Highlight: Refining the ENSO 2023-24 Outlook by including multivariate ENSO index. Executive summary: The JUL/AUG multivariate ENSO index (MEI) failed to indicate evolution of an El Nino climate despite the robust oceanic El Nino presence across the equatorial East Pacific. Consequently, adjustments of the ENSO 2023-24 forecast considering MEI indicate that despite a robust warm equatorial East Pacific SSTA regime continuing to strengthen during Q4/2023, the attendant El Nino climate is likely moderate. Discussion: A vigorous El Nino has formed in the eastern equatorial Pacific as identified by a traditional measuring index of ENSO phase, the Nino34 SSTA. HOWEVER, the response of the atmosphere to equatorial warming is weak. Multivariate ENSO index (MEI), a measure of the atmosphere’s reaction to equatorial SSTA patterns, indicates neutral ENSO for JUL/AUG (Fig. 1). The atmospheric response to the equatorial SSTA pattern is the one that counts. Consequently, the season 1-3 ahead forecast is based on a MEI analog (Fig. 2). The reason for the lack of atmospheric response, based on MEI, is not well-understood. The preliminary explanation (by Climate Impact Company) is the traditional consolidation of anomalous warmth in the tropical East Pacific is not happening in 2023 as many other sectors of the global oceans are warmer than normal plus the usual cooling in the equatorial West Pacific during El Nino is weak. In fact, the subsurface equatorial Pacific remains warmer than normal which is not typical of vigorous El Nino episodes. The Nino34 SSTA analog forecast indicates oceanic El Nino is vigorous (Fig. 3). However, El Nino 2023-24 will feature a moderate El Nino climate likely to fade back toward La Nina one year from now. Fig. 1: Comparing Nino34 SSTA and multivariate ENSO index since January 2020 reveals MEI has consistently lagged Nino34. Fig. 2: ENSO analog forecast projecting atmospheric ENSO using MEI. The warmer solution is based on a consensus between the MEI analog and warmer Nino34 forecast. Fig. 3: ENSO analog forecast based on conventional Nino34 SSTA.

Fig. 1: Climate Impact Company ENSO forecast through August 2026 with annotated timing of events. ENSO outlook discussion: El Nino 2023 continues to strengthen following a robust +1.30 Nino34 SSTA in August. The Nino34 SSTA has entirely reversed from a potent cool La Nina signature 1 year ago to the impressive current warm signature. Oceanic El Nino is now classified as moderate-to-strong intensity. The Climate Impact Company constructed analog forecast is based on 7 analogs, 4 occurring during the cool (ENSO) cycle (2023 is similar) and 3 occurring during the warm cycle (Fig. 1). The analogs are in reasonable agreement on the observed Nino34 of the past 12 months, the onset and strengthening of El Nino in 2023, the peak of El Nino 2023-24 occurring in December, and weakening to a La Nina in October 2024. ENSO phase is in-and-out of La Nina for 2025-26. The “caveat” forecast is a 1-in-7 chance of El Nino continuing next year and 2026. The analog forecast implies El Nino 2023-24 is not as strong as the 3 most intense on record (1982-83, 1997-98, and 2015-16). Reasons for the lack of super intensity despite the extremely warm Nino SSTA regions is suggested by the Northeast Pacific SSTA regime and trend in upper ocean heat. The super El Nino of 2015-16 was created due to merging of a Northeast Pacific marine heat wave (MHW) and warm waters in the southeast Pacific tropics associated with El Nino. Once each warm zone merged, an intense El Nino formed. In 2023 a MHW north and northeast of Hawaii expands to the West Coast of North America earlier this summer season. However, the warmth reaching the coast while still present (Fig. 2) has cooled significantly during the past 30 days (Fig. 3). However, waters just off the California Coast are warmer than normal and continue to warm in recent weeks. Unclear is whether a robust warm signature in the coastal Northeast Pacific is sustained and spreads southward into the tropics. Additionally, upper ocean heat across the equatorial Pacific while still robust has weakened in August (Fig. 4). Strong El Nino’s usually have consolidation of upper ocean heat east of the Dateline at this stage of a developing strong event while west of the Dateline cools. Fig. 2-3: Northeast Pacific SSTA regime and 30-day change. Fig. 4: Equatorial Pacific upper ocean heat during the past 4 months. Conclusion: El Nino 2023-24 continues to develop and oceanic El Nino is now classified as moderate-to-strong in intensity. The Climate Impact Company constructed analog forecast projects additional strengthening with peak intensity in December followed by weakening early in 2024. El Nino 2023-24 ends by next May and is followed by the return of a weak La Nina later next year. El Nino 2023-24 is not likely to strengthen to levels observed with the most intense El Nino’s on record.  

Headline: Marine heat waves boast major influence on the northern hemisphere JUL/AUG upper air patterns. Fig. 1: Daily global SSTA analysis identifying marine heat waves and their trend. Discussion: Currently, Climate Impact Company is monitoring 8 marine heat waves (MHW). Most prominent is MHW NEP23A in the Northeast Pacific (Fig. 1). The core of MHW NEP23A has strengthened to the north of Hawaii. However, the eastward extension to the West Coast of North America earlier during summer 2023 is now weakening. The merging of NEP23A and warm SSTA in the Southeast Pacific associated with El Nino remains in question. If the warm SSTA regions join, the likelihood of the 2023-24 El Nino reaching similar intensity to the 1982, 1997, and 2015 super El Nino’s is likely. If the two warm SSTA zones do not join, El Nino 2023-24 is not as intense as the 3 historic episodes. The Northwest Pacific MHW remains intense with not much change in strength. The MHW is well-correlated to the semi-permanent high-pressure ridge east-northeast of Japan (Fig. 2). Additionally, the upper ridge over Northwest Canada is associated with the Northeast Pacific MHW. In the North Atlantic, the quick-developing MHW in the Labrador Sea earlier this summer is now weakening. However, the MHW in this region is well-correlated to an amplified upper ridge present in JUL/AUG in this same region. Additionally, the MHW’s in the vicinity of U.K. and off the Northwest Coast of Africa are attached to the semi-permanent upper ridge roasting Southern Europe during summer of 2023. In the southern hemisphere, semi-permanent MHW’s located on the South Pacific, near and east of New Zealand, and east of Argentina have weakened during the winter 2023 season. However, each is forecast to regain strength as the southern hemisphere warm season arrives. Also note the ongoing and developing El Nino plus the emergence of positive phase Indian Ocean dipole (+IOD). Fig. 2: The northern hemisphere 500 MB anomalies for JUL/AUG 2023 identifying location of high-pressure ridge areas well correlated with marine heat waves.