Winter weather patterns across the Northern Hemisphere middle-to-high latitudes are influenced by several larger-scale phenomena in the atmosphere and ocean. Variability in the strength and position of the stratospheric polar vortex, a large cyclonic circulation that forms 30-50 km above the surface of Earth in the polar regions during the winter months, is one of these players. When this vortex is disturbed, the jet stream in the troposphere weakens and moves southward, allowing cold air outbreaks and severe winter storms to impact the middle latitudes. For these, making skillful long-lead (2-6 week) predictions about the state of the stratospheric polar vortex has several scientific and socioeconomic benefits. For decades, scientists have known that waves traveling upward from the troposphere into the polar stratosphere are the primary mechanism for affecting the polar vortex. This project provided a new view about the nature of "wave pulses" (i.e., occurrences of one or more of these waves) and how dynamical interactions between the stratosphere and troposphere have changed over recent times. 

First, we produced an algorithm to identify wave pulses through analyzing poleward heat transport (a proxy for vertical wave motion) throughout the troposphere and lower stratosphere. From 1979-2019, there were over 100 observed wave pulse events, with multiple pulse events (i.e., events with more than one wave in succession) exhibiting much stronger heat transport than single pulse events (i.e., only one wave). Moreover, all but one of the documented major breakdowns of the stratospheric polar vortex (also known as major sudden stratospheric warmings) were associated with either a single or multiple wave pulse event from our database. Hence, identifying the occurrence of these wave pulses would provide a 1-2 week lead to these important events for predicting future winter weather patterns. Before either single or multiple wave pulse events, higher-than normal heights exist across Northern Eurasia and the North Atlantic along with a "trough-ridge" pattern over the North Pacific (Figure 1). However, these circulation features are weaker and fade rather quickly during single pulse events versus remaining fairly strong and persistent during multiple pulse events. Additionally, multiple pulse events tend to lead to an overall weaker polar vortex, which subsequently produces a stronger and longer-lasting impact on surface winter weather patterns. Therefore, although both types of wave pulse events have similar circulation patterns beforehand, knowing the persistence of those patterns can help us predict a single or multiple pulse event.

Along with studying wave pulses, the team also investigated how interactions between the troposphere and stratosphere have changed over the last several decades. Using a clustering algorithm, the team identified two distinct patterns of polar vortex weakening: (a) the classic sudden warming pattern (i.e., complete breakdowns of the vortex); and (b) a displaced, "stretched" polar vortex whereby the vortex changes shape from circular to elliptical and extends over North America. The identification of this second "stretched" polar vortex pattern is important for long-lead predictions of winter weather conditions for three reasons:

a)     Cold winter temperatures in central and eastern North America are most sensitive to the "stretched" vortex pattern.

b)    This pattern forms when the upward-moving waves are reflected back down into the troposphere, rather than being absorbed in the stratosphere during sudden stratospheric warming events. This difference adds another perspective to our understanding of wave pulse events and stratosphere-troposphere dynamical interactions.

c)     The "stretched" vortex pattern has occurred more frequently over the last 20 years than earlier periods.

Finally, the research team examined other diagnostics associated with the polar vortex to understand its changes since the middle twentieth century. The findings indicate that the polar vortex has experienced higher intraseasonal variance in its size over the last 70 years along with having further excursions southward into the middle latitudes. These changes are hallmark signatures of more frequent extreme winter weather conditions across the middle latitudes, as has been observed over the last 20 years. Also, the time series of these metrics show individual shorter time periods throughout the record of relative strengthening and weakening trends in the vortex embedded in the long-term trend. Hence, the stratospheric polar vortex undergoes its own internal variability unrelated to larger climate signals and forcings.

The overall results of this project presented new perspectives on how stratosphere-troposphere coupling via wave pulses can affect the strength and position of the stratospheric polar vortex. In turn, knowledge of these interactions offer windows of opportunity for more skillful long-lead forecasts of extreme winter weather in the middle latitudes.

This project supported two graduate students and one undergraduate student in conducting various aspects of the work. Moreover, the team developed a publicly-accessible educational video about polar vortex dynamics and variability. Team members also contributed weekly to a public blog discussing stratospheric dynamics, polar vortex variability, and 14-45 day winter weather forecasts across the Northern Hemisphere.

Last Modified: 11/24/2021
Modified by: Jason C Furtado