Hydrographic Climatology in the Gulf of St. Laurence: Its Recent Trends and an Estuarine Regime of Its Interannual Variability

Combining hydrographic data from the NOAA NODC World Ocean Database and the archive of the Bedford Institute of Oceanography we construct a hydrographic climatology of the Gulf of St. Lawrence (GSL) and analyze interannual to multidecadal variability of its principal water masses. Our analysis is based on the assumption that buoyancy is a primary forcing mechanism defining thermohaline fields and driving circulation in GSL. We estimate mean hydrographic structures as well as annual anomalies and their temporal trends on several transects orthogonal or tangential to the principal pathways of buoyancy-driven flows in GSL. Data spanning warm seasons (May through October) of 1950 through 2014 are analyzed. In the Cabot Strait, the surface layer exhibits freshening while the bottom layer (affected by the Atlantic water influx) becomes warmer and saltier. The latter tendency can be traced along the axis of the entire Laurentian Channel and becomes even stronger further inland (e.g., off Anticosti Isl.). This enhancement in two-layer estuarine exchange flow is likely related to the freshening effect of the melting ice further north advected into the Gulf. Indeed, the near-surface coastal box off Nova Scotia occupied by the coastal buoyancy current originating from the St. Laurence estuary does not exhibit similar freshening, in line with the decreasing riverine discharge. Interannual variability of temperature and salinity also exhibits patterns of estuarine exchange: near-bottom salinity indicative of the return flow from the Atlantic into GSL correlates with the St. Laurence River discharge. The coastal buoyancy current responds to variations in the freshwater discharge by offshore expansion while the salinity variations near the coast do not show a significant relationship with the discharge. Accelerating departure of hydrographic anomalies from their record-mean trends is observed over the last 10-15 years.