Observed Enhanced Mid-Depth Mixing on the Antarctic Continental Slope Drives Heat Flux Into the Winter Water Layer.

Turbulent mixing plays a key role in regulating heat and salt distribution in polar oceans, influencing water-mass transformation and ice-ocean interactions, yet direct observations of mixing south of 60°S remain scarce. As a result, the magnitude and drivers of vertical mixing in the Southern Ocean, particularly under and near sea ice, remain poorly constrained. Here, we present microstructure turbulence observations collected over three consecutive austral summers (2023-2025) in the King Haakon VII Sea (the eastern part of the Weddell Gyre, Southern Ocean). We observe enhanced sub-surface mixing over the continental slope, with mean dissipation rates an order of magnitude higher than typical values in the open ocean below the mixed layer.

This elevated mean dissipation on the continental slope is strongly influenced by a single episodic extreme event, reaching dissipation rates of up to 3 × 10⁻⁶ W kg⁻¹ mid-depth. Excluding the extreme event, we still observe enhanced mixing on the slope, with mean dissipation about three times higher than the open ocean values below the mixed layer. The observed elevated dissipation is associated with peaks in velocity shear and occurs when tidal inversion models predict periods of large tidal acceleration during the spring-tide cycle. Comparisons with internal-tide mixing model outputs further suggest that the enhanced slope mixing is driven by tides interacting with the steep continental slope.

Based on our observations, the continental slope mixing acting on the local temperature gradients produces a mean upward heat flux of approximately 3 W m⁻² into the base of the Winter Water layer, with peak values of 10 W m⁻². The extreme mixing event occurred within the Winter Water itself, where temperature gradients are weak. However, if the same extreme turbulence were to occur on the stronger thermal gradients at the base of the Winter Water layer, it could generate vertical heat fluxes of up to 124 W m⁻² from the underlying warm waters into the Winter Water layer.

Model-based estimates further suggest that tidal mixing along the Antarctic continental slope could drive a circumpolar mean heat flux of approximately 9 W m⁻² into the base of the Winter Water. These results highlight continental slope mixing as a mechanism for upper-ocean heat redistribution, with implications for Antarctic sea-ice formation, melt processes, and polar ocean heat budgets more broadly.