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, which quenches the thirst of the harsh summers, is caused by the movement of cloud bands from the equator towards the north.
A recent study from the Indian Institute of Science (IISc) has shown that contrary to previous understanding, the strength of the cloud band 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.
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 wet and dry spells are controlled by the Boreal Summer Intraseasonal Oscillation (BSISO, also called monsoon intraseasonal oscillations) which also brings a band of clouds 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.
Since its discovery in 1979, 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," explains 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 . "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."
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," says PN Vinayachandran, Professor and Chair of CAOS and corresponding author of the study. "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."
Vinayachandran adds 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.
The researchers believe that these findings will help improve the efficiency of current climate models used to forecast seasonal and sub-seasonal rainfall.
