Extended Range ENSO and IOD Forecast

May 2025 Climate Impact Company ENSO/IOD 2-Year Analog Forecast Through May 2027

Issued: Sunday May 11, 2025

Highlight: Analog forecasts are disagreeable to dynamic models in 2025; Favored is potential for weak El Nino and +IOD. ENSO phase extremes potential indicated for 2026.

Executive summary: Climate Impact Company projects ENSO and IOD regimes through the next 2 years based on analogs years. Analog years are selected from the past 3 decades. Analog years taken prior to the mid-to-late 1990’s are from different ENSO cycles and a cooler ocean surface away from the tropics and therefore are considered not, in any way, representative of the modern seas surface temperature anomaly (SSTA) regimes used to produce (and explain) climate regimes and their forecasts. An oceanic and atmospheric ENSO phase forecast is provided. The oceanic projection favors the possibility of El Nino late this year while the atmospheric ENSO forecast is neutral phase. Dynamic models favor neutral ENSO phase although tilted toward La Nina returning if any deviation. Interestingly, the IOD analog forecast favors the positive phase for 2025. Dynamic models favor negative phase. -IOD is required for any La Nina potential and dynamic models lean in that direction while the analog projection is just the opposite solution. In 2026, the ENSO analog forecast is neutral but indicates near equal potential for a strong la Nina or strong El Nino while IOD is neutral. The analog forecast slow favors a major ENSO event for the first half of 2027.

Discussion: Based on analogs, ENSO is favored to stay in neutral phase through next year and into 2027. The Nino34 SSTA analog, the most convectional index used, is largely attached to the oceanic character of ENSO. The Nino34 SSTA analog forecast through May 2027 is interesting despite the (mostly) neutral projection (Fig. 1). Unlike dynamic models which (also) project neutral ENSO for 2025 but lean toward La Nina generation if any change, the analog forecast is in favor of weak El Nino risk later in 2025. At that time, 4 of the 5 analog years project El Nino development. The onset is during the late North Atlantic tropical cyclone season. The reaction of the atmosphere to the Pacific Ocean equatorial sea surface temperature (and subtropical latitude) is measured effectively by multivariate ENSO index (MEI). The atmospheric character of ENSO is cleanly in the neutral phase for 2025 with near equal weak El Nino/weak La Nina (analog) alternatives (Fig. 2). Based on a consensus of the Nino34 and MEI analogs, neutral ENSO phase is favored for 2025 with a “caveat” forecast favoring weak El Nino later in 2025 disagreeable to dynamic models.

In 2026, both the Nino34 and MEI analog forecast are neutral. However, the interesting part of the forecast is the extremes. Nino34 and MEI analog forecasts indicate a 16.6 to 20 percent chance of a strong El Nino and about twice as high probability of a moderate (or stronger) La Nina.

In 2027, the analog forecast increases the risk of an extreme ENSO event with a near equal split on El Nino or La Nina.

The analog years were regressed 12 months and taken from the past 30 years only due to the steadily rising ocean surface temperatures unique to the past several decades within the period of (ENSO) record. The warming has accelerated during the past decade. Therefore, analogs from the past 10 years may have higher validity as to what is ahead 1-2 years from now. Given that possibility, the 2 most recent analog years average to El Nino development for 2026 lasting into 2027.

The Indian Ocean dipole (IOD) constructed analog forecast through May 2027 is based on 7 analog years. The majority of dynamic global SSTA forecasts project development of -IOD during Q3/2025. If so, the risk of La Nina returning increases. The analogs are increasingly in the face of the dynamic models. The IOD analog forecast for 2025 is neutral to weak positive phase (Fig. 3). Two of the analog years suggest a strong +IOD pattern. The analog years indicate only 1 firm -IOD forecast. Since February, the IOD pattern has bounced between neutral and weak positive phase. So, no sign of -IOD development for now therefore the analog forecast for 2025 holds. In 2026, the consensus of analogs favors weak -IOD the second half of the year. The extreme risk is minimal (14%) and favors the negative phase. In 2027, neutral IOD is favored.

Fig. 1: The Climate Impact Company Nino34 analog forecast to determine oceanic ENSO phase through May 2027.

Fig. 2: The Climate Impact Company multivariate ENSO index forecast to determine atmospheric ENSO phase through May 2027.

Fig. 3: The Climate Impact Company Indian Ocean dipole forecast through May 2027.