El Nino and La Nina events typically tend to develop during the period Apr-Jun and reach their maximum strength during Oct-Feb. El Nino/La Nina cycle normally lasts for 9-12 months. However, these norms have been defied more than once and the events have lasted for 2 years or even longer. Since 1950, when the reliable records began, La Nina endurance has far exceeded the El Nino. La Nina has lasted for three consecutive monsoon seasons but El Nino has mostly collapsed during the second successive monsoon. There have been four sessions of triple dip La Nina: 1954-56, 1973-76, 1998-2000 and the recent one between 2020-22.
On the other hand, El Nino had the longest run of 24 months from Mar-Apr-May 1957 to Feb-Mar-Apr1959. The recent one in the history of Indian monsoon was 18 months long from Sep-Oct-Nov 2014 to Mar-Apr-May 2016. There were back-to-back droughts in 2014 and 2015. La Nina of 2016, preceded by super El Nino of 2015, defied the norm of delivering a bumper monsoon. As against the ‘above normal’ forecast of most weather agencies, the season ended at the lower end of the normal (97% of LPA), barely managing to save the grace.
ENSO: The timings and intensity of El Nino/ La Nina events vary widely. Also, El Nino is believed to have stronger correlation of corrupting Indian monsoon, whereas, La Nina has faltered on more occasions to deliver healthy monsoon. El Nino, the oceanic component of ENSO, is always read in conjunction with the Southern Oscillation, the atmospheric arm of ENSO. The large scale coupled interaction of El Nino and Southern Oscillation, together cause the climatic chaos, around the globe. Strong El Nino events are associated with drought in India, Indonesia, Philippines, Australia, Brazil, Northeastern South America and with altered patterns of tropical storms. There is no such authentic list of large scale climatic pattern variation leading to extreme weather events, in different parts of the globe with La Nina.
The equatorial sea surface temperatures (SST’s) are above average in Nino 4, Nino 3.4, Nino 3 region. Nino 1+2 has consistently recorded negative anomalies in the month of April 2024. However, during the last week, all indices have reversed the falling trend and warmed up marginally. Quarterly value of ONI for the season Feb-Mar-Apr is unlikely to drop below 1°C. For La Nina, the quarterly index necessarily has to drop below 0.5°C. Therefore, ENSO neutral is unlikely before May end or June. Thereafter, it is likely to stay neutral till end of July.
IOD: Indian Ocean Dipole develops in boreal summer (Jun-Jul-Aug) and generally peaks during the fall (Sep-Oct-Nov). Like El Nino/ La Nina, IOD also has slippages, with respect to onset and decay. In the recent decades, two extreme positive IOD events occurred, in 1997 and 2019. Both the events saved the Indian monsoon from the scare of drought. The 1997 was a super El Nino year but ended with normal rainfall of 102% of LPA. The year 2019 was a devolving El Nino and started terribly with huge deficit of 33% rainfall in June. Courtesy, strong positive IOD, the latter half of the monsoon witnessed soaking rainfall of 115% and 153% of LPA in August and September 2019. Monsoon finished with an all time high of this century at 110% of LPA.
The IOD index for the week ending 28Apr2024 was +0.68°C, which is above the positive IOD threshold of +0.4°C. IOD index has been positive, maintaining above the threshold for the straight seventh week, since 17March 2024. Typically, a positive IOD event is considered underway, once the index is sustained above +0.40°C for about 8 weeks, subject to favorable atmospheric indicators. A positive IOD event is likely to develop, earlier than usual, in the Indian Ocean. It is expected to peak in July and start narrowing, dropping below the threshold in Oct-Nov 2024.
MJO: After staying dormant for over two weeks, MJO pulse is expected to turn stronger and more coherent in May 2024. The MJO signal is likely to shift eastward rapidly over the Western Hemisphere in Phase 8 & 1, during second week of May. Commencing third week, it is likely to sail over the Indian Ocean, albeit with diminishing magnitude. The forecast is low after 3rd week.
ENSO neutral is still struggling to establish against the continuing El Nino. It may take another 2-3 weeks for the Nino indices to drop below the threshold of +0.5°C. Development of positive IOD is a cheerful sign for the monsoon. However, weak MJO in the second half of May, may not support expeditious streaming of cross equatorial flow.