Now, a team from Fudan University, China, has used 40 state-of-the-art climate models (CMIP6) to assess how well the models capture the three types of positive IOD events and to estimate how they will change this century. The study finds a systematic bias in how the models represent this ocean region—and warns that future risk assessments built on them may be misled. The findings were recently published in Atmospheric and Oceanic Science Letters.
Based on which pole is stronger, the researchers sorted positive IOD events into three groups: west-dominated (Type-W), east-dominated (Type-E), and comparable (Type-C). Observations for 1950–2023 contain 14 positive IOD events, spread across the three groups. In the models, however, Type-E events crowded out the rest. Over the 1930–2014 historical period, the 40-model average produced about 12 Type-E events per century, against about seven in observations, and made them almost twice as strong. Barely 11 of the 40 models could reproduce a west-dominated event at all. The researchers traced the bias to the models’ eastern Indian Ocean, which is colder in the simulations than in reality. One type of event comes at the expense of another: the more Type-E events a model produces, the fewer west-dominated and comparable events it produces.
Looking ahead, under the highest-emission scenario (SSP5-8.5), the frequency of Type-E events rises from about four per 31 years at the start of this century to about 7.2 per 31 years by 2100—an increase of 80 percent—and 30 of the 40 models agree. Comparable events also become more frequent before tapering off after mid-century, while west-dominated events show little change. The driver is an uneven warming of the Indian Ocean. The western basin heats up faster than the southeast, and this “west-fast, east-slow” pattern makes it easier for cold anomalies—and so for Type-E events—to form.
“The regional impacts of east-dominated events are particularly far-reaching—they invigorate tropical cyclone activity over the Northwest Pacific, intensify extreme rainfall over East Africa, and raise drought risk in Indonesia and Australia,” said Zhen-Qiang Zhou, corresponding author of the study and Associate Professor at the Department of Atmospheric and Oceanic Sciences, Fudan University. ”
Just as important, when we assess future climate risk we should not simply count positive IOD events—we need to know which type they are, because the regional impacts differ markedly,” Zhou added.
Next, the team plans to examine how the different types of positive IOD events affect climate in different regions, and to test whether current climate models can reproduce those impacts, so that future climate risk assessments can be made more reliable.
