El Niño Continues to Ramp Up

The latest updates from NOAA (see PDF) and the Australian Bureau of Meteorology confirm that the El Niño event that began in earnest this spring continues to build. NOAA reports that sea-surface temperatures (SSTs) in the closely watched Niño3.4 region of the eastern tropical Pacific were 1.2°C above average last week, well into the range associated with moderate El Niño events (+1.0°C to +1.5°C). Based on another marker, the Multivariate ENSO Index, this El Niño event is already into the “strong” category. All of the Niño monitoring regions had SSTs of at least 1.2°C above average last week, making for the most widespread oceanic warmth since the landmark El Niño event of 1997–98. Policymakers and investors are already taking note of the potential implications of the intensifying El Niño for agriculture and the economy.
 

Figure 1. Sea-surface temperatures on June 8 show the hallmark of El Niño, with a band of warmer-than-average temperatures, extending along the equator across the central and eastern tropical Pacific and poleward near the South American coast. Two animations showing the emergence of this El Niño pattern can be found at Tom Yulsman’s ImaGeo blog (Discover Magazine). Image credit: NOAA/NESDIS.
 
 
Computer models are in firm agreement that El Niño conditions will strengthen further during the latter part of 2015. All eight of the international models tracked by BOM show Niño3.4 readings of 1.5°C or higher by October (see Figure 2), and several exceed 2.0°C, suggesting that the strongest event since 1997–98 may well be in the cards. Some models predicted that a significant El Niño would emerge in mid-2014, but that didn’t happen, largely because the atmosphere failed to respond to oceanic shifts that often kick off El Niño. This time, the atmosphere and ocean are much more in sync, so we can put more trust in the current model outlooks—especially now that we’re past the “spring predictability barrier” that makes early-year forecasts of El Niño so tough. In today’s update, NOAA is calling for a greater than 90% chance that El Niño will continue through the northern fall of 2015, and around an 85% chance it will last through the winter of 2015-16.
 

Figure 2. Projections of sea-surface temperature in the Niño 3.4 region (5°S - 5°N, 120°W - 170°W) for October 2015 in terms of departure from average (degrees C), as compiled from eight international climate models and released by the Australian Bureau of Meteorology in May. Each model’s projection on the graph above is based on the average of multiple runs in an ensemble. The threshold for El Niño conditions for Australia is 0.8°C above average, as shown on the chart, while the NOAA threshold (not shown) is 0.5°C. Image credit: BOM.
 
 
A wet footprint that’s already shown up
El Niño appears to have played a role in record-smashing rainfall over the south-central states last month, the main factor behind the single wettest month in U.S. history. Satellite imagery averaged for May shows a band of enhanced moisture sweeping from the Niño regions of the central and eastern tropical Pacific into the southern United States (see Figure 3). That upper-level moisture was pushed along by an strong subtropical jet stream and juxtaposed with unusually rich low-level moisture from the Gulf of Mexico. Kevin Trenberth (National Center for Atmospheric Research) sees a probable link between El Niño and the May rains. “It doesn't mean El Niño caused the pattern, but it did change the odds and helped it persist,” Trenberth told me. “Then the global warming component, whereby all SSTs are higher, adds to the moisture, and this is what helps break the records.”
 

Figure 3. Departures from the average amount of outgoing longwave radiation (OLR) for the period May 11 through June 9, 2015, as measured by satellite. Negative values denote outgoing radiation being blocked by clouds and precipitation. Strongly negative values (magenta) can be found over the central and eastern Pacific Ocean, where El Niño’s oceanic signature is based. An extension of low OLR values runs through Mexico into the western half of the United States. Image credit: NOAA Earth System Research Laboratory.
 
 
What will summer bring us?
On average, El Niño’s impacts on U.S. weather are much stronger in winter than in summer. As NOAA’s Anthony Barnston puts it in a recent ENSO Blog post, “while El Niño is the 800-pound gorilla in winter forecasts in the U.S., it is more like a tame, 6-pound Chihuahua in summer.” Barnston cites two reasons for this:
 
—El Niño affects climate in midlatitudes by adding energy to the Hadley circulation, a vast loop that sends tropical air upward and poleward and higher-latitude air downward and equatorward. The Hadley loop is weaker in the half of the world where summer is occurring, due to the reduced equator-to-pole temperature contrast. This means an El Niño event has less ability to torque climate in the summer hemisphere.
 
—Weaker north-south temperature gradients in summer also mean that the circulation patterns driving midlatitude weather are weaker and smaller-scale than they are in winter, again reducing the ability of El Niño to influence things.
 
The main tendency in U.S. weather during El Niño summers, albeit a weak one, is for above-average precipitation. In particular, the southwestern states may continue to see the effects of a Northeast Pacific hurricane season amped up by El Niño (which produces favorable atmospheric conditions in that basin) as well as by very warm SSTs. Already, remnant moisture from Hurricanes Andres and Blanca has led to periods of unusually high amounts of water vapor across the Southwest and bursts of rainfall that are uncommon in late spring for places like Phoenix, and today brought the birth of Tropical Storm Carlos (see this morning’s update from Jeff Masters. Meanwhile, in the Southern Hemisphere, El Niño will boost the odds of drought in Indonesia and parts of Australia over the next several months (see Figure 4). Toward the end of 2015, El Niño’s influence on U.S. climate should grow, and there’s a reasonable shot at a wetter-than-average winter in southern California, assuming that model forecasts of a strong El Niño prove accurate. For more on how the balance between the subtropical and polar jet streams is affected by El Niño events of various intensities, see this explainer by Daniel Swain at California Weather Blog.
 

Figure 4. Temperature and precipitation patterns that are typical of El Niño conditions during northern summer. Image credit: climate.gov, based on originals from the NOAA Climate Prediction Center.
 
 
Looking further out: La Niña in 2016-17?
It’s far too early for dynamical models to peer beyond the current El Niño event, but if history is any guide, there’s a better-than-average chance of La Niña developing sometime in 2016. El Niño events usually last just one year—two at most—so the current event should be on the wane by early- to mid-2016, as suggested by the model guidance in Figure 2. When a significant El Niño pattern weakens, there can be a rebound effect that allows cooler-than-average water to upwell into the eastern tropical Pacific and revitalized trade winds to push it westward, leading to a La Niña event. NOAA’s historical database of Niño3.4 readings, which goes back to 1950, shows that most El Niño events of at least moderate strength were followed by La Niña conditions within a year of their demise. A classic example is the strong 1998-99 La Niña that quickly followed the powerful 1997-98 El Niño. That said, each El Niño event is different, and as every smart investor knows, past performance is no guarantee of future results. We’ll just have to wait and see if this El Niño—which took the better part of a year to get rolling—proves equally quirky on its way out the door.
 
Bob Henson