South Asian monsoon

Abstract

As the word “monsoon” (derived from an Arabic word meaning seasons) indicates, the South Asian (SA) summer monsoon is part of an annually reversing wind system (Figure 2.1(b, e) (Ramage, 1971); (Rao, 1976)). The winds at low levels during the summer monsoon season are characterized by the strongest westerlies anywhere at 850 hPa over the Arabian Sea, known as the low-level westerly jet (LLJ) (Figure 2.1e), and a large-scale cyclonic vorticity extending from the north Bay of Bengal (BoB) to western India known as the “monsoon trough” (Figure 2.1(e) (Rao, 1976)). The easterly jet (Figure 2.1(f)) centered around 5oN and the Tibetan anticyclone centered around 30oN are important features of upper level winds over the monsoon region during northern summer. Millions of inhabitants of the region, however, attach much greater importance to the associated seasonal changes of rainfall. Wet summers and dry winters (Figure 2.1 (a, d)) associated with the seasonal changes of low-level winds are crucial for agricultural production and the economy of the region. The monsoon, or the seasonal changes of winds and rainfall, in the region could be interpreted as a result of northward seasonal migration of the east-west oriented precipitation belt (Tropical Convergence Zone, TCZ) from southern hemisphere in winter to northern hemisphere in summer (Gadgil, 2003). The largest northward excursion of the rain belt takes place over the Indian monsoon region where it moves from a mean position of about 5oS in winter (Figure 2.1 (a)) to about 20oN in northern summer (Figure 2.1(d)) (Waliser and Gautier, 1993).