Diagram showing the polar stratospheric vortex in strong and weak states. Graphic: Marlene Kretschmer / The Washington Post

By Chris Mooney
22 September 2017

(The Washington Post) – More and more, we are learning that climate change can lead to some pretty strange and counterintuitive effects, especially when it comes to the wintertime.

For instance, scientists have pointed out for a number of years that warmer seas, and a wetter atmosphere, can actually fuel more snowfall in massive nor’easters affecting the U.S. East Coast.

More controversial still is an idea called “Warm Arctic, Cold Continents.” This is the notion that as the Arctic warms up faster than the middle latitudes, it may sometimes cause a displacement of the region’s still quite frigid air to places that aren’t so used to it. In other words, even as the planet warms, masses of cold air could also become more mobile and deliver quite a shock at times when outbreaks occur in more southerly latitudes.

In both November and December of 2016, for instance, temperatures at the North Pole surged tens of degrees above normal while at the same time a huge mass of abnormally cold air descended over Siberia. Capital Weather Gang reported that in November, during one of the excursions, Siberian temperatures were “up to 60 degrees below normal.”

Now, a new study in the Bulletin of the American Meteorological Society makes the case that in January and February — later in the winter than those events — another, perhaps related change is occurring. This one involves the notorious “stratospheric polar vortex,” a loop of extremely cold and fast-flowing air, high in the atmosphere, that tightly encircles the Arctic in the freezing dark of polar winter. This vortex can sometimes develop outward bulges, allowing for a more southerly invasion of air.

The study, led by Marlene Kretschmer of the Potsdam Institute for Climate Impact Research in Germany, sought to find patterns in the stratospheric polar vortex over the past 37 years, categorizing its behavior into seven states, ranging from a tight loop around the Arctic to “a weak distorted vortex.” And it determined that the stronger and more defined vortex has been occurring less frequently, while distorted states have been growing more common — a change linked to colder temperatures over Eurasia.

“This study provides quite some evidence that the cooling trend over Eurasia was at least partly affected by the weakening of the stratospheric polar vortex,” said Kretschmer. [more]

One of the most bizarre ideas about climate change just found more evidence in its favor

ABSTRACT: Over the last decades, the stratospheric polar vortex has shifted towards more frequent weak states which can explain Eurasian cooling trends in boreal winter in the era of Arctic amplification. The extra-tropical stratosphere in boreal winter is characterized by a strong circumpolar westerly jet, confining the coldest temperatures at high latitudes. The jet, referred to as the stratospheric polar vortex, is predominantly zonal and centered around the pole; however, it does exhibit large variability in wind speed and location. Previous studies showed that a weak stratospheric polar vortex can lead to cold-air outbreaks in the mid-latitudes but the exact relationships and mechanisms are unclear. Particularly, it is unclear whether stratospheric variability has contributed to the observed anomalous cooling trends in mid-latitude Eurasia. Using hierarchical clustering, we show that over the last 37 years, the frequency of weak vortex states in mid to late winter (January and February) has increased which were accompanied by subsequent cold extremes in mid-latitude Eurasia. For this region 60% of the observed cooling in the era of Arctic amplification, i.e. since 1990, can be explained by the increased frequency of weak stratospheric polar vortex states, a number which increases to almost 80% when El Niño/Southern Oscillation (ENSO) variability is included as well.

More-Persistent Weak Stratospheric Polar Vortex States Linked to Cold Extremes



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