January 29 - February 3, 2019 | Polar Vortex | Extreme Cold
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A Sudden Stratospheric Warming event.
"At altitudes of less than 10 kilometers above Earth’s surface, planetary-scale waves form and propagate upward into the stratosphere, where they dissipate. This leads to a weakening of the polar vortex, a confined region of strong eastward winds that form during wintertime at high latitudes. As the polar vortex weakens, polar stratospheric temperatures increase"
"Hemisphere-scale weather patterns in the wintertime Northern Hemisphere troposphere and stratosphere are associated with changes in an index called the Northern Annular Mode (NAM) [Thompson and Wallace, 1998]. In the troposphere, the NAM is characterized by a pressure anomaly over the polar region, with an opposite-signed anomaly near 50°–55°N. That is, high-pressure anomalies over the North Pole are coupled with low-pressure anomalies farther south and vice versa. This pattern is related to stronger eastward winds during positive NAM phases (i.e., for a negative polar pressure anomaly) and westward wind anomalies during negative NAM phases. In the stratosphere, the NAM describes the strength of the polar vortex. Negative NAM phases are associated with weak stratospheric polar vortices, like those that occur during SSWs.
NAM anomalies often move downward from the stratosphere to the tropopause (the boundary between the troposphere and the stratosphere) over the course of about 10 days and can then significantly alter extratropical weather patterns during the following 2 months. Knowledge of this downward movement can extend the range of weather forecasts.
Outside the tropics, an SSW can displace extratropical cyclonic storm tracks toward the equator, among other consequences. This displacement increases the probability that storms will pass over the United Kingdom and southern Europe, and it increases the probability of record-breaking cold temperatures and snowfall in eastern North America [Kidston et al.,2015]. Although atmospheric reanalyses and climate model simulations clearly illustrate the downward propagation of the NAM anomalies, we do not yet fully understand the mechanism responsible for the stratospheric control of tropospheric weather patterns."
Pedatella, N. M. (2018, March 21). How Sudden Stratospheric Warming Affects the Whole Atmosphere. Retrieved from https://eos.org/features/how-sudden-stratospheric-warming-affects-the-whole-atmosphere
Very informative. An important read!
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