During the La Nina event, the changes in the Pacific Ocean temperature affect the pattern of tropical rainfall from Indonesia to the west coast of South America. El Nino/ La Nina events tend to develop during the period Apr-Jun. They tend to reach their maximum strength during Dec-Feb. However, there are no strict bounds and the phenomenon can persist anytime through the year, like the current event. El Nino/ La Nina effects are usually strong during summer monsoon over the Indian Sub-Continent, as they control the streaming of monsoon systems over the Indian Seas. The persistence of these phenomena during the winter season is coupled with the positioning of the Jet Stream. During La Nina years, the narrow core of very strong westerly winds have a tendency to shift southward and in which case, the northern regions become wetter and colder and the southern one’s drier and warmer. Going by the historical records, La Nina does not have a robust correlation for any stringent weather pattern. Factually, during La Nina conditions, the winter rainfall (Jan+Feb), more often remained less than the average and exceeded only on a couple of occasions.
ENSO: According to the ENSO forecast issued by IRI in November last, ENSO neutral conditions are forecast (52% prob) for Nov-Dec- Jan 2025, while the likelihood of La Nina emerging has decreased to 48%. It is a close call between the two. For the subsequent quarter Dec-Jan-Feb 2025, the probability of SST’s reaching La Nina threshold is 50% while the likelihood of ENSO – Neutral conditions is estimated at 49%. Again, it is a close call. These appear to be borderline cases and have the chance to swing, either way. Even, when the event starts, there is always a lag time between its appearance and impact, more so, in case of water, which has a long memory. In any case, from Jan-Mar 2025 to the end of the forecast period in Jul-Sep 2025, ENSO neutral conditions are favoured. It means, that if La Nina conditions build-up, it will have a small window and their effect may start becoming indiscernible before the impact is realized.
There was cooling across the equatorial sea surface of the Pacific Ocean all along the Nino region. All the four Nino indices have simultaneous drops, not very commonly seen in the last several weeks. Two things need to be observed. The Nino indices still remain far from the threshold needed. The downward trend noticed this week needs to be watched for consistency. The atmospheric wing of ENSO is holding the hand of Nino indices. The Southern Oscillation Index remains sufficiently positive in Nov 2024, nearly a repeat of October 2024. If all goes well, La Nina may usher in sometime in Jan 2025.
IOD: The Indian Ocean Dipole for the week ending 01stDec2024 was -0.19°C. This is the first time in the last 8 weeks that the index has crossed the threshold mark of -0.4°C and turned neutral. With this, the occurrence of negative IOD events has been averted. Now, the countdown starts afresh for either of the events, neutral or positive. A typical IOD event life cycle has neutral values in the month of December and start turning positive in the early spring of the Northern Hemisphere.
MJO: The Madden-Julian Oscillation is expected to propagate steadily eastward, across the Maritime in Phase 5 to Western Pacific in Phase 6. As against the earlier scare of losing amplitude, the pulse is likely to retain its magnitude and remain healthy, all through the 2nd and 3rd week of December. The growing La Nina response and revert of neutral IOD may withdraw the decline of the MJO signal and let it go strong over the Western Pacific. Though, there are no active storms anywhere around the globe, but there are no official bounds either, to restrict formation. MJO may still help perturbation to grow, more so, over the equatorial region of the East Indian Ocean and West Pacific basin.
The speculation of evolving La Nina is losing material support. Even if it comes up, a very short and weak stint, may not have far-reaching consequences. The La Nina window is likely to close as quickly as it opens, early next year.