La Nina Likely To Return Later In 2021. Could be a vigorous event!

Executive summary: The Climate Impact Company July 2021 ENSO forecast reveals potential for a moderate-to-strong La Nina event returning later in 2021. The moderate La Nina strength atmosphere has lingered despite oceanic ending of La Nina in April. For this reason, the atmosphere is likely to regenerate conditions to propel the return of La Nina for quarter 4 of 2021.

Fig. 1: Multivariate ENSO index maintains a moderate La Nina signature despite Nino34 indicating an ending to La Nina in April.

Discussion: Despite the oceanic component (Nino34 SSTA) signaling an end to La Nina by April, the multivariate ENSO index (MEI) maintains a moderate La Nina climate (Fig. 1). Why? The velocity potential anomaly which identifies areas of subsidence (lack of clouds) and convection (enhanced thunderstorm activity) in the tropical atmosphere remains in a classic El Nino-like pattern (Fig. 2) with negative VPA/converging air to create convection over marginally warm SSTA of the equatorial West Pacific (Fig. 3) while positive VAP/diverging air to create subsidence is stretched across the still relatively cool SSTA of the eastern equatorial Pacific.

Fig. 2: Velocity potential anomalies for APR/MAY/JUN 2021. Analysis provided by the International Research Institute for Climate and Society.

Fig. 3: SSTA anomalies for APR/MAY/JUN 2021. Analysis provided by the International Research Institute for Climate and Society.

Forecasts: The persistent La Nina-like MEI supports some dynamic models forecasting La Nina to return later in 2021. Climate Impact Company is in agreement with these outlooks. The two top indicators of La Nina’s return (Fig. 4-5) are the NCEP CFS V2 and POAMA (models). Each model is AGGRESSIVE with building moderate-to-strong La Nina for quarter 4 of 2021.

Fig. 4: NCEP CFS V2 Nino34 SSTA forecast indicates a vigorous La Nina returning later in 2021.

Fig. 5: Australia Bureau of Meteorology Nino34 SSTA forecast indicates vigorous La Nina returning later in 2021.

Implications: A moderately supportive environment to propel another busy season for North Atlantic tropical cyclones could become even more intense! Look for a longer lasting and very active mid-to-late tropical season in the North Atlantic with considerable risk to the U.S. coastline.

The West U.S. drought continues and could expand to the Great Plains. Meanwhile the eastern half of the U.S. is exposed to wet weather due to tropical influences. Winter could reverse much drier in the Southeast States while the North-central U.S. turns cold and snowy. The Northwest could see a wetter pattern for wintertime.

Elsewhere, Australia and Indonesia look wet with a combined -IOD and returning La Nina regime. There’s a chance of much-needed rains for Brazil while the focus of drought could shift to Argentina.