Executive summary: La Nina 2020-22 is most likely to hang on in 2022 and last into early 2023. La Nina should weaken but last into a third year for only the 5th time since 1950. Forecast confidence remains below average, but the trend is firmly toward lingering La Nina. The global atmospheric climate defined by multivariate ENSO index more strongly projects a lingering La Nina climate for 2022. La Nina climate during JUN/JUL/AUG supports a wet Indian Monsoon, wet climate across Indonesia, wet across northern South America and dryness for Argentina. The effects are less dramatic in the northern hemisphere except for enhancement of both North Atlantic and West Pacific tropical cyclone activity.
Fig. 1: The Climate Impact Company ENSO analog forecast through February 2023 is indicated.
Discussion: Taking a look at ENSO 2020-22 using the Nino34 SSTA index and comparing with the few analogs available dating back to the 1950’s reveals the most likely following ENSO phase for the following 10 months (through February 2023) is a weaker version of La Nina (Fig. 1). The analogs are in the La Nina category except 2012-13 which flipped to an El Nino episode. Since 1950, only 5 long duration (3 years) La Nina episodes have occurred and 2020-23 is potentially the first of this century.
Research on long duration ENSO events, particularly La Nina has revealed the event is triggered by the intensity of the previous ENSO regime acting as a balancing compensation sometimes referred to as a “see-saw” effect across the tropical Pacific. The lengthy La Nina episodes following the historic 1982-83 and 1997-98 El Nino regimes are cited as examples. However, the El Nino preceding the current long-duration La Nina was weak. Only the 1954-56 La Nina regime followed a comparatively weak El Nino. The 1970’s featured a decade of mostly La Nina climate. There is no historical explanation as to why the current La Nina has lasted 2 years and may extend to 3 years.
One possible explanation for the 2020-22 long duration La Nina is the persistent convection across the equatorial West Pacific (Fig. 2). The rising air currents invite replacement air from the east in the form of enhanced trade winds from the tropical East Pacific. The enhanced trade winds are continually up-welling cool water to maintain a La Nina condition. Interestingly, current Madden Julian oscillation (MJO) forecasts indicate the pattern cited may break temporarily through the middle third of May.
Fig. 2: Outgoing longwave radiation anomalies for 2020, 2021 and 2022 (so far). The blue zones represent convection while orange zones represent subsidence.
The 2022 ENSO phase forecast is officially continuation of La Nina although weaker. The multivariate ENSO index (MEI) which identifies the atmospheric response to equatorial SSTA and SLPA patterns has been strongly indicating a La Nina climate. Consequently, the weaker Nino34 SSTA ahead signals that La Nina is barely hanging on while MEI is likely to maintain a stronger La Nina signature as long as the OLRA pattern cited for 2020-22 continues.
A collection of forecast models for ENSO phase identifies a restrengthening La Nina in May (Fig 3). By September, all models are very close to the NOAA La Nina threshold of -0.5C (Fig. 4). The Australia Bureau of Meteorology uses -0.8C for the La Nina threshold.
Fig. 3: A collection of ENSO phase forecast models and their Nino34 SSTA assessment for May 2022. NOAA identifies La Nina threshold at -0.5C.
Fig. 4: A collection of ENSO phase forecast models and their Nino34 SSTA projection for September 2022.
Finally, a key diagnostic to project future ENSO phase change is the trend of subsurface equatorial Pacific upper ocean heat. In 2022 so far, the equatorial East Pacific subsurface has maintained a cool regime which implies continuation of La Nina. Once warmth near and west of the Dateline shifts eastward, the La Nina episode erodes. There is potential for that scenario in 2022 therefore constant close monitoring is required.
Fig. 5: Subsurface equatorial Pacific Ocean upper ocean heat schematic from the Australia Bureau of Meteorology.