Research Document - 2016/056

Physical Oceanographic Conditions in the Gulf of St. Lawrence in 2015

By P.S. Galbraith, J. Chassé, C. Caverhill, P. Nicot, D. Gilbert, B. Pettigrew, D. Lefaivre, D. Brickman, L. Devine, C. Lafleur

Abstract

An overview of physical oceanographic conditions in the Gulf of St. Lawrence (GSL) in 2015 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. The annual average freshwater runoff of the St. Lawrence River measured at Québec City was below normal and its combination with rivers flowing into the Estuary (RIVSUM II) was about normal in 2015 (-1.0 SD and +0.1 SD respectively). The spring freshet was below-normal at -1.1 SD and -0.5 SD, but its timing was normal. The cold winter of 2015 created a thick surface mixed layer over the GSL with near-freezing temperatures, but a sea ice cover that was only slightly above-normal. The August cold intermediate layer (CIL) showed warmer minimum temperature (+0.8 SD) and thinner volume colder than 1°C (-1.5 SD) than normal in spite of the cold winter conditions. Including the colder temperatures from the June survey led to a near-normal CIL minimum temperature index (+0.3 SD). Sea-surface temperatures averaged over the Gulf were generally below normal until July and above normal from August to September 2015, leading to a near-normal May-November average (+0.3°C, +0.5 SD). Record highs were nevertheless reached in September averaged over the Gulf (+1.8°C, +2.3 SD) and in the following regions: Estuary (+1.9°C, +2.3 SD), Northwest Gulf (+1.9°C, +1.9 SD), Anticosti Channel (+2.1°C, +2.1 SD) and Magdalen Shallows (+1.4°C, +2.0 SD). The timing of summer warming onset was near-normal while fall cooling was later than normal (+1.7 weeks, +1.4 SD). Deep water temperatures have been increasing overall in the Gulf, with inward advection from Cabot Strait where temperature had reached a record high (since 1915) in 2012 at 200 m. Temperature averaged over the Gulf at depths from 150 to 300 m all show record highs in 2015 and attained values above 6°C at 250 and 300 m for the first time since 1915. The bottom area covered by waters warmer than 6°C increased in 2015 in Anticosti Channel, Esquiman Channel and Central Gulf, reaching a record high value in Anticosti Channel and Esquiman Channel while reducing its bottom habitat area in the temperature range of 5–6°C.

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