El Nino events are typically associated with increased rainfall in parts of southern South America, the southern United States, the Horn of Africa and Central Asia. In contrast, El Nino may trigger severe droughts over Australia, Indonesia, South Asia and northern South America. No two El Nino events depict similar results and sometimes large variations can not be ruled out.
ENSO: Since Feb 2023, monthly sea surface temperature (SST) anomalies in the central-eastern equatorial Pacific Ocean have warmed significantly. These anomalies were half a degree Celsius below average in Feb 2023 and rose around half a degree Celsius above average by May 2023. Currently, the entire Nino region is warmer by more than 1°C. Nino 3.4 region, the representative region of El Nino has reached a value of 1.6°C, for the 2nd consecutive week.
Earlier, there was uncertainty because of weak ocean-atmosphere coupling . However, the coupling seems to have established now, albeit towards end of the monsoon season. The atmospheric response to the ongoing El Nino event remains the most coherent climate signal. Southern Oscillation Index (SOI) has dropped from +0.3 in June to -0.3 in July and -0.8 in August 2023. The warming of the sea surface works to decrease the atmospheric pressure above it by transferring more heat to the atmosphere. The strengthening and weakening of trade winds is also a function of changes in the pressure gradient over the tropical Pacific.
IOD: The Indian Ocean Dipole index was +1.13°C for the week ending 10 Sep 2023. This is the 4th week above the positive IOD threshold (+0.4°C), with the index rising from +0.79°C on 21 Aug 2023 to 1.05°C on 05Sep2023. The increasing magnitude of the IOD index is indicative of a likely positive IOD event this year. The index needs to sustain the value above the threshold mark for 8 weeks or more. A typical positive IOD weakens the ill effects of El Nino.
MJO: A weakened Madden-Julian Oscillation pulse had moved to the Maritime Continent. Uncertainty among the dynamical models is high about the progression and amplitude of MJO. GFS ensemble supports a weak MJO, with amplitude in the inner ring, in phases 2&3, for the next about 2 weeks. Monsoon rainfall activity over the centre and eastern parts may accentuate. This may help reduce the seasonal rainfall deficit.
Due to ongoing weather activity in association with the well-marked low-pressure area, which earlier developed over the Bay of Bengal, the rains will continue over Madhya Pradesh, Maharashtra, Gujarat and East & South Rajasthan. This may delay the withdrawal of southwest monsoon from Rajasthan, to start with.