Increased warming of the Indian Ocean near the equatorial region is slowly transforming the annual monsoon rains, which is very critical for the economy of the country. India receives most of its rainfall from the southwest monsoon and the retreating northeast monsoon. The rains, which irrigate 60% of India?s farms, are crucial for rural growth as increased soil moisture from the rains is very important for key winter crops such as wheat and rapeseed. About half of India?s farm output comes from crops sown during the four-month-long monsoon season.
A decline in the number of monsoon depressions, weakening of the monsoon current and the increasing occurrence of heavy one-day rains is slowly but steadily having an adverse impact on the monsoon activity of the Indian subcontinent, meteorologist PV Joseph, a former director of the India Meteorological Department (IMD) told FE.
While the average rainfall in the country has not changed in the last 100 years, there are certain pockets in the country that are experiencing declining rainfalls, Joseph said. ?Certain states like Kerala, Orissa and the eastern parts of Madhya Pradesh are witnessing decreasing rainfall. In some parts of Kerala the decrease in the last 100 years is as high as 30%,? he said.
In contrast, some states like Maharashtra are getting more rainfall from the monsoon, he added. During the monsoon, heavy to very heavy rainfall is increasing in some areas and rainfall of lower intensity is decreasing in other areas. These trends compensate each other in terms of net rainfall, but they can be disruptive to normal agriculture activity.
?The central equatorial region of the Indian Ocean is warming at an alarming rate. During the last 20 years, the rate of increase has become higher,? Joseph said. The sea surface temperature of the equatorial central Indian Ocean has increased by about 1.5 degrees Celsius, which was much higher than anywhere else in the global tropics, he said. The sea surface temperatures over both the Arabian Sea and the Bay of Bengal had increased also during the last 50 years. The all-India average air temperature too had increased by 0.6 degree Celsius in the last century. This was comparable to the global average.
?It has led to an adverse impact on the Indian monsoon by creating an area of increasing rainfall near the equator, which would weaken the monsoon heat engine,? Joseph said. The observed trend is that the monsoon current flow has weakened and there has been a decline in the number of monsoon depressions, he added.
Monsoon depression is a low-pressure circulation that develops in regions where climatologically there are narrow elongated areas of weak low pressure (monsoon troughs). The term is most frequently used to describe weak cyclonic disturbances that form over the Bay of Bengal and generally track northwestward over the Indian subcontinent. Because of its large size and its typical slow track toward the west or northwest, once over land it can produce copious amounts of rain and cause widespread flooding. Because winds in a monsoon depression rarely exceed 65 kmph and their circulation is strongest above the ground, they can persist for many days over land with little weakening and can be prolific rain producers.
?Tracking the data for the last 100 years, we can find that monsoon depression have steadily decreased over the years. Earlier, if there were 12 monsoon depression annually during the June-September period, now there are only four depressions,? Joseph said.
Another disturbing occurrence is the high one-day precipitation or freak rains, he said. Mumbai saw more than 94 cm of rain near the airport during July 2005, which can be best described a freak phenomenon. The intensity of rainfall of around 94 cm in 24 hours has been rarely observed in the country over a century and it was concentrated over a small area extending from Dharavi, Santacruz (944 mm), Bhandup (815 mm) and Vihar-Powai Lake (1,045 mm).
The Intergovernmental Panel on Climate Change’s (IPCC) scientific assessments of climate change states that El Nino ? an abnormal rise in sea surface temperature over the equatorial central Pacific Ocean, one of several factors that can delay or cause a failure of the monsoon ? seems to be becoming stronger, more common and are no longer disappearing completely. El Nino is already known to cause droughts and it will be fair to say that global warming may act to exacerbate this extreme event.
The changes in the monsoon pattern is seen as slow but irreversible and one that could change the socioeconomic structure of the region.