Asymmetry in the response of a stratified coastal embayment to wind forcing

Abstract

We investigate mechanisms that lead to asymmetry in the response of a stratified coastal embayment following the onset of a uniform, steady wind that is blowing both along the axis and out of the bay. We focus on bays on the east coast of Newfoundland where the typical duration of wind events is 5 days and stratification representative of June conditions yields a first baroclinic mode wave speed of 0.51 m s-1. We use several numerical models ranging from a linear, reduced gravity model with a single baroclinic mode, to a nonlinear, prognostic, primitive equation model (CANDIE). We investigate the effect of factors such as continuous stratification, vertical mixing, nonlinearity, and realistic bottom topography. If the linear dynamics of only the first baroclinic mode is considered, the response of the idealized bay to 5 days of steady wind forcing is symmetric about the axis of the bay. Continuous stratification allows for higher-order vertical modes. These slower modes increase the response time of the bay, yielding asymmetry in the circulation pattern after 5 days of constant wind forcing. Model results using realistic geometry demonstrate that realistic bottom topography has little effect on near-surface circulation on the 5 day timescale. Adding nonlinearity allows a significant cross-bay transport of upwelled water and leads to the characteristic along-bay pattern of the surface isotherms evident in observations and can also lead to the separation of the coastal jet from the upwelling favorable shore.