Date of Award

2015

Document Type

Open Access Thesis

Department

Marine Science

First Advisor

Alexander Yankovsky

Second Advisor

Howie Scher

Abstract

The Gulf of St. Lawrence (GSL) is characterized by a general circulation of an estuarine type, where fresher/lighter water flows seaward above saltier/heavier water flowing landward. In an estuarine regime, the inflowing deep layer water compensate for a loss of mass due to the surface layer outflow. A major element of the GSL estuarine circulation is the coastal buoyancy-driven current fed by the St. Lawrence River discharge. In addition, some surface water is advected into the GSL as a branch of the Labrador Current (LC). Recent climatological changes in the North Atlantic have increased the melting rate of the Greenland ice sheets, resulting in a freshening of the LC. NOAAs’ National Oceanographic Data Center (NODC) hydrographic data, comprising salinity and temperature measurements in the GSL from 1950-2010 warm seasons, were used to construct three climatological transects across major branches of the estuarine circulation in the GSL. Each transect was then subdivided into three vertical layers representing distinct water masses in the GSL: a fresh and warm surface layer, a cold intermediate layer (CIL), and a warmer, salty Atlantic water bottom layer. The surface layer was further subdivided into boxes inside and outside of the coastal buoyancy-driven current. A climatology was constructed on a bi-monthly basis in order to assess and remove effects of the seasonal cycle on the annual anomaly estimates. Linear trends for temporal evolution of annual temperature and salinity anomalies were estimated. We vi found a basin wide warming and salting of the bottom layer. Minimal change was found for the CIL. The surface layer showed a cooling and salting trend within the buoyancy-driven coastal current, while a warming and freshening trend was found in the rest of the surface layer; a signal of changes in the LC. Salinity trends in the buoyancy costal current cannot be deducted from annual variations of the St. Lawrence River discharge and could be explained by increased mixing. Entrainment of ambient water into a buoyancy current implies enhanced seaward transport, which leads to stronger near-bottom landward advection of Atlantic water into the GSL. The observed increase of stratification has strong implications for the GSL ecology. Our analysis suggests that global change trends can be amplified in semi-enclosed basins due to advective processes.

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