Velocity Observations in the West Passage of Narragansett Bay: A Partially Mixed Estuary

Narragansett Bay is a weakly stratified estuary comprised of three connecting passages of varying depths. The vertical distribution of horizontal velocity was observed in the West Passage using moored current meters. The instantaneous motion was characterized by semi-diurnal tidal currents of amplitude 25–60 cm s−1. These currents exhibited a phase advance with depth (total water depth=12.8 m) ranging with lunar phase from 0–3 h. The net current time series obtained by filtering out motions at tidal and higher frequencies were found to be an order of magnitude less than the instantaneous motion and well correlated to the prevailing 2–10 m s−1 winds. For periodicities of 2–3 days, the coherence between the longitudinal components of wind and net near surface current was as high as 0.8 with the current lagging the wind by about 3 h. The mean near surface speed, obtained by averaging over one month, was 1.2±1.6 cm s−1. The large error bounds were a result of the large variability of the net current time series (and not a result of inadequate sampling). A measure of this variability due to day-to-day changes in weather is given by the root mean square deviation of the net current time series or 2.6 cm s−1. The net transport of water through the West Passage was observed to be seaward or landward over the entire water column for several days duration, with typical wind induced transport fluctuations of ± m2 s−1. Hence, a net communication of water exists between the East and West Passages with water flowing either way in response to the wind. Wind is concluded to be the dominant mechanism driving the net circulation in the West Passage of Narragansett Bay. This is in contrast with the classical views of gravitationally convected net estuarine circulation.