Impact of Global Warming on Tropical Cyclones and Monsoons

Abstract

Tropical cyclone is one of the most hampered natural hazard in the North Indian Ocean. The North Indian Ocean is divided by the Indian sub continent into two ocean basin one is Bay of Bengal and the other one is Arabian Sea. Bay of Bengal is the most vulnerable to cyclones than Arabian Sea. Recent studies suggest that cyclone activity over the North Indian Ocean (NIO) has changed over the second half of the 20th Century (Mooley, 1980; Rao, 2002; Knutson & Tuleya, 2004; Emanuel, 2005; Landsea, 2005; IPCC, 2007; Muni Krishna, 2009; Yu and Wang, 2009). General features include a poleward shift in strom track location, increased strom intensity, but a decrease in total storm numbers and also the ocean response in the weak of cyclone. Sea surface temperature (SST) is a fuel to tropical cyclones for their genesis and intensification. Global warming heats both the sea surface and the deep water, thus creating ideal conditions for a cyclone to survive and thrive in its long journey from tropical depression to Category Four or Five superstorm. SST increasing is so fast and high in the equatorial Indian Ocean compared with other the oceans. It has increased 0.6°C over the NIO since 1960, the largest warming among the tropical oceans. Recent increase in frequency of severe tropical cyclones is related to the increase in SST in response to global warming. Higher SSTs are generally accompanied by increased water vapour in the lower troposphere, thus moist static energy that fuels convection. The large scale thermodynamic environment (measured by Convective Available Potential Energy, CAPE) become more favorable for tropical cyclones depends on how changes in atmospheric circulation, especially subsidence, affect the static stability of the atmosphere, and how the wind shear changes (IPCC, 2007). Despite an increase in SST over the Bay of Bengal (Sikka 2006), observational records indicate for a decline in the number of depressions over the Bay of Bengal since 1976 (Xavier and Joseph 2000), and various factors are attributed to this trend that includes weakening of the low-level westerly flow over the Arabian Sea (Joseph and Simon 2005), decrease in the horizontal and vertical wind shears as well as in moisture and convection over the Bay of Bengal (Mandke & Bhide 2003; Dash et al., 2004). Vertical wind shear and high static stability has an adverse influence on tropical cyclone formation and on cyclone strength and longevity (Gray, 1968; Hebert, 1978; DeMaria, 1996; Shen et al., 2000; Garner et al., 2009). Joseph & Simon (2005) indicate that low level jet stream associated with Indian summer monsoon over the NIO is weakening in recent years, which reduces the vertical easterly shear and thus it is favorable for the formation of more