An assessment of the seasonal mixed layer salinity budget in the Southern Ocean

The seasonal cycle of mixed layer salinity and its causes in the Southern Ocean are examined by combining remotely sensed and in situ observations. The domain-averaged terms of oceanic advection, diffusion, entrainment, and air-sea freshwater flux ( evaporation minus precipitation) are largely consistent with the seasonal evolution of mixed layer salinity, which increases from March to October and decreases from November to February. This seasonal cycle is largely attributed to oceanic advection and entrainment; air-sea freshwater flux plays only a minimal role. Both oceanic advection-diffusion and the freshwater flux are negative throughout the year, i.e., reduce mixed layer salinity, while entrainment is positive year-round, reaching its maximum in May. The advection-diffusion term is dominated by Ekman advection. Although the spatial structure of the air-sea freshwater flux and oceanic processes are similar for the steady state, the magnitude of the freshwater flux is relatively small when compared to that of the oceanic processes. The spatial structure of the salinity tendency for each month is also well captured by the sum of the contributions from the air-sea freshwater flux, advection-diffusion, and entrainment processes. However, substantial imbalances in the salinity budget exist locally, particularly for regions with strong eddy kinetic energy and sparse in situ measurements. Sensitivity tests suggest that a proper representation of the mixed layer depth, a better freshwater flux product, and an improved surface salinity field are all important for closing the mixed layer salinity budget in the Southern Ocean.