Concurrent Development of Benthic Storms and Bottom Mixed Layers underneath an Eddying Surface-Concentrated Zonal Jet

Benthic storms are episodes of strong bottom currents and sediment resuspension that occur at abyssal depths in the ocean. They are often observed in regions with strong, eddying surface currents, such as the western North Atlantic and the Argentine Basin. Deep cyclonic and anticyclonic eddies induced by surface current instabilities have been postulated to accelerate abyssal currents and generate benthic storms. Here, using a primitive-equation model with high vertical resolution, we conduct idealized numerical experiments of the unforced instability of a surface-concentrated, eastward-flowing jet in a zonal channel. We find that the jet, initially in strict thermal wind balance, becomes spontaneously unstable as a result of baroclinic instability, meanders, and eventually develops a complex eddy field with regions of strong ageostrophic flow. Associated with jet meandering, a train of cyclonic and anticyclonic eddies form along the jet and migrate in the same direction as the parent current but much slower. These eddies extend over the whole water column (4000 m depth), consistent with the tendency for eddy barotropization in geostrophic turbulence. They accelerate bottom currents, which are found to be more vigorous in a flat channel than in a channel with a meridional bottom slope. Over the course of the numerical integrations, bottom mixed layers with quasi-uniform potential temperature develop at various locations in the channel. These layers have thicknesses of O(100) m on a flat bottom and O(10) m on a sloping bottom. Our study yields preliminary insights into the fundamental role of surface current instabilities on near-bottom processes, calling for further investigations on downward energy transfer and near-bottom dissipation.