The interaction between the atmosphere and ocean in the tropical belt of the Pacific Ocean has a tremendous role in altering global weather and climate patterns. The developments in the Pacific are largely observed by the complex dynamical models. Statistical models can also capture some of the precursors of such developments. Keeping in mind the degradation of models with longer leads, expert analysis adds further value in interpreting the implications of evolving situations. All the forecasting techniques try to incorporate the effects of ocean-atmosphere interactions within the climate system.
ENSO: The equatorial Pacific Ocean has remained the most mysterious this season. An unsettled pattern, more so, in the ONI part of the Nino region, has been the highlight since the commencement of the monsoon season. The SST anomaly of Nino 3.4 had dropped to 1°C on 01st April 2024, from its highest mark of 2.3°C during the active phase of El Nino 2023. It has taken over four long months to drop below zero this week, albeit with some upheavals mid-way through this period. Not only the ocean, but even the atmosphere has also been perplexing, leaving large differentials from week to week and month to month. The Southern Oscillation Index (SOI) after observing an average value of zero-zero in June 2024, has precariously overshot to -0.7, the highest ever since March 2024. The fairly warm ocean surface, supported by the wavering atmosphere together, has defied the anticipated timelines of La Nina early in the monsoon season.
The Oceanic Nino Index (ONI) is based on SST departures from average in the Nino 3.4 region and is a principal measure for monitoring, assessing and predicting ENSO. Nino 3.4 anomaly has fallen below zero, for the first time since March 2023. La Nina is characterized by a negative ONI less than or equal to -0.5°C. To be classified as a full-fledged La Nina episode, these thresholds must be exceeded for a period of at least 5 consecutive overlapping 3-month seasons. The beginning has been made with the ONI dropping to 0.4°C for the first three-month season of April-May-Jun 2024. The Climate Prediction Center (CPC) considers La Nina conditions to occur when the monthly Nino 3.4 SST departure meets or exceeds -0.5°C along with consistent atmospheric features. These anomalies must also be forecasted to persist for three consecutive months. SOI, the atmospheric component of ENSO continues to be a grey area with the index value still negative, for July 2024.
IOD: The tropical Indian Ocean usually witnesses a dipolar pattern of SST anomalies, which is commonly referred to as the Indian Ocean Dipole (IOD). The IOD phenomenon has profound global impacts, yet its seasonal prediction remains a large challenge. The year-to-year variability of IOD has usually been linked to the ENSO. The relationship between EL Nino or La Nina and the intensity of IOD is not linear. El Nino and La Nina also do not have a set pattern in the equatorial Pacific. Studies have shown that the longitudinal positions of El Nino and La Nina in the Pacific have a strong correlation with the intensity of IOD. Different from westward-displaced La Nina events that can drive robust negative IOD events, eastward-displaced La Nina events do not yield significant IOD anomalies due to excessively cold conditions prevalent in the eastern Pacific. Since the La Nina conditions are yet to be established in the Pacific, its flavour is not yet known at this juncture. However, the IOD index is refusing to lift for the fourth consecutive week, so the La Nina event may be displaced eastward. In such a case, the IOD index may remain suppressed during the second half of the monsoon season. The IOD is currently neutral. The latest value of the IOD index for the week ending 04th Aug 2024 was -0.15°C.
MJO: The Madden-Julian Oscillation is currently indiscernible. Some of the models favour the brief appearance of weak MJO pulses in the Western Hemisphere and Africa in Phase 8 & 1, during the first half of August. A few dynamical models MJO index forecasts are in good agreement of its canonical propagation across the Indian Ocean during week 2-3.
After the initial setback at the time of onset, the southwest monsoon has undergone a prolonged active phase in July and spilled over to August. Normally, the monsoon exhibits its natural variability and goes into a lean phase after staying active for 10-15 days. This feature was missing this season so far. There is a chance for the monsoon trough to shift north of its normal position early next week. The rainfall activity may become subdued over the southern and central parts during the later half of August 2024.