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)

Advertisements
%d bloggers like this: