Subtidal currents from a shipboard acoustic Doppler current profiler in tidally dominated waters

Abstract

As part of a comprehensive study of inner shelf dynamics in the vicinity of a major estuary, we test the performance of a shipboard acoustic Doppler current profiler (ADCP). We compare velocities from the ADCP with those from current meters located 6 and 10 m below the surface in water shallower than 30 m. We find the ADCP suitable to estimate subtidal currents of O(10 cm s −1) embedded in a tidal flow exceeding 100 cm s −1. We obtain the former currents by subtracting predicted tidal currents which we computed from empirical models. Three different models all result in similar subtidal flows; none of them appears superior. However, spatial and temporal averaging of the velocity data are necessary to reduce noise in the ADCP record to acceptable levels. Our subtidal flow estimates are reliable, robust and physically meaningful. We find that bottom friction influences the tidal currents over the entire water column. We also find a narrow, baroclinic, subtidal jet exiting the estuary on the right looking seaward. This jet is the source of the local buoyancy-driven coastal current.