Baroclinic Effects on Tidal Propagation and Estuarine Circulation: an Idealized Modeling Analysis

This study evaluates the baroclinic effects on the propagation of a semidiurnal tide and on the estuarine circulation by applying a three-dimensional Regional Ocean Modeling System (ROMS) to an idealized estuary. Different riverine flows and tidal amplitudes are considered to generate various types of estuaries (from strongly stratified to well mixed) in the estuarine parameter space, which is based on the freshwater Froude number (Fr) and mixing number (M). Two groups of scenarios, with and without density difference between riverine flow and ocean water, are carried out. Tidal wave characteristics and residual currents of each scenario are investigated. In strongly stratified estuaries, the baroclinic effects result in a typical bidirectional estuarine circulation, with the surface water flowing downstream and bottom water flowing upstream; if no density variations are considered, the residual current will flow downstream. In well mixed estuaries, surface water goes upstream due to Stokes Drift and bottom water goes downstream as a compensating flow, which can be found in both barotropic and baroclinic simulations. In partially mixed estuaries, without considering the density variations, the estuarine circulation patterns are similar to those in well mixed estuaries; the baroclinic effects will generate a triple-directional estuarine circulation, characterized by a downstream flow below the tidal wave trough and an upstream flow close to the bottom; between the wave crest and trough the residual current is directed upstream due to the Stokes Drift. Tidal amplitude is increasingly amplified from estuary mouth to head in all scenarios, and the intensity of this amplification is larger in baroclinic simulations. The strongest baroclinic effects on the tidal propagation occur in partially mixed estuaries, where the tidal amplitude can increase up to 30% due to baroclinic effects.