May 2021 ENSO Update: Why Forecasters Declared La Niña Over

In May 2021 NOAA's forecasters declared that La Niña had ended. They put the chance that neutral conditions would last through the summer at about 67%, and were far less sure about the fall, when the odds of a second La Niña winter hovered around 50–55%.

Still cool, but no longer La Niña

The ocean alone seemed to say otherwise. In April 2021 the Niño 3.4 index, the sea surface temperature of a key stretch of the tropical Pacific, was 0.75 °C below average in NOAA's main data set, ERSSTv5. That is past the La Niña threshold of 0.5 °C below average. ("Average" by then meant 1991–2020.)

Sea surface temperature in April 2021 compared with the 1981–2010 average; this map still used the older averaging period. Image from NOAA Climate.gov.

But ENSO (the El Niño–Southern Oscillation) is more than a patch of unusually warm or cool water. The atmosphere matters just as much: it reinforces the ocean's anomaly and carries ENSO's effects around the world. During La Niña the Walker circulation, the loop of air over the tropical Pacific, strengthens, bringing fewer clouds and less rain to the central Pacific and more to the far western Pacific and Indonesia. That pattern held for months, then faded by early April.

Outgoing long-wave radiation anomalies over the past year: purple where clouds and rain were above average, orange where below. La Niña’s dry signature over the central Pacific is the large orange area that ends in April 2021. Image from NOAA Climate.gov.

Other signs weakened too. The stronger-than-average trade winds eased over most of the Pacific, and both the Southern Oscillation Index and the Equatorial Southern Oscillation Index, which measure ENSO's atmospheric side, pointed to neutral. So the team called La Niña over, despite the cool water.

Crossing ±0.5 °C is not a switch that makes the atmosphere respond at once, as the months before the previous El Niño showed; the ocean–atmosphere system is too complicated for that. Beneath the surface, meanwhile, the Pacific was warmer than average and a downwelling Kelvin wave was travelling east, leaving little cool water to refresh the surface in the coming months.

What might come next

The question everyone wanted answered was what would follow in the fall. ENSO has a strong link with the Atlantic hurricane season (June to November): El Niño tends to mean fewer tropical storms and hurricanes, and La Niña more. The previous year's extremely active season had been influenced by La Niña.

• El Niño looked unlikely, at about 8%. It has followed a first-year La Niña only twice since 1950, and most computer models agreed.
• Warm water below the surface can give an early hint of El Niño, but April's figure of 0.6 °C ranked only 12th of 43 years on record, and the February–April average was about zero. Springs like that have been followed by every kind of fall.
• La Niña or neutral was harder to call. Many models suggested a second-year La Niña, which is common in the historical record, but the spring predictability barrier meant that forecasts made in spring are less skilful than those made at other times of year.

Model forecasts for the Niño 3.4 index from the North American Multi-Model Ensemble; the darker band holds 68% of forecasts, the lighter 95%. Built from University of Miami data. Image from NOAA Climate.gov.

For August to October, the heart of the hurricane season, forecasters put the chance of La Niña at 42%, slightly below the 50% chance of neutral. ENSO is only one influence on hurricanes, with Atlantic Ocean temperatures among the others, and NOAA's 2021 hurricane outlook was due on May 20.

Sources

• NOAA Climate.gov, "May 2021 ENSO update: bye for now, La Niña!," ENSO Blog; rewritten in hubnx's own words. Images from NOAA Climate.gov, used with its attribution; graphs credited to outside researchers are left out.