BENGALURU: It is not just winds and local convection that play a role in deciding the intensity rainfall, but also the density of the clouds travelling from the oceans. A study by researchers from the Indian Institute of Science (IISc) shows that the the strength of a cloud band (collection of clouds) plays a key role in their movement and the density of rains in the monsoon.
Professor and Chair of Centre for Atmospheric and Oceanic Sciences (CAOS) and co-author of the study paper, PN Vinayachandran, said that it also plays a role in the intensity of rain that is received in short spells. He said that the propagation has not changed, but the amount of rainfall over the years has increased owing to increased moisture-holding capacity of clouds.
The study titled- ‘Equatorial convection controls boreal summer intraseasonal oscillations in the present and future climates’ shared on Monday was also published in NPJ-Climate and Atmospheric Science- journal in February. In the study, researchers said that India receives 80% of its annual rainfall from June to September.
The wet and dry spells during this season are controlled by the Boreal Summer Intraseasonal Oscillation (BSISO, also called monsoon intraseasonal oscillations). They bring clouds from the equator to the Indian subcontinent. The duration of the wet spell is determined by the size and strength of the cloud band, Vinayachandran explained. Researchers studied data from the last 16 years.
Aditya Kottapalli, PhD student at CAOS and co-author of the paper said, “We have shown that if the cloud band in the equator is weak to start with, then it cannot propagate northwards, unlike what previous literature stated.”
The IISc team examined gaps in predictions among existing climate models and combined projections from highly efficient models to determine what factors drive the movement of BSISO. They found that the robust northward propagation of BSISO occurs only when the equatorial cloud band is strong.
This strong cloud band increases moisture in the atmosphere over the subcontinent, via stronger winds, and triggers northward propagation.
“It was found that the air-sea interaction in the equatorial Indian Ocean played a major role in driving wet spells in India. This is likely to change in future because the atmosphere would be warmer,” Vinayachandran said.