Origins of Mesoscale Mixed Layer Depth Variability in the Southern Ocean

Mixed-layer depth (MLD) exhibits significant variability, which is important for atmosphere-ocean exchanges of heat and atmospheric gases. Origins of the mesoscale MLD variability at the oceanic mesoscale in the Southern Ocean are studied here in an idealized Regional Ocean-Atmosphere Model (ROAM). The main conclusion from the analysis of the upper-ocean buoyancy budget is that, while the atmospheric forcing and oceanic vertical mixing on average induce the mesoscale variability of MLD, the three-dimensional oceanic advection of buoyancy counteracts and partially balances these atmosphere-induced vertical processes. The relative importance of advection changes with both season and the average depth of the mixed layer. From January to May, when the mixed layer is shallow, the atmospheric forcing and oceanic mixing are the most important processes, while the advection plays a secondary role. From June to December, when the mixed layer is deep, both atmospheric forcing and oceanic advection are equally important in driving the MLD variability. Importantly, buoyancy advection by ocean eddies can lead to both local shoaling and deepening of the mixed layer. The role of the atmospheric forcing is then directly addressed by two sensitivity experiments in which the mesoscale variability is removed from the atmosphere-ocean heat and momentum fluxes. The results from these experiments confirm that while the mesoscale MLD variability is controlled by mesoscale atmospheric forcing in summer, the intrinsic oceanic variability and surface forcing are equally important in winter. As a result, MLD variance increases when mesoscale anomalies in atmospheric fluxes are removed in winter and oceanic advection becomes a dominant player in the buoyancy budget. This study emphasizes the importance of oceanic advection and intrinsic ocean dynamics in driving mesoscale MLD variability, and demonstrates the importance of MLD in modulating the effects of advection in the upper-ocean dynamics.