Direct observations of along-isopycnal upwelling and diapycnal velocity at a shelfbreak front

Abstract

By mapping the three-dimensional density field while simultaneously tracking a subsurface, isopycnal float, direct observations of upwelling along a shelfbreak front were made on the southern flank of Georges Bank. The thermohaline and bio-optical fields were mapped using a towed undulating vehicle, and horizontal velocity was measured with a shipboard acoustic Doppler current profiler. A subsurface isopycnal float capable of measuring diapycnal flow past the float was acoustically tracked from the ship. The float was released near the foot of the shelfbreak front (95-100-m isobath) and moved 15 km seaward as it rose from 80 to 50 m along the sloping frontal isopycnals over a 2-day deployment. The float's average westward velocity was 0.09 m s-1, while a drifter drogued at 15 m released at the same location moved westward essentially alongfront at 0.18 m s-1. The float measured strong downward vertical velocities (in excess of 0.02 m s-1) associated with propagation of internal tidal solibores in the onbank direction from their formation near the shelf break. The float measured large upward vertical velocities (in excess of 0.001 m s-1 ≃ 100 m day-1) as the pycnocline rebounded adiabatically after the passage of the internal tide solibore. The directly measured mean along-isopycnal vertical velocity was 17.5 m day-1. Intense mixing events lasting up to 2 hours were observed in the shelfbreak front at the boundary between cold, fresh shelf water and warm, salty slope water. Diapycnal velocities of up to 3 × 10-3 m s-1 were measured, implying a diapycnal thermal diffusivity as large as 10-2 m2 s-1, indicative of strong mixing events in this coastal front. © 2004 American Meteorolgical Society.