As January ended, Milwaukee—along with a huge swath of the country—plunged into the deep freeze, the result of Arctic air breaking out into the mid-latitudes in North America. The extreme cold was a factor in more than 20 deaths and may result in about $5 billion in economic losses according to AccuWeather.
It is somewhat counter-intuitive that our sub-zero temperatures may have been the result of climate change and warming global temperatures. During the past 50 years, the Arctic has warmed twice as fast as the rest of the planet. Warmer temperatures caused more ice to melt and exposed more heat-absorbing dark ocean water that contributed to warmer temperatures at the poles. A warmer Arctic means that the polar vortex winds are weakened and disrupted by the reduced temperature difference between the Arctic and lower latitudes.
The jet stream, which normally swirls below the polar vortex, is also weakened by these decreased temperature differences, which cause the jet stream to meander and to drag frigid Arctic air into the lower latitudes. Conversely, the meandering jet stream also drags warmer lower-latitude air to the polar region, compounding the polar vortex disruption and causing more polar warming. Paradoxically, even with a warming climate, these extreme temperature polar vortex weather events at mid-latitudes are becoming more frequent, according to a 2018 study published in the Journal of the American Meteorological Society.
We know that when Lake Michigan freezes over, this generally results in less evaporation, which causes lake levels to rise. With more polar-vortex events likely, wouldn’t this lead to higher lake levels and cooler lake (and air) temperatures in southeastern Wisconsin? Maybe, maybe not, says Steve Vavrus, senior scientist at the Nelson Institute Center and expert on global climate change, Arctic climate, Wisconsin climate and extreme weather.
“All other things being equal, we do expect less evaporation when the lakes freeze over, but in order for them to have frozen over, there must have been a lot of cold air beforehand and thus a lot of evaporation,” Vavrus says. “Also, frozen lakes mean less lake-effect snowfall, so that results in less meltwater eventually going into the lakes during spring. The first of these effects favors higher lake levels, while the latter favors lower levels.”
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