Research Document - 2012/023
Physical Oceanographic Conditions in the Gulf of St. Lawrence in 2011
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 2011 is presented. Both winter and annual air temperatures were above normal by 1.7°C (+0.9 SD) and 0.7°C (+0.7 SD), respectively. The annual runoff measured at Québec City was above normal in 2011 (+1.4 SD), seventh highest since 1955, with May having the fourth largest monthly runoff since 1955. Near-surface water temperatures were above normal in winter and typically normal for the remainder of all year until November, when they were above normal. Maximum sea-ice volume within the Gulf was 14 km³, close to the record low of 11 km³ recorded a year earlier, consistent with the above-normal mixed layer temperatures and with very warm air temperatures in December 2010 and January 2011. The duration of the 2010–11 ice season was much shorter than normal, beginning later than normal. No ice was exported onto the Scotian Shelf. Winter inflow of cold and saline water from the Labrador Shelf occupied the Mécatina Trough over the entire water column during the winter. Its volume was above normal in March, at 2200 km³ (+0.5 SD), and the percentage of cold water it represented was 29% (+1.5 SD), higher than normal. The winter cold mixed layer volume (T < 0°C) in the Gulf, excluding the Estuary, was 13 300 km³. While this matches the lowest volume recorded since 1996, it still corresponded to 40% of the total water volume of the Gulf. However, it was very warm, on average about 0.5 to 1°C above the freezing point. This is the second time in 16 years of winter surveys that such high temperatures were recorded. The cold intermediate layer (CIL) minimum temperature index for summer 2011 was +0.17°C, similar to observations in 2006 and the warmest since 1983. On the Magdalen Shallows, none of the bottom area was covered by water with temperatures < 0°C in September 2011, similar to conditions in 2005-07, 2009-2010. 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 2011 than in 2010 and had a generally higher core temperature everywhere. Conditions in March 2011 were characterized by a thin and warm mixed layer. The CIL remained below normal in thickness and above-normal in minimum temperature throughout the remainer of the year, especially in the Estuary and Central Gulf. The CIL at Rimouski station was, overall, the thinnest and warmest recorded since monitoring began there in 1991. The deep waters in the Estuary were colder and fresher than normal in 2011. Very warm and saline waters occupied Cabot Strait all year at 250 m, the depth of the temperature maximum. Gulf-wide average temperatures were above normal at 150 m and normal at 200 to 300 m and salinity averages were normal from 150 to 250 m and above normal at 300 m. Temperature at 300 m increased significantly when averaged over the Gulf (by 1.3 SD). At Cabot Strait the 300 m temperature anomaly was positive by 1.2 SD; the highest since 1994. Salinity at 200 m and 300 m increased overall by 0.9 and 2.4 SD respectively, but increased at Cabot Strait to reach +1.2 SD at 200 m and +1.5 SD at 300 m, the latter being a high since 1986. Salinities in Central Gulf changed from -1.6 SD in 2010 to +1.2 SD in 2011, an increase of 2.8 SD. The 300 m waters of the Estuary were expected to cool between 2010 and 2011 but instead they warmed slightly. The warm anomaly present since 2010 at Cabot Strait should progress up the channel towards the Estuary in the next two years. By November the surface mixed layer was anomalously thick but more importantly very warm, warmer in fact than that observed during fall 2010 which were preconditions of the low ice cover of winter 2011.
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