On the observed wind-driven circulation response in small semi-enclosed bays

Abstract

This study analyzes horizontal and vertical wind-driven circulation responses in small semi-enclosed bays, the associated offshore dynamic conditions and the relative importance of each term in the momentum balance equations using a multiplatform observational system. The observational platform consists of 3 ADCPs and a land-based radar monitoring the velocity field within the bay and in the contiguous offshore area. The wind-driven patterns in the bay can switch from a barotropic cyclonic or anti-cyclonic circulation to a 2-layers baroclinic mode response as a function of the wind regime (its direction and magnitude). For the baroclinic mode, the vertical location of the inflection point in the velocity profile can vary according to the proximity of the boundary current to the entrance of the bay. The influence of offshore meteo-marine conditions in the inner-bay dynamics is evidenced under strong to mid wind conditions, and is almost inexistent under negligible wind. The momentum balance analysis as well as the non-dimensional numbers evidence the impact of wind stress, coastline shape, stratification, and the nonlinear advective terms. Advection can be at the same order of magnitude as pressure gradient, Coriolis or wind stress terms, and greater than the bottom stress terms. The nonlinear terms in the momentum equations are frequently neglected when analyzing wind-driven circulation by means of in-situ data or analytical models.