In the Southern Ocean, mixed layer variability plays a key role in water mass transformation, air–sea interactions, and primary productivity. While the seasonal cycle of air–sea heat flux is the primary driver of mixed layer depth variability, strong lateral buoyancy gradients and wind-forced processes can significantly alter stratification at the submesoscale. Submesoscale processes influence mixed layer stratification through mixed layer eddies, which restratify the upper ocean, and through cross-frontal Ekman buoyancy fluxes, where down-front winds erode stratification. However, observational studies of these processes are largely limited to spring and summer, when mixed layers are shallow and submesoscale motions expected to be weaker.

Here, we use an unprecedented year-long dataset of high-resolution glider measurements in the subpolar Southern Ocean to investigate the role of submesoscale fronts in controlling mixed layer depth across seasons. Mixed layer eddies act to restratify the water column year-round, but with low mean equivalent heat fluxes (~40 ± 210 W m⁻²). Ekman buoyancy fluxes are more intermittent but can reach amplitudes comparable to or exceeding surface heat fluxes. When these fluxes balance or exceed the net heat flux, they can drive mixed layer depth variations of up to 50 m over a single day. Our results reveal a strong seasonality in submesoscale processes, which modulate mixed layer depth and may influence primary productivity in this high-nutrient low-chlorophyll region.