Seasonal Variation and Governing Dynamics of the Mixed Layer in the Indian Sector of the Southern Ocean

The dynamics involved in the variability of temperature, salinity, and density distributions in the upper layer of the Indo-Pacific sector of the Southern Ocean were studied using Acoustic Doppler Current Profiler and Conductivity Temperature Depth data from a mini triangle trans-ocean buoy network mooring deployed at 60 degrees S and 140 degrees E between January 15th and 19th December 2012, together with hydrographic casts, reanalysis outputs, and satellite imagery. Spectral analysis confirmed that near-inertial internal waves are a major factor transporting energy downward to the lower boundary of the mixed layer (ML), with 12.5 J m(-3) or 45% of the mean surface energy being able to reach deeper than the ML depth during one of the analyzed events. During a period of net heat loss from the ocean surface to the atmosphere, most of the energy was concentrated below the maximum shear layer, at around 80 m depth. Internal wave events were found to be linked to scalar diffusivity increases of up to two orders of magnitude. A box model clarified that surface cooling plays the most significant role in mixing surface waters during winter, but it is not enough to explain variability below 100 m depth, where horizontal advection and eddy enhancement are also important in modulating warm water flux. Our results elucidate how wind energy might enhance mixing, even in ice-covered regions, while influencing depths greater than previously thought.