El Nino advisory is issued when El Nino or La Nina are observed and expected to continue. Advisory is in vogue, starting early days of last year, since triple dip La Nina staged collapse. For La Nina ‘Watch’, the Climate Prediction Center (CPC) issues a bulletin indicating conditions favourable for development of La Nina with a notice period of six months. Final El Nino/ La Nina advisory is issued when these conditions have ended. Going by the current perspective, Advisory and Watch, will go hand- in- hand for the next 4-6 weeks, till ENSO turns neutral.
ENSO: Since 1950, there have been six episodes of ‘Super El Nino’. El Nino during 1965-66, 1972-73, 1982-83, 1997-98, 2015-16 and 2023-24 crossed the threshold mark of 2°C and therefore, classified as Super El Nino. Historically, Super El Nino’s have been followed by strong La Nina, except in 1966 when ENSO neutral conditions prevailed from the season April-May-June and lasted till August-September-October 1966. IOD also remained neutral, through the monsoon and winter season in 1966. Post ‘Super El Nino’, all the monsoon seasons recorded normal or above normal rainfall, except in 1966, an ENSO neutral year, when it finished as a drought year, at 87% rainfall of LPA.
Southern Oscillation Index has turned positive and is all out to support ENSO-neutral. It also means that, in the atmospheric-ocean coupling, atmosphere has turned benign. Now, the ocean has to fight the battle all alone, for El Nino. Volatility of the Nino indices continue, as usual in the tropical Pacific. All four indices have recorded warming in their respective regions, during the last week. It may be construed that El Nino is unlikely to ‘vanish in thin air’ and resistance may go on till next month.
IOD: The IOD and the ENSO have complimentarily affected the ISMR. Indian Ocean Dipole plays an important role as a modulator of the Indian monsoon rainfall and influences the correlation between the ISMR and ENSO. During La Nina conditions, the ENSO induced anomalous circulation over the Indian region is supported by the positive IOD induced anomalous meridional circulation cell. The two major tropical phenomena in the Indo-Pacific sector, when in phase, act in tandem to enhance monsoon rainfall.
The IOD index for the week ending 07 April 2024 was +0.64°C. Like the Nino 3.4 index, Dipole Mode Index is also wavering over the last 4 weeks. Earlier, the IOD index rose from 0.81°C on 24 March 2024 to +0.95°C on 31 March 2024. Within next one week, it crashed to 0.64°C. The index is expected to stabilize in the next 2-3 weeks. Reliable signals, concerning health of monsoon, could possibly start, early next month.
MJO: As per CPC wave analysis, while propagating eastward from the Western Hemisphere, the MJO pulse has encountered strong planetary waves. These are long and low frequency water waves near the equator and adversely impact the trade wind pattern. This has weakened the signal and slowed down the propagation. This could, as well, suppress the ongoing sub surface cooling in the Pacific basin, further delaying the transitioning of El Nino state. Possibly, this feature has restricted the drop of Nino indices, which have rather shown a marginal rise all along the equatorial Pacific, during last week. Indian seas are likely to stay inactive and therefore, wait gets longer for the maiden tropical cyclone of pre-monsoon season. As such, no cyclone has come up in the Indian seas in the month of April for the last four years. The last cyclone to form in April was ‘Fani’, between 26 April – 04 May 2019. It made landfall, close to Puri in Odisha.
The positive IOD can weaken the impact of El Nino. Negative IOD with El Nino can ruin the monsoon. Positive IOD and La Nina is a rare combination and can steer the monsoon safely. Last, it happened in 2007 when monsoon closed above normal with 106% rainfall of LPA. Similar alignment is expected this season when La Nina and positive IOD may go hand in hand. Monsoon riding on the back of La Nina and IOD is expected to have a safe landing.
