The research team led by Professor Chunzai Wang from the South China Sea Institute of Oceanology, Chinese Academy of Sciences, has revealed that the Indian Ocean Dipole (IOD), a major climate mode in the tropical Indian Ocean, consists of two distinct types with different physical mechanisms and climate impacts. The study, published in National Science Review , provides new insights into IOD diversity and its influence on regional climate.
The IOD is one of the most important air-sea coupled climate modes in the tropical Indian Ocean and plays a key role in global climate variability and regional climate anomalies. Traditionally, the IOD has been characterized by the difference in sea surface temperature (SST) anomalies between the western and eastern tropical Indian Ocean and has been regarded as a single dipole mode. However, whether distinct spatial types of IOD exist and whether they have different formation mechanisms and climate impacts have remained unclear.
To investigate the diversity of IOD structures, the research team analyzed typical IOD events identified by the traditional IOD index. “We initially expected the IOD to exhibit a single dominant SST anomaly pattern, as it has traditionally been described, but we were surprised to find that it actually consists of two distinct spatial types,” said Professor Chunzai Wang, corresponding author of the study. By combining multiple approaches, including empirical orthogonal function combinations, the research team identified and defined two types of IOD: the west-east type IOD, characterized by opposite SST anomalies between the western and eastern Indian Ocean, and the central-east type IOD, characterized by opposite SST anomalies between the central and eastern Indian Ocean.
Further analysis revealed that the two IOD types undergo different air-sea coupling processes. The west-east type IOD is strongly influenced by wind-thermocline-SST feedback, with enhanced easterly winds and thermocline changes reinforcing the east-west temperature gradient. As a result, it typically reaches its peak in autumn. In contrast, the central-east type IOD develops earlier and exhibits weaker air-sea coupling and thermocline responses, leading to distinct seasonal evolution.
The two IOD types also have distinct climate impacts. The west-east type IOD shows a stronger connection with East African autumn rainfall, whereas the central-east type IOD has a weaker influence because its convection response is centered over the central Indian Ocean. “Traditional IOD events contain signals from both types, which may contribute to uncertainty in the relationship between the IOD index and regional climate responses,” said Dr. Jiayu Zheng, first author of the study. “Separating the two types reveals a more consistent relationship between the west-east type IOD and East African rainfall variability.”
This study advances the understanding of IOD diversity by demonstrating that the Indian Ocean Dipole is not a single climate mode but consists of two distinct types with different physical processes and climate impacts. The findings provide a new perspective for interpreting IOD variability and assessing its regional climate effects. Notably, the research also found that positive west-east type IOD events have become more frequent in recent decades and can trigger extreme rainfall events that lead to flooding over East Africa, highlighting the urgent need to better understand IOD variability and its regional climate impacts.
National Science Review
Data/statistical analysis