Dynamics of a Buoyancy-Driven Coastal Jet: The Gaspé Current

Abstract

A primitive equation ocean model is applied to the process study of the Gaspé Current and cyclonic circulation over the northwestern Gulf of St. Lawrence (NWG). The model is driven by river discharge and barotropic boundary flows. Two types of model domains are used: an idealized basin with a flat bottom and piecewise coastline, and a realistic basin with model-resolved NWG bathymetry. The model domains are initially filled with horizontally uniform but vertically stratified waters. The river discharge is expressed in terms of lower salinity and a weak barotropic inflow in the upper waters at the estuary head. The early developments of the estuarine plume and coastal current system driven by the river discharge are qualitatively similar in both basins. After a short-period adjustment, a buoyant plume is developed near the estuary mouth, with a surface-intensified coastal current advecting the estuarine water seaward in the direction of Kelvin wave propagation. The coastal current initially follows the coastline closely but later becomes unstable with backward-breaking waves developed along the outer edge of the current. The kinetic energy analysis reveals that the plume–current system is baroclinically unstable with the transient motions resulting primarily from the mean available potential energy. With the river discharge at the head as the only driving force, the offshore front of the estuarine plume expands continuously seaward, leading to a large-scale anticyclonic circulation over the NWG. The addition of a barotropic westward jet along the Quebec shore, however, is able to restrain the seaward expansion of the offshore front of the plume, and therefore form a large-scale cyclonic motion over this region.