Enhanced near-bottom circulation and mixing driven by the surface eddies over abyssal seamounts. (November 2022) - Xie, Xiaohui
Abstract: Wind-driven geostrophic currents have been considered as one of the major energy sources for stimulating abyssal mixing that shapes the ocean overturning circulation. However, the mechanism governing the energy transfer from surface geostrophic currents to small dissipation scales available for abyssal mixing remains elusive. In this study, we identify a potential energy transfer pathway from surface geostrophic eddies to abyssal small-scale turbulence based on observations and numerical simulations of flows over seamounts in the northwestern Pacific Ocean. Our analyses reveal the generation of strong near-bottom anticyclonic circulations following the passage of surface geostrophic eddies across seamounts. These circulations drive downslope Ekman transports and convective instabilities, leading to enhanced turbulent mixing and thickened bottom mixed layers. Altimetry observations suggest that the energy transfer from surface eddies to deep currents and subsequent turbulence may be a dominant mechanism that accounts for 25–40% energy decay of mesoscale eddies passing seamounts. Strong near-bottom anticyclonic circulations associated with the surface geostrophic eddies are likely common for many seamounts and other topographic slopes in the global ocean, and may therefore play a significant role in the oceanic energy balance and water mass transformation in the abyssal ocean.