Research Document - 2010/035

Physical Oceanographic Conditions in the Gulf of St. Lawrence in 2009

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

Abstract

An overview of physical oceanographic conditions in the Gulf of St. Lawrence in 2009 is presented. Air temperatures were close to normal when averaged from January to March. Air temperatures were in general either normal or above normal for the remainder of the year. The monthly averaged freshwater runoff measured at Québec City was normal overall in 2009 but consisted of above-average runoff in July compensated later by lower runoff in the fall. Near-surface water temperatures in the Gulf were above normal in all regions except the Northwest Gulf and the Estuary in June and in every region in August. Maximum sea-ice volume within the Gulf and on the Scotian Shelf was 65 km³, a value that is below normal using updated ice volume estimates for 1971-2000. The duration of the 2008-09 ice season was longer than normal in the Estuary, normal in the central Gulf and Cabot Strait, and shorter elsewhere. This was mostly associated with the variability of the first occurrence of ice. Winter inflow of cold and saline water from the Labrador Shelf occupied the Mécatina Trough over the entire column in winter 2009. The spread of the intrusion was confined a bit closer to the coast compared to 2008 conditions, leading to an overall smaller volume of 1270 km³, which is similar to the 2002 observations. The winter cold mixed layer volume in the Gulf, excluding the Estuary, was 14 000 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. The cold intermediate layer (CIL) index for summer 2009 was -0.42°C, which is similar to observations in 2002, 2004, 2005 and 2007. This is an increase of 0.32°C since 2008. On the Magdalen Shallows, almost none of the bottom area was covered by water with temperatures < 0°C in September 2009, similar to conditions in 2005, 2006 and 2007. 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 2009 than in 2008 and had a generally higher core temperature everywhere. In the northern Gulf, the area covered by low temperature water (< 1°C) decreased in 2009 relative to 2008 conditions. Temperatures in March 2009 were characterized by a very thick cold layer, including a thick intrusion of Gulf CIL waters into the Estuary. By June 2009, CIL temperatures returned to normal with a warming trend that continued into August, especially on the Magdalen Shallows. By October–November, CIL conditions were normal in most regions except the estuary and Northwest Gulf, where the CIL and the surface mixed layer were anomalously deep. Overall, temperature and salinity were generally normal from 150 m to 200 m, and slightly lower than normal at 250 and 300 m. Temperature and salinity at 300 m decreased for a third consecutive year, from 2008 to 2009. The lower-than-normal Gulf-wide water temperatures at 300 m were composed of normal waters in the Estuary and northwest and colder waters in the centre and coming into the Gulf at Cabot Strait. This cold anomaly has propagated inward in the last few years and is expected to continue toward the Estuary during the next few years. 

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