Strength of cloud band influences movement and density of rains during Indian monsoon, finds IISc study

A recent study from the Indian Institute of Science (IISc) has shown that contrary to previous understanding, the strength of a cloud band [a nearly continuous cloud formation] plays a key role in its movement as well as the density of rains that the Indian subcontinent receives during the wet spells of the monsoon.

A recent study from the Indian Institute of Science (IISc) has shown that contrary to previous understanding, the strength of a cloud band [a nearly continuous cloud formation]plays a key role in its movement as well as the density of rains that the Indian subcontinent receives during the wet spells of the monsoon.

The monsoon rains have long remained the lifeblood of India, providing the lion’s share of the water used for drinking and irrigation. The yearly arrival of the rains is caused by the movement of cloud bands from the equator towards the north.

An IISc release said that India receives 80% of its annual rainfall during the summer monsoon months, between June and September, which are marked by several wet and dry spells along with strong winds. These spells are controlled by the Boreal Summer Intraseasonal Oscillation (BSISO), also called monsoon intraseasonal oscillations, which also brings a cloud band from the equator over to the Indian subcontinent, putting an end to the dry spell. The duration of the wet spell is determined by the size and strength of the cloud band.

Most theories have suggested that regardless of the strength of disturbance at the equator, the cloud band propagates northward. “Most of the existing literature says that even if you put any small instability, it should always propagate northwards. What we have shown is that this is not the case. If the cloud band in the equator is weak to start with, then it cannot propagate north,” said Aditya Kottapalli, PhD student at the Centre for Atmospheric and Oceanic Sciences (CAOS), IISc, and first author of the study published in npj Climate and Atmospheric Science.

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 the BSISO. They found that the robust northward propagation of the 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. 

“Research teams at CAOS have been pursuing the topics of interaction between ocean and atmosphere for a long time, using data sets and model simulation. What we found is that air-sea interaction in the equatorial Indian ocean plays a major role in driving wet spells in India. This is likely to change in the future because the atmosphere would be warmer,” said P.N. Vinayachandran, Professor and Chair of CAOS and corresponding author of the study.

Prof. Vinayachandran added that in the future, the background moisture – how much water vapour is already present before the rains begin – will increase over most of this region, resulting in stronger wet spells. The associated rainfall during these wet spells is expected to increase by 42% to 63% over India and the adjoining seas.