The influence of the “tidal stress” on the residual circulation

Abstract

The residual current field in the sea is defined as the mean velocity field over a time sufficiently long to cancel transitory wind currents and tidal oscillations. The hydrodynamic equations governing the residual circulation are established and it is shown that, in the regions of intensive tides, the tidal motion has a cogent influence on the residual flow pattern. This effect which arises from the non linear terms is equivalent to the application of a “tidal stress” which combines with the wind stress to drive the water motion. The tidal stress is calculated in the Southern Bight of the North Sea from the results of a numerical tidal model and the residual circulation is computed. The comparison with the circulation obtained when neglecting the tidal stress shows determinant differences enforcing the theory.