Meteorological, Sea Ice, and Oceanographic Conditions in the Labrador Sea during 2020

In the Labrador Sea, the coldest and freshest North Atlantic basin south of the Greenland-Iceland-Scotland Ridge, wintertime surface heat losses result in the formation of dense waters that play an important role in ventilating the deep ocean and driving the global ocean overturning circulation. In the winter of 2020, the central Labrador Sea lost more heat through surface cooling than in the previous winter. However, the surface heat loss remained near normal for a third straight year, contrasting a 27-year record high in 2015 and above-normal losses in 2016 and 2017. The 2020 winter (Dec–Mar) North Atlantic Oscillation (NAO) index was above normal and the highest after reaching its record high in 2015. However, the sea level pressure pattern was not associated with strong westerly winds along the Labrador coast. This led to, respectively, near-normal and above-normal winter and spring air temperatures in the Labrador Basin domain. Both winter and spring sea surface temperatures in the Labrador Basin were above normal. Winter sea ice extent was below normal in the Davis Strait, Northern Labrador Shelf, and Labrador Shelf regions. Spring sea ice extent was also below normal in all three regions. With respect to temperature anomalies averaged annually over the central Labrador Sea, the upper 100 m layer was the coldest in the 2002–2020 period in 2015 and 2018. After 2018, this layer attained above-normal annual temperatures in 2019–2020, reaching a 2011–2020 temperature high in 2019, then slightly cooling yet remaining above normal through 2020. The intermediate, 200–2000 m, layer of the Labrador Sea started to cool immediately after hitting a record warm point of the 1972–2020 period in 2011. This cooling trend was mainly driven by strengthening and progressively deepening winter convection in 2012 and during 2014–2018. The key factor that contributed to the recurrent deepening of convective mixing in the three straight winters following the winter of 2015 was not as much air sea heat exchange as it was the water column preconditioning set by convective mixing in the previous years. Such multiyear persistence of deepening winter convection, continuing through the winter of 2018 when it exceeded 2000 m in depth, resulted in the most voluminous, densest, and deepest formation of Labrador Sea Water since 1994. The situation changed in 2019, with the depth of winter convection largely ceasing to exceed 1400 m in that and the following two years. The intermediate layer has been warming since 2019 with the seawater density trend eventually reversing to negative. Even though wintertime mixing reached marginally deeper in 2020 (by 100 m or so), and the intermediate layer slightly cooled, the negative density trend prevailed. Between 2018 and 2020, the annual mean intermediate layer density reduced by about 0.01 kg/m3. Overall, the changes in the depth of winter convection and intermediate layer properties between these years imply that the effect of the water column preconditioning on winter convection has weakened since 2018. Vertical distributions of dissolved oxygen and chlorofluorocarbons (CFCs) – CFCs, industrially Freons, are the anthropogenic gases that are commonly used as tracers of convectively-formed water masses spreading in the ocean – in the central Labrador Sea, based on quality controlled drift-corrected measurements assembled since 1990, follow very closely the multiyear events of recurrently persistent renewal of dense deep Labrador Sea Water in the Atlantic Ocean. Bedford Institute of Oceanography North Atlantic model simulations suggest that the transport of the Labrador Current generally decreased between 1995 and 2014, increased between 2014 and 2019, and slightly decreased in 2020. The Atlantic meridional overturning circulation index based on this model demonstrates a general weakening trend from the mid-1990s until 2004, then slight strengthening lasting until 2011, then weakening again until the overturning weakest point was reached in 2019. The overturning circulation started to strengthen in 2020, but it is too early to associate this short-term increase with a reversal of the current negative trend in the Atlantic meridional overturning circulation.

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