Science Advisory Report  2008/016

State of the Ocean 2007: Physical Oceanographic Conditions in the Gulf of St. Lawrence

Summary

• Air temperatures ranged from normal to cooler than normal for most of the year in the western parts of the Gulf, contrasting with the very warm conditions in 2006. However, the eastern regions were only significantly cooler than normal in April and May. October was very warm almost everywhere, and December was cool. Averaged over the whole Gulf for the entire year, air temperature was normal in 2007 and only slightly above normal (by 0.3 × standard deviation) when considering only the January to March period.
• The monthly averaged runoff measured at Québec City was consistently below normal in 2007. The annual mean was 1.5 standard deviations below normal, which is comparable to values observed from 2001 to 2003 but higher than the record low conditions observed from 1962 to 1965.
• Near-surface waters were anomalously warm in the St. Lawrence Estuary in January and February. After a rather uneventful spring, the summer maximum temperatures occurred earlier than usual everywhere in the Gulf (early August instead of mid-August). Earlier-than-usual cooling followed the maximum, and temperatures were below normal for the rest of the year except for October in the Estuary. Near-surface water temperatures were much cooler in 2007 than in 2006 in all regions of the Gulf.
• Near-surface waters in the northeast were consistently lower than normal in May and September and variable to below-normal during other months. Near-surface waters in the southern Gulf were either normal or warmer than normal from May to July and generally lower than normal from August to the end of records in October.
• On the Magdalen Shallows, there was (almost) no bottom area covered by water with temperatures < 0°C in September 2007 (as for 2005 and 2006), which contrasts with the cold period observed in the 1990s. Waters colder than 1°C covered slightly more of the bottom in 2007 than in 2006, but the opposite can be said for waters colder than 3°C.
• Maximum sea-ice volume within the Gulf and on the Scotian Shelf was below normal by 1.5 times the standard deviation but still much higher than recorded in 2006 (the lowest recorded value since 1969).
• Winter inflow of cold and saline water from the Labrador Shelf occupied the Mécatina Trough from top to bottom in winter 2007 (up to 235 m in depth). The spread of the intrusion had an area similar to that of 2006, but its volume was much larger (2850 km³), similar to that observed in 2004.
• The winter cold mixed layer volume was nearly normal at 13100 km³, higher than the 1996–2007 average but only by 0.4 × the standard deviation. This cold-water volume corresponded to 39% of the total water volume of the Gulf.
• The CIL minimum temperature index for summer 2007 was -0.23°C, comparable to conditions observed in 2004. This was a large decrease in the index of 0.44°C after three consecutive years of warming and brought it close to the 1971–2000 time series average of –0.32°C. This was fairly well predicted following the March 2007 survey of the cold mixed layer water volume in the Gulf. Part of the decrease of the CIL index was due to the larger-than-normal winter intrusion of Labrador Shelf water from the Strait of Belle Isle.
• Regional patterns of the CIL show that the layers for T < 1°C and < 0°C were much thicker everywhere except the Magdalen Shallows in 2007 than in 2006, and that the CIL had a generally lower core temperature as well. The exception was the Mécatina Trough, where core temperatures were similar to 2006 conditions, presumably due to the similar area extent of the cold Labrador Shelf water intrusion during both winters.
• Seasonal and regional patterns of water temperatures: 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 Mécatina Trough, and warm deep waters in the Estuary and the northwest Gulf. This overall pattern persisted in the August–September mean conditions, except for the then-normal CIL in Mécatina Trough and warm deep waters also found in Anticosti Channel. However, by October–November, conditions were about normal everywhere except for anomalously warm near-surface mixed layers in the northwest Gulf and warm near-surface waters on the Magdalen Shallows and in Cabot Strait.
• Temperature and salinity were generally normal at 150 m to 300 m after a decrease at most depths from 2006 to 2007. At 300 m, the near-normal temperatures were composed of warmer waters near the Estuary (regions 1 and 2), near-normal temperatures in the centre (region 6) and colder waters coming into the Gulf at Cabot Strait (region 7).
• Dissolved oxygen in the deep waters of the St. Lawrence Estuary remained hypoxic at 20%, having decreased very slightly in 2007 compared with 2006 observations.
• 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 winter cold surface layer.

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