Tidal eddy motions in the western Gulf of Maine, Part 2: Secondary flow

Abstract

The kinematics and dynamics of the tidal circulation in the western Gulf of Maine (GoM) region are investigated with focus on the secondary circulation. This study is motivated by previous research suggesting the formation and evolution of transient tidal eddy motions in a high-density scallop region off Chatham, MA. Three-dimensional flow velocity and surface elevation fields were obtained using the QUODDY finite-element coastal ocean circulation model in the barotropic mode and forced by the five most important tidal constituents in the region (M2,N2,S2,K1 and O1). The secondary flow kinematics related to the primary tidal flows feature time/space-varying convergences and divergences that are affected by the associated transient tidal eddy motions. Interestingly, the upwelling and downwelling in the study region were not dominated by the secondary circulation. Rather, the model results show that instantaneous vertical motions close to the coast and close to the bathymetric slope are mainly controlled by the divergence/convergence of the primary flow. The instantaneous secondary flow dynamics are mainly controlled by a balance between pressure gradient and Coriolis forces. Off Chatham, the surface maximum strength of the secondary flow calculated by the model is consistent with theoretical predictions of 0.025m/s. The mechanisms controlling the long-term average tidal secondary circulation, which is relevant for biological transport, are discussed.