The Xiamen Symposium on Marine Environmental Sciences (XMAS 2027)

Turbulent mixing above rough topography is crucial for the upwelling of deep water and the closure of the meridional overturning circulation. Recent theoretical studies hypothesize that such upwelling may occur predominantly within a thin bottom boundary layer (BBL) along the sloping seafloor. Here, we present moored observations from the Seamount Boundary Layer Turbulence experiment (S-BLTE), featuring high-resolution (1 Hz) temperature measurements, which reveal tide-modulated turbulent mixing and water transformation over a deep seamount in the South China Sea (SCS). We demonstrate that the thickness of the BBL increases with proximity to the seamount and exhibits significant spatio-temporal variability. Turbulent mixing varies from 10^-5 to 10^-2 m²s^-1 detected up to 280 m height above the seafloor (~1540 m depth). Episodic enhanced mixing events, triggered by shear and convective instabilities (Richardson number less than 0.25 and water overturns), could intensify mixing rate up to 10^-2 m²s^-1 during transitions between flood and ebb tides. This turbulence mixes dense up-slope waters with overlying lighter down-slope waters, driving net upwelling along the boundary of seamount. This process results in a diapycnal upwelling velocity of ~10 m d⁻¹ and a net downward heat flux of up to 20 W m^-2 over a tidal period. Our findings substantiate the view that seamounts could be the stirring rods of the ocean, providing observational evidence for the closure of the abyssal meridional overturning circulation.