ENSO-Neutral And Weak MJO Offer No Push For Monsoon: Monsoon Entry Starts Mid-May
May 10, 2025, 4:15 PM | Skymet Weather Team
Image: AI-Skymet
The ENSO-neutral phase is typically seen as a transition period between El Niño and La Niña. In the month of March 2025, the SST anomaly in the Niño 3.4 region had reached -0.1°C, and in April 2025, it further dropped to touch the zero-zero mark. The latest weekly value of the Niño 3.4 index is -0.1°C.
The index value in the last about 4-6 weeks has been much warmer than the La Niña threshold of -0.5°C. It means cool waters that had characterized La Niña in the previous month have now faded completely. Of course, the atmospheric component was sluggish to change over. Now, that also seems to have aligned, and the average Southern Oscillation Index (SOI) for April 2025 is 0.4 as against its previous mean of 1.7 in March 2025. The retention of SOI within +/- 0.5 is indicative of the return of ENSO-neutral.
ENSO: During the last 4 weeks, SSTs mostly became near average across the equatorial Pacific. The positive SST anomalies have been neutralized in the central and eastern Pacific Ocean. Even the negative subsurface temperature anomalies weakened in the central and east-central Pacific Ocean.
The Niño 3.4 index has varied between +0.2 and -0.2°C for the last about 8 weeks. The recent value of the index is -0.1°C as against its previous week's value of 0.0°C. As on expected lines, the Oceanic Niño Index (ONI) for the quarter Feb-Mar-Apr has dropped to -0.2°C as compared to its mark of -0.4°C for the previous quarter. It means that ONI remained within the La Niña threshold of -0.5°C for just two quarters, Nov-Dec-Jan and Dec-Jan-Feb, making it the shortest La Niña event since the time records began in 1950. While the ONI had a marginal increment of just 0.1°C for each quarter between Jun-Jul-Aug 2024 and Dec-Jan-Feb 2025, the rate of warming has been double that in the last two quarters. The ONI for the next quarter of Mar-Apr-May is estimated to be zero-zero.
IOD: The Indian Ocean Dipole is neutral. The latest value of the index for the week ending 04 May 2025 was +0.5°C. The index had neared the threshold of 0.4°C last week and remains above the positive IOD threshold for the second consecutive week. For a full-fledged event, the index essentially needs to be sustained above the threshold for at least 6-8 weeks. Despite the recent positive values of the index, most models, including BoM, predict an overall neutral state of IOD until at least August. Caution needs to be exercised, as the skill for IOD forecasts made at this time of the year has historically been low for forecasts beyond a month ahead.
MJO: The Madden-Julian Oscillation pulse is likely to meander within the unit circle, propagating incrementally over the Indian Ocean and the Maritime Continent for the next two weeks. The models are largely unsupportive of a coherent MJO reemerging in the later half of May. The fading away of La Niña and the return of ENSO-neutral are raising the forecast uncertainty for the couple of weeks to come. The southwest monsoon may arrive a little before time over the South Andaman Sea on its own; the MJO may not support expeditious advancement of the current further ahead over the North Andaman Sea and Central Bay of Bengal.
No storm is expected to develop over the Indian Seas in the next two weeks. The month of April also was storm-free. Any deep convection over the Bay of Bengal is unlikely. Yes, a small perturbation may come up over the east-central Bay of Bengal, next to the Arakan Coast, facilitating amplification of cross-equatorial flow around the middle of May 2025. This may initiate the entry of the monsoon over the Andaman Sea, a little before time.
