On the spatial scales of a river plume

Abstract

We report observations of the structure of the front that surrounds the plume of the Connecticut River in Long Island Sound (LIS). Salinity, temperature, and velocity in the near‐surface waters were measured by both towed and ship‐mounted sensors and an autonomous underwater vehicle. We find that the plume front extends south from the mouth of the river, normal to the direction of the tidal flow in LIS and then curves to the east to parallel the tidal current. The layer depth at the front and the cross‐front jumps in salinity and near‐surface velocity all tend to decrease as distance from the source increases. This is qualitatively consistent with the prediction of layer models. In the across‐front direction, the plume layer depth increases from zero to the asymptotic value within a few times the plume depth (∼5 m). Vertical motion is generated in this zone, and there is evidence of overturning. Farther from the front, the high‐frequency salinity standard deviation decays exponentially with a length scale of 30 m. Assuming that the salinity fluctuations are a consequence of turbulence, we find that the rate of turbulent kinetic energy dissipation decreases exponentially in the across‐front direction with a decay scale LG ≈ 15 m. Estimates based on AUV‐mounted shear probes are consistent with this estimate. We present an explanation of the physics that determines LG and provide a simple formula to guide the choice of resolution in models that are designed to resolve the frontal structure.