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

Abstract:

The role of mesoscale currents in the ocean mixed layer (OML) heat budget and air-sea coupling is analyzed using a regional high-resolution atmosphere-ocean coupled model with a realistic atmospheric model and a model of a zonal ocean flow. The results show that the seasonal variation of the OML depth determine the relative importance of the surface heat flux and OML-integrated heat advection in the heat budget: the heat advection dominates over the surface heat flux in determining the OML heating/cooling in winter months when OML depth is deep, whereas the surface heat flux becomes more important during summer.

Mesoscale anomalies are defined here using two different spatial filters, in order to explore the scale dependence in the mesoscale air-sea coupling. For short scales, the OML-integrated mesoscale advection is shown to induce Sea Surface Temperature Anomalies (SSTAs), while the large-scale heat advection acts to weaken them. For large-mesoscale anomalies, in contrast, the OML-integrated advection determines the heating/cooling of the OML, but does not exhibit a clear spatial correlation with SSTAs. The difference is explained by the fact that the large-mesoscale SSTAs are characterized by the stationary meanders, whereas the OML heating/cooling and heat advection are characterized by much shorter spatial scales. For both large-mesoscale and small-mesoscale anomalies, the negative correlation between SSTAs and the heat flux out of the ocean 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.