Updated synopsis of La Nina enunciates its continuation through Northern Hemispheric winter 2022-23. ENSO probability is retained with over 90% chance during Oct-Nov and decreases to mid 50’s in Jan-Feb 2023. So far, there was consensus among the dynamical and statistical models, that La Nina will attenuate by the fall of the year. Forecaster’s opinion is now aligned with statistical models but not without the element of uncertainty, as to, how long La Nina will last and when is the transition to ENSO neutral likely.
Negative Sea Surface Temperature(SST) anomalies persisted for fairly long in the equatorial Pacific Ocean. Against the earlier predictions of smoothening of -ve anomalies, they actually have strengthened across most of the equatorial Pacific during the last 2 weeks.’ Back to Back’ La Nina event is heading for one of the longest in recent times.
ENSO: Nino 3.4 index, the principal measure of ENSO conditions is retaining the cool of SST in the Central Pacific. The index is moderately strong and the temperature is hovering close to -1°C , for the last 6 weeks.
IOD: Indian Ocean Dipole is related with year to the year variability of weather. It is akin to ENSO, with the difference that the phenomenon takes place in the Indian Ocean, rather than in Pacific. However, the impact and duration of ENSO are more powerful and longer than the Indian Ocean Nino – IOD. Like ENSO, IOD has 3 phases: neutral, -ve and +ve. Neutral IOD corresponds to the Sea Surface Temperature within +/- 0.4°C of the average.
Once the IOD goes into a particular phase, it invariably remains in that state for a few months and many times for the season. This year during the southwest monsoon, it persistently remained in the -ve phase and registered the record lowest value since 2016. The latest Indian Ocean Dipole index value for the week ending 18th Sep 2022 was -0.99°C.
MJO: Madden Julian Oscillation is a global scale feature of the tropical atmosphere. Its evolution and impact is more evident in the Indian Ocean and West Equatorial Pacific Ocean. Numerical models suggest weak MJO activity for the next 2 weeks. The index is likely to stay in the unit circle and may move to the equatorial belt of the Eastern Hemisphere. There is a fair amount of uncertainty about its progression and timelines.
Southwest monsoon is going to conclude shortly, on 30th September. Moderate La Nina conditions prevailed throughout the season. The influence of the cool Pacific has been amply manifested as adequate rains over most parts, barring few pockets. The impact of negative IOD events was overcome by powerful ENSO. The season is expected to close as ‘above normal'.