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Research Document 2021/045

Physical Oceanographic Conditions in the Gulf of St. Lawrence during 2020

By Galbraith, P.S., Chassé, J., Shaw, J.-L., Dumas, J., Caverhill, C., Lefaivre, D. and Lafleur, C.

Abstract

An overview of physical oceanographic conditions in the Gulf of St. Lawrence (GSL) in 2020 is presented as part of the Atlantic Zone Monitoring Program (AZMP). AZMP data as well as data from regional monitoring programs are analysed and presented in relation to long-term means, shifted from 1981-2010 to 1991–2020 in this edition of the report. The annual average freshwater runoffs of the St. Lawrence River measured at Québec City and its combination with rivers flowing into the Estuary (RIVSUM II) were above normal. Sea-ice maximum volume was below normal, but the winter mixed layer volume was near normal. The August cold intermediate layer (CIL) was warmer than normal but the seasonally averaged minimum temperature index was near normal. Surface water temperatures in the Estuary reached an unusual series record high in July; upwelling and mixing that occurs at the head of the Laurentian Channel usually keeps Estuary surface water cool, but North-easterly winds appear to have created a circulation that capped the mixing region with warmer waters. The overall seasonal average May-November SST for the Gulf was near normal in spite of an above normal seasonal August maximum (+0.7°C, +0.9 SD). Surface water temperatures were then at a record low in September in the Estuary and Northwest Gulf, caused by strong vertical wind mixing. This mixing warmed the bottom waters of the Magdalen Shallows such that the area covered by water with temperatures <1°C in September decreased to a near record low. Deep water temperatures have been increasing overall in the Gulf since 2009, with inward advection from Cabot Strait. Gulf-wide average temperature at 150 m was lower than the 2015 record highs but above normal at 3.7°C (+1.6 SD). New series record highs (since 1915) were set at 200, 250 and 300 m, at 5.7°C (+1.2°C, +1.9 SD), 6.6°C (+1.1C, +2.5 SD) and 6.8°C (+1.1°C, +2.7 SD) respectively. Bottom area covered by waters warmer than 6°C was at a record high in the Northwest Gulf, the Northeast Gulf, and in Centre and Cabot Strait, and some 7–8°C habitat appeared for the first time in the Northeast Gulf.

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