LINGERING LA NIÑAS MAY HELP FORECASTERS SPOT COSTLY WEATHER PATTERNS TWO YEARS AWAY ~ The Washington Post

Sea surface temperature difference from average as simulated Thursday by the American model. Blue shades, indicating cooler-than-average temperatures, over the central and eastern tropical Pacific Ocean, are indicative of a La Niña pattern. (earth.nullschool.net)

By Bob Henson

Dec. 10, 2020

Weather forecasters are keeping a close eye on a potent La Niña event expected to peak in the winter. This sprawling ocean-atmosphere feature is expected to help produce a mild winter of 2020-2021 over much of the United States, together with strengthening drought across the Sun Belt well into the new year.

La Niña is also inspiring a growing set of researchers to look beyond the traditional limits of seasonal prediction. That’s because La Niña tends to hang around longer than El Niño, often returning or restrengthening for a second or even a third northern winter.

That asymmetry points to the potential for outlooks that may — at least in some years — extend well beyond the one-year limit of standard practice. Such forecasts could give more than a year’s notice of the likelihood of impacts such as a drought or enhanced hurricane activity.

Left: Places in the contiguous United States where the forecast odds from NOAA for December 2019 to February 2021 favor unusual cold (blue) or unusual warmth (red). Darker colors denote higher odds of unusual warmth or cold, but not necessarily a more intense outcome. Areas in white have equal chances of an unusually cold, warm or near-normal winter. Right: The same, except for drier (brown) or wetter (green) conditions. These outlooks are quite similar to the classic features observed during a La Niña winter, as shown below. (NOAA/Climate.gov)

The players and their impacts

El Niño and La Niña refer to a warming and cooling, respectively, of the uppermost part of the eastern tropical Pacific Ocean, together with a broad set of atmospheric extensions. This entire semi-cyclic phenomenon is known as the El Niño-Southern Oscillation, or ENSO.

ENSO events tend to emerge in the northern fall, peak by winter and fade in the spring. It’s often clear by late summer that either La Niña or El Niño will predominate during the next few months. About a third of all northern winters see neutral conditions, with neither El Niño or La Niña present.

Precipitation across many parts of the world, especially surrounding the Pacific, is affected by La Niña conditions in the tropical Pacific. Although the strongest shifts vary somewhat from one La Niña to the next, they remain fairly consistent in the regions and seasons shown on the map above. (IRI)

Both El Niño and La Niña influence how much it rains or snows and how warm or cold it gets across broad areas — and even the amount of hurricane activity in the Atlantic and Pacific — so knowing what to expect from ENSO is of keen use to agricultural interests, energy traders and others.

Even outlooks that are only modestly better than chance in a single year can still provide long-term value to large-scale economic sectors.

“If you’re betting just on a single year, it’s not always going to be a safe bet, but over the long term we see these impacts pan out more often than not,” said Michelle L’Heureux, a climate scientist at the National Weather Service’s Climate Prediction Center. L’Heureux leads the creation of ENSO outlooks that are issued jointly around the middle of each month by the Weather Service and the International Research Institute for Climate and Society at Columbia University’s Earth Institute.

The unforeseen arrival of a record-setting El Niño event in 1982-1983, including destructive floods in California, kicked off efforts by the National Oceanic and Atmospheric Administration to provide advance warning of ENSO events. The tropical Pacific was soon slathered with dozens of buoys to monitor ocean and air temperature. The development of a long-range computer model known as the Climate Forecast System — which includes interactions among ocean, atmosphere, land and sea ice, going out up to nine months — allowed NOAA to launch its quantitative ENSO outlooks in the early 2010s.