Hyderabad: Warming of western Indian Ocean due to climate change is affecting southwest monsoon rainfall over central India. It is decreasing by 1.49 mm per day per year.
This connection has been established in a recently published paper by S. Vishnu Nair, a research scholar from Indian National Center for Ocean Information Services (INCOIS), Hyderabad.
Mr Nair in his paper has established a link between the warming of the western Indian Ocean, decreasing trend in monsoon depressions (MD) forming over Bay of Bengal and decrease in monsoon rains occurring over central India, an area that includes Madhya Pradesh, Chattisgarh, Jharkhand, Bihar and some portions of Uttar Pradesh Odisha and northwest Andhra Pradesh. Monsoon rainfall in these parts is connected to the formation of MD over Bay of Bengal.
Scientists had established earlier that the western Indian Ocean is warming up drastically. In the past century it warmed up by 1.2 degree Celsius as compared to 0.8 degree Celsius rise in global ocean surface warming in the same period.
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Through his research Nair observed that the reduction in number of monsoon depressions over Bay of Bengal is mainly due to decrease in Relative Humidity(RH), one of the major contributing factors affecting formation of depressions apart from Vertical Sheer, Absolute Vorticity and Sea Surface Temperature.
RH is formed over sea because of mainly two reasons: evaporation of sea water and moisture being carried from one region to another through wind, known as advection. Advection is a major factor as it contributes nearly 85% towards formation of RH.
In the case of Bay of Bengal, advection occurs from western Indian Ocean. That is from where winds carrying moisture first travel towards Somalia's coast and then take a u-turn and travel over the Arabian Sea to reach Bay of Bengal.
Reduction of RH over Bay of Bengal is mainly due to reduction in moisture in this wind.
Nair says that due to the rise in ocean temperature of western Indian Ocean region there is a rise in precipitation or rainfall in the region surrounding the area. Because of this as the wind travels from western Indian Ocean to the Bay of Bengal, its moisture content keeps decreasing.
The drop in moisture content in turn results in decrease in formation of monsoon depressions that leads to formation of clouds and hence less rainfall is experienced over central India which is highly dependent on such depressions for rainfall.
