Turbulent Thermal-Wind Driven Coastal Upwelling – Current Observations and Dynamics

Abstract A remarkably consistent Lagrangian upwelling circulation at monthly and longer time scales is observed in a 17-year time series of current profiles in 12 m of water on the southern New England inner shelf. The upwelling circulation is strongest in summer, with a current magnitude of ∼1 cm s−1, that flushes the inner shelf in ∼2.5 days. The average winter upwelling circulation is about half the average summer upwelling circulation, but with larger month-to-month variations driven, in part, by cross-shelf wind stresses. The persistent upwelling circulation is not wind-driven, it is driven by a cross-shelf buoyancy force associated with less dense water near the coast. The cross-shelf density gradient is primarily due to temperature in summer, when strong surface heating warms shallower near-shore water more than deeper offshore water and to salinity in winter, caused by fresher water near the coast. In the absence of turbulent stresses, the cross-shelf density gradient would be in a geostrophic, thermal-wind balance with the vertical shear in the along-shelf current. However, turbulent stresses over the inner shelf due to strong tidal currents and wind stress cause a partial breakdown of the thermal-wind balance that releases the buoyancy force, which drives the observed upwelling circulation. The presence of a cross-shelf density gradient has a profound impact on exchange across this inner shelf. Many inner-shelves are characterized by turbulent stresses and cross-shelf density gradients with lighter water near the coast, suggesting turbulent thermal-wind driven coastal upwelling may be a broadly important cross-shelf exchange mechanism.