Warming of Bay of Bengal and the Indian Ocean is likely to weaken the India monsoon further in the near future, which could be accentuated by land mass changes across the country, says a new study by the National Institute of Oceanography at Goa published in a peer-reviewed journal.
The study is significant as it reviewed the under sediment cores derived from the Krishna Godavari basin of the Bay of Bengal to understand how the monsoon rainfall pattern has changed in the past 2,000 years. Most of the studies on Indian monsoon pattern are based on temperature and rainfall variations available with the India Meteorological Department (IMD) since 1901 for land and sea.
“The intensity of the monsoon depends on the difference in temperatures on the Eurasian landmass and the waters of the Indian Ocean during the spring each year. If the landmass is warm and the sea waters are cool we will have a strong monsoon. But if the land is warm and the sea is also relatively warm, the monsoon will be weak,” Dr Divakar Naidu who led the two-year long study, said.
In the study, researchers have modelled the decade-wise variations in the monsoon rains over the subcontinent over the last two thousand years. The study, published in the journal Nature-Scientific Reports has found that decades that reported warm sea surface temperature corresponded with weaker monsoon rains due to bigger variation in land and sea temperature which prevents adequate cloud building.
Variation in the Indian monsoon rains has a direct bearing on the livelihoods of more than two billion people on the Indian subcontinent and much of South and South East Asia. Monsoon rains is the main water source for agriculture in half of India with irrigation facilities being limited.
“The Indian Ocean is warming and it has a negative effect on the Indian monsoon because (when) the thermal contrast between Indian Ocean and Eurasia decreases and the rainfall decreases accordingly,” he added. This has also been corroborated by weather data available with IMD since 1901, the study said.
From the study, he said, it could be easily concluded that the global warming scenarios will have a “negative effect” on the Indian Monsoon on a decadal time scale.
A layman may think that warmer air should hold more water and therefore, global warming should be good for Indian monsoon.
“It is actually counter intuitive,” Naidu said.
But in the case of the Indian monsoon it is not so, he said, adding that warmer earth and sea restrictions movement of clouds to low depression areas to cause rainfall. Sudden, spikes in sea water temperatures also mean lesser continuous longer period rainfall. “That is the reason that India is getting more high intensity shorter period rainfall which leads to higher water run-off,” he said.
K J Ramesh, former director-general of IMD, said the study is clear indicator of how climate change was impacting Indian monsoon on the longer term basis. “There is clear evidence that warming of sea surface temperatures have reduced intensity of monsoon rains in several places in India, especially the north-east, where the dip in average annual rainfall is 6-8% since 1980s,” he said.
The study was done as part of the project to build models that can project annual monsoon rainfall for future. The researchers studied variations in the monsoon intensity over the past 2,000 years in order to build models that will be able to offer insight into future variations in the monsoon especially with warmer oceans associated with global warming, the study said.
“Instrumental records (means rain gauge data) are available only for the last 150 years. In order to improve our forecasting we need to have long term rainfall records. We have reconstructed monsoon variability by using marine sediment cores. We examined the zooplankton trapped in these sediment cores for oxygen isotopes, which in turn would offer insight into the flow of fresh water into the KG-basin. When we plotted this corresponding to the sea surface temperatures we found the strong correlation,” Dr Naidu said.
The study covered the time span of the last 2000 years with a time resolution which is the first of its kind monsoon record from the marine side, he added.