Upwellings and downwellings on the edge of the Gaspé Current: Observations and processes

Abstract

The Gaspé Current carries the relatively fresh surface waters of the St. Lawrence estuary along the Gaspé peninsula to the Gulf of St. Lawrence. It presents complex dynamics including the development of meanders. A density front separates the fresher surface waters of the estuary from the saltier surface waters of the gulf. Over the past 40 years, upwellings and downwellings observed on the edge of this front have been the subject of several investigations. Tang (1982) proposed an analytical model featuring a permanent circulation consisting of an upwelling offshore of the front and a downwelling below the front. Using a unique set of in situ observations made with CTD (Conductivity Temperature Depth) profilers and ADCP (Acoustic Doppler Current Profiler) data collected between September 1991 and June 1993, upwellings and downwellings were inferred using a water masses analysis. Even if their presence is predominant (∼ 80% of the total number of sampled transects), they are not a permanent feature over the entire length of the Gaspé Current front, and their positions relative to the front vary. This data set also made it possible to determine via the ω equation that these vertical currents mostly result from frontogenesis and frontolysis (strengthening and weakening of the front). A new method to combine the ω equation with the nonlinear Ekman pumping shows that wind-driven vertical motions were generally weaker and much shallower than those due to frongenesis/frontolysis. This analysis shows that upwellings and downwellings are dependent on favorable conditions leading to the establishment of Gaspé Current instabilities and to frontogenesis or frontolysis.