Typically, La Nina events are accompanied by negative IOD-related SST anomalies in the tropical Pacific. The relationship, however is non linear. Since the year 2000, there have been seven La Nina years, including triple dip La Nina ( 2000, 2007, 2010, 2016, 2020,2021,2022). IOD was marginally positive on two occasions, 2000 & 2007 and negative on rest of the five occasions. IOD was strongly negative during the last triple dip La Nina, across the monsoon period. Strength of IOD forecast has been lowered from April to May 2024. Therefore, a strong positive IOD and peak La Nina may not resonate together. As such, there is always a lag between ENSO and IOD changes and its influence on weather. Most models indicate low to moderately positive IOD in the core monsoon months of July and August and turning nearly neutral in September. But, remember that historical skills of IOD forecasts, beyond spring is low, at this time.
ENSO: While the El Nino occurrence is stated as 2-7 years, no such periodicity is established for La Nina. Even, the constancy of El Nino appears to be bit vague. The intermittency seem to be varying widely and the phenomenon is neither periodic nor predictable. There has never been a gap of seven years between the two El Nino events, at least in the last three decades.
El Nino is transitioning towards ENSO neutral. Equatorial SST’s are above average in the Western and Central Pacific Ocean and below average in the East-Central and Eastern Pacific Ocean. Since mid-March 2024, below average SST’s have emerged in the Eastern Pacific and have expanded gradually westward. Now, indices have turned negative in the Nino 3 and Nino 1+2 significantly. Only, Nino 4 is warmer than the threshold of 0.5°C and the ONI indicator has dipped to 0.2°C, with a further decline likely. As per Climate Prediction Center (CPC), a transition from El Nino to ENSO neutral is likely in the next month. Historically, strong El Nino like the last one in 2023, are followed by short neutral phases, before portaging to La Nina. La Nina may develop in June-August 2024 (49% chance) or July-September (69% chance).
IOD: The Indian Ocean Dipole is currently neutral. The most recent value of the index (+0.20°C) is the 3rd consecutive weekly value within the neutral threshold. Earlier, the index consistently remained above the threshold of +0.4°C, for seven weeks, raising hopes of initiating a positive IOD event. Typically, a positive IOD event is considered underway, once the IOD is sustained above +0.4°C for 8 weeks or more.
A strong positive IOD has more often negated the ill effects of ENSO, if any. IOD of 1994 was responsible for stronger than normal monsoon that summer. Again, positive IOD was able to reject the negative relationship between strong El Nino and ISMR in 1997. Despite El Nino, monsoon finished with normal rainfall of 102% of LPA. Recently, the emergence of strong positive IOD, late in the season, evidenced how interaction between ISMR, IOD and ENSO are complex and highly influenced by internal dynamical processes. Positive IOD with La Nina is the best possible combination for a prolific monsoon and the sketch is somewhat similar, this monsoon season.
MJO: The Madden-Julian Oscillation is the largest element of intra-seasonal variability in the tropical atmosphere. Unlike a standing pattern like the ENSO, the MJO is a travelling pattern that propagates eastward at approximately 4-8m/s, through the atmosphere above the warm parts of the Indian and Pacific Ocean. The overall circulation pattern manifests itself most clearly as anomalous rainfall.
The positioning of MJO in phase 2 &3 during the monsoon trigger the tropical disturbances over the Indian seas. An MJO with fair amount of amplitude can spark off bursts of monsoon. MJO has a cycle of about 30-60 days going around the equatorial belt. It may visit the Indian Ocean, only 3-4 times during the entire southwest monsoon season. Currently, the MJO is positioned over the Indian Ocean in phase-3 and likely to transit to the Maritime Continent in phase-4. MJO is favourable for setting off monsoon stream across the Indian seas.
Monsoon onset over the mainland is getting closer. All the three parameters ENSO, IOD, MJO influence the seasonal rainfall appreciably, throughout the monsoon run. While ENSO and IOD are the static pattern, the transitory MJO pulse has the potential to trigger monsoon systems over the Indian seas. It also carry the capacity to enhance convection and prolong the wet phase of the monsoon.