Weak La Niña Expected to Persist into 2017

After a few months of on-again, off-again prospects for a La Niña in 2016-17, NOAA pulled the trigger on Thursday. In its monthly El Niño/Southern Oscillation (ENSO) Diagnostic Discussion issued on Thursday, NOAA placed its alert system into La Niña Advisory mode. A La Niña Advisory means that La Niña conditions are now present and expected to continue--in this case, through winter 2016-17.

The NOAA advisory follows a cooling trend that’s persisted for months across the central and eastern equatorial Pacific, including the Niño3.4 region. The core definition of La Niña is a cooling of sea surface temperatures (SSTs) over the Niño3.4 area to at least 0.5°C below the seasonal average. To be considered full-fledged La Niña conditions, the SST shift must be accompanied by La Niña-supportive trends in the overlying atmosphere, such as a strengthening of Pacific trade winds and increased shower and thunderstorm activity in the western Pacific. SSTs entered La Niña territory back in July, but the atmospheric cofactors were less consistent and more ambiguous until the last month or so. The nagging uncertainty, which was backed up by less-than-enthusiastic computer model runs, led to NOAA’s issuing a La Niña Watch in April, implementing a La Niña Advisory in July, reverting to a La Niña Watch in August, cancelling that watch in September, and reissuing it in October. In a NOAA ENSO Blog posted Thursday, Emily Becker outlines the step-by-step process guiding NOAA’s decision-making on La Niña. According to Becker, “The atmospheric response overall is fairly weak, going along with the borderline cooler sea surface temperatures of this La Niña…but it’s been consistent for a few months, meaning that we are seeing a change on seasonal timescales, and it’s time to formally welcome La Niña conditions!”

Australia’s Bureau of Meteorology (BOM) issued a La Niña Watch in April and has stuck with it ever since, including its November 8 update. The agency uses a more stringent definition than NOAA for La Niña: SSTs in the Niño3.4 region must be at least 0.8°C below average, together with other cofactors. An interesting retrospective analysis from BOM points up how uncertain this year’s picture has been: if advisories had been issued by the agency starting in 1980, this year’s La Niña Watch would be the most protracted La Niña or El Niño Watch of the entire period, based on the conditions that unfolded.


Figure 1. Sea surface height anomalies (departures from average) during the northern-autumn phase of the last four La Niña events, including the one now beginning. Higher sea surface heights (red and white areas) are associated with warmer upper-ocean temperatures. Image credit: NASA imagery, compilation courtesy Jan Null, @ggweather, Golden Gate Weather Services.

La Niña competes with a warming Pacific
El Niño and La Niña events are playing out in a Pacific Ocean that’s warming up, in line with the planet-wide warming associated with human-produced greenhouse gases. During the “super” 2015-16 El Niño event, we saw SSTs jump to record- or near-record levels over a vast part of the northern and eastern Pacific, well beyond the equatorial region where El Niño is focused. Now we’re seeing a narrow ribbon of La Niña cooling squeezed by a continuation of warm SSTs on either side (see Figures 1 and 2). Traditional indices of El Niño and La Niña don’t consider SSTs poleward of 5°N and 5°S, except right off the coast of South America, so the Niño3.4 values for La Niña don’t incorporate the very warm conditions elsewhere in the tropical and subtropical Pacific. However, the NOAA indices do take into account long-term warming within the immediate Niño3.4 region, since they are calculated against 30-year averages that are updated every 5 years.


Figure 2. The Niño3.4 region is one of the few parts of the Pacific Ocean where temperatures in early November 2016 have been below average. Image credit: NOAA/CPC and climate.gov.

The outlook into next year
La Niña 2016-17 isn’t looking like a spectacular event. Model runs released this week from the North American Multi-Model Ensemble (see Figure 3 below) are in general agreement that this should bottom out as a weak La Niña event over the next several months, with SSTs gradually returning to the neutral range by late winter or early spring. In their joint predictions from early November, forecasters at NOAA and the International Research Institute for Climate and Society (IRI) call for neutral conditions to be the most likely state by the first quarter of 2017 (see Figure 4 below). Hints of a possible return to El Niño in 2017-18 are already showing up in the longest-range model runs. The NOAA/IRI outlook gives odds twice as high for El Niño (about 30%) as for La Niña (about 15%) by summer 2017, although neutral conditions remain the most likely outcome.


Figure 3. Predicted departures from average sea-surface temperature in the Niño3.4 region of the tropical Pacific, based on various models in the North American Multi-Model Ensemble. The NMME website includes more details on the various models that make up the NMME. Image credit: NOAA/CPC.


Figure 4. The likelihood of La Niña (blue), El Niño (red), or neutral (green) conditions for overlapping three-month periods into mid-2017, as determined by forecasters at NOAA’s Climate Prediction Center and the International Research Institute for Climate and Society. Image credit: IRI.

Will warmth win out over northern La Niña chill?
During northern winter, La Niña tends to favor relatively dry, mild weather over the southern U.S. and northern Mexico, with relatively cool conditions across the far northern U.S. and western Canada and wetter-than-average winter for the U.S. Pacific Northwest. Many of these favored conditions have already popped up, with October producing record-high precipitation over the Pacific Northwest and extreme drought over the Southeast. What you wouldn’t expect from La Niña is unusual warmth encompassing virtually all of North America, as we’ve seen in recent weeks. As always, La Niña and El Niño intersect with other factors. This is the warmest year in global recordkeeping, and warmth has been sharply focused over North America this autumn. Meanwhile, it’s been quite cold and snowy across northern Eurasia (more on that in a future post). It remains to be seen when or even if the high-latitude circulation will shift to bring some of this intense cold toward North America. Most of the NMME model runs from early November suggest that temperatures could end up above average for the entire winter across much or most of North America. For at least the next few days, the continent’s autumn warm wave should stay remarkably widespread, with especially dramatic mildness persisting over the Arctic and unusually sharp cold centered over Russia (see Figure 5 below).


Figure 5. Departures from average temperature for the five-day period from Friday, November 11, 2016 to Wednesday, November 16, as projected by the GFS model. Image credit: ClimateReanalyzer.org, University of Maine.

Supermoon!
The dry, mild pattern across much of North America will provide optimal conditions for viewing the moon’s closest encounter with Earth in almost 70 years. This “supermoon” will peak on Sunday and Monday nights, as the moon’s closest approach will be at 6:15 am EST Monday. The moon will appear about 14 percent larger and up to 30 percent brighter than it does at its furthest distance from Earth. EarthSky has excellent background on the event. A supermoon of this caliber is about as rare as the Chicago Cubs making it to the World Series, as the last one was in January 1948 (though less dramatic supermoons are fairly common). The next Earth-Moon encounter this close won’t be until November 2034, so if the weather cooperates where you are, be sure to take advantage of the chance to appreciate nature at its finest.

Have a great weekend, everyone!

Bob Henson


Figure 6. This image approximates the look of the Nov. 14, 2016, full moon with data from NASA's Lunar Reconnaissance Orbiter. Credit: NASA/GSFC Scientific Visualization Studio.