Cyclonic storms have been disrupting the Indian summer monsoon for some years now with an increase in the number of cyclones in the Arabian Sea. A recent study by researchers at the ST Radar centre of Cochin University has noted that these are disrupting the onset of the monsoon in Kerala and also its further advancement in the country.
The analysis done as part of the study showed that the jetstreams associated with the monsoon, namely the low level jetstream (LLJ) and tropical easterly jetstream (TEJ) have been altered by the cyclones formed within eight days before or just after the monsoon onset over Kerala.
The onset of the summer monsoon over Kerala is considered as a period of unique and imminent transition in the weather pattern over Indian subcontinent and is marked as the start of the rainy season.
It is also associated with abrupt or gradual changes in the large-scale atmospheric and ocean circulation as well as an abundance of atmospheric moisture content over the Arabian Sea (ARB) and the Bay of Bengal (BOB) Studies have pointed out that tropical cyclones (TC), can alter the large-scale circulation pattern and fluxes between the atmosphere and ocean in the tropical region. The North Indian Ocean (NIO), consisting of the BOB and the ARB, has two cyclonic seasons, namely pre-monsoon (April-mid June) and postmonsoon (Oct-Dec). The premonsoon cyclonic activities generally occur before the onset of southwest monsoon.
It has been seen that the frequency of tropical cyclones prior to the monsoon onset has increased in the Arabian Sea while that over the Bay of Bengal has decreased in the recent decades. In the 40 year period, there were 10 years in which cyclones formed in the Arabian Sea close to the onset phase of monsoon.
The new study shows that Arabian Sea cyclones pull the moisture towards the centre of the cyclone, which results in the reduction of rainfall along the west coast. Weaker than normal LLJs is found to affect sea surface temperatures (SST). As a result, the SST over the south Arabian Sea is higher in a cyclone year than in a non-cyclone year. This showed that the Arabian Sea cyclones which form close to monsoon onset over Keala can change the thermodynamic structure of the south Indian region thus adversely affecting the Indian Summer Monsoon during its onset phase.