The different stratospheric influence on cold-extremes in Eurasia and North America: Summary

The polar vortex, a low-pressure area near the Earth’s poles, influences winter weather in mid-latitude areas like Canada and France. When the vortex weakens, the risk of cold spells has been proven to increase in Eurasia, but the effects on North American weather is not clear. A group of meteorologists has studied nearly thirty years worth of data on the polar vortex and its effects. This study has found two patterns that stem from a fluctuating polar vortex but affect different regions differently. The first pattern has zonal symmetry and is associated with atmospheric wave activity that travels upwards. This leads to cold-air outbreaks in Europe and changes in the atmospheric circulation of the Arctic and North Atlantic. The second pattern consists of downward traveling atmospheric waves that reflect over Canada, which leads to cold spells in Central Canada and the Great Lakes. The findings of this study suggest that these reflective mechanisms depend on the strength of the polar vortex and where the wave activity fluxes. Finding out the causes of these patterns and waves on a sub-seasonal to seasonal basis can help meteorologists predict cold spells in the mid-latitudes.

The Polar Vortex

Changes in the polar vortex influence the circulation of the troposphere, which affects the weather. Weak polar vortex states, like a sudden stratospheric warming(SSW), can induce widespread cold spells in the mid-latitudes, which would impact many populated areas, but these are not easy to predict. Understanding how the troposphere affects the stratosphere in these situations would help meteorologists predict these changes on a sub-seasonal to seasonal (S2S) timescale, which would be very helpful to many countries.

SSWs have been classified by their spatial properties, wave force, or wave strength, but these classifications do not tell us the effect it has on the troposphere. Recently, some other meteorologists classified SSWs by the connection between the wave activity and the polar vortex, which led to the classifications of absorbed and reflecting waves. These are the two patterns described earlier, and they form the basis of the study conducted here.

Absorbed waves are characterized by a several-week-long pulse of upward-propagating waves, which increase the temperature of the polar cap. The NAM, a stratospheric ring of pressure over the poles drops down to the troposphere, which leads to a negative phase of the NAO. The North Atlantic Oscillation is a weather phenomenon that consists of fluctuations in the two pressure zones that affect the weather. A negative phase leads to cold snaps in the mid-latitudes.

The North Atlantic Oscillation

Reflecting waves are characterized by shorter pulses of waves that move upwards and then reflect downwards. A stronger polar vortex reflects these waves, which influences the flow of the troposphere. This leads to a negative phase of the Western Pacific Oscillation and North Pacific Oscillation, which are the Pacific version of the North Atlantic Oscillation. This wave activity strongly affects North American weather, although it is not clear how. To find out, a group of meteorologists studied the spatial patterns of the polar vortex that are linked to cold spells. Specifically, they focused on how it affects the tropospheric circulation during Eurasian and North American winters. They also analyze the time series, or a series of data points in the order of when they occurred, using causal effect networks to determine the different potential causes of wave reflection.

The scientists of this study clustered the data of the daily height of polar anomalies from 1979 to 2018 in January and February. These two months were chosen because the greatest polar vortex changes occurred at this time. They merged the clusters together based on the distance between the data, and then calculated the mean until there was one final cluster. They also constructed climate indices for different parts of the world, like the Western Pacific. Then, they used CEN to analyze the correlation between the different pieces of data.

The analysis found five clusters show variability in the lower stratosphere polar vortex. They are ordered by the mean polar height. The first cluster represents an extremely strong polar vortex. The second and third show weaker and less concentrated polar vortex patterns. The fourth and fifth clusters of data represent the weak polar vortex states that these scientists wanted to analyze.

The Five Clusters

The fourth cluster shows positive anomalies over Siberia and negative anomalies over Canada and the North Atlantic. This pattern is detected on fourteen percent of all winter days, with 48 occurrences lasting an average of seven days each. It shows a disrupted polar vortex, but only four out of forty-eight of these disruptions were from the major SSWs that the scientists were trying to connect to these occurrences. The fifth cluster represents a more disturbed polar vortex, with positive anomalies over the entire polar cap. Sixteen percent of all winter days show this pattern. There are thirty-two occurrences, each lasting around twelve days each. Most of them occur with SSWs. The two clusters also show distinct wave patterns. The fourth cluster shows upward traveling waves over Siberia and downward traveling waves over Canada. Cluster five shows upward traveling waves throughout the northern hemisphere, with the strongest waves in Canada and the North Atlantic.

In conclusion, a group of scientists tried to find a correlation between sudden stratospheric warmings and changes in the polar vortex. This could help meteorologists predict cold spells during the wintertime. To find this connection, they collected data from almost thirty years of winters and put them into clusters. Five clusters were found to represent different polar vortex states, with the fourth and fifth being the weakest. Of the two, the fifth cluster of data found the strongest relationship between SSWs and a weak polar vortex. Hopefully, this data will help meteorologists accurately predict cold spells, and make our lives easier.

Original Article

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