El Nino and La Nina events, as a norm, tend to develop during the period Apr-Jun and they reach max strength during Oct-Feb. Typically, these events persist for 9-12 months and occasionally last up to two years or even longer. Ignoring these standards, weather agencies across the globe predicted likely La Nina conditions, after the cessation of strong El Nino 2023-24. At best, the La Nina was expected to settle down during the southwest monsoon 2024 and continue till the Northern Hemisphere winters of 2025. US’s National Oceanic Atmospheric Administration (NOAA) issued a La Nina watch in May 2024, indicating that La Nina conditions were expected soon. However, most global agencies got the La Nina forecast wrong, this year.
In anticipation of the La Nina event, boreal winters were forecast to be harsh this season. Following La Nina, a bountiful winter rainfall is also prognosticated. Assumingly, La Nina conditions will last till early spring 2025 (NH), and turn neutral before the commencement of the Southwest Monsoon next year.
In one of the earlier narratives, based on statistical records, Skymet differed from any such likely consequences attached to La Nina. The reasons for this difference of opinion were attributed to the following.
- La Nina does not have as strong a correlation as El Nino, for altering the weather pattern of the Indian Sub-Continent in different seasons.
-The Density of water is higher at lower temperatures (0°- 4°C) and decreases at higher temperatures. With this, the depth of cooler water in the case of La Nina remains shallow. Contrarily, during El Nino, the layer of warm water goes quite deep. The impact and correlation of these phenomena, therefore vary, being soft in the case of La Nina and mostly robust for El Nino.
-The La Nina event commenced during the fall of the year only once, in 2005-06, since 1950. And, the winter season rainfall (Jan + Feb) was recorded below normal.
-The winter rainfall during La Nina years has mostly succumbed, ending below normal in the last two decades, with an exception in 2022. Therefore, the rainfall may not be generous during the upcoming winter months.
ENSO: La Nina is expected to be weak and truncated, this season. The possibility of “No Show” cannot be ruled out, either. La Nina can only be confirmed if the Oceanic Nino Index (ONI) drops consistently below the threshold mark of -0.5°C. Nino 3.4, the indicator and measure of ONI and in turn La Nina, is nowhere near this mark. Forecasters, including at NOAA, had anticipated a clearer shift to La Nina by mid-year, but predictions fell short. This “spooky weather” as described in NOAA’s latest blog, reflects the challenges of predicting precise ENSO patterns amid changing climate.
The Nino regions in the equatorial Pacific Ocean keep shuffling and are nowhere near settling down to support expeditious La Nina conditions. The Nino 3.4 index has touched the threshold mark of -0.5°C twice in the last 8 weeks but only to revert back to warmer conditions. This week, the index continue to fall short of the mark. The latest value of the Nino 3.4 index for the week ending 11 Nov 2024 was -0.3°C. The mean value of the index over the last 8 weeks stands at -0.37°C. It needs, not only to reach -0.5°C but also remain steady above that mark for a sufficient period.
IOD: The latest value of the Indian Ocean Dipole for the week ending 10th Nov 2024 was -0.69°C. This is for the fifth consecutive week that the index has been below the threshold of -0.4°C for negative IOD. The IOD index will meet or exceed the -VE threshold in Nov 2024. To be classified as a negative IOD event, the index needs to be below the threshold for a sustained period of at least 6-8 weeks. The IOD forecast skill is mostly poor beyond one month ahead.
MJO: The Madden-Julian oscillation has completed a full circle around the globe. The pulse is slowly shifting over the Indian Ocean and is likely to propagate eastward over the Maritime Continent in the fourth week of Nov 2024. MJO's position over the Indian Ocean in Phases 2 & 3 favours an uptick in the tropical cyclone development. However, the models are not supporting aggressively any storm formation over the next ten days. Despite some strong hunches, the chances of any storm formation over the Indian Seas are placed only at 20% to 40%.
The Formation of La Nina remains dubious and inconclusive in terms of timings, intensity and duration. At best, it could be a borderline event and persist during the winter months, Jan & Feb 2025. ENSO neutral is likely to return during early spring and may last through the monsoon 2025.