If it appears that the weather has become more volatile recently, then appearances may not always deceive. And there may be a reason behind the wild swings in weather—likely having to do with rapid changes in the Arctic.
First, some background: Each hemisphere on Earth is divided into three main regions—the tropics, the midlatitudes, and the polar regions (in the Northern Hemisphere, the Arctic). Each region occupies 30 degrees of latitude. But because Earth is a sphere, the tropics, which occupy the widest band of the sphere, are much greater in area than the midlatitudes and even more so than the polar regions.
In addition, because the tropics face the sun more directly and the polar regions more obliquely, the tropics are the warmest regions on Earth and the polar regions the coldest. It’s long been a tenet of climate science that the tropics exert the greatest influence on Earth’s climate system simply because they are the largest and most “energetic” region on Earth, given that the tropics are also the warmest.
In contrast, the Arctic, which is the smallest region and the coldest—and therefore the least energetic—has been assumed to be much less likely to exert any noticeable influence outside its domain. In other words, what happens in the Arctic stays in the Arctic.
Another tenet of climate science has been that as Earth warms, the coldest regions and the coldest seasons should warm the fastest. These ideas are known as Arctic amplification and winter amplification, respectively. The conventional wisdom is that, as Earth warms, it does not warm evenly, but rather, the coldest air masses warm more quickly than the warmest air masses. Thus, the differential heating of the air masses should lead to more muted or diminished variability in temperature swings or extremes.
The climate science establishment stubbornly clings to these tenets, believing that only the tropics can influence the weather in the midlatitudes and that temperature swings will become less extreme. But the public and the media have been much quicker to adopt more “heretical” ideas: first, that the Arctic has had a substantial impact on our weather through the vacillations and meanderings of the “polar vortex,” and second, that temperature extremes have become more, not less, extreme—a situation described by the term “weather whiplash.”
So how is it possible that the Arctic can influence our weather here in the midlatitudes despite its deficit both in area and energy content? Based on arguments from thermodynamics, it’s hard to conceive how the Arctic can compete with the tropics when the amount of energy is so much greater in the tropics. But this thinking neglects the importance of dynamics on the weather, and with respect to dynamics, the Arctic can be a force comparable to that in the tropics. For example, we know the speed of the jet stream is simply a function of the temperature difference between the tropics and the polar regions. And this relationship is not dependent on or standardized by relative area size and is also not sensitive to the absolute value of the temperature. So a one-degree change in the Arctic is equal to a one-degree change in the tropics in their relative impact on the jet stream.
Even the climate establishment agrees that temperatures are changing more rapidly in the Arctic when compared with other regions on Earth. Therefore, it’s likely that the Arctic is the region on Earth exerting the biggest influence on any changes in the velocity of the jet stream. The jet stream is a fast ribbon of air that carries with it weather systems, and our weather is derived from the speed, path, and configuration of the jet stream.
It’s not controversial to argue that the Arctic may influence the speed of the jet stream. But it’s the path and configuration of the jet stream that likely have a bigger impact on our weather, and it’s certainly very controversial to argue that the Arctic is influencing the jet stream’s path and configuration. Also, the jet stream mainly zips along over the midlatitudes, and therefore it’s not obvious how changes in the Arctic modulate the path and configuration of the jet stream, which resides mostly outside the Arctic.
To advance the idea that the Arctic is exerting a significant impact on the jet stream, the polar vortex is required. The jet stream resides in the troposphere (the lowest layer of the atmosphere—an altitude where planes fly) and mostly in the midlatitudes. But in winter, there appears another fast ribbon of air that resides in the stratosphere of the polar regions, aptly named the polar vortex. Empirical studies clearly show that variability in the jet stream and in the polar vortex are coupled (although the theory as to why is still unclear). So the speed, position, and configuration of the jet stream usually follow or mimic those of the polar vortex.
And since the polar vortex resides in the Arctic, it seems plausible that Arctic change influences the polar vortex and, by proxy, the jet stream. Furthermore, it’s the proximity of the Arctic to the polar vortex that provides the Arctic with a dynamic advantage over the tropics in influencing the polar vortex. Indeed, changes in the Arctic have been of such magnitude that they’ve more than compensated for the relatively low energy of the Arctic—so much so as to impact the polar vortex.
The media has been quickest to recognize the connection between Arctic sea ice and the polar vortex, where melting sea ice has resulted in a weakened and more meandering polar vortex. A weakened polar vortex results in more persistent and severe winter weather (see figure 1 below). AER’s research has shown that, in addition to sea ice variability, the polar vortex is also sensitive to greater snow cover in Siberia and weather patterns or waves across the high latitudes. Siberian snow cover in the autumn has increasingly reinforced the weakened and more meandering polar vortex caused by less sea ice.
The extended polar vortex brings warmer air to the Arctic and colder air to the midlatitudes, thereby increasing the chances of severe winter weather across the industrialized nations throughout the Northern Hemisphere. It also increases temperature extremes because cold air from the Arctic and warm air from the tropics are more efficiently mixed in the atmosphere. This process leads to weather whiplash.
But the proof is in the pudding: an analysis of the relative importance of the tropics versus the Arctic on midlatitude weather over the past 30 years. AER researchers analyzed the atmosphere for the most fundamental trends in wind and temperature and then compared those trends with the known influence of the tropics and the Arctic on those same two variables. To represent the tropics, the El Niño Southern Oscillation (ENSO) was the preferred metric; and to represent the Arctic, both Arctic sea ice and Siberian snow cover were analyzed. At the end of the experiment, the results were clear and decisive: the influence of the Arctic was more easily discernible than that of the tropics, and the competition wasn’t even close. Finally, we also measured the amplitude of temperature swings in the atmosphere, which have increased rather than decreased, as previously argued. The analysis showed the greatest increase in variability in the entire atmosphere has occurred in the region of the polar vortex, further strengthening the dynamic link between the vortex and weather.
The focus of the climate community has been squarely on the tropics when trying to anticipate weather in the midlatitudes. And yet, incredible changes have taken place in the Arctic, the most striking of which has been the rapid disappearance of sea ice. At the same time, the mild snowless winters promised by climate change swiftly gave way to extended cold snaps and historic snowstorms. To both the media and public, a connection was quick and obvious. The cold has not been persistent but rather punctuated by large swings in temperature and unseasonable warmth. The temperature swings have been so pronounced as to be called weather whiplash. But while the public made the connection, the climate science establishment remained skeptical, ignoring the observations and instead clinging to dogma. The debate will likely continue, even as it becomes clearer that what happens in the Arctic does not always stay in the Arctic.
This study was published in the peer-reviewed journal Geophysical Review Letters (J. Cohen, Geophys. Res. Lett., 43, 2016, doi:10.1002/2016GL069102) and supported by the National Science Foundation under grants PLR-1504361 and AGS-1303647.
Strong and weak polar vortex. Plot shows strong polar vortex from December 2015 (left) and weak polar vortex (vortex is split into two pieces) from March 2016 (right). Shown are 10 hPa geopotential heights (decameters; contours) and temperature anomalies (degrees C; shading). In the free atmosphere, geopotential heights are strongly related to temperatures. In the strong polar vortex case, all the cold temperatures are wrapped up with the polar vortex over the Arctic. In the weak polar vortex case, the cold air spills out of the Arctic to lower latitudes, while temperatures warm over the Arctic.