Science Advisory Report 2009/019

State of the Ocean 2008:
Physical Oceanographic Conditions in the Gulf of St. Lawrence

Summary

• Air temperatures were close to normal when averaged from January to March, contributing to an ice cover volume that was also close to the climatological mean. Air temperatures were in general either normal or above normal for the remainder of the year.
• The monthly averaged runoff measured at Québec City was normal overall in 2008 but consisted of above-average runoff in summer compensated later by lower runoff in the fall. The high summer runoff contributed to higher-than-normal stratification
• Near-surface water temperatures were generally above normal throughout the Gulf for the months of May, July and November and were also above-normal on the Magdalen Shallows in June and in the northern Gulf from August to October. In August the northern parts of the Gulf saw positive anomalies while the southern parts experienced negative anomalies. This lead to the unusual occurrence that the waters around Prince Edward Island and in Northumberland Strait had higher temperature in July than in August 2008.
• On the Magdalen Shallows, a large bottom area was covered by water with temperatures < 0°C in 2008, similar to the cold period observed in the 1990s and contrary to conditions present in September 2005, 2006 and 2007 when no such waters were observed.
• Maximum sea-ice volume within the Gulf and on the Scotian Shelf was 81 km³, which is near normal.
• Winter inflow of cold and saline water from the Labrador Shelf occupied the Mécatina Trough from top to bottom in winter 2008 (up to 235 m in depth). Its volume of 1850 km³ was similar to the 2001 and 2006 observations.
• The winter cold mixed layer volume was 13 700 km³, a value higher than the 1996–2008 average by 0.8 SD. This cold-water volume corresponded to 41% of the total water volume of the Gulf.
• The CIL index for summer 2008 was -0.70°C, which is comparable to the very cold conditions observed in 2003. This was a large decrease in the index of 0.47°C since the summer of 2007. This is likely attributable to the very cold air temperatures in March 2008, late ice melt and delayed spring warming. Higher than normal summer stratification may also have played a role.
• Regional patterns of the August and September CIL show that the layers for T < 1°C and < 0°C were much thicker in most parts of the Gulf in 2008 than in 2007 and had a generally lower core temperature everywhere..
• In the northern Gulf, the area covered by low temperature water (bins from < -1°C through 0°C to 1°C) increased in 2008 relative to 2007 conditions.
• Seasonal and regional patterns observed in water column temperatures are summarized as follows: Temperatures in June 2007 were characterized by a very thick and cold CIL in most regions except the Estuary and by warm deep waters in the Estuary and the northwest Gulf. This overall pattern persisted in the August–September mean conditions. By October–November, CIL conditions were still thick and cold, and waters above the CIL were anomalously warm.
• Overall, temperature and salinity were generally normal from 150 m to 300 m, with the exception that temperature was slightly lower (by 0.6 SD) at 150 m due to the thick CIL. Temperature and salinity in this depth range decreased for a second consecutive year, from 2007 to 2008. The near-normal Gulf-wide water temperatures at 300 m were composed of warmer waters in the Estuary, near-normal temperatures in the northwest and centre, and colder waters coming into the Gulf at Cabot Strait.

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