Geostrophic approximation in the St. Lawrence estuary

Abstract

A 1965 survey of currents and geostrophic currents in the St. Lawrence estuary is described. An innovation employed in the survey was to moor the strings of oceanographic bottles in the cross-section and trip them simultaneously. A tidal oscillation was detected in the vertical shear of the geostrophic current as well as in the vertical shear of the axial and cross-channel current components. The observations qualitatively confirm predictions from a simple theory that is presented for geostrophic response on one-and two-layer canals. The theory suggests that the period of resonant cross-channel oscillation is an important time scale since current fluctuations of much longer periods reflect accurately in the geostrophic current, while fluctuations of shorter periods may appear as considerable distortions in the geostrophic current. From this, it is concluded that a single determination of geostrophic current may represent neither the instantaneous nor the long-term average current. The average geostrophic current over a time interval longer than the resonant period may, however, represent the average current over the same interval. DOI: 10.1111/j.2153-3490.1970.tb01936.x