Enhancement of Alongshore Freshwater Transport in Surface-Advected River Plumes by Tides

Abstract

Abstract A recent numerical study by Isobe showed that imposing alongshore tidal forcing on buoyant coastal discharge enhances the net freshwater transport in the coastal currents. The mechanisms for this transport enhancement are studied using a three-dimensional, primitive equation ocean model [Regional Ocean Modeling System (ROMS)]. Lagrangian drifters are used to trace the freshwater transport paths. It is found that the river plume bulge circulation largely follows the rigid-body motion (i.e., constant vorticity). The buoyant fluid near the bulge’s outer edge is thinner and faster, behaving as a baroclinic jet. The bulge currents then split after impinging on the coast. The outer fluid feeds the downshelf-flowing coastal currents, while the inner fluid recirculates to form the bulge. The coastal current transport estimated from the present and prior studies corresponds well to a baroclinic jet theory, with the incident angle of bulge currents at the coast being a key parameter. Without tides, the bulge is approximately circular. The incident angle measured with respect to the cross-shore axis is small. With tides, the convergence of tidal momentum fluxes near the upshelf plume front leads to a positive pressure anomaly, which acts to compress the bulge shoreward. As a result, the incident angle increases, which in turn enhances the downshelf momentum input, thus increasing the freshwater transport in the coastal currents. Finally, the parameter space for coastal current transport in the presence of tidal forcing is explored with a conceptual model. A few observational examples are given.