I was running around Manhattan the night of December 22, and the temperature was 13C. Being a bratty Californian, I admit I was thrilled to not be shivering. But the responsible part of my brain also found it disconcerting.
It's even moreso when you look at the numbers. Discovery News reports:
Bellingham, Wash., for example, saw a high of 60 (15.5C) in the first week of the year, while the mercury soared to a balmy 44 (7C) in Fargo, 61 (16C) in New York City, 72 (22C) in parts of Colorado and 79 (26C) in Tucson. In some regions of the Midwest, temperatures are higher than average. And snow covers just 19 per cent of the country at the moment, compared to a usual coverage of about 50 per cent at this time of year.
The immediate culprit is a super weird jet stream, which influences weather across North America. The jet stream is fuelled by the North Atlantic and Arctic Oscillations, which are climate patterns determined by differences in sea-level pressure. A positive pressure difference, which is what we've been experiencing in the past few months, draws warm air from the southwest over the eastern United States instead of cold air from the Arctic which is what happened last year and why tons of snow dumped on us.
Piling on the weather weirdness, La Niña conditions have pushed warm water toward Australia, decreasing Pacific Ocean temperatures off the US to lower than normal. That also means less precipitation, because colder water is less likely to evaporate.
A lack of snow is one more temperature raiser: snow reflects the sun, adding moisture to the air and lowering the temp. But the naked earth is free to absorb solar radiation, making the ground toasty.
What about climate change, you ask? Stu Ostro, senior director of weather communications at The Weather Channel told me in an email that melting ice in the Arctic, caused by increasing temperatures over the past few decades, could be influencing those extreme pressure differences:
The weather oscillations over the North Atlantic Ocean and in the Arctic have been off-the-charts extreme in recent years, likely driven at least in part by the precipitous decline in the volume of Arctic sea ice. This is something which warrants close attention and further study, and what happens with atmospheric patterns in the far north also directly affects weather to the south of there. An example of that is the past three winters including this one.
Those positive oscillations will likely head in the opposite direction in the next few weeks, weather experts say, meaning the northern hemisphere won't completely miss out on a more a normal, nippy winter.