The Polar Vortex Has Been Acting Strangely…
This Jan-Feb, a powerful phenomenon known as the polar vortex has been rampant. Its intensity has frequently hit record levels for the time of year, particularly in Feb.
This has been driving a very unsettled run of weather across the northern US and northern Europe. Those regions have been contending with wet and windy weather more frequently than usual.
Usually, the polar vortex begins to gradually wind down as Feb progresses, with more in the way of ‘quiet’ weather to lead us into spring – but this year it’s remained about as strong as it was for most of Jan.
By contrast, it was in a weak state for much of Dec. Even so, the winter mean is likely to be among the highest on record since 1950. The excellent graphic below (courtesy of Dr H. Attard) tells this story via the west-east wind speeds high up in the stratosphere, at 65°N latitude.
And It’s Not Stopping…
Notice the red line. This is the latest Global Forecasting System prediction out to mid-March. Only a small decline in strength – the polar vortex doesn’t know it’s spring! This fight back against the seasonal weakening (it really doesn’t like sunlight!) keeps it record-strong or close to it.
Now, here’s the thing – this situation proves even more unusual when considering that the past winter occurred during the minimum of the solar cycle (i.e. the lowest point within a repeating 11-year oscillation in the sun’s energy output).
Solar Minimum: Not the Vortex’s Strong Point…?
A solar minimum winter typically sees a similarly weal Dec-Feb polar vortex To demonstrate this…
Check out the plot below for the Arctic Oscillation (AO, which correlates strongly with polar vortex strength) – not only is the mean (horizontal bar within the box) lower than for any other category, but the spread of the middle-50% of observations (box limits) doesn’t reach as high either.
Box-whisker plot, showing the ‘shape’ of the data for the historical years, categorised between four different positions within the solar cycle. ‘Falling’ means that solar activity was falling toward the minimum during that Dec-Feb, while ‘Rising’ means it was rising toward the maximum.
can also see that the highest on record for a solar minimum year is only +1.0 sigma, the lowest of the four categories. The minimum years also saw the lowest on record (-1.4 sigma).
This year’s raging polar vortex looks mightily out of place against this background.
An Atypical March for Solar Minimum?
The solar cycle also some relationship with surface temperature patterns in March.
For example, when solar activity is at the minimum (as it is now), there’s usually an increased likelihood of a chilly March across most of Europe and the western half of the USA, (see below, left).
I stress usually because this tends to be with a polar vortex state that we don’t have this year… and when you look at how March tends to be after a strong Dec-Feb polar vortex (below, right), there are significant differences.
Maps of Europe (top) and N. America (bottom) showing mean air temperatures (at 2 m elevation), compared to the long-term average (1981-2010), for March during solar minimum years (left) and following winters with a top-ten mean polar vortex strength across Dec-Feb.
‘I stress usually because this tends to be with a polar vortex state that we don’t have this year… and when you look at how March tends to be after a strong Dec-Feb polar vortex, there are significant differences.’
Notice how about half of Europe has a slight warmer than average signal instead of cooler than average. There’s also a generally warmer look for the US, especially in and around Texas.
This contradiction lowers forecast confidence as to how March 2020 will pan out for many parts the Northern Hemisphere.
There are notable exceptions, however, such as eastern Europe and the north-western US. Here, both indicators point toward a chilly March (increased likelihood of frosts, ice and snow).
This is often the way with long-range forecasting; relatively clear signals for some regions, to which meteorological knowledge may be applied to anticipate what may happen elsewhere.
James Peacock MSc
Head Meteorologist at MetSwift