Secondary circulation associated with a shelfbreak front

Evidence for secondary circulation associated with a shelfbreak front is obtained from a high-resolution, cross-shelf section of hydrographic, optical and velocity fields. Convergence in the bottom boundary layer on the inshore side of the front and subsequent upwelling into the interior is evident by a mid-water region of suspended bottom material emanating from the foot of the front and extending to within 35 m of the surface, 80 m above bottom. Downwelling on the offshore side of the front in the upper water column is inferred from a 20-m downward bend of the subsurface phytoplankton layer. These observations are in agreement with recent model predictions for secondary circulation near an idealized shelfbreak front. Convergence in measured cross-shelf velocity at the foot of the front is consistent with upwelling of bottom material detected there. An estimate of 9+/-2 m day(-1) of upwelling on the inshore side of the shelfbreak front is obtained, implying a transit time from the bottom to the surface of 10-16 days.