Is -IOD Potential for Q3/2025 Dependent on ENSO Flipping Back Toward La Nina?

Discussion: Currently, the Indian Ocean dipole (IOD) is +0.45C based on the difference between the very warm western Indian Ocean tropics and the less warm southeast Indian Ocean tropics (Fig. 1-2). The IOD is unsettled, favoring a weak positive phase for much of FEB/MAR/APR following an -IOD episode reaching moderate intensity during OCT-24 through JAN-25 (Fig. 3). The dynamic models are favoring a shift from the positive phase to neutral or negative phase during mid-year lasting into September. Based on the dynamic models forecast, weakening of the positive phase should begin noticeably in May. However, the Climate Impact Company analog forecast, based on an 8-month regression, indicates neutral phase ahead with the only deviation outside of neutral favoring the positive phase during Q3/2025 (Fig. 4). The +IOD analog years are 1997 and 2018 when El Nino developed. The global SSTA forecasts for AUG/SEP/OCT 2025 vary widely but generally avoid El Nino development. Instead, a common forecast is neutral ENSO with weak La Nina tendencies as indicated by the National Multi-model Ensemble (Fig. 5). The NMME (and other) global SSTA models suggest above normal trade winds develop across the equatorial East Pacific, eastern tropical Indian Ocean, and eastern tropical Atlantic Ocean by AUG/SEP/OCT, necessary to up-well the cool SSTA indicated for each basin. To compensate, trade winds shut off across tropics of the West Pacific and East Indian Ocean which allows warm SSTA to develop. In this set of circumstances, -IOD generates as waters in the southeast Indian Ocean warm while the western tropics of the Indian Ocean cool. In summary, if neutral ENSO expected during the middle third of 2025 trends toward La Nina, the likelihood of -IOD for Q3 and possibly Q4 of 2025 increases. If La Nina does not develop or a weak El Nino forms, the risk of -IOD development fails.

Fig. 1-2: The daily SSTA analysis across the western tropical Indian Ocean and southeast tropics of the Indian Ocean when compared with each other yield a moderate positive phase Indian Ocean dipole.

Fig. 3: Observed daily IOD compared to various climatology percentiles with the 5-month annotated IOD forecast (from ECM, CFS, and IMME).

Fig. 4: The Climate Impact Company 2025 analog forecast of IOD using an 8-month regression.

Fig. 5: The NMME global SSTA forecast for AUG/SEP/OCT 2025 with annotated tropical wind fields suggesting evolution of -IOD.