Marufish World of Disaster Prevention2020-2021 La Niña event has concluded

WMO, May 2021

The 2020-2021 La Niña event has concluded, according to both oceanic and atmospheric indicators. The latest forecasts from the WMO Global Producing Centers of Long-Range Forecasts indicate that neutral conditions are likely to dominate the tropical Pacific through the boreal summer: with a 78% chance of neutral for May-July, decreasing to 55% by August-October. ENSO-neutral remains the most likely outcome for the rest of the calendar year. The outlook for the second half of the year, however, contains considerable uncertainty with some suggestions of either a transition to El Niño or a re-emergence of La Niña conditions later this year. National Meteorological and Hydrological Services will closely monitor changes in the state of El Niño/Southern Oscillation (ENSO) over the coming months and provide updated outlooks.

In summary:

La Niña has ended in May 2021, based on both oceanic and atmospheric indicators.
Model predictions and expert assessment indicate a relatively higher likelihood for ENSO-neutral conditions to prevail over the next five months, with a probability of 78% during May-July 2021 and 55% during August-October 2021.
For the second half of the year, model predictions differ considerably on whether ENSO-neutral will remain, La Niña conditions redevelop, or El Niño conditions develop. ENSO-neutral conditions are currently predicted as more likely than either El Niño or La Niña.
Sea surface temperatures in the eastern-central Pacific are predicted to be below- to slightly above-average during May-July 2021, in the range of -0.6 to +0.3 degrees Celsius. For August-October 2021, they are predicted to range between -0.8 to +0.8 degrees Celsius deviation from the average.

Full May 2021 Update

La Niña conditions were established in August-September 2020, according to both atmospheric and oceanic indicators. The sea surface temperature anomalies in the central/eastern-central equatorial Pacific reached peak magnitude during October-November 2020. In 2021, the sea surface temperature anomalies in the eastern-central Pacific weakened, leading to the currently prevailing weak cool anomalies that are not cold enough to meet the La Niña threshold. Cooler than normal sub-surface anomalies also weakened considerably in February and began to show positive anomalies in March.

Since late March, active surface-wind variability allowed the remaining cool surface anomalies to decay more slowly than the sub-surface ocean adjustment would suggest. The equatorial sea surface temperatures off the South American coast have been quite variable and are currently much colder than the rest of the tropical Pacific, which may present localized effects.

Enhanced trade wind anomalies and stronger-than-average upper-level westerly winds were present in the tropical Pacific since mid-2020. The enhanced trade wind anomalies near the surface are now weak and lack spatial structure. The upper-level westerly winds are also much weaker. Cloudiness and rainfall indicators have returned to normal. No longer present are the below-average cloudiness over the central and west-central tropical Pacific, nor is the rainfall around the Maritime Continent above average any longer. The Southern Oscillation Index (represented by standardized Tahiti minus Darwin sea-level pressure difference) also returned to the neutral range in March and has remained there. These ocean patterns and corresponding atmospheric anomalies are typical of the transition out of a La Niña event. Past observations show that the ENSO conditions typically transition sometime during April to June.

The recent conditions from April and May are the starting point for climate models from the WMO Global Producing Centres of Long-Range Forecasts to produce global-scale ENSO forecasts for the coming months. The predictions for May-July 2021 indicate a 78% likelihood that neutral conditions will remain in place, a 19% likelihood for La Niña, and only a 3% likelihood for El Niño conditions. The range of possible central-eastern Pacific sea surface temperatures predicted for May-July 2021 span the values of -0.6 to +0.3 degrees Celsius. Through the August-October 2021 season ENSO-neutral is the most likely outcome, with probabilities over 50%, and central-eastern Pacific sea surface temperature values predicted to be within -0.8 to +0.8 degrees Celsius deviation from average. After that, while ENSO-neutral conditions are more probable than either El Niño or La Niña, the uncertainty is greater. Forecasts at long leads, and in particular those that extend through the boreal spring, tend to be less accurate. The so-called “spring prediction barrier” has nearly passed, but greater clarity is yet to emerge in the forecasts for the end of the year. At forecast lead times extending through the end of the year, nearly half indicate the persistence of ENSO-neutral conditions, and the remaining half indicate either a re-emergence of La Niña or development of El Niño.