Mesoscale Air-Sea Coupling and Mixed Layer Depth Variability in the Southern Ocean

In this study, we analyze the importance of ocean dynamics and atmospheric forcing in the mesoscale air-sea coupling and mixed layer depth (MLD) variability in the Southern Ocean, using an idealized regional ocean-atmosphere coupled model and comprehensive global climate models. First, we study the role of mesoscale heat advection in the Southern Ocean mixed layer (ML) heat budget, using a regional high-resolution coupled model, with a realistic atmospheric and an idealized ocean component. We conclude that heat advection by oceanic currents creates mesoscale anomalies in Sea Surface Temperature (SST), while the atmospheric turbulent heat fluxes dampen these SST anomalies. This relationship between SST anomalies and advection depends on the spatial scale, the strength of the currents and the MLD. Second, we study the origins of MLD variability in the Southern Ocean, using the upper-ocean buoyancy budget in the same regional model. Oceanic mesoscale currents are shown to play an important role in the upper-ocean buoyancy budget. The main conclusion is that, while the atmospheric forcing and oceanic vertical mixing induce the MLD variability, the oceanic advection of buoyancy counteracts these atmospheric effects. Third, we study the response of MLD variability to mesoscale ocean currents and atmospheric noise in the Southern Ocean by analyzing a suite of coupled global climate simulations and interactive ensemble experiments. We assess the impact of ocean mesoscale currents by examining the difference between the eddy-resolving and non-eddy-resolving simulations. The time-mean MLD in the eddy-resolving simulations is significantly deeper than in the non-eddy-resolving simulations, which is likely due to overly re-stratifying eddy parameterization in the non-eddy-resolving simulations. The study aims at understanding mechanisms of the upper-ocean variability and air-sea interactions at the oceanic mesoscale. The progress in this direction will help to interpret and improve those coupled atmosphere-ocean models that do not resolve mesoscale processes.