As of mid-August 2024, ENSO neutral conditions persist in the west equatorial Pacific Ocean. The oceanic and atmospheric conditions also align with an ENSO-neutral state. There is very little agreement between IRI ENSO prediction and CPC ENSO outlook. The IRI predicts ENSO–neutral conditions for Aug-Oct and Sep-Nov 2024. Borderline La Nina conditions are forecasted during Oct-Dec and Nov-Jan but with weak elevated probabilities. After a pause in the spring of 2025 (Northern Hemisphere), the forecast shows a return of ENSO–neutral conditions. The CPC ENSO outlook is broadly inclined towards La Nina onset in Sep-Nov 2024 and its continuation until Jan-Mar 2025. It has a clear preference for ENSO - a neutral state during the boreal winters and spring of 2025. It means, there is a mismatch in the onset window of La Nina. The IRI is favouring the onset of La Nina beyond the official timelines of the Indian monsoon.
The official CPC ENSO probability forecast and the objective model-based IRI ENSO probability forecast, are often quite similar. However, exceptions are always there and the difference becomes noticeable on a few occasions, as is the case in point now. There can be several reasons for the discord. One reason is related to the fact that the models are not run at the same time that the forecasters make their assessment. So, the initial conditions may vary. The second plausible reason could be that models have quite strong biases and the human forecasters may smoothen out the discrepancies, if any, with their experience and judgement.
ENSO: The sea surface temperature in the central equatorial Pacific Ocean has cooled gradually, since Dec 2023. This cooling is being sustained by deep water surfacing in the east and central Pacific Ocean. Accordingly, ENSO-neutral conditions have prevailed for the last few weeks. However, contrary to the normal expected trend, the extent and magnitude of cool water have decreased in the last three weeks. This has extended the neutral state of the ocean and delayed the onset of La Nina, expected earlier during the first half of the monsoon season.
There are marginal changes in the temperature anomalies across the Nino region in the equatorial Pacific Ocean. The central parts of the Pacific Ocean hosting Nino 3.4 and Nino 3 have maintained a temperature anomaly between +/- 0.3°C, since May 2024. The cooling rate has been rather slow and the Nino 3.4 has observed a neutral range of temperature (within +/-0.5°C) for the last 4 months. The latest value of Nino 3.4 is -0.1°C and shows no immediate signs of collapsing to -0.5°C or less, the necessary conditions for initiating La Nina.
IOD: The Indian Ocean Dipole has generally remained negative, albeit above the threshold of -0.4°C, since June 2024. The latest value of the index for the week ending 18 Aug 2024 was +0.33°C. The rise of 0.3°C in the last one week needs to be ascertained, to rule out any chance of aberration. Most models indicate the IOD is likely to remain neutral till the fall of the year. However, the IOD forecast skill has historically been low at this time of the year for forecasts beyond two months ahead.
MJO: The Madden-Julian Oscillation is currently sailing over the Indian Ocean with moderate amplitude, in Phases 2&3. The models suggest that the pulse will become weak, as it propagates to the Maritime Continent during the last week of August 2024. There may not be any outright support from MJO for the development of any monsoon system over the Bay of Bengal. However, eastward propagating MJO greatly increase the chances for tropical cyclone development over a large area extending from the South China Sea to the Philippines Sea, around the first week of September.
ENSO-neutral conditions are lingering and the onset of La Nina, anytime soon, is presumptive. Monsoon will enter its retreating phase in September, Therefore, La Nina, more or less, will become inconsequential for the overall state of monsoon 2024. The monsoon current, by itself, seems to be energetic enough to pull out a decent number and close on a satisfactory note.
Image Courtesy: Pixabay
