Forced Oscillations near the Critical Latitude for Diurnal-Inertial Resonance

Abstract

Oscillations at, or close to, the inertial frequency are widely observed in shelf seas where frictional damping is weak. In the vicinity of latitudes 30°N and S, such motions may become significantly enhanced by a resonance in which the local inertial frequency coincides with that of diurnal forcing. Under these conditions, regular daily variations in wind stress tend to produce large anticyclonic motions that may extend throughout the water column as shown in the analytical theory of Craig. Here the authors examine new observations from a location close to the critical latitude on the Namibian shelf in the Southern Hemisphere. The measurements cover almost the full depth (175 m) of the water column by using upward and downward looking ADCPs suspended in midwater. The observed flow involves a steady drift to the north (maximum ?11 cm s?1 at 40 m) but the kinetic energy budget is dominated by anticyclonic circular motions with speeds in the surface layers exceeding 40 cm s?1 at times. Comparably energetic motions (speed >35 cm s?1) were found in the lower layers of the stratified water column where, below 70 m, there was a consistent phase shift, relative to the near-surface motion, of ?180°. During the observation period, the winds at the nearest land station, 130 km distant, exhibited significant diurnal variation with a stress magnitude of up to 0.1 Pa and almost equal, and in-phase, components of coast-normal and coast-parallel wind stress. The principal features of the observations are interpreted in terms of an analytical model of two uncoupled layers in which the bottom layer is forced by through the coast-normal pressure gradient set up by the oscillatory wind stress. This pressure gradient is of comparable magnitude, but opposite in phase, to the surface forcing and this accounts for the relatively energetic phase-shifted motions in the lower layers. In areas of low tidal energy close to the critical latitude, diurnal oscillations of the kind observed here should be considered as an important candidate source for vertical mixing.