During the last 4 weeks, above average equatorial sea surface temperatures (SST’s) increased across the central and eastern Pacific Ocean. SST’s near Ecuador and Peru have risen exponentially. As of end-July 2023, the previously weak El Nino conditions in the central-eastern equatorial Pacific have strengthened gradually to a weak-to-moderate El Nino. Key oceanic and atmospheric variables are consistent with weak El Nino. However, Southern Oscillation Index (SOI) still remains in the ENSO-neutral range. The respective value of SOI for the last quarter were: Apr : 0.2, May : -1.0, June : 0.3. The SOI is a standardized index based on the observed sea level pressure (SLP) differences between Tahiti and Darwin, Australia. The SOI is one measure of the large scale fluctuations in air pressure occurring between the western and eastern tropical Pacific. Prolonged periods of negative SOI values invariably coincide with abnormally warm ocean water across the eastern tropical Pacific Ocean. For reasonably strong El Nino and its adverse effect on monsoon rains, SOI necessarily need to be in sync with oceanic variables. It is expected to follow suit in the coming weeks.
ENSO: Central and eastern Pacific SST’s are exceeding El Nino threshold. Further warming is likely to raise the chances of El Nino event during monsoon, continuing in to autumn and winters and weakening during the spring of 2024. There has been incremental rise in the Nino indices since the last observation. The SST anomaly for Nino 3.4 during the Apr-May-Jun season was +0.51°C and or the month of June 2023, it was +0.88°C. Average monthly SST for the month of July is likely to be +1°C. With this, quarterly ONI of Nino 3.4 region for May-Jun-Jul is likely to be +0.75°C approximately. This will be a big leap from the previous quarter of Apr-Jun 2023.
IOD: The Indian Ocean Dipole is currently neutral. The IOD index for the week ending 23 July 2023 was +0.01°C, which is within neutral bounds ( between -0.4°C and +0.4°C). IOD had remained negative neutral for the last 4 weeks with its lowest value of -0.31°C on 09 July 2023. IOD has now turned nearly zero for the first time after 18 June 2023, when its weekly value was 0.0°C. Numerical models indicate a gradual rise of this index breaching the threshold mark of +0.4°C during 2nd half of August. A strong positive IOD has the potential to dilute the ill effects of El Nino.
MJO : Madden Julian Oscillation is currently weak. Madden Julian Oscillation signal is likely to become increasingly coherent during opening phase of August 2023. GEFS is increasingly showing renewed activity of MJO transiting from Maritime Continent to the Western Pacific during 2nd week of August. If it materializes this may contribute to El Nino development by weakening trade winds. However, there are some models which do not indicate any significant strengthening of MJO in the next 2 weeks.
In the equatorial Pacific Ocean, sub surface temperatures are currently warmer than average. The traditional and equatorial SO indices are remaining in the ENSO-neutral stage. It means that the atmosphere is not coupled firmly with the warm ocean surface of equatorial Pacific. This aspect may linger in the development of El Nino. After running a marathon in July, monsoon is likely to enter in to a weak phase, more so after 04thAugust. Monsoon trough may shift north of its normal position, close to the foothills. This feature runs the risk of dragging monsoon in to a prolonged weaker phase.
