Summary: Extreme winter weather poses several threats to different sectors including energy, supply chain, and even agriculture. The stratospheric polar vortex, a seasonal cyclonic (counterclockwise) area of low pressure high in the Arctic atmosphere, is one important harbinger of extreme winter weather patterns across the Northern Hemisphere. Two main mechanisms for this link are: (1) the classic paradigm, which examines variability in the strength and shape of the vortex, and (2) a newer perspective involving stratospheric wave reflection, whereby tropospheric waves upwell into the stratosphere and are reflected back down, causing more immediate changes in the jet stream. Part of my focus during my Sabbatical is working with Salient’s data-driven approach to subseasonal forecasting to quantify and detect these important features for enhancing extreme winter weather forecast skill.

What is the Stratospheric Polar Vortex?

The Arctic stratospheric polar vortex (SPV) is a large, cold, low pressure center that resides 10-50 km above the Earth’s surface over the North Pole. The feature forms near the start of the autumn season, when the Arctic enters complete darkness and the air cools rapidly. As a result, strong west-to-east winds spin counterclockwise around this vortex high up in the Arctic atmosphere. Oftentimes, the vortex extends downward and couples with the jet stream in the troposphere, which determines the storm track and temperature patterns across the Northern Hemisphere. Because of this dynamical connection, changes in the structure and strength of the SPV can ultimately result in changes in winter weather patterns in the troposphere.

Classic Link between the SPV and Enhanced Subseasonal Forecasts

Stratosphere-troposphere interactions have been known about for many decades, but the use of the SPV as an important signal for subseasonal weather information took over twenty years ago. Research by Baldwin and Dunkerton (2001) was among the first to show that changes in the strength (and structure) of the SPV circulation can propagate downward and also change the orientation of the jet stream. Their classic “dripping paint” plot (shown below) has two main findings: (1) Same-signed circulation anomalies (represented here by changes in geopotential height) can propagate downward into the troposphere; and (2) circulation changes in the lower stratosphere and throughout the troposphere can persist for weeks beyond the initial stratospheric disturbance. Herein lies the key to why monitoring and predicting the state of the SPV is important for subseasonal winter weather forecasts. For example, evidence of a very weak SPV (or sudden stratospheric warming) often increases probabilities of extreme cold air outbreaks and stormy weather for Europe, parts of East Asia, and sometimes North America. Indeed, Salient has already demonstrated how their data-driven approach can improve cold season forecasts during sudden stratospheric warming episodes.

Stratospheric Wave Reflection - Another Link between the Stratosphere and Troposphere

There is another important way in which variability in the polar stratospheric circulation can cause changes in tropospheric winter weather patterns. This mechanism is called stratospheric wave reflection. In brief, the way that the troposphere “talks” to the stratosphere during the winter is through vertically moving atmospheric waves. These waves carry with them momentum and heat. In the middle and high latitudes of the Northern Hemisphere, once waves enter the stratosphere, they can either: (1) travel right through the stratosphere and dissipate at higher altitudes; (2) break near the edge of the SPV and disrupt its circulation (i.e., the “classic” paradigm); or (3) get reflected and return back to the troposphere. The conditions for wave reflection require there to be a small layer in the lower to middle stratosphere in which zonally averaged westerly winds decrease with height (i.e., negative wind shear). When the waves encounter this layer, much like a mirror, they are reflected back downward into the troposphere, where they then interact and change the shape of the polar jet stream, oftentimes in favor of the Alaskan Ridge weather regime.

Wave reflection episodes occur over relatively short timescales – i.e., the course of a couple of weeks versus several weeks for weak/strong vortex episodes. But, their impacts can be extremely significant, especially for North America. The winter of 2013/14 was a classic winter for stratospheric wave reflection. This winter featured several bouts of extreme cold across much of central and eastern North America, resulting in record energy use and severe damage to agriculture across much of the Midwest. In a study I did with other colleagues, we showed that during the winter of 2013/14, the subseasonal weather patterns favored several episodes of stratospheric wave reflection, resulting in the extended periods of severe cold. Indeed, work by my research group indicates that nearly a third of the severest cold air outbreaks to affect the central US since 1950 has ties to stratospheric wave reflection.

The Bottom Line

The state of the stratospheric polar vortex is one important piece of skillful subseasonal predictions of extreme winter weather. Why? Because variability in its strength and overall structure provides forecasts of opportunities for subseasonal weather forecasts - i.e., periods of enhanced subseasonal skill for temperature and precipitation patterns across the hemisphere, both for weak and strong vortex episodes. 

For North America, specifically, relevant industries (energy, agriculture, transportation) and traders should also understand the importance of stratospheric wave reflection episodes, which have clear ties to severe winter weather.  Though dynamical models currently feature some success detecting both types of stratosphere-troposphere interactions, dynamical models are only skillful out to 10-14 days ahead –  that is, the dynamical models have several inherent biases that limit their prediction prowess for stratosphere-troposphere interactions. 

Salient’s rigorous, data-driven and novel approach to forecasting extends skillful probablistic weather predictions through the full subseasonal lead time window (weeks 1-5), with constant forecast accuracy improvements as a top R&D priority. #forecastfurther

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