Weather
Will next season be better? The annual question, asked after both exceedingly deep and depressingly shallow winters, represents not only the hopes and dreams of skiers everywhere, but also one of the biggest challenges in meteorology and atmospheric science. Long-term weather forecasts are notoriously inaccurate. The Farmer’s Almanac Winter Forecast, probably the most famous seasonal weather forecast, does about as good as flipping a coin. Monthly and seasonal outlooks from NOAA’s Climate Prediction Center perform slightly better than chance, but on timescales longer than a few weeks, they offer very little skill in forecasting precipitation. A recently published two–part research article from Florida State University and the Nanjing University of Information Science and Technology has identified a new approach to seasonal weather forecasting based on the Stratospheric Polar Vortex.
Over the North Pole, high up in the atmosphere, is a giant swirling mass of air known as the stratospheric polar vortex. Though most of our weather happens in the troposphere, roughly the first ten miles above the surface, air patterns higher up in the stratosphere can have significant influence over what happens lower down. When the stratospheric polar vortex is strong, it does not interact very much with the jet stream lower in the atmosphere, and cold arctic air tends to stay locked up in the arctic. But, when the vortex weakens, it can cause fluctuations in the jet stream that lead to arctic air getting pushed further south into North America, bringing with it frigid temperatures. The strength of the stratospheric polar vortex varies from year to year, and has been linked to long periods of extremely cold temperatures in North America, like in 2019 and 2021. Other season-to-season variations, like the temperature of the Pacific Ocean near the equator, can have significant influence over the weather on a seasonal timescale. The El Niño Southern Oscillation is the most famous, and the most used to generate seasonal winter weather forecasts. Other oscillations like the Madden-Julian Oscillation or the Arctic Oscillation, as well as the interactions between these different processes, have been used to try and generate seasonal weather forecasts by atmospheric scientists and mountain managers alike, but often with limited results. Big Sky probably would like us to forget about their A.I.-based foray into seasonal weather forecasting that brought a prediction of above average snow totals made last September.
Using more than 40 years of atmospheric data, the team at Florida State and Nanjing found that certain attributes of the stratospheric polar vortex follow recognizable patterns from year to year. The wind speed at 60 degrees N and the total air mass of the vortex were found to follow elliptical orbits throughout a one year period running from July to June. These two measures of the intensity of the vortex varied from year to year, but always formed roughly elliptical shapes when plotted together, with different lengths, widths, and angles from year to year. The forecasting method is based on this elliptical orbit that the zonal winds and the total air mass follow from year to year. Essentially, data collected from July through October traces out part of the ellipse that the vortex is likely to follow for that year, then forecasters can trace out the rest of the path through the winter months to determine how strong or weak the stratospheric polar vortex will be. This method does not lead to predictions for how much snow will stack up at the base of Palisades Tahoe or when the Back Bowls at Vail will have enough snow to open, but it can help guide forecasters thinking about how frequently storms are likely to form in North America. Knowing that there are likely to be less powder days over a given season may help skiers decide if it’s worth calling in with the powder flu, or if there should be plenty more storms on the way throughout the season.
Estimating the strength of the stratospheric polar vortex throughout the winter from the elliptical pattern between the polar winds and the total air mass offers a distinct advantage over other methods of seasonal forecasting, the forecast errors are balanced throughout the winter. Numerical weather prediction relies on complex simulations of what the atmosphere might look like further and further into the future. Operational models like the Global Forecast System offer dependable accuracy for the first five days, then deteriorating performance, to the point where they become unreliable around the ten day mark. Other numerical models focused more on seasonal forecasting may offer useful forecasts on slightly longer timescales, but they also have growing errors for forecasts further and further into the future. The authors of the Florida State-Nanjing study reported only modest forecasting skill for November and December, but that the skill actually increased as time went on, peaking in about March. If the shape of the ellipse predicted in October is correct, then the evolution of the vortex over the entire season will be close to the prediction.
- Related: Antarctica’s Ice Sheet Is…Growing?
The atmosphere is awash with a complex balance of short-term and small-scale weather systems, and longer, much bigger atmospheric cycles that make seasonal weather forecasting a formidable challenge. As researchers learn more about connections between long-term climate oscillations like the Arctic Oscillation, El Niño, the Madden-Julian Oscillation, and others, seasonal weather forecasts may grow steadily more accurate. The Stratospheric Polar Vortex is an important driver of seasonal weather for North America, and new methods for forecasting its influence represent important advances in our understanding of the atmosphere. As we finish out the last little bit of the current ellipse, which has brought unseasonably warm temperatures for much of the winter, let’s hope that the next few months begin tracing a much weaker path for the stratospheric polar vortex in hopes that more powder days and deeper snowpacks will follow next winter.
