Research Document - 2008/001
Physical Oceanographic Conditions in the Gulf of St. Lawrence in 2007
By Galbraith, P.S., D. Gilbert, R.G. Pettipas, J. Chassé, C. Lafleur, B. Pettigrew, P. Larouche and L. Devine
An overview of physical oceanographic conditions in the Gulf of St. Lawrence in 2007 is presented. Air temperatures ranged from normal to cooler than normal for most of the year in the western parts of the Gulf; however, the eastern regions were only significantly cooler than normal in April and May. Averaged over the whole Gulf for the entire year, air temperature was normal. The monthly averaged runoff at Québec City was below normal during all months of 2007. Near-surface water temperatures were much cooler in 2007 overall than in 2006 in all regions of the Gulf. Near-surface waters were warm in the St. Lawrence Estuary in January and February. Summer maximum surface temperatures occurred earlier than usual, followed by earlier-than-usual cooling. Surface temperatures were generally below normal for the rest of the year except for October in the Estuary. On the Magdalen Shallows, there was (almost) no bottom area covered by water with temperatures < 0°C in September 2007. Maximum sea-ice volume within the Gulf and on the Scotian Shelf was below normal but still much higher than the volume recorded in 2006. Winter inflow of cold and saline water from the Labrador Shelf occupied the Mecatina Trough from top to bottom. The spread of the intrusion had an area similar to that of 2006, but its volume was much larger and similar to that observed in 2004. The winter cold mixed layer volume was 13100 km³, slightly above the 1996–2007 average, and corresponded to 39% of the total water volume of the Gulf. The higher winter volume of cold water compared with 2006 conditions led to a decrease of 0.44°C in the Cold Intermediate Layer (CIL) index, reaching -0.23°C in summer 2007, which is comparable to conditions observed in 2004. The index saw a large decrease after three consecutive years of warming. Regional patterns of the CIL show that the layer for T < 1°C and < 0°C was much thicker in the northern half of the Gulf in 2007 than in 2006 and had a generally lower core temperature almost everywhere as well. Seasonal and regional patterns of water column temperatures in June were generally close to the 1971–2000 climatology at all depths, except for the very thick and cold CIL in the Anticosti Channel and warm deep waters in the northwest. This overall pattern persisted from August to September, but by late fall conditions were about normal everywhere except for the anomalously warm near-surface mixed layers in the northwest and warm near-surface waters on the Magdalen Shallows and in Cabot Strait. Averaged annually for the entire Gulf, the temperature and salinity from 150 m to 300 m were normal in 2007. Spatially, at 300 m, this was composed of warmer than normal waters near the Estuary, near-normal temperatures in the centre and colder than normal waters coming into the Gulf at Cabot Strait. The outlook for 2008 from the March 2008 survey is for a slight cooling of the CIL index to -0.47°C resulting from a thicker cold winter surface layer.
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