El Nino ‘advisory’ pertains to conditions when El Nino is observed and expected to continue. La Nina ‘watch’ indicate conditions favorable for development of La Nina with a notice period of six months. The El Nino/La Nina phenomenon is the biggest cause of year to year difference in weather in many regions. In La Nina years, the east to west Pacific trade winds are stronger, pushing warm surface waters to the west and drawing up deeper, cooler waters in the east. El Nino events happen when the trade winds wane, allowing the warm waters to spread back eastward, smothering the cooler waters and leading to rise in global temperatures.
The buzz is really loud that India is likely to embrace abundant rainfall during monsoon this year, under the reduced influence of El Nino and growing stature of La Nina conditions in the Pacific region, after May. There is an excitement and why not. But, the expectations of generous monsoon attributable to La Nina, need to be qualified. The state of Indian Ocean carry lot of weight and remains accountable to qualify any such predictions. Unlike strong correlation of El Nino corrupting Indian monsoon, La Nina does not enjoy robust relationship of steering the monsoon to unerring safety, on its own. Bumper monsoon gets assured when La Nina sails on the back of strong and positive Indian Ocean Dipole. The two together, essentially need to remain in phase and work in tandem, for unfailing returns.
ENSO: The oceanic and atmospheric conditions across the tropical Pacific still indicate continuation of El Nino event. The Pacific warming is resolute and any sudden collapse is unlikely. The Southern Oscillation Index is yet to dilute completely and the monthly average value is marginally negative (-0.2). The index had earlier turned positive in March 2024. It may be construed that the oceanic conditions and atmospheric indicators of ENSO are not yet ‘done and dusted’.
The remnant effect may linger longer and ENSO could turn neutral by end of May or start of June, coinciding with the scheduled onset of monsoon. A positive IOD event may develop earlier than usual but La Nina is unlikely to be in place, to have a joint push at the start of the monsoon. It is feared that La Nina and IOD may not be in sync, when it is needed the most.
During the last four weeks, above average SST’s have weakened across central portion of Nino region. Nino 3.4 is very close to the threshold value and Nino 3 has dropped below for the first time since 17Apr2023. Nino 4 is consistently warmer than normal and Nino 1+2 is unsteady, as it is normally observed during the transition period. Stable indices are likely by the end of this month.
IOD: Positive IOD increases precipitation to above average in East Africa and Western Equatorial Indian Ocean, while it decreases rainfall in Indonesia and Australia. Even with the latest simulation techniques, IOD’s are difficult to predict several months in advance. For Indian monsoon, IOD prediction is needed from Summer to Autumn (NHS). However, it faces ‘winter prediction barrier’, like the El Nino has ‘spring barrier’.
The Indian Ocean Dipole index for the week ending 05 May 2024 was +0.24°C. The index has slipped below the average after remaining positive for seven consecutive weeks. It is rather difficult to reason out this collapse from +0.68°C on 28Apr 2024 to +0.24°C on 05 May 2024. A uniform pattern can be expected, on stabilization of cross equatorial flow over the Indian Ocean.
MJO: The Madden Julian Oscillation is likely to remain weak, as it tracks eastward over the Maritime Continent and further on to Western Pacific. The MJO pulse will become stronger over the Western Hemisphere & Africa in Phase 8&1, around middle of May. It may trigger speedy cross equatorial flow over the western parts of Indian Ocean. However, its diminishing amplitude in the last week of May over the Indian Ocean may not give desired push to streaming of monsoon.
Scheduled monsoon onset is drawing nearer. The Pacific and Indian Ocean remain elusive. In the process of changeover, ENSO and IOD signal remain concealed and indeterminate. At this stage, it is difficult to read monsoon with clarity and precision.
