Observations of a Remarkably Consistent New Form of Coastal Upwelling on the New England Inner Shelf

The relevance of recent advances in our understanding of inner-shelf dynamics to seasonal and interannual time scales is unclear because of the lack of long-term observations. Seventeen years of current profiles in 12-m of water and concurrent wind and surface gravity wave measurements collected at the Martha’s Vineyard Coastal Observatory on the New England inner-shelf are used to characterize the cross-shelf current variability and associated dynamics on seasonal to interannual time scales. Monthly averages of the depth-averaged cross-shelf flow are always offshore and are equal in magnitude but opposite in direction to the estimated depth-average Stokes velocity driven by surface gravity waves. The depth-average Lagrangian (Eulerian + Stokes) cross-shelf velocity is small, generally less than 0.5 cm/s. In summer, the monthly-mean depth-dependent Lagrangian cross-shelf circulation is remarkably consistent, with an offshore flow of 1 cm/s in the upper water column and an onshore flow of -1 cm/s in the lower water column. This weak but persistent upwelling circulation plays a dominant role in maintaining cooler inner-shelf temperatures during the summer by fluxing heat offshore and balancing the surface heat flux that would otherwise warm the inner shelf. This persistent upwelling circulation only occurs when there is a relatively strong thermal wind shear in the along-shelf current associated with a persistent cross-shelf density gradient in summer. I hypothesize that a reduction in the along-shelf current shear driven by tidal mixing results in reduced Coriolis force in the thermal wind balance and as a consequence the unbalanced portion of the cross-shelf density gradient drives the consistent summer upwelling circulation.