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Uneasy Truce Between El Nino And La Nina

April 6, 2024 12:42 PM |

Tropical Pacific Sea Surface Temperatures (SST’S) have been steadily loosing heat but the indices remain above the El Nino threshold. Exception being, Nino 1+2, which has turned negative. The rapid cooling there can be attributed to the proximity of the coastline. Also, the whole segment lies south of equator which possibly is cooler than the corresponding areas in the north. The atmospheric indicators are consistent with decaying El Nino. The Southern Oscillation Index (SOI) has turned positive. It is getting aligned with ENSO neutral conditions.

ENSO: There is no complete consensus between the international models on the prospects of ENSO conditions in the upcoming months. Yes, there is broad agreement on likely development of La Nina through the monsoon months and stretching further to the fall of the year. Yet, there is a bit of ambiguity on the exact timings of La Nina. ENSO forecast has historically had their lowest skills, issued in March and April and more so in the latter month. Model skills start becoming better from May. But then, it is too late for the forecasters to commit on the prospects of monsoon rainfall.  Meteorologists will have to live with this limitation and die the cast.

Nino 3.4 region is the reference for ONI, which is principal measure for monitoring, assessing and predicting ENSO. The SST anomaly for Nino 3.4 during the Dec-Feb season was 1.8°C and for the month of February 2024, it was 1.53°c. This further dropped to 1.3°C in March 2024. The most recent weekly anomaly on 01stApr2024 was 1.0°C, indicating that the tropical Pacific is gradually loosing heat, diluting El Nino conditions.

IOD: The relation between ENSO and IOD is quite complex.  While, some researchers are of the view that IOD in the Indian Ocean can evolve without the ENSO forcing from the tropical Pacific. But some of them argue that on some occasions, ENSO can force the IOD. Notwithstanding, IOD significantly influences climate signals, including of Indian monsoon.

The true values of IOD index are in excess of model projections. IOD index for the week ending 31stMarch  was +0.95°C. It has risen sharply  from its negative score of -0.42°C on 03rd March 2024. If these values sustain, then it is defying the norm that IOD events are typically unable to form  between December and April. IOD is likely to stay positive and work in tandem with La Nina to better the monsoon prospects.

MJO: Over the next two weeks, Madden-Julian Oscillation is quickly transiting from the Indian Ocean to Maritime Continent first, and then head for Western Pacific. MJO pulse remains weak, with amplitude  largely confined to the inner circle. It leaves least possibility of any tropical cyclone  evolving over the Indian seas.

La Nina conditions alone, do not bail out the southwest monsoon completely. Theoretically, La Nina is associated with good rainfall during the monsoon season. But, the latest findings based on research paper by Center for Climate Change Research, the relationship between ENSO and ISMR , which was prominent over South India, parts of West and North India from 1901-1940, has weakened between 1941 and 1980. The relationship weakened further after 1980, thus certain La Nina years experienced less than normal rainfall. The last casualty of La Nina year was year 2000 when the season ended with below normal rainfall, 92% of LPA.  Even, La Nina of 2016 could not secure a bountiful monsoon, barely averted the scare of below normal and stopped at the lower margin of normal, 97% of LPA. Yes, La Nina after a super El Nino has historic record of steering monsoon to safety, more often than otherwise.

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