Entrainment-driven modulation of Southern Ocean mixed layer properties and sea ice variability in CMIP5 models

The evolution of the upper Southern Ocean hydrographic structure in response to the representative concentration pathways 4.5 (RCP 4.5) forcing scenario is analyzed using model data drawn from the coupled model intercomparison project phase 5 (CMIP5) archive. A robust freshening trend is evident, associated with an increase in stratification and decoupling of the upper ocean as the mixed layer gains buoyancy at a faster rate than the underlying ocean. The magnitudes of the individual terms of the salinity and heat budgets are evaluated. Convection-driven entrainment from the thermocline into the mixed layer is found to play a significant role in modulating the mixed layer salinity, whilst the heat budget of the mixed layer is dominated by a primary balance between atmospheric warming and the entrainment-modulated supply of oceanic heat from below the mixed layer. The relationship between oceanic heat storage below the mixed layer, ice thickness and atmospheric temperature is investigated, and a very disparate response noted amongst the models considered here. Based on this analysis, we hypothesize that the balance between the entrainment-modulated supply of oceanic heat from below the mixed layer and the heat supplied by the atmosphere may play an important role in determining the simulated sea ice variability.