Long‐term current and temperature observations on the Middle Atlantic Shelf

Abstract

Nearly 2200 days of current and temperature data were collected at a midshelf location in the Middle Atlantic Bight between June 1974 and March 1977. These data were examined for the average conditions and seasonal cycles of water circulation and temperature and some statistical properties of their variation at higher frequency. The average flow is found to be toward the south‐southwest at about 5 cm/s near the surface, diminishing to about 1 cm/s near the bottom. The occurrence of energetic wind‐driven transient current events which can exceed a 2‐month duration makes it impossible to determine a clear seasonal pattern in the sequence of monthly mean flows. There is, however, a clear seasonal pattern in the distribution of higher‐frequency fluctuations. Storm wind‐induced transient currents of 3‐ to 10‐day duration appear prominently in winter records. Inertial currents appear selectively in the summer, that is, in parts of the water column well insulated from the bottom by a strong thermocline. For low‐frequency motions (periods from 3 to 10 days) in both summer and winter records the preferred directions of motion throughout the water column appear to be consistent with equilibrium (mean) Ekman veering arguments. The water column response was found also to be most sensitive to the component of wind stress parallel to the bathymetry. The temperature follows a well‐known seasonal cycle of heating and stratification. During the unstratified seasons, very little high‐frequency temperature variation is observed, but during summer, thermal oscillations due to daily heating and inertial and semidaily tidal frequencies (3/3, 4/3, and 6/3 cpd) and a 5/3‐cpd oscillation appear prominently.