Two contrasting summer monsoon seasons in the recent past decade: An observational study

摘要

India experienced two extreme summer monsoons, 2007 (active monsoon) and 2009 (weak monsoon) in the recent past decade. The characteristic features of these two contrasting Indian summer monsoons have been presented. The country received 11.8% excess and 17.1% deficit rainfall during 2007 and 2009 monsoon seasons, respectively. These large deviations in rainfall encourage us to study the influence of meteorological factors on rainfall activity. The distributions of rainfall over India, latent heat flux, wind, Vertically Integrated Moisture Transport (VIMT), and Vertically Integrated Moisture Divergence (VIMD) values in the layer 1000-300 hPa for the domain, 0°-40°N, 40°-120°E in the two contrasting monsoon seasons are evaluated. In active monsoon, (ⅰ) predominant low level southwesterly flow over the Arabian sea and deflection of winds over the Bay of Bengal, strengthening of the tropical easterly jet stream, wide area extent of easterlies in the upper troposphere, the maximum strength of easterlies in the upper troposphere (150 hPa) is observed over the area around 10°- 15°N and 70°-75°E; (ⅱ) position of subtropical ridge is more northward, i.e., 32°N; (ⅲ) a predominant moisture transport in the layer 1000-300 hPa from Southern Hemisphere, Arabian Sea and the Bay of Bengal to the Indian mainland, westward/northward transport of moisture and area coverage of larger quantum of moisture flux is seen; and (ⅳ) India experienced more number of mesoscale systems. The reverse is true for weak monsoon. Positive (neutral) Indian Ocean Dipole (IOD) and La-Nina (El-Nino) conditions lead to active (poor) summer monsoon conditions over India in 2007 (2009) year. Highlights: The inter-annual variations in summer monsoon rainfall over India exhibited two extreme summer monsoons 2007 (active) and 2009 (weak) during recent past decade, 2001-2010.Dominant lower and upper tropospheric circulations over monsoon region are evident in the active monsoon.The incursion of subtropical westerlies in the mid as well as upper troposphere inhibits the rainfall activity over India in the weak monsoon season.A predominant moisture transport from the surrounding oceanic area as well as Southern Hemisphere to Indian mainland is seen in the active monsoon.The moisture divergence over the surrounding oceanic area is high in active monsoon.