Impact of Cross-Equatorial Meridional Transport on the Performance of the Southwest Monsoon over India

Water vapour transport over the Indian Ocean has been computed for the 30-year period (1979-2009). The monthly evolution of meridional moisture fluxes across different sections is presented. March and April clearly indicate the north to south flow of moisture across the equatorial region. During May there is intensification in the northward cross-equatorial moisture transport, which may act as a precursor of the rainy season. During the monsoon season maximum transport occurs in June with values of 1.24 ×10¹¹ and 5.58 ×10¹² tonnes/day for moisture and air flux across the equator respectively, which occurs in the lower atmospheric level between 1000 and 650 hPa. Our finding clearly shows that during the monsoon season across the equatorial cross-section major transport occurs between 42°E and 60°E. Analysis of moisture transport over two regions, i.e. (i) 6°S-6°N and 42- 60°E and (ii) 1.5°S-1.5°N and 42-60°E for two good (1988, 2008) and two bad (1987, 2009) monsoon years shows that during 1987, which was a drought year, the amount of moisture crossing the equator was less by about one order of magnitude compared to 1988. While during 2008, which was a normal/good monsoon year, the amount of moisture transported was almost three times compared to 2009. This clearly indicates that the moisture transport in May can be used as a predictor of monsoon performance.