On the validity of thermal wind balance in alongshelf currents off the New Jersey coast

The validity of thermal wind balance (TWB) as an approximation for the observed vertical shear in coastal currents (depth 10-30 m) is analyzed based on the ADCP and CTD transects conducted off the New Jersey coast in early August of 1996. At that time, the study area was forced simultaneously by a brief upwelling-favorable wind event and the arrival of a buoyant plume from the Hudson Estuary. As a result, both downwelling- and upwelling-favorable alongshelf currents resided within the same area, the former inshore and the latter offshore. The very intense pycnocline (buoyancy frequency ∼0.06 s−1) was shaped as a dome and thus the TWB shear of both signs was observed within the same across-shelf transects and under the same wind conditions. In average, the geostrophic (TWB) shear overestimated the observed by a factor of 2. The maximum of the ageostrophic shear (which is the observed minus TWB shear) was within the sloping pycnocline and not near the bottom (where one would expect to find a strong frictional shear). For both upwelling and downwelling-favorable currents, the geostrophic shear had a sharp maximum in the pycnocline (localized within a few meters), while the ageostrophic shear tended to smooth the actual velocity profile, counteracting the TWB shear in the pycnocline and adding to it (changing sign) below. The result was a more linear velocity distribution with depth compared to the TWB prediction. The significant reduction of the TWB shear in the pycnocline described in this study was likely related to the turbulence production by the internal wave breaking. The wind-induced stresses were trapped in the shallow, ∼5 m deep surface layer, and were inhibited below by a strong pycnocline. Nonfrictional mechanisms (inertia or momentum advection) did not contribute significantly to the ageostrophic shear.