doi:10.1038/nindia.2018.41 Published online 30 March 2018
The popular perception that the Indian summer monsoon is a gigantic breeze that blows from land to sea triggered by the difference in their surface temperatures, “needs to be abandoned”, says a leading monsoon meteorologist in a new study1.
Instead, seasonal migration of a zone of converging trade winds near the Equator may offer a better explanation to the monsoon phenomenon, says Sulochana Gadgil from the centre for atmospheric and oceanic science at the Indian Institute of Science in Bengaluru.
The popular ‘land to sea’ breeze theory, which has been around for over 300 years, does not explain the observations of monsoon variability, she says. According to her analysis, the intense sun and warm water of the equator heats the air in the Inter-tropical Convergence Zone (ITCZ) – a belt of converging trade winds and rising air that encircles the Earth near the Equator. This raises the humidity of the air and makes it buoyant. As the air rises it expands and cools, releasing the accumulated moisture in a series of thunderstorms.
Between June and December, the Indian subcontinent is under the sway of the monsoon – summer monsoon from June to September and northeast monsoon from October to December. Most monsoon studies have been on the summer monsoon, responsible for about 80% of the annual rainfall over India. Within the summer monsoon season there is considerable spatial and temporal variation in rainfall, which has major impact on agriculture.
Prediction of monsoon variability therefore assumes great importance. However, such predictions can be made only if the basic system responsible for the monsoon is known, Gadgil says in her report.
The prevalent hypothesis is associated with a stronger gradient of temperature between land and ocean. Gadgil tested this hypothesis using rainfall and land surface temperature data from the Indian Meteorological Department and sea surface temperature data from Hadley Centre for Climate Change in Exeter, UK. Predictions based on this hypothesis failed to correlate with observed monsoon variability.
However, observed variability in monsoon rainfall was consistent with her alternative hypothesis that looks at the monsoon phenomenon as a manifestation of the seasonal migration of ITCZ.
"These new ideas are not accepted yet,” says Jayaraman Srinivasan, chairman of Divecha Centre for Climate Change at IISc. “But they will be accepted in the future," he told Nature India. The seasonal mean monsoon is not controlled by the contrast in temperatures between the land and the sea but by the energy available to drive the monsoon, he adds.
According to Madhavan Rajeevan, secretary for India’s Ministry of Earth Sciences, The land-ocean thermal and pressure gradient is a very important component of the monsoon system but it alone does not explain everything about monsoons.
1. Gadgil, S. Land–sea breeze or the ITCZ? J. Earth. Syst. Sci. 127 (2018) doi: 10.1007/s12040-017-0916-x