After Short-term La Nina, Neutral ENSO Will Persist

Highlight: 2-year forecast reveals lack of strong ENSO events.

Executive summary: The monthly Climate Impact Company ENSO analog forecast reveals a short-term La Nina which ends quickly, by early spring 2022 and is followed by mainly neutral ENSO through northern hemisphere summer 2023.

Fig. 1: Climate Impact Company constructed analog Nino34 SSTA forecast regressed 2 years and forward forecast for 2 years implies lack of intensity.

Discussion: The Nino34 SSTA pattern for 2019-2021 is remarkably similar to 2009-2011, 2006-2008 and 1997-1999. Consequently, an analog projection forward 2 years through JUN/JUL/AUG 2023 is generated and yields reasonable reliability (Fig. 1). Results render a short-term moderate La Nina event followed by quick weakening to neutral ENSO by next April and followed by neutral phase continuing through northern hemisphere summer 2023. The analogs suggest any deviation would favor El Nino late next year.

During the past 20 years the mid-latitude SSTA pattern has been consistently quite warm. Theorized is the mid-latitude warmth is diminishing the frequency and intensity of ENSO events. During the past 20 years 5 ENSO events have generated (3 La Nina and 2 El Nino) while in the previous 20 years 8 ENSO events occurred including both record-strength La Nina and El Nino episodes. The analog forecast indicating lack of intensity of ENSO the next 2 years (except for the short-term La Nina) is reasonable.

Caveats? Of course! In a rare occurrence, the atmosphere has maintained a La Nina signature in 2021 as defined by multivariate ENSO index (MEI) and to a certain extent southern oscillation index (SOI) while Nino34 SSTA indicated a La Nina ending last March (Fig. 2). On its own, the persistent -MEI pattern suggests that returning La Nina during Q4/2021 may last longer than indicated by the analog in 2022. The Australia Bureau of Meteorology (ABOM) has indicated the persistent La Nina climate as defined by -MEI was caused by the relative very warm SSTA north of Australia compared to the relative cool SSTA in the eastern tropical Pacific Ocean. Presence of a negative Indian Ocean Dipole also contributed to the -MEI in 2021 according to ABOM.

Fig. 2: Multivariate ENSO index has maintained a La Nina signature in 2021.

In recent weeks warm SSTA north of Australia has shifted westward into the eastern half of the equatorial Indian Ocean. Trade winds have increased near the Dateline. Consequently, up-welling cool subsurface water is most apparent near and just east of the Dateline in the equatorial Pacific Ocean. If this pattern continues, a La Nina Modoki will generate for upcoming northern hemisphere winter. La Nina Modoki is difficult to analog due to their very infrequent and short duration character. The last full-throttle La Nina Modoki peaked during February 1989 when global SSTA were much cooler. Historically, the influence of La Nina Modoki on the global climate is difficult to quantify due to the lack of occurrence in the warming SSTA pattern of the past 2 decades.

Summary: Conditions are set for evolution of La Nina during quarter 4 of 2021. Recent diagnostics indicate La Nina Modoki is likely. The specific effects of La Nina Modoki on climate are not well-understood due to the lack of episodes during the warming of mid-latitude SSTA of the past 20 year. La Nina is forecast to end by late Q1/2022 with neutral ENSO to follow. Expected is a general lack of significant ENSO events through northern hemisphere summer 2023.