During the 2021 winter, several cold extreme events were noticed, mainly over Asia and North America. However, would this type of events be expected when the global warming, associated with an increasing frequency of events such as heat waves or heavy precipitation, is becoming a more relevant phenomenon?
The global warming is unequal according to different regions, and the Arctic warms at a rate two times faster than the global average. This phenomenon, known as Arctic Amplification (AA), is associated mostly with the retreat of sea-ice in summer, but also with an increase of the snow cover in northern Europe and Asia between October and January.
Recently, the AA has been associated to more vigorous winters in mid-latitudes, which might be one of the causes of the cold waves registered in January and February 2021 over Asia, Europe, and the United States. For example, in the state of Texas this might have been one of the costliest natural phenomena ever as the lack of preparation of the energy sector to such cold conditions led to the interruption of electrical energy supply.
But how is AA related with extreme events in mid-latitudes during winter? One of the hypotheses is related with the polar vortex in the stratosphere (layer above the troposphere, between 15 and 50 km height). This vortex is known as a band with strong winds in the stratosphere that encircles the North Pole. When the vortex is strong and stable, the cold air stays confined in the Arctic and in the mid-latitudes the air is warmer than normal. However, when the polar vortex weakens, for example due to a sudden stratospheric warming, it can move to southern latitudes or split into two. In the Earth’s surface, the weakening of the vortex impacts the movement of colder air towards the mid-latitudes and warmer air towards the Arctic.
A recent study in the United States was based on a machine learning technique, which consists in applying algorithms that enable the automatic recognition of patterns, to understand the relationship between changes in the Arctic vortex with extreme winters in mid-latitudes. Thus, reanalyses (global data that contain historical information about the atmosphere, land surface and ocean) between 1980 and the beginning of 2021 were used, to identify different variability patterns of the polar vortex from October to December. Several patterns were identified in three different classes: normal polar vortex; polar vortex stronger than normal (strong low-pressure system), representing around 39% of the days; and polar vortex weaker than normal (weaker low-pressure system), representing around 38% of the days.
During the last 40 years, a higher frequency of weaker than normal patterns has been noticed, concurrently with a lower frequency of stronger patterns. This means that there is a higher number of events where the polar vortex is weaker, during autumn and winter, which promotes the propagation of colder air towards the mid-latitudes. As a result, AA might be related with more extreme cold events in the mid-latitudes during winter. If the patterns of weakened polar vortex continue to be observed, it is possible that in the future there is a higher frequency of extreme cold events during winter in more southern regions, where the population and infrastructures are not prepared for this type of phenomenon.
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Source: Cohen J., Agel L., Barlow M., Garfinkel C. I. & White I. Linking Arctic variability and change with extreme winter weather in the United States. Science 373, 1116–1121 (2021). DOI: 10.1126/science.abi9167.
Author: Carolina Viceto
