During the autumn and winter of 2019, Australia has experienced devastating bushfire seasons. This is related to a climate system known as the positive Indian Ocean Dipole (pIOD).

The IOD is conventionally described using the Dipole Mode Index (DMI) which measures two poles located in the western and eastern Indian Ocean. A pIOD event is associated with warm sea surface temperature (SST) anomalies in the equatorial western Indian Ocean (the western pole) and cold SST anomalies occur in the east (the eastern pole).

Understanding how the IOD will respond to a warmer climate is difficult because models disagree on how pIOD SST may respond when using the conventional DMI.

Using the DMI assumes all models have similar pIOD patterns. To accurately capture pIOD diversity in models, two separate indices should be used, an S-index for strong events, and an M-index for moderate pIODs. Strong pIOD events are dominated by cooling in the eastern equatorial Indian Ocean while moderate events by broadscale warming in the western Indian Ocean.

Based on the new indices and CMIP5/6 models, a research published in Nature Climate Change showed that there was an opposite response between moderate and strong pIOD events. This opposite response occurs due to mean state changes over the tropical Indian Ocean.

Faster warming in the western than the eastern Indian Ocean is supported by a southeasterly wind trend. While the lower troposphere warms faster than the surface, increasing atmospheric stability.

Ekman pumping is the main forcing for warm SST anomalies over the tropical western Indian Ocean. This process weakens in the future climate and leads to decreased M-index variability.

"For strong pIOD events, the faster mean warming in the west and northwest tropical Indian Ocean is favorable for anomalous atmospheric convection. This in turn facilitates anomalous easterly winds along the equator and enhances equatorial nonlinear zonal and vertical advection, and these processes are favorable for cold SST anomalies," said one of the authors, Dr. YANG Kai from the Institute of Atmospheric Physics of the Chinese Academy of Sciences.

Prof. HUANG Gang, another author of the paper, emphasized that "although there is a decrease in moderate pIOD events, extreme events similar to major floods and drought in 1997 and 2019 are expected to occur more frequently in a warmer climate."

Projected opposite response of moderate and strong pIOD. (Image by IAP)