Whither the Monsoon?
Roxy Mathew Koll, Indian Institute of Tropical Meteorology
The South Asian monsoon is one of the strongest phenomenon in the tropical climate system, and affects the lives of over two billion people via its effects on food security, water resources and power generation. Traditionally, monsoon has been defined as the seasonally reversing winds, accompanied by changes in rainfall. During summer, these winds flow southwesterly from the Indian Ocean to South Asia, providing up to 90% of the total annual rainfall. These winds are driven by the differential heating between land and sea, which is dependent on the seasonal migration of solar insolation. Monsoon can be considered as a huge movement of water from the oceans to the landmass.
The South Asian monsoon as we see now shaped up when the Indian subcontinent collided with the Eurasian continent about 20-30 million years before. The location of the Indian peninsula in the north Indian Ocean enhanced the land-sea temperature difference, resulting in the monsoon as we see now. Now the term monsoon is used to define similar phases of rainfall accompanying changes in the wind circulation—such as the African, Australian and the American monsoons.
The monsoon has a variability on different timescales such as the intraseasonal, seasonal and interannual scales, and exhibits long-term trends too. Despite the advancement of science in understanding the monsoon, its complexity spanning several earth system components, the spatiotemporal variability and teleconnections (e.g.: El Niño) makes it a challenge to comprehend and predict it successfully—the seasonal forecast skill of present day models are at 0.5-0.6 (on a scale of 0-1). Global warming conditions accentuate the need of realistic future projections by climate models, but we are yet to be there as these models do not show a great skill in simulating the present day monsoon. Recent studies suggest that extreme rain/weather events are increasing over the monsoon domain. At the same time, past data indicate that the summer monsoon circulation and rainfall has weakened, due to a rapidly warming Indian Ocean which has reduced the land-sea temperature difference. Whither the monsoon? What will be the fate of the population and biodiversity in view of a monsoon climate which is changing? These are questions which remain, but require immediate attention to!