Extremely long Kelvin-Helmholtz billow trains in the Romanche Fracture Zone

Abstract

In the Atlantic Ocean, the densest water mass Antarctic Bottom Water "AABW" can only cross the Mid-Atlantic Ridge from its southwestern to northeastern basins in limited, because deep, conduits. At the southwestern entrance of one of these, the equatorial Romanche Fracture Zone, AABW crosses a sill at 4550 m depth in a 7 km narrow channel before plunging into the deep. At the sill-slope, the rapidly flowing AABW causes shear-induced turbulent mixing with the overlying water masses. We present an excerpt of 1 Hz sampled, half-yearlong moored observations from 99 high-resolution temperature sensors that demonstrate and quantify the turbulence details. On top of quasi-steady shear flow, an internal tide modulates the mixing. Together, they constitute a means for an extremely long train of >250 consecutive Kelvin-Helmholtz billows in a day that vary between 5 and 100 m in vertical scale.