Executive summary: The upper ocean heat in the equatorial East Pacific is collapsing rapidly. This climate diagnostic is a key signature for ENSO phase change toward La Nina. The cooling subsurface indicates there is potential for a significant La Nina event. La Nina will enhance the North Atlantic tropical cyclone season activity/intensity for mid-to-late season, bring welcome rainfall this winter to the Northwest States while the Southeast could become a drought target for 2022. Wet climate also appears in drought-stricken Brazil. La Nina should keep Australia in drought avoidance for a second consecutive year.
Fig. 1: Subsurface equatorial Pacific Ocean temperature anomalies east of the Dateline indicate rapid loss of upper ocean heat supportive of La Nina.
Fig. 2: Intense cooling of the subsurface equatorial East Pacific in early August.
Fig. 3: Multivariate ENSO index versus Nino34 SSTA for the past 1 year.
Observation discussion: The upper ocean heat in the equatorial East Pacific is decelerating rapidly! The upper ocean heat has plummeted to almost the peak cool signature of last year’s La Nina episode (Fig. 1). The primary cool zone has intensified rapidly in the Nino34/Nino3 SSTA regions in early August (Fig. 2). The cooling is no surprise thanks to a robust +1.6 La Nina-like southern oscillation index (SOI) observed in July and the coinciding -1.5 very La Nina-like multi-variate ENSO index (MEI). The La Nina climate has persisted all year despite the oceanic component of La Nina 2020-21 ending last spring (Fig. 3). The Australia Bureau of Meteorology has stated the La Nina-like atmosphere may have been contributed to by the warm SSTA of the eastern Indian Ocean (negative Indian Ocean Dipole). While that statement may be true of the past 1-2 months (when -IOD developed), the lingering La Nina-like MEI has persisted since last year.
Forecast discussion: Given the rapid cooling of the subsurface equatorial East Pacific the Climate Impact Company constructed analog forecast of the Nino34 SSTA region is adjusted cooler as La Nina is projected slightly more intense than the previous outlook (Fig. 4). La Nina onset is likely in October with peak intensity in January and fade back to neutral ENSO in MAR/APR 2022. The 2021-22 La Nina is not quite as intense or lengthy as the 2020-21 La Nina episode.
Other forecast models are in general agreement with CIC forecasting weak to moderate La Nina developing by quarter 4 of 2021 and strengthening by December 2021 (Fig. 5). The Japan Meteorological Agency maintains neutral ENSO. If the subsurface cooling of August continues the next 1-3 months, La Nina 2021-22 will be stronger than forecast.
Fig. 4: Climate Impact Company Nino34 SSTA constructed analog forecast.
Fig. 5: The Australia Bureau of Meteorology Nino34 SSTA forecasts for December of all ENSO models indicate weak-to-moderate La Nina except neutral phase continuing according to the Japan Meteorological Agency.