Discussion: La Nina 2020-22 has entered a 3rd year. Although Nino34 SSTA briefly shifted to neutral phase one year ago, multivariate ENSO index has steadily shown a moderate-to-strong La Nina climate since Q2/2020. Four lengthy La Nina precedents from the past 35 years indicate that historically, La Nina fades after 2 years (Fig. 1). What follows is mixed with 2 analog years staying in neutral phase while 1 analog year surges into a strong El Nino and another returned to weak La Nina. In 1+ years the analog maintains an equal split between strong La Nina or El Nino plus 2 years staying in neutral phase. Clearly, the analog (statistical) forecast method has limited reliability at this time.
Fig. 1: Nino34 analog projection based on the last 4 lengthy La Nina events.
The limited statistical forecast skill also implies longer-term ENSO projections are not reliable now. Consequently, reliance is on dynamic models and confined mostly to shorter term projections. A collection of all Nino34 SSTA forecast by the Australia Bureau of Meteorology reveals a borderline weak La Nina consensus for September 2022 (Fig. 2). Interestingly, toward the end of 2022, the consensus is for a slightly stronger La Nina (Fig. 3). If La Nina managed to hang-on to the middle of 2023, a tie for longest La Nina on record (1973-76 since 1950) occurs. Following the 1973-76 La Nina, the long-term ENSO cycle shifted to the warm phase and that decadal pattern (1977-1997) produced many El Nino episodes.
Fig. 2-3: Australia Bureau of Meteorology collection of Nino34 SSTA forecasts to determine ENSO phase in September and November.
Fig. 4: The tropical Pacific SSTA observations for July 9, 2022.
Although La Nina has recently weakened, regeneration later in 2022 is likely due to the persistent tropical SSTA pattern across the Pacific Ocean. Although Nino34 SSTA trends toward normal, the collective central and eastern equatorial Pacific is much cooler than the extremely warm SSTA north of Australia (Fig. 4). The SSTA pattern described is a classic La Nina SSTA-driven climate signature. This pattern is likely to continue and/or become more enhanced as a negative Indian Ocean Dipole (-IOD) emerges northwest of Australia the next few months. During -IOD the surface water northwest of Australia warms and should contribute to the pattern already established.
Convection is stronger than normal across the warm SSTA region and compensated for by subsidence across the cooler equatorial waters near the Dateline. The subsidence inspires high pressure and trade winds off the northwest coast of South America to up-well cool subsurface waters and La Nina is maintained. La Nina 2020-22 probably continues until the -IOD pattern fades.
Summary: La Nina 20-22 has entered a third year. Historic precedent indicates La Nina should be ending. Dynamic models indicate a weakening La Nina in the short-term re-emerges later this year. Although Nino34 SSTA may shift briefly to neutral phase, the tropical Pacific SSTA pattern indicates a La Nina climate will continue and lead to regeneration of cooling trade winds in the equatorial East Pacific as La Nina regenerates later this year. If La Nina lingers into next year there is risk of a reverse to El Nino which could be strong for later next year or 2024.