Research Document - 2011/045

Physical Oceanographic Conditions in the Gulf of St. Lawrence in 2010

By P.S. Galbraith, J. Chassé, D. Gilbert, P. Larouche, D. Brickman, B. Pettigrew, L. Devine, A. Gosselin, R.G. Pettipas, and C. Lafleur

Abstract

An overview of physical oceanographic conditions in the Gulf of St. Lawrence in 2010 is presented. Air temperatures reached record highs when averaged from January to March and from October through December as well as annually. The monthly averaged freshwater runoff measured at Québec City was normal overall in 2010 but was unusually high during winter and fall, and the spring freshet was almost absent. Near-surface water temperatures in the Gulf were normal or above normal all year and in all regions except for the Mécatina Trough and Esquiman Channel in June. Maximum sea-ice volume within the Gulf and on the Scotian Shelf was 11 km³, a record low since 1969. The duration of the 2009–2010 ice season was shorter than normal and associated with the early ice melt. Winter inflow of cold and saline water from the Labrador Shelf occupied the Mécatina Trough over the entire column in winter 2010. The spread of the intrusion was confined close to the Strait of Belle Isle, leading to an overall small volume of 809 km³. However, this intrusion volume represented 29% of the unusually small volume of mixed layer waters that were colder than -1°C. The winter cold mixed layer volume in the Gulf, excluding the Estuary, was 13 900 km³, a value higher than the 1996–2009 average by 0.7 SD. This cold-water volume corresponded to 42% of the total water volume of the Gulf. However, it was very warm, on average about 1°C above the freezing point. This is the first time in 15 years of winter surveys that such high temperatures were recorded. The cold intermediate layer (CIL) index for summer 2010 was -0.04°C, which is similar to observations in 2000. This is an increase of 0.38°C since 2009. On the Magdalen Shallows, none of the bottom area was covered by water with temperatures < 0°C in September 2010, similar to conditions in 2005, 2006, 2007, and 2009. In other regions of the Gulf, very few areas had bottom temperatures below 0°C. Regional patterns of the August and September CIL show that the layers for T < 1°C and < 0°C were much thinner in most parts of the Gulf in 2010 than in 2009 and had a generally higher core temperature everywhere. Conditions in March 2010 were characterized by a very thick winter mixed layer, although very warm, including a thick intrusion of Gulf waters into the Estuary. By June 2010, the CIL thickness returned to near-normal but still had above-normal minimum temperatures. The CIL warming rate appeared to be slower than usual because core temperatures were closer to normal in certain regions by August and more so by November. The warm deep waters in the Estuary in 2009 were replaced by colder-than-normal waters by June 2010. Colder-than-normal deep waters also occupied the northwest Gulf at that time. Very warm waters occupied Cabot Strait in June at 250 m—the depth of the temperature maximum—and there is a hint that the top portion of this water mass was sampled during the March survey. The warm deep waters were still present in Cabot Strait in August as well as in November. Gulf-wide average temperatures were below normal at 200 to 300 m and salinities were below normal from 150 to 300 m. Temperatures at 300 m increased marginally overall but significantly (by 1 SD) at Cabot Strait, where the anomaly is now +1 SD. Salinity at 200 m and 300 m decreased overall by 0.6 SD but increased at Cabot Strait to reach +0.6 SD at 200 m. The 300 m waters of the Estuary are expected to cool further during the next two years, but it will be interesting to follow the warm anomaly present in 2010 at Cabot Strait as it progresses up the channel toward the Estuary. The surface mixed layer in November was anomalously thick but more importantly very warm, warmer in fact than in November 2009 which were the preconditions for the record conditions of March 2010.

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