The role of mesoscale ocean currents in the mixed-layer heat budget and air-sea coupling in the Southern Ocean

Ocean mixed layer (OML) modulates the air-sea heat exchange by changing the effective heat capacity of the surface water and responding to Sea Surface Temperature Anomalies (SSTAs). Cooler SSTs generally lead to deeper OML, which is manifested by the negative correlation between the OML depth and SSTAs observed in most of the ocean. However, this simple relationship is broken in several parts of the Antarctic Circumpolar Current, where the mean currents and eddy activity are both strong and heat advection plays a big role in the mixed-layer heat balance. This property is observed in both comprehensive climate-model simulations and high-resolution regional simulations of a sector of the Southern Ocean.

In this study, the importance of mesoscale advection in the OML heat budget and air-sea coupling is explored using a regional high-resolution atmosphere-ocean coupled model with a realistic atmospheric component and an oceanic model of a zonal flow. The results show that the OML heat budget is dominated by the heat advection by both the large-scale and mesoscale currents and by the heat exchanges at the base of OML. On average, the OML-integrated mesoscale heat advection is shown to induce SSTAs, while the large-scale heat advection acts to weaken them. The negative correlation between SSTAs and air-sea heat flux anomalies further demonstrates that these mesoscale current-induced SSTAs drive the anomalous air-sea heat exchange, with the warmer SSTAs releasing heat to the atmosphere, and vice versa. Therefore, neglecting the mesoscale currents in low-resolution climate model may lead to errors in the simulated SST variability and air-sea heat exchange.