[Column] A negative Indian Ocean Dipole Mode will occur this summer

The tropical Pacific condition is now in a normal state with no El Niño or La Niña conditions (e.g., as reported by the U.S. NOAA.). The canonical type of La Niña event (characterized by below normal sea surface temperature (SST) in the central and eastern tropical Pacific Ocean) has dissipated. However, a Modoki type (marked by above normal SST in the western and eastern tropical Pacific Ocean and below normal SST in the central region) appears to persist. The Application Laboratory’s dynamical prediction system (SINTEX-F) predicts that the La Niña Modoki-like conditions will end in late spring, and that the tropical Pacific will return to a neutral state by summer.
The tropical Indian Ocean warrants a close monitoring this summer. Although there is still considerable uncertainty in the prediction, the SINTEX-F predicts that a negative Indian Ocean Dipole (IOD) will occur this summer (see Figure 1 below). If it occurs as predicted, it will be another negative IOD event since the consecutive negative IOD events of 2020, 2021, and 2022. Interestingly, the second half of 2024 had negative IOD-like conditions, highlighting a recent trend toward more frequent negative phases. A negative IOD typically brings increased rainfall and flood risks to regions like Indonesia and Australia, while heightening the likelihood of drought conditions in East Africa. Notably, during the negative IOD events in 2020, 2021, and 2022, Japan also experienced significant rainfall events in July and August - suggesting potential impacts that extend beyond the Indian Ocean basin.
We need to continue to pay attention to the tropical oceans that cause extreme weather and climate events around the world. The SINTEX-F predictions in the Application Laboratory are updated monthly. Please refer to the SINTEX-F homepage, and APL Virtualearth for the latest information.

What is the Indian Ocean Dipole Mode?

This phenomenon is seen in the tropical Indian Ocean, and like the El Niño phenomenon, it is a phenomenon in which the ocean and atmosphere vary in tandem. It occurs every few years, from summer to autumn. Indian Ocean Dipole phenomena can have a positive or negative phase. When a negative dipole mode event occurs, SSTs are higher than normal in the southeastern part of the tropical Indian Ocean and lower than normal in the western part. These temperature fluctuations cause convective activity, which is active in the tropical Eastern Indian Ocean even under normal conditions, to become even more active, resulting in heavy rainfall and flood damage in Indonesia and Australia. On the other hand, droughts are more likely to occur in East Africa, and three consecutive years of the negative dipole mode events in 2020, 2021, and 2022 caused severe droughts in many areas of East Africa, severely threatening food and drinking water security (For more information, see the press release “On the predictability of the extreme drought in East Africa during the short rains season~Key roles of the negative Indian Ocean Dipole~ ").
Active international research continues on the remote impact of the Indian Ocean Dipole on the weather/climate in the mid-latitudes (including Japan) and their relationships with El Niño and La Niña (Modoki) phenomena. Recent studies have reported that the frequency of tropical cyclones (including typhoons) tends to decrease near Okinawa and Taiwan in the summer when negative dipole mode events occur (For details, see the press release “Seasonal predictability of tropical cyclone frequency around Okinawa and Taiwan―Key roles of the Indian Ocean Dipole―”). If the frequency of tropical cyclones decreases, there is concern that the stirring and cooling effect of the ocean caused by them will weaken, and that the high temperature stress will be so strong that large-scale coral bleaching will occur.

Schematic diagram of a negative dipole occurrence. The shading represents anomalous sea surface temperatures, with red indicating higher-than-normal temperatures and blue indicating lower-than-normal temperatures. White patches indicate enhanced convective activity, and arrows indicate anomalies in ocean wind direction.

Can the occurrence of Indian Ocean Dipole Mode events be predicted in advance?

The Application Laboratory has been promoting prediction research on the Indian Ocean Dipole Mode for more than 20 years using the SINTEX-F, and has achieved internationally pioneering results. Considering its impact on society, prediction information is distributed on a quasi-real-time basis (see SINTEX-F website for details). Although it is considered difficult to predict dipole mode events that occur in summer–autumn from spring, the SINTEX-F demonstrated that some events could be successfully predicted (for example, the SINTEX-F successfully predicted the occurrence of a very strong positive dipole mode event in 2019 from the autumn of the previous year. (For more information, see the press release “El Niño Modoki in the tropical Pacific Ocean was key to successfully predicting the 2019 Super Indian Ocean Dipole phenomenon”).
Although there is still a large uncertainty in the prediction, the SINTEX-F predicts that a negative Indian Ocean Dipole Mode event will occur this summer (Figure 1). This summer, we need to pay close attention to the tropical Indian Ocean.

Figure 1: Indian Ocean Dipole Mode Index, defined as the east-west difference in SST anomalies in the tropical Indian Ocean, in ºC). A value below -0.5ºC indicates a negative dipole mode event. The black line shows the observation, and the colored lines show the prediction issued on April 1, 2025. 24 members predictions were conducted using the SINTEX-F2, taking in initial sea surface and undersea observational data and slightly changing the initial conditions and model settings in various ways (we call them ensemble predictions, purple line: ensemble mean, light blue or green line: individual ensemble member). Although there is a large uncertainty in the predictions, the ensemble members' mean values have dropped above -0.5ºC since July, and many ensemble members predict the occurrence of a negative Indian Ocean dipole mode event in summer.