Direct Observations of Along-Isopycnal Upwelling and Diapycnal Velocity at a Shelfbreak Front

Support from NationalScience Foundation Grants OCE-9806650 andOCE-9813641 is gratefully acknowledged. Additionalsupport for JAB during the analysis phase was providedby NSF Grants OCE-9907854 and OCE-0001035 andfor DH from the State of Rhode Island and ProvidencePlantations.

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.