EQSOI - A Better Representation Of Equatorial Circulation

Sea Surface Temperature variability in the tropics can significantly impact the global climate through atmospheric circulation. El Nino / La Nina events, which are identified by SST fluctuations from the central to the eastern equatorial Pacific (Nino 3), are widely known examples of this.

In addition, SST variability in the western tropical Pacific (Nino West) and the tropical Indian Ocean (IOBW) may also have significant effects on climate conditions around the world. Indian Ocean Dipole events also largely impact climate conditions of Southeast Asia, East Asia, and Indian Ocean rim countries.

ENSO comprises an oceanic arm derived from the ocean surface temperature and an atmospheric wing, represented by the low-level circulation. The Southern Oscillation is the atmospheric component of ENSO. This component is an oscillation in surface air pressure between the tropical eastern and western Pacific Ocean waters. The Southern Oscillation Index (SOI) is computed from the fluctuations in the surface air pressure between Tahiti (in the Pacific) and Darwin, Australia (on the Indian Ocean). El Nino episodes have a negative SOI, meaning there is lower pressure over Tahiti and higher pressure in Darwin. La Nina episodes have the reverse configuration. Although the SOI has long station records, its reliability is limited due to the latitude of both Darwin and Tahiti being well south of the equator, so that surface air pressure at both locations is less directly related to ENSO. All the Nino indices are measured within very close margins of the equator.

To overcome this effect, a new index was created, named the Equatorial Southern Oscillation Index (EQSOI). To generate this index, two new regions centered right on the equator were defined. The western region is located over Indonesia, and the eastern one over the equatorial Pacific, close to the South American coast. However, the database on EQSOI is less comprehensive.

All the Nino indices have remained zero or above for the last six weeks. It means there is no longer any cooling of sea surface likely in the weeks ahead. A possible drop in the temperature is expected from the quarter Aug–Sep–Oct.

IOD: The Indian Ocean Dipole is neutral. The latest value of the IOD index for the week ending 20 Jul 2025 was -0.04°C. The index is maintaining the anomaly of below 0°C. For the months of June–July, the index remained within the threshold of ±0.4°C. The neutral state of IOD is likely until at least August. Skill for IOD forecasts made at this time of the year has been historically low beyond 2–3 months ahead.

MJO: The Madden–Julian Oscillation is likely to meander over the Western Pacific in Phase 6 for the next about one week. Thereafter, the pulse is expected to propagate eastward into the Western Hemisphere in Phases 7 & 8, around the start of August, albeit with low amplitude. The Western Pacific is likely to remain an active basin for stormy activity. Earlier, during last weekend, Typhoon Whifa ravaged Hong Kong, Hainan, and struck Vietnam en route through the Gulf of Tonkin. Two more storms wait in the wings to move across the Philippines Sea, South China Sea, and Sea of Japan.

Western Pacific storm activity has a strong bearing on the Indian monsoon. The remnant of Typhoon Whifa entered the Bay of Bengal. The system will energize the monsoon stream. Later, it may drag the monsoon trough close to the foothills and possibly lead to the first ‘break-in-monsoon’ of the season.