Convergence and Downwelling at a River Plume Front

Abstract

The small-scale structure of the circulation and hydrography at the frontal boundary of the Connecticut River plume in Long Island Sound has been resolved using a novel combination of instruments: a towed acoustic Doppler current profiler (ADCP) and a rigid array of current meters and conductivity?temperature sensors. Observations were made during the latter half of the eastward ebb tide, when the river plume was well established and the front was moving to the west at approximately 0.3 m s?1. Two across-front transects revealed a horizontal convergence rate in the across-front velocity components at 0.6 m of 0.05?0.1 s?1. This was associated with a salt-induced horizontal density gradient of 10?2 kg/m4. Observations obtained during a period in which the towed ADCP was caught in the zone of maximum surface convergence showed significant downwelling with a near-surface maximum of 0.2 m s?1. Vertical velocities of this magnitude are consistent with observed magnitudes of the convergence rate at 0.6 m assuming volume flux continuity and weak alongfront variations. Quantitative comparison of the velocity observations to the model of Garvine show reasonable agreement, though the vertical distribution of vertical shear and stratification could be improved. Within 20 m of the front, observations reveal regions of strong subsurface horizontal gradients in density and velocity that are not well described by the model. This limitation is a consequence of the assumption of similarity in the vertical structure of the fields. Both the magnitude and the across-front variation of the vertical velocity component observed agree with the theoretical predictions. The authors conclude, however, that the fundamental dynamics in the model are adequate to describe the general structure of the plume layer thickness.