Science Advisory Report 2021/026

Oceanographic Conditions in the Atlantic Zone in 2020

Summary

• Monthly average sea surface temperatures were generally normal to above normal in ice-free areas until May when they were below normal to normal. After being mostly near-normal in June, temperatures increased to mostly above-normal in July, including a series record (since 1982) in the St. Lawrence Estuary. Temperatures remained normal to above normal in August but reached a series record low in the Estuary in September, after strong winds. While temperatures remained below normal in the Estuary, the Gulf and parts of the Scotian Shelf in October, they were above normal on the Labrador and Newfoundland Shelf, reaching series records in 3MNO in October. The year ended with December record highs on the Scotian Shelf.
• Sea surface temperatures averaged over the ice-free months were normal to above normal across the zone except for the Northern Gulf. They were above normal on the Newfoundland and Labrador Shelf for the first time since 2014.
• Winter average sea ice conditions were below normal in the Gulf of St. Lawrence and also for the first time since 2013 on the Newfoundland and Labrador Shelf.
• Summer cold intermediate layer (CIL) metrics indicated normal to warmer-than-normal conditions across the zone. The CIL and sea ice index for the zone was the 3^rd warmest of the time series (since 1980), after 2011 and 2006.
• Bottom temperatures were above normal across the zone except in NAFO Divisions 2J and 3K where they were near normal; There were however no measurements in 3Ps and 3LNO due to a cancelled survey. Warm conditions included 100+ year record highs in the deeper waters of the northern Gulf of St. Lawrence and in Cabot Strait at 300 m, as well as a series record on the Magdalen Shallows in waters deeper than 30 m in September. The zonal average index was highest of the time series (since 1980).
• At the high-frequency stations, 0-50 m seasonal average and bottom temperatures were mostly above normal, including a series record at Rimouski Station.
• The Labrador Current weakened to normal during 2019 and 2020 on the Newfoundland and Labrador slope, and has been below normal fairly consistently since 2014 on the Scotian slope.
• Deep nitrate inventories were above or near normal on the Newfoundland and Labrador Shelf and in the northern Gulf of St. Lawrence, but mainly below normal in the southern Gulf of St. Lawrence and across the Scotian Shelf, including a record low on Browns Bank.
• Annual chlorophyll a inventories were above normal over most of the Newfoundland and Labrador Shelf and Gulf of St. Lawrence, but near or below normal on the Scotian Shelf.
• The onset of the spring phytoplankton bloom was highly variable across the Atlantic Zone, but mostly close to normal. It was later than normal in the Labrador Sea, the latest on record on the Central Scotian Shelf and the earliest on record on Georges Bank. The magnitude of the bloom was mostly above normal on the Newfoundland shelf, including a series record in Hibernia, normal to above normal in the Gulf, and variable on the Scotian Shelf with a record low on the Central Scotian Shelf. Bloom duration was variable and close to normal in most regions across the zone but included a record high on Georges Bank.
• The zooplankton community shift observed in recent years (2014–2019), characterized by lower abundance of the large energy-rich copepod Calanus finmarchicus and higher abundance of small copepods and non-copepods, moderated in 2020 with increases in Calanus finmarchicus and declines in some small copepods, although the overall abundance of copepods and non-copepods remained elevated. There were below normal abundances of copepods in the Gulf of St. Lawrence and a record minimum on the Halifax Line section. There were below normal abundances of Pseudocalanus spp. in most regions with a record low in the Southern Gulf and on the Cabot Strait section, but a record high on the Bonavista section. Non-copepods were at a record low at Halifax 2 station but at a record high at Prince 5. Seal Island section saw record high abundances of copepods, non-copepods and Calanus finmarchicus.
• Zooplankton biomass was generally normal to above normal on the Newfoundland and Labrador shelves and below normal to normal elsewhere.
• Near-bottom pH and aragonite saturation are generally much lower in the Gulf of St. Lawrence than on the Grand Banks and Scotian Shelf. Near-bottom aragonite is under-saturated throughout most of the Gulf of St. Lawrence, including the shallow waters of the Southern Gulf. From 2019 to 2020, near-bottom pH across the Newfoundland Shelf and Gulf of St. Lawrence has shown a general decline, with undersaturation state for aragonite occurring in 2020 on the Grand Banks and in the Avalon channel on the Newfoundland shelf.
• New record low concentrations of deep dissolved oxygen were measured in the Lower St. Lawrence Estuary.
• In the Labrador Sea, convection reached the depth of 1600 m and possibly deeper, exceeding depths observed in 2019. Despite the NAO index being relatively high, the convection depths were noticeably shallower than the depths reported for the 2015-2018 period (which reached 2000 m). Winter and spring air temperatures over the Labrador Basin were near normal and above normal. Both winter and spring sea surface temperatures in the Labrador Basin were above normal, while sea ice extent was below normal.
• In the Labrador Sea, the late timing of the mission, in July rather than in May, induced a bias in the in situ measurements that prevented comparisons with the biochemical and surface observations collected in most other years. However, compared to those collected around the same time of year, 2020 appeared to be a warmer than normal year as in 2003. Nutrients and chlorophyll a concentration were lower than the seasonal average with the exception of nutrients in the surface layer in the Central Labrador Sea and chlorophyll a concentration on the Greenland Shelf.

This Science Advisory Report is from the March 22–26, 2021 Twenty-third Annual Meeting of the Atlantic Zone Monitoring Program (AZMP) held via video-conference. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

Science Advisory Report 2021/026 (PDF, 5,061 KB)

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