Polar Warming Spawns More Severe Winter Storms

So there’s a lot of groundbreaking work going on in the climate sciences right now. And a major focus is evidence that winter polar warming events are increasingly connected to blizzards and storms in places like Europe and North America. Storms that are both historically powerful and that occur with greater frequency.

(A historic nor’easter produces major flooding on the U.S. East Coast even as a blizzard pounds the UK in early March. Were these extreme storms linked to human-caused climate change and related rapid polar warming? A new scientific study says — yes. Image source: NASA Worldview.)

A new study led by pioneers in the emerging field of climate change attribution for extreme weather events (including the notable Dr. Jennifer Francis), finds:

Recent boreal winters have exhibited a large-scale seesaw temperature pattern characterized by an unusually warm Arctic and cold continents… Using a recently developed index of severe winter weather, we show that the occurrence of severe winter weather in the United States is significantly related to anomalies in pan-Arctic geopotential heights and temperatures.

In particular, the authors discovered thatwinter storms were two to four times more likely when the Arctic is abnormally warm, compared to when it was abnormally cold (emphasis added).”

Stronger, More Frequent Storms

This is a rather big deal for a number of reasons. First, it’s an observational confirmation of earlier scientific work predicting just these kinds of extreme weather instances due to polar warming and related climate change. Second, it’s another indicator that human-caused climate change is pushing us into a period of much stormier weather for the North Atlantic region during fall and winter.

(A new study in the journal Nature finds that winter storms in the U.S. are two to four times more likely when the Arctic is abnormally warm than when it is abnormally cold. Due to human-caused climate change, the Arctic is now warming up at a rate two times faster than the rest of the globe (emphasis added). Image source: Atmospheric and Environmental Research.)

With the new NASA global temperature data set out, I thought we’d take this opportunity to apply a bit of context to apparent stormy changes we see at present in winter weather patterns.

The first bit that I’d like to be crystal clear about is that the Arctic, overall, has become much, much warmer than usual during winter. That this warming spike occurs in the context overall global warming. And that this polar warming is increasingly associated with severe weather events in the middle latitudes and especially over the land and North Atlantic mid latitude zones.

The above graph shows polar temperature anomalies from the surface (1000 mb/2 meter) of the Earth to the top regions of the atmosphere (10 mb/25 kilometers). Along the bottom of the graph, we have a list of extreme weather events. Analyzing the graph we find that major polar warming associated with extreme temperature increases at the bottom of the atmosphere all the way through to the stratosphere correlate with recently more frequent historic blizzards and nor’easters in the regions mentioned.

Polar Warming Flushing Cooler Air into the Middle Latitudes

In previous posts, I used the ground-breaking scientific research of Dr. Jennifer Francis and others as a basis to analyze how energy transfer into the polar zone in the form of heat build-up has generated these extraordinary temperature extremes. How this ramping heat is associated with polar amplification — an aspect of human-caused climate change. And how these warming events can have upstream (Jet Stream) impacts that increase storminess in the middle latitudes.

(From January [top] to February [bottom] the pole heats up and extreme weather events ensue. Image source: NASA.)

But let’s take this analysis a step further to look at, as January progressed into February, where it got warmer, where it got colder, and where the big storms fired off.

The maps above show global temperature anomalies (NASA) for January (top) and February (bottom). And looking at those maps we find that the polar region heated up significantly from already warm ranges of 4 to 6.9 degrees Celsius above average during January to an amazing 4 to 12.3 C above average during February.

As this relative polar warming increased during February, the NASA maps show that colder than normal temperatures expanded over North America through Canada and parts of the Northern U.S. even as a cold spell began to blossom in Europe. Cold pools that were fed by Arctic air shunting southward as the Polar Vortex collapsed and remnant continental troughs emerged.

NASA’s zonal anomaly measures provide further evidence for this trend.

(Major northern polar warming from January [top] to February [bottom] is clearly visible in NASA’s zonal anomalies maps. Note that despite cold air excursions into North America and Europe, most zonal regions are warmer to much warmer than average.)

For here we find that as temperatures spiked from 4.5 degrees Celsius above average in the polar region of 80 to 90 degrees north latitude during January to an amazing 11 degrees above average during February, the region of 45 to 70 N dipped from 1 to 3 C warmer than average to 0.8 to 2.5 C warmer than average.

Note that the zonal middle latitude continental cooling is moderated by both the relatively warmer oceans and by very strong ridge zones running through these regions. But that the trough regions over both Europe and North America produced locally frigid temperatures and related instances of extreme weather.

Putting all these maps together from top to bottom we find that the polar warming events coincided both with mid latitude cooling even as we saw extreme snowfall in Canada and Montana, historic cold and snowfall in Europe and the UK, record flooding in the Central U.S., and record heat along the U.S. East Coast. We also find that the developing deep trough over Canada due to the expulsion of polar air southward in turn produced the succession of instabilities that would later spawn 3 very severe nor’easters off the U.S. East Coast during March.

Of course, all of these severe weather events are happening in the context of months that are around 1 degree Celsius warmer than 1880s averages globally. That January was the fifth hottest on record and that February was the sixth hottest on record during a La Nina that, all things being equal, should cause the world to be cooler than average.

But as we can see clearly here, all things are not equal — human-caused climate change is a big spoiler.

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Why are So Many Powerful Nor’Easters Striking New England?

A major nor’easter is pummeling states from New York through Maine today with heavy snow, near hurricane force winds, and high surf. The storm is expected to dump 1-2 feet of snow over this region even as it pounds coastlines that have already been raked by two other major storms during the past two weeks.

It would be relatively unusual to see one storm of such intensity striking this region during any given March. But as the third in a two-week-long parade of extreme events, the presently intense storm pattern is starting to look more than a little outlandish.

So what the heck is going on? In a couple handfuls of words — influences related to human-caused climate change are spiking East Coast storm intensity while setting in place a general pattern that causes these storms to repeatedly fire.

(Over the past 11 days, three major nor’easters have struck the U.S. East Coast. Why have these storms been both so strong and such a persistent feature? Image source: RAMMB/CIRA. H/T to Chris Dolce.)

The Most Recent of Three Powerful Nor’Easters

Presently, the most recent strong storm has an intensity of 970 mb and features winds gusting to hurricane force just off-shore with gusts of up to 69 mph along the coast. Pressures are expected to drop into the upper 960s — making it about as powerful as the system that produced major flooding in parts of New England on March 2nd.

For reference, storm intensity measured by pressure in the range of 970 mb is about as strong as a category 2 hurricane. This is a rough comparison as hurricanes tend to be more intensely concentrated even as nor’easters tend to have broader if more diffuse impacts. But it’s a marker for the high level of atmospheric energy the system is now pumping out and how potentially damaging it could ultimately become.

The storm is thus strong enough to produce record and historic impacts. This is notable enough by itself. But the fact that we have had three systems of similar strength in just 11 days over what is practically the same region is concerning.

(Global warming fuels increased convection as lands waters pump out more heat and moisture. At times, this can result in some unexpected instances of atmospheric pyrotechnics.)

Specifically, on March 7 a 989 mb system raked the same region with gale force winds and instances of intense thundersnow (see above tweet by NOAA). And on March 2nd, a sprawling storm that dipped to around 975 mb generated massive waves and significant coastal flooding.

Atmospheric Train Wreck

Looking for causes, we need to go all the way back to February. At that time, a big polar warming event was taking place. In the upper levels of the atmosphere over the pole, the stratosphere was warming up. But at the same time, surface temperatures at the pole were rising to above freezing. In some locations near Northern Greenland, readings were pushing as high as 63 F above average.

High amplitude Jet Stream waves were eating away at the typically faster polar circulation patterns even as they were helping to inject much warmer than normal air into the Arctic and pull its resident cold air out. Eventually, all this heat running into the various layers of the Arctic atmosphere drove the polar vortex to collapse. This, in turn, resulted in cold Arctic air being ejected south and west into Europe. This massive jet stream dip, in eddy-like fashion produced a large, countervailing high pressure ridge over Greenland.

(A deep trough that has consistently lingered over the U.S. East Coast and helped to spawn storm after powerful storm, was initially generated by a very intense polar warming event linked to human-caused climate change. Image source: Earth Nullschool.)

The rippling upper level jumble of winds backed all the way to the U.S. East Coast — forming a deep and persistent trough. The trough funneled numerous disturbances slowly through the region. And it was both the trough’s persistence and depth that enabled strong storms to form repeatedly even as they set off such long-lasting and intense impacts (see Dr Jennifer Francis’s related work on how polar amplification impacts the Jet Stream here).

Much Warmer than Normal Ocean Waters

Though polar amplification — which is another term for how global warming spurs the poles to heat up faster than the rest of the world — helped to generate the upper level features in the atmosphere that would consistently generate storms running across the U.S. East Coast, widespread warmer than normal ocean waters helped to give these storms more fuel.

In the Gulf of Mexico, sea surface temperatures have consistently ranged between 0.5 and 3 C above normal since February. These warm ocean waters contributed to severe floods over the Ohio River Valley at that time by pumping record levels of atmospheric moisture into the storms running south.

(Much warmer than normal sea surface temperatures dominate throughout the Gulf of Mexico and just off the U.S. East Coast. These warmer than normal waters — warmed by climate change — are providing fuel for the powerful nor’easters of recent weeks. Image source: Earth Nullschool.)

As the Jet Stream dip became more oriented toward the East Coast during March, storms that would ultimately blow up over the Atlantic at first got a big plug of moisture from the extra evaporation flowing off that warmer than normal Gulf. But it was over the Atlantic Ocean that the storms would really start to fire. There, ocean temperatures were ranging between 0.5 and as high as 9 C above normal over parts of the Gulf Stream.

Such very warm sea surfaces provide a lot of fuel in the form of moisture and related convection. And, in particular, we saw some rather amazing instances of convective lift during the recent March 2nd and 7th storms as they tapped that incredible Atlantic Ocean heat and moisture.

Conditions in Context

So to sum up, an extreme polar warming event driven in large part by human-caused climate change set up conditions that generated a persistent trough over the U.S. East Coast. This trough was both deep and long-lasting. As low pressure systems moved into the trough zone, they were able to tap abnormal levels of heat and moisture rising off of the Gulf of Mexico and Atlantic Ocean near the coast in order to bloom to abnormally powerful intensity. Both of these factors — Arctic warming and warmer than normal sea surface temperatures — would not have been as acute or intense without the extra push to the climate system that human forced warming provides. As a result, we are seeing a very strong climate change related signal in the present severe storm pattern.

(UPDATES TO FOLLOW)

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