What to Expect from El Niño: How Much Snow Back East?

Some of the impacts from El Niño across the United States are fairly straightforward: hurricane suppression in the Atlantic, for example. Then there’s snowfall in the Northeast, where El Niño is just one of several big factors at work. We touched on the complexities of this topic in Tuesday’s roundup post on typical El Niño impacts during North American winter. (Jeff Masters posted an update earlier today on tropical activity in the Atlantic and Pacific, including an area of interest being tracked in the eastern Atlantic.)

Two main elements to watch in the upcoming winter are:

Temperature. Strong El Niños tend to spread above-average winter readings from the Midwest across the Northeast. With temperatures often borderline along the big East Coast cities during heavy snow, this outcome would push the region toward more rain and less snow, all else being equal. That said, it’s entirely possible for a northeastern winter to average on the milder-than-usual side while still featuring a few periods that are cold enough for big snow. The blockbuster El Niño events of 1982-83 and 1997-98 brought temperatures across the entire Midwest and Northeast well above average, yet the snowstorm of February 10-12, 1983, pummeled big cities along the East Coast with widespread 1-to-2-foot amounts (see Figure 1). The most destructive winter weather event of 1997-98 was actually a multi-day ice storm in early January 1998 that paralyzed Montreal and parts of far northern New York and New England for days (see Figure 2).

The North Atlantic Oscillation. This semicyclic atmospheric pattern describes whether the flow from eastern North America to Europe is a strong, west-to-east channel (a positive NAO) or a more wavy, variable path (a negative NAO). See Figure 3 below. Heavy snow becomes more likely along the eastern seaboard when a negative NAO is present, as the more-variable polar jet stream allows storm systems to dig more sharply along and near the coast before heading poleward as nor’easters. Since the NAO doesn’t have a strong oceanic connection like El Niño does, it varies much more from week to week and month to month.


Figure 1. Snowfall map for the February 10-12, 1983, snow event (the “Megapolitan storm”), which arrived during one of the two strongest El Niño events since 1950. This storm was rated as 6.25 (“crippling”) on the Northeastern Snowfall Impact Scale. Image credit: NOAA National Centers for Environmental information.



Figure 2. A pedestrian walks past fallen branches and trees in Ottawa, Canada, on January 8, 1998. Arriving in the midst of a very strong El Niño event, this ice storm paralyzed much of southeastern Canada and the far northeast United States. Image credit: Dave Chan/AFP/Getty Images.


Figure 3. On average, the surface pressure near Iceland is relatively low (L), while the pressure near the Azores Island is relatively high (H). During a negative phase of the North Atlantic Oscillation (left), this pressure difference weakens. During a positive phase of the NAO (right), the difference becomes even stronger than usual. The variation in pressure patterns influences the strength and location of the jet stream and the path of storms across the North Atlantic. Image credit: climate.gov, adapted from AIRMAP by Ned Gardiner and David Herring, NOAA.


Uncharted territory
The powerful El Niño now taking shape will face off this winter with a recent tendency toward cold and snow across the northeast U.S. During the northern winter of 2015, eastern North America was the only large, populous land mass on Earth that saw temperatures much below average (see Figure 4 below). Over the better part of a decade, winter storms and intense cold have buffeted the northeast U.S. and southeast Canada more often than one might expect in a warming climate. For the NOAA-defined Northeast region, three of the last six winters have seen December-through-February temperatures running below the 20th-century average. Since 2009, Boston has experienced its snowiest winter on record (2014-15) and its eighth snowiest (2010-11), while Washington’s Dulles International Airport has scored its third, fourth, sixth, and ninth biggest one-day snowfalls of the last 53 years.


Figure 4. Eastern North America was the only major region on the globe with temperatures well below average for the period December 2014 – February 2015. Image credit: NOAA National Centers for Environmental Information.


There’s been spirited debate among weather and climate scientists over exactly what’s happening and why. Some researchers, including Jennifer Francis of Rutgers University, have analyzed evidence for a weaker, more variable polar jet stream, possibly associated with Arctic sea ice loss. Others, such as Dennis Hartmann of the University of Washington, emphasize the role of warmth across parts of the tropical Pacific in generating an atmospheric “bridge” that extends to the cold, snowy Northeast. Given that the processes at work aren’t universally agreed on, and we’re still months away from winter, there’s no single diagnosis on what lies in store for the Northeast, but I checked in with several experts to see what they’ll be looking for with the upcoming El Niño. 


According to Francis, we may be wise to expect the unexpected this time around. “In many ways, we are sailing in uncharted waters, because humans have never experienced a strong El Nino in combination with such a warm Arctic and low sea-ice cover,” Francis told me. Another wrinkle: the Pacific Decadal Oscillation, which was in a predominately negative mode from the late 1990s into the early 2010s, has turned strongly positive over the last few months. Positive PDO regimes tend to favor El Niño over La Niña and generally boost global temperatures.

Should the plentiful supply of North Pacific tropical cyclones continue into this fall, said Francis, “it may lead to episodic injections of tropical energy into midlatitudes, which tends to throw large kinks into the jet stream that create highly amplified, persistent ridge/trough patterns like the Ridiculously Resilient Ridge/Terribly Tenacious Trough configuration that has dominated the past two years and caused a variety of extreme weather.” She added: “Recent research suggests that large ridges are intensified by the abnormally warm Arctic, making them even stronger and more persistent. Will this pattern continue into fall/winter 2015? Like I said, we have no chart for the path ahead.”

The Siberian connection
For more than a decade, Judah Cohen (Atmospheric and Environmental Research) and colleagues have been predicting U.S. winter weather based on the state of Siberian snow cover in the autumn. Cohen's hypothesis is that heavier-than-normal autumn snowpack in Siberia can trigger atmospheric waves that disrupt the polar vortex over a period of weeks, leading to a negative NAO tendency by wintertime. The resulting U.S. winter outlooks for Jan-Feb-Mar successfully anticipated the very mild readings of early 2012 and the widespread eastern cold of early 2013 and 2015. (For more background, see this Capital Weather Gang interview with Cohen from last December.)

Cohen pointed to the winters of 2002-03 and 2009-10 as good examples of snowmaking teamwork between moderate El Niño events and a negative NAO, reflected on both sides of the North Atlantic. The snowy winter of 2002-2003 included the “President’s Day II” storm, which plastered East Coast cities from Washington to Boston with 15” to 30” of accumulation, and 2009-10 brought the infamous Snowmageddon as well as several other major winter storms. In both of these cases, Cohen added, the El Niño warming was focused toward the central Pacific, a state known as El Niño Modoki. During the stronger, more classic El Niño of 1997-98, the NAO also tended negative, but mild weather dominated the Northeast, and it wasn’t an especially snowy winter.

“For now, I see little reason to anticipate or favor one NAO phase over the other for this upcoming winter,” Cohen told me. However, he did note that Siberian snow cover tends to run above average during El Niño events. Also, the relative lack of of late-summer/autumn sea ice in recent years over the adjacent Barents and Kara Seas may be allowing extra oceanic moisture to contribute to heavier autumn snows in Siberia, which suggests an enhanced likelihood of negative NAO periods.

Overall, says Cohen, “I have always felt and continue to believe that the relationship between ENSO and winters in the northeastern U.S. is relatively weak and the phase of the NAO is more important. But that may not be true when El Nino is in record-strong territory, especially if the NAO is relatively weak.”


A bridge from the western Pacific to the eastern U.S.
UW’s Dennis Hartmann questions the idea that Arctic sea ice loss is playing the lead role in the recent cold across eastern North America. “This would be remarkable given the small area of the Arctic Ocean and its presence at the tail end of the atmospheric and oceanic chain moving energy from the tropics toward the poles,” wrote Hartmann in Geophysical Research Letters earlier this year. In that paper, Hartmann pointed to another player on the field: the North Pacific Mode. You can think of the NPM as a third sibling in the family that includes the El Niño/Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Each of these explains a portion of the multi-year ups and downs in sea-surface temperature and favored weather patterns across the midlatitude North Pacific, which in turn helps shape weather downstream across the United States. ENSO varies most on timescales of a year or two, while the PDO and NPM are most influential on decade-plus timescales. The NPM is a pattern of warming along the U.S. West Coast and into the North Pacific, with a timescale similar to the Pacific Decadal Oscillation but a different spatial fingerprint. See Figures 5 and 6 for more about the nature of all three siblings.


Figure 5. The three EOFs (empirical orthogonal functions) that represent the bulk of monthly variation in sea-surface temperature (SST) over the ocean area from 30°S to 65°N and 120°E to 105°W. EOF1 is related to the El Niño-Southern Oscillation (ENSO), EOF2 to the Pacific Decadal Oscillation (PDO), and EOF3 to the North Pacific Mode (NPM). The period examined is from January 1900 to July 2014, with positive values in red and negative values in blue. Image credit: American Geophysical Union, with permission, from “Pacific sea surface temperature and the winter of 2014,” Dennis Hartmann, doi:10.1002/2015GL063083.


Figure 6. Time series showing the trends over time in the EOFs depicted in Figure 5 above, expressed here as principal components (PCs) over the Pacific Ocean from 30°N northward. PC1 represents the El Niño-Southern Oscillation (ENSO), PC2 the Pacific Decadal Oscillation (PDO), and PC3 the North Pacific Mode (NPM). The NPM is often strongly positive during the build-up to an El Niño event. The time period shown is from January 1979 to January 2015. Image credit: American Geophysical Union, with permission, from “Pacific sea surface temperature and the winter of 2014,” Dennis Hartmann, doi:10.1002/2015GL063083.


In his 2015 GRL paper, Hartmann linked a positive NPM to to the resilient Western ridge and tenacious Eastern trough of 2013-14. According to Hartmann, the NPM has remained positive throughout 2015 after its big positive jump in 2013 and 2014 (see Figure 6). “It appears the NPM anomalies are controlled by the SST gradient along the equator, especially by the warm conditions west of the dateline and slightly north of the equator,” said Hartmann. Even as the tropical Pacific has gradually shifted into an El Niño configuration over the last year-plus, the patch of unusually warm water identified by Hartmann and colleagues has remained fairly stable. However, said Hartmann, “I expect a large El Niño will dominate and wipe out or reverse the NPM. I don't see a collaboration developing.” Moreover, like Francis and Cohen, Hartmann opted not to offer any prediction on whether the East Coast will see big snow this winter in tandem with the upcoming big El Niño.

Watch for an upcoming post from Jeff Masters that will take a global look at potential El Niño impacts. Meanwhile, I'll have a full update on the tropics by midday Friday.

Bob Henson