stronger restratification induced by vertical eddy and turbulent heat transport, making STMW colder and heavier. With more realistic representation of FOA and MOA coupling, the eddy-present/rich coupled global climate models reproduce the observed STMWs much better than the eddy-free ones. Such improvement in STMW production cannot be achieved even with the oceanic resolution solely refined but without coupling to the overlying atmosphere in oceanic general circulation models. Our findings highlight the need to resolve FOA and MOA coupling to ameliorate the common severe underestimation of STMW and associated climatic and biogeochemical impacts in earth system models.