If Negative Indian Ocean Dipole Develops Q3/2025, La Nina Follows

Climate Impact Company Weekly ENSO Climate Diagnostics

Issued: Monday May 5, 2025

Highlight: ENSO diagnostics remain mixed; La Nina regeneration later this year occurs if negative Indian Ocean dipole forms.

Fig. 1-4: The current tropical Pacific/Indian Ocean(s) daily SSTA analysis and 30-day change (upper) and subsurface equatorial Pacific upper ocean heat anomalies and NOAA seasonal ENSO phase probabilistic forecast.

Discussion: ENSO phase is important to tropical cyclone season activity and the monsoon patterns in the northern hemisphere during the warm season. The ENSO regime has featured consistent on again/off again phase change ahead signals sometimes pointed toward El Nino and sometimes favoring La Nina regeneration all of 20205 (so far). Currently, the daily SSTA analysis reveals warm SSTA in the tropical Indian Ocean and Maritime Continent while east of the Dateline a neutral ENSO condition in control (Fig. 1). The 30-day (SSTA) change reveals the strong cooling of waters off the northwest coast of South America where an El Nino like warming occurred in FEB/MAR (Fig. 2). The most recent warming in the eastern equatorial Pacific is near and just east of the Dateline, a new development. In the subsurface, often an excellent indicator of an ENSO phase change, waters have warmed significantly during the past 10 days east of the Dateline while robust warming remains intact west of the Dateline (Fig. 3). The latest seasonal Nino34 SSTA probability forecast from NOAA reveals a likelihood of returning La Nina later this year (Fig. 4). The tropical SSTA regime required to regenerate La Nina is an eastward shift of warm SSTA in the tropical Indian Ocean toward Maritime Continent which ignites negative phase Indian Ocean dipole (-IOD). The consolidated warming of the ocean surface across the Maritime Continent region increases convection (thunderstorm activity) compensated for by subsidence and increased trade winds near and east of the Dateline (Fig. 5). In summary, the opinion of Climate Impact Company, is that -IOD generation – which is expected – will regenerate La Nina for later this year which would enhance seasonal tropical cyclone activity in the North Atlantic basin.

Fig. 5: If -IOD develops (warming SSTA northwest of Australia), convective currents across Maritime Continent increases which draws increased trade winds from the east to replace the rising air. Trade winds upwell cool water causing La Nina to regenerate.