Australian Bureau of Meteorology has a standard index of +/- 0.8°C above/ below the normal sea surface temperature (SST) for El Nino and La Nina conditions, respectively. As per BOM, El Nino has ended and the ENSO has turned neutral. However, as per Climate Prediction Center (CPC-NOAA), the threshold value is +/- 0.5°C.
The ocean and atmospheric conditions across the tropical Pacific indicate continuation of an El Nino event, albeit with a declining intensity. The strengthening and weakening of El Nino in October 2023 and March 2024, respectively has been captured nicely in the ocean surface temperature across the equatorial Pacific.
ENSO: The SST anomaly for Nino 3.4 region during the Jan-March 2024 season was +1.52°C and for the month of March2024, it was +1.24°C. The most recent weekly anomaly in the Nino 3.4 region, as on 22ndApril 2024 was +0.7°C. It is indicative of steady cooling in the tropical Pacific. Negative sub-surface anomalies have intensified and extended further westward in the equatorial Pacific Ocean, near the surface east of 120°W. Meanwhile, warm sub-surface temperature anomaly still prevails over central to eastern Pacific region from surface to 50 mtr depth.
Nino 3.4 index, the representative of ONI shows a steady fall. It seems to be reaching the threshold of +0.5°C in another 2 weeks or so. The 30 day Southern Oscillation Index stands within the ENSO-neutral range. The trade winds in the lower levels are mostly normal across the stretch of tropical Pacific. ENSO neutral is likely by second week of May, the time for strengthening cross equatorial flow.
IOD: The Indian Ocean Dipole is a climatic pattern affecting the Indian Ocean and its brim countries. Its extent of influence stretches from Indonesia to Australia on the eastern side and African Coast to mainland India, during the monsoon season. Various combinations of IOD and ENSO can lead to climatic chaos. The most favorite for the Indian monsoon is positive phase of IOD, which can rescue monsoon, when in distress, as it happened in 2019. The rare combination of La Nina and positive IOD during the monsoon months lead to increased threat of coastal flooding and associated impacts. The IOD index for the week ending 21stApril 2024 was +0.56°C, nearly a repeat of last week value of +0.57°C. IOD values start stabilizing from second half of April and reliably depict sign of the event from May onwards.
MJO: SST anomalies are declining in all the Nino regions. With widespread drop in the upper ocean heat content, the global tropical circulation has become less organized. The Madden-Julian Oscillation has continued to weaken and will remain indiscernible for the coming week. The RMM (Real-time Multivariate MJO) forecasts generally Favour weak and incoherent intra-seasonal activity during the next couple of weeks, on account of MJO. As such, the global tropics remain rather quiet, with respect to the tropical cyclone activity, around this time of the year. With weak MJO providing little or no support, the tropical cyclone is unlikely over the Indian seas.
ENSO is likely to turn neutral in May. Whenever, there is change over, transition period from El Nino to La Nina or otherwise, is approximated as 6-8 weeks, and at times could be longer. Accordingly, a full fledged La Nina is unlikely before 3rd-4th week of July. La Nina, once in place, is expected to peak towards fall of the year and start decaying during early winters of next year.