During the last 4 weeks, above-average equatorial SST’s increased across the central and eastern Pacific Ocean. SST’s near Ecuador and Peru remain strongly above average, maintaining >/=3°C for the last 5 consecutive weeks. El Nino pattern seems steadily making incremental rise.
ENSO : El Nino and Southern Oscillation, together known as ENSO, is a periodic fluctuation in the sea surface temperature (El Nino) and the air pressure of the overlaying atmosphere (SO) across the equatorial Pacific Ocean. The negative phase of SOI represent below normal air pressure at Tahiti and above normal air pressure at Darwin (both Australia). Prolonged period of negative SOI coincides with warm ocean water across the eastern tropical Pacific and is typical of El Nino episode.
Central and eastern Pacific sea surface temperatures (SST’s) exceed threshold mark. As per numerical models, El Nino will grow stronger with an absolute share of 100% in the quarter Jul-Aug- Sep. La Nina and neutral components remain negligible till fall of the year. As anticipated, the quarterly Oceanic Nino Index (ONI) for May-Jun-Jul has risen to +0.8°C, an increase of 0.3°C from the previous quarter of Apr-May-Jun. All the Nino indices have been maintaining positive values above the threshold since early June 2023. SOI also seem to be shifting to the negative regime steadily. The respective SO index value for the last 4months were: Apr +0.2, May -1.0, Jun +0.3, Jul -0.3. With El Nino and Southern Oscillation getting aligned, the ocean and atmosphere are expected to respond in tandem to evolve perfect El Nino event.
IOD: The Indian Ocean Dipole is currently neutral. The IOD index value for the week ending 06Aug2023 was +0.20°C, which is within the neutral bounds( -0.4°C and +0.4°C). IOD has maintained a negative phase fairly long, for nearly 3 years, 2020, 2021 and 2022. As a norm, 2 successive positive or negative IOD are rare during the Indian monsoon. Out of 10 episodes, the ratio of positive, negative and neutral IOD is 3:3:4 respectively. In 2023, the index value had been fluctuating between +ve and -ve neutral during 1st and 2nd quarter. In any case, IOD values are not reliable from Jan to May, on account of low accuracy of models. As such, the model skills, unlike El Nino, are rather poor during this period. Also, the correlation of IOD vis-a vis monsoon performance remains rather low and is in contrast to the El Nino connection. Model survey suggest development of moderately +ve IOD during end of August and September, crossing the threshold value of +0.4°C during 2nd half of August.
MJO: There is no significant uptick in the amplitude of MJO and it continue to remain in the unit circle. MJO has been sailing over the Western Pacific in phase 7&8. West Pacific Ocean has hosted back to back typhoons: Khanun and Lan. Also, a Cat IV equivalent tropical cyclone Dora is moving across central Pacific and an invest area of potential cyclone is located over the East Pacific Ocean. The models largely favour a weakened and disorganized MJO through late August. Indian seas may remain muted and unlikely to host any strong monsoon system supported by MJO.
A strong El Nino is emerging in the Pacific waters. ENSO is likely to push and impact monsoon rainfall during the 2nd half of season. IOD may still fall short of neutralizing ill effects of El Nino.