Numerical Simulations with WRF to Study the Impact of Sea Surface Temperature on the Evolution of Tropical Cyclones Over Bay of Bengal

Abstract

Tropical cyclones (TCs) belong to the class of severe weather systems associated with strong ocean-atmospheric coupling. They are highly disastrous weather phenomena that cause damage to the life and physical infrastructure in tropical maritime countries. The Indian coastal lands, especially the east coast, are highly vulnerable to the TCs in the post-monsoon season of October to December. Weak vertical wind shear, high sea surface temperature (SST) (>26.5 °C), pre-existing disturbance like tropical easterly waves are some of the favorable conditions (Anthes, Tropical cyclones: their Evolution, structure and effects, American Meteorological Society, Science Press, Ephrata, p 208, 1982; Gray, Mon Weather Rev 96:669–700, 1968) favouring the development and sustenance of TCs in this season in the North Indian Ocean (NIO). SST is a crucial influential parameter for the formation and development of TCs. A threshold SST value of 26.5 °C was defined by Gray (Mon Weather Rev 96:669–700, 1968) for the genesis and further development of TCs. A TC is characterised with an outward diverging motion in the upper atmosphere and converging motion at the surface. Higher upper air divergence facilitates further deepening through enhanced convergence at the lower levels. The upper ocean provides heat energy to the overlying atmospheric boundary layer and for the deepening process by enhancing the convection. Earlier studies have shown that TCs experience sudden intensification when they entered oceanic areas with higher SSTs. There are several observational and modelling studies which explained the upper-ocean response to TCs (e.g. Price, J Phys Oceanogr 11:153–175, 1981; Price et al., J Phys Oceanogr 24:233–260, 1994; Shay and Elsberry, J Phys Oceanogr 17:1249–1269, 1987; Sanford et al., J Phys Oceanogr 17:2065–2083, 1987; Bender and Ginis, Mon Weather Rev 128:917–946, 2000; Shay et al., Mon Weather Rev 128:1366–1383, 2000). Among a number of factors, the TC intensification is partly controlled by the surface heat and moisture fluxes that feed the energy to the storm and the dissipation by the roughness to the winds at the sea surface (Chen et al., J Atmos Sci 70:3198–3215, 2013). The upper ocean heat content and surface temperature are influential in the ocean–atmosphere interaction and the supply of energy through sensible and latent heat fluxes for the development and sustenance TCs. Emanuel (Nature 436:686–688, 2005) and Webster et al. (Science 309:1844–1846, 2005) suggested that the rising SSTs in the Atlantic Ocean are related to the increase in the hurricane activity. A number of studies have shown that the strong surface wind in a TC drives sea surface waves and underlying ocean currents and enhance upper-ocean turbulent mixing, cools the SST, and result in a cold wake behind (Price, 1981), which, in turn, provides a negative feedback on TC intensity (Schade and Emanuel, J Atmos Sci 56:642–651, 1999; Chan et al., J Atmos Sci 58:154–172, 2001). It has been reported that warmer SST associated with large ocean heat content causes TC intensification (Hong et al., Mon Weather Rev 128:1347–1365, 2000; Shay et al., Mon Weather Rev 128:1366–1383, 2000; Bright et al., Geophys Res Lett 29:1801, 2002), whereas negative SST anomalies associated with cold-core eddies or TC-induced cold wake weakens TC systems (Walker et al., Geophys Res Lett 32:L18610, 2005). A few workers (Knutson and Tuleya, J Climate 17:3477–3495, 2004; Michaels et al., Geophys Res Lett 33:L09708, 2006) using computer models have shown that increase in SST due to green house warming leads to increase in hurricane intensity. Michaels et al. (Geophys Res Lett 33:L09708, 2006) reported that the strong relationship of TC intensity with SST is not clear at the upper range of SSTs. Though SST is known to influence the intensity of TCs, it is still not clear how it influences the movement of the storms. It is necessary to investigate the air–sea interaction processes associated with SST parameter and upper ocean heat content by analysis of the air–sea fluxes through numerical experiments. In this study, an attempt is made to understand the role of SST on the movement and intensity of the TCs by performing numerical simulations with a mesoscale atmospheric model.