Kicking the Refrigerator Door Open: How Warming the Polar Stratosphere Means Cold Weather for the SW
In this month’s Southwest Climate Podcast, Mike Crimmins, a climatologist at the University of Arizona and regular blog contributor to SWCCN, explained the mechanism behind the cold surge that passed through the Southwest in mid-January bringing record-cold temperatures to parts of the region. Crimmins introduced the phrase “sudden stratospheric warming,” a phrase that intrigued me. Normally when we have a cold snap, such as the one in February 2011, we hear about the influence of the Arctic Oscillation (AO), a climate fluctuation characterized by differences in the atmospheric pressures over the Arctic and surrounding regions (see CLIMAS feature article and previous blog). But sudden stratospheric warming is a different and unfamiliar attribute, so I decided to delve a little deeper.
First, I was surprised to learn that sudden stratospheric warming (SSW) events occur in about half of all Northern Hemisphere winters—and some of these events affect our weather here in the Southwest; I had no idea! Even more interesting, they appear to have become more frequent over the past decade, possibly related to the rapid decline in Arctic sea ice from global warming (Climate Central).
So what exactly are these events and how do they affect our weather? First, let’s explore what normally happens over the Arctic. During the winter season, a low pressure system of fast-flowing cold air circulates in a cyclonic pattern over the North Pole from the surface to the stratosphere, which is the upper layer of the atmosphere. This is known as the polar vortex. When the vortex is strong, the cold air stays contained, or to use Crimmins’ analogy, “the refrigerator door is closed.” In this case, temperatures are usually mild in the U.S. and Europe. But when the vortex weakens, the cold air is released and spills southward, like a refrigerator door being opened, bringing much colder air to the U.S. and Europe. The Atmospheric and Environmental Research website has a very illustrative animation that shows what a strong and weak vortex looks like over the Arctic.
There are a couple possible reasons for why the vortex might weaken. First, since the two phases—positive and negative—of the AO represent differences in atmospheric pressure between the Arctic and surrounding regions, it influences the strength of atmospheric winds. During the negative phase, high pressure exists over the Arctic, making the winds slacken and allowing cold air to spill south. During a SSW event, another possible reason for a weak vortex, the polar stratosphere warms and high pressure builds over the Arctic, weakening the vortex and kicking the refrigerator door open. In the event that occurred this winter, Crimmins explains that the polar stratosphere warmed because of a giant superstorm that formed off the coast of Japan in late December to early January. The energy from this storm propagated upward into the atmosphere, warming the stratosphere and weakening the vortex.
According to Climate Central, as the stratosphere warms and cold air spills south, the jet stream over the middle latitudes becomes wigglier, with deeper troughs and ridges. This is what allowed that cold air to travel all the way to the Southwest. It usually takes a little while for changes in the Arctic to effect the mid-latitudes, therefore when the stratosphere started warming in early January, forecasters had a few days to predict the cold temperatures we experienced. The Climate Prediction Center has a pretty neat graphic that shows the strength of the vortex since October, alongside the index of the AO (figure 1). You can see that around January 6 it started weakening substantially and is still pretty weak, meaning we might still be in store for some more cold weather in the next few weeks.
So why has this phrase, which is so common among research and forecast meteorologists, become much more mainstream because of this one event in mid-January? According to Crimmins, it’s because the SSW event this year was of epic proportions. He says that meteorologists were stunned at how quickly it occurred and how strong it was, “it [was] by all accounts a record since we’ve been keeping observations of the upper atmosphere.” This, combined with the fact that these events have been becoming more frequent over the past decade—possibly related to melting Arctic sea ice which also set a record last year for the largest melt extent—could mean that we in the Southwest should start getting used to these cold weather snaps because they may become more and more frequent in the coming years.
*For a more in depth discussion of this topic, including how the Arctic Oscillation and SSW interact, listen to this month’s episode of SWCCN’s new Southwest Climate Podcast. And if after listening you have any questions you’d like to hear answered in next month’s episode, just shoot me an email (you can find my address here).