Observations of wind influence on exchange flows in a strait of the Chilean Inland Sea

Abstract

A 100-day time series of velocity profiles, sea level and wind velocity at a strait in the Chilean Inland Sea was analyzed to examine the effects of wind forcing on the mean two-layer exchange. Measurements took place in the Meninea Constriction of the Moraleda Channel during a period dominated by northerly winds. The mean flow in the strait, and in general in the Moraleda Channel, showed net surface northward outflow and net bottom southward inflow that likely resulted from the dynamical balance between pressure gradient and friction. The influence of tidal mixing on mean exchange flows was further suggested by isopycnals intersecting the bottom. The same momentum balance between pressure gradient and friction, applied with temporally varying sea level slopes, satisfactorily described the subtidal modifications to the mean exchange flows produced by wind forcing. The use of sea level slopes to explain the subtidal variability of velocity profiles at the Meninea Constriction was justified by the strong correlation between sea level slopes and wind stresses (0.84). In fact, the vertically integrated dynamics was essentially explained by the balance between wind stress and barotropic pressure gradient for northerly winds. Addition of bottom stress improved the dynamical explanation during periods of weak or southerly winds. This dynamical response was confirmed by the first two empirical orthogonal functions of the record. Two-layer exchange flows were weakened by northerly winds as depicted by the first empirical function mode, which was unidirectional throughout the water column in response to depth-integrated dynamics. The second empirical function mode was related to the depth-dependent response that followed the wind-induced sea level set-up.