Rain some, lose some

Climate scientist Govindasamy Bala was curious about studies that indicated the summer rainfall over Kerala had been decreasing in recent years, and research that revealed a rainfall variation across the southern Indian region. The scientist from the Centre for Atmospheric and Oceanic Sciences (CAOS) at the Indian Institute of Science wanted to build a theoretical and scientific understanding of this. 

This curiosity drove him — along with his student Tresa Thomas — to examine 30 climate models (bit.ly/Rain-Models). Studying multiple models helped them to even out the biases of individual models. Their key finding from these models is that while the entire global ocean warms, this warming is not uniform.

In climate studies, models of the Earth system are crucial because of the time-scales over which climate change occurs. Experiments are conducted as simulations. The newer models of the Earth-ocean-atmosphere system are significantly more granular now than they were when many studies were conducted. Earlier, most models had grid sizes of hundreds of kilometres. This system glossed over local features such as cloud systems that could extend over a few kilometres or microscopic aerosols. So, these smaller features were represented using parametrisations, which vary from model to model.

The first work to tangibly discuss variation in the Indian summer monsoon was a high-resolution model by K. Rajendran and A. Kitoh in 2008. Rajendran points out that the models in the third and fourth assessment reports (ARs) of the Intergovernmental Panel on Climate Change were of too low a resolution to enable scientists to correctly map out the meanderings of monsoon clouds over the Kerala coast, for example. "We calculated and found that the relevant features will start showing only when the resolution goes less than 60 km," says Rajendran, Director of the KSCSTE Institute of Climate Change Studies. The resolution was about 250 km in the third AR and around 100 km in the fourth report, he says. In a subsequent work, published in 2012, they, along with other researchers, noted that the summer monsoon over the Western Ghats was likely to weaken due to climate change (bit.ly/Monsoon-Circulation). A key observational work on this subject — the Kerala Monsoon — carried out by S. Abhilash and others was published in 2018 (bit.ly/Kerala-Study).

Earlier studies predicted an increase in rainfall at the end of the 21st century, except over the southern half of the west coast of India (Kerala coast), eastern India, and Jammu & Kashmir. But Bala believed that the scientific understanding of why the rainfall over the Indian subcontinent had this patchy behaviour needed to be derived. For that, he and Thomas used 30 climate models. "Looking at multiple models is better as the results are usually 'robust' and 'reliable'," says Bala.

Their study finds that in the Pacific Ocean, warming is more in the eastern tropical Pacific than in the western tropical Pacific. This situation resembles the current El Niño, which occurs once every 4-7 years. El Niño years are usually associated with below-normal summer monsoon rainfall in India.

However, the Indian Ocean does a balancing act. According to the suite of models, warming is projected to be larger in the western tropical Indian Ocean and the Arabian Sea, and relatively lower in the eastern tropical Indian Ocean. The east-west orientation of this is opposite to what happens in the Pacific. Climate scientists call this the positive phase of the Indian Ocean Dipole (IOD). The IOD occurs once in a few years in the current climate. It is commonly observed that a positive phase of IOD usually brings plentiful rainfall to India during the summer monsoon season. This, again, contrasts with the effect of the El Niño. 

The Indian Ocean Walker Circulation, with ascending motion and clouds in the eastern tropical Indian Ocean, and descending motion and clear skies in the western tropical Indian Ocean, is also weakened. This weakening is similar to the response during a positive IOD event today. These atmospheric circulation anomalies cause significant changes in rainfall in tropical monsoon regions, such as the South Asian monsoon region.

So, even though the monsoon circulation weakens, Bala's study finds an increase in summer monsoon rainfall over India because of a large increase in the water vapour content in the atmosphere in a warmer world. "The increase in rainfall over India is also partly attributed to the 'biasing' of the Indian Ocean basin toward a positive IOD in a warmer world," Bala says.

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Interview with Govindasamy Bala