Seasonal variability of upper ocean heat content in Drake Passage

[1] Mixed-layer depth (MLD) is often used in a mixed-layer heat budget to relate air-sea exchange to changes in the near-surface ocean temperature. In this study, reanalysis heat flux products and profiles from a 15 year time series of high-resolution, near-repeat expendable bathythermograph/expendable conductivity-temperature-depth (XBT/XCTD) sampling in Drake Passage are used to examine the nature of MLD variations and their impact on a first-order, one-dimensional heat budget for the upper ocean in the regions north and south of the Polar Front. Results show that temperature and density criteria yield different MLD estimates, and that these estimates can be sensitive to the choice of threshold. The difficulty of defining MLD in low-stratification regions, the large amplitude of wintertime MLD (up to 700 m in Drake Passage), and the natural small-scale variability of the upper ocean result in considerable cast-to-cast variability in MLD, with changes of up to 200 m over 10 km horizontal distance. In contrast, the heat content over a fixed-depth interval of the upper ocean shows greater cast-to-cast stability and clearly measures the ocean response to surface heat fluxes. In particular, an annual cycle in upper ocean heat content is in good agreement with the annual cycle in heat flux forcing, which explains 24% of the variance in heat content above 400 m depth north of the Polar Front and 63% of the variance in heat content south of the Polar Front.