What to Expect from El Niño: North America

We’re now well into the ramp-up phase of what promises to be one of the top three El Niño events of the last 60-plus years. Sea-surface temperatures (SSTs) in the Niño3.4 region--an area straddling the eastern tropical Pacific--are the most widely accepted index for the oceanic evolution of El Niño. NOAA announced in its weekly ENSO update on Monday (see PDF) that Niño3.4 SSTs were running 1.6°C degrees above the seasonal average for the week ending Monday. While this is down slightly from a peak of 1.7°C the week before, Michelle L’Heureux reminds us in NOAA’s ENSO Blog that minor weekly variations aren’t worth getting too worked up about. The latest value still keeps the current El Niño in the “strong” category (Niño3.4 SSTs at least 1.5°C above average). Unusually warm waters now extend from the South America coast westward to the International Date Line in a classic El Niño signature (see Figure 1), with widespread above-average SSTs at least partially related to El Niño extending northward across much of the northeast Pacific. For much of 2014, the atmosphere failed to respond to several brief warmings of the eastern tropical Pacific, but now both ocean and air are locked into the synchrony that builds and sustains the strongest El Niño events. Westerly winds bursts continue to kick up across the tropical Pacific, pushing warm water downward and eastward in the form of lumbering, downwelling Kelvin waves that push toward the shores of South America, where they act to suppress the normal upwelling of cooler water.


Figure 1. Sea surface temperatures for the week ending July 22 were more than 1°C above average from the eastern tropical Pacific northward through much of the northeast Pacific, with pockets of 2 - 4°C above average evident near the equator. Image credit: NOAA National Centers for Environmental Information.


Figure 2. Precipitation departures from average for the period November through March for the five events characterized as “strong” (SSTs in the Niño3.4 region of at least 1.5°C above average for at least three overlapping three-month periods). The final two events, 1982-83 and 1997-98, are characterized by Jan Null as “very strong,” with SST departures of more than 2.0°C above average. Image credit: Jan Null, Golden Gate Weather Services, from data and graphics generated by NOAA/ESRL/PSD and CIRES-CU.



The only El Niño events in NOAA's 1950-2015 database comparable in strength to the one now developing occurred in 1982-83 and 1997-98. A single pair of cases is a thin framework on which to build any projections of what El Niño may bring across North America this winter. However, three other episodes since 1950 are rated as “strong” (Niño3.4 readings topping the SST threshold of +1.5°C for at least three overlapping three-month periods). Many of the far-flung atmospheric responses to El Niño become more reliable the stronger the event, so it’s wise to look especially closely at these cases, rather than simply averaging across all El Niño events.

Later this week, Jeff Masters will take a look at the global consequences of El Niño for weather and climate. In today’s post, we’ll focus on North America, which has some of the world’s clearest tie-ins to El Niño--not surprisingly, since we’re located just north of the oceanic heart of the phenomenon.

Drought-easing rains for California? Likely, but not certain
Some of the keenest interest in El Niño lies with Californians, who are suffering through Year 4 of an extreme drought that’s left Sierra snowpack in tatters and pushed statewide average temperatures far above anything on record over the last few months. The state needs a very wet winter just to get soil moisture back to near-normal levels, and a good deal more than that to bring California’s reservoirs and groundwater close to their long-term average. "It takes years to get into a drought of this severity, and it will likely take many more big storms, and years, to crawl out of it," said NASA’s Jay Famiglietti at an American Geophysical Union talk last December.

Like other strong El Niño events, this one will almost certainly last just one winter. But at least for the coming wet season, it holds encouraging odds of well-above average precipitation for California. During a strong El Niño, the subtropical jet stream is energized across the southern U.S., while the polar jet stream tends to stay north of its usual winter position or else consolidate with the subtropical jet. This gives warm, wet Pacific systems a better chance to push northeast into California. During 1997-98, downtown San Francisco scored its largest number of days with measurable rain (119) and its second wettest rainfall season (47.22”) since records began in 1849, coming in behind only 1861-62 (49.27”). The 1982-83 event was the fifth wettest in San Francisco annals, with a wet-season total of 38.17”. In downtown Los Angeles, the 1982-83 and 1997-98 seasons came in as fifth and sixth wettest, respectively, with 31.25” and 31.01”. Records began in L.A. in 1877.

Californians will need to be patient, as the biggest drenchings from a strong El Niño can take till the midwinter peak of the wet season to arrive (December can actually be drier than average). The 1997-98 season didn’t produce much more than sporadic storms until January in northern California and February over the state as a whole. The story was similar in 1982-83, which brought California its biggest storms after New Year's. This was before regular monitoring of El Niño, so scientists and the public didn't even know that a wet winter was in the cards. Jack Williams, who founded the USA TODAY weather section when the newspaper debuted in 1982, has said he doesn't recall writing a single article about El Niño in the winter of 1982-83. Things were different in 1997-98, when ocean monitoring systems caught the development of El Niño months ahead of its U.S. impact and word spread widely through traditional media and the burgeoning World Wide Web (and via Chris Farley in a brief but unforgettable “Saturday Night Live” skit).

With hopes for drought relief running so high in California, it can’t be stressed enough that El Niño shifts the odds but doesn’t guarantee the roll of the meteorological dice in any particular winter. On the plus side, the heavy rains that often accompany a strong El Niño don’t necessarily translate into major flooding damage. That threat hinges largely on the timing, intensity, and location of individual storm systems, which can cause problems during La Niña or El Niño alike.


Figure 3. A luxury home in the Orange County suburb of Laguna Niguel slips down a hillside eroded by heavy El Nino generated rains earlier on March 19, 1998. Two homes and seven condominiums were destroyed in the slide. Image credit: Vince Bucci/AFP/Getty Images.


Milder and drier a good bet for Pacific Northwest, Northern Plains, western Canada
The altering of the polar and subtropical jet stream tracks fostered by El Niño can leave a big chunk of North America in the lurch, with relatively tranquil weather that tends to be warmer and drier than average. Unusually mild weather can overspread most of Canada---1998 was the nation’s warmest year on record, though 2014 didn’t pan out that way)---and the mildness often extends across the northern tier of US states from Washington to the Great Lakes. (The winter of 1997-98 was the second warmest in U.S. history.) It won’t necessarily be bone-dry in normally damp places like Washington or British Columbia, but anything less than average precipitation wouldn’t be good news for that region, which has seen wildfires taking advantage of a warm winter with little snowpack followed by a very dry spring. Cliff Mass (University of Washington) does cite one potential benefit to the Pacific Northwest this winter: “Big windstorms avoid strong El Niño years. Similar to vampires and garlic.”

Rockies snowfall: The south usually wins out
Thanks to the jet-shifting effects noted above, snowfall tends to be below average in the Northern Rockies and above average in the Southern Rockies during strong El Niños. The north-south split extends to Colorado, where northern resorts such as Steamboat Springs typically lose out to areas like the San Juan and Sangre de Cristo ranges across the southern part of the state. Along the populous Front Range from Denver to Fort Collins, El Niño hikes the odds of a big snowstorm, especially in the spring and autumn. About half of Boulder’s 12” – 14” storms occur during El Niño, and the odds of a 20” or greater storm are quadrupled during El Niño as opposed to La Niña. See this UCAR writeup for more detail.

Rainy and cool across the Gulf Coast
According to NOAA, the single most reliable El Niño outcome in the United States, occurring in more than 80% of El Niño events over the last century, is the tendency for wet wintertime conditions along and near the Gulf Coast, thanks to the juiced-up subtropical jet stream. (The same upper-level jet also tends to suppress Atlantic hurricane activity by fostering subsidence and stable air and boosting the upper-level wind shear that inhibits tropical cyclone formation). Severe weather is often associated with El Niño during the winter months across the southeast fringes of the nation, a finding reinforced in a 2015 study led by John Allen (International Research Institute for Climate and Society) that we discussed in a March post. The study found that the risk of tornadoes across south Texas and Florida is roughly doubled during El Niño. Florida's worst outbreak on record occurred on February 22-23, 1998, during the intense 1997-98 El Niño. A total of 12 tornadoes killed 42 people, mainly in a swath running along Interstate 4 through central Florida.


Figure 4. A resident of a Kissimmee, Florida, residential complex picks up some of her belongings from what is left of her home after a tornado leveled her house and ripped through the neighborhood on February 23, 1998. Image credit: Roberto Schmidt/AFP/Getty Images.


A dry pocket in the Midwest
One fairly localized but distinct product of El Niño is a tendency for drier-than-usual winters across the lower Midwest, especially in the Ohio Valley. A typical winter brings a stream of low-pressure centers approaching the lower Midwest from either the southwest or northwest. The split stream favored by El Niño tends to push these lows either well north or well south of the Ohio Valley, leaving the area with better-than-usual odds of relatively mild temperatures and light precipitation during the core of winter.

What about the Northeast US?
Some of the bigger snowstorms on record for the mid-Atlantic and Northeast have accompanied El Niño events, but the influence of El Niño is highly conditional on other factors. The blockbuster El Niño events of 1982-83 and 1997-98 pushed temperatures across the entire Midwest and Northeast well above average, yet the ”Megapolitan” snowstorm of February 10-12, 1983, pummeled big cities along the East Coast with widespread 1-to-2-foot amounts. The most destructive winter weather event of 1997-98 was actually a multiday ice storm that paralyzed Montreal and parts of far northern New York and New England for days. And the infamous Snowmageddon of 2009-10 (see Figure 5) occurred during a moderate El Niño. One crucial element is the state of the North Atlantic Oscillation (NAO), a semi-cyclic atmospheric pattern that 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). Heavy snow during El Niño becomes much more likely along the eastern seaboard when a negative NAO predominates. Winter hasn’t been especially kind to the northeast U.S. and eastern Canada over the last few years, with frequent negative NAO periods leading to major winter storms and intense cold buffeting the region more than one might expect in a warming climate. In a follow-up post to this one, we took a closer look at the imminent face-off between a powerhouse El Niño and the recurrent tendency over the last few years toward cold and snow in parts of the Northeast.


Figure 5. Walkers struggle through the snow in front of the U.S. Capitol Building on February 6, 2010, in Washington, DC. The blizzard that came to be known as Snowmageddon dumped 20” to 35” of accumulations from the Washington area into southern New Jersey, paralyzing the region and snapping power lines. Image credit: Mandel Ngan/AFP/Getty Images.


Villain or welcome guest?
When all is said and done, a strong El Niño can actually be a net benefit to the US economy. A detailed analysis by climatologist Stanley Changnon, published in 2004 in the Bulletin of the American Meteorological Society, estimated that U.S. weather associated with the big 1997-98 El Niño event led to direct losses of about $4 billion but direct gains of around $19 billion. Moreover, Changnon reckoned that hundreds more lives were saved by the lack of intense winter cold than were taken by El Niño-related storminess. Seth Borenstein and Frank Bajak make a good case in an AP story that the United States is one of the largest beneficaries of El Niño relative to other parts of the globe.

Based on NOAA data and analyses, Jan Null (Golden Gate Weather Services) has created an handy graphical guide to U.S. temperature and precipitation effects from weak, moderate, strong, and very strong El Niño and La Niña events, including composites as well as individual Nov-Mar seasons. Looking these over will give you a good sense of where El Niño’s fingerprints are crystal clear or on the muddy side. You’ll also find helpful writeups on NOAA’s ENSO Blog outlining typical U.S. El Niño impacts, including entries by Mike Halpert (June 2014) and Tom Di Liberto (June 2015).

Bob Henson