Nearshore currents off Long Island

Currents were observed for a 25-day period in September 1975 at 11 km south of Long Island, where the water is 32 m deep, at three levels by using electromagnetic current meters. Tidal currents were found to be moderately strong, of the order of 20 cm s−1. Nontidal flow is caused by wind stress, horizontal density contrasts due to fresh water influx, and a longshore surface level gradient sloping down south-westward. The time-averaged flow has simple characteristics, adequately described by classical Ekman models in frictional equilibrium. By suitable changes in the averaging period, quantitative estimates of a bottom friction coefficient and a longshore pressure gradient are deduced from the data. The bottom friction coefficient agrees with what one would estimate from boundary layer theory for roughness elements of about 70-cm height. The longshore pressure gradient deduced from the behavior of currents agrees with the evidence of geodetic leveling.