ABSTRACT

Keeping in mind the global warming issue in the Bay of Bengal region, we have investigated the factors maximum sustained intensity, surge height, the annual frequency, Accumulated Cyclone Energy (ACE) and Power Dissipation Index (PDI) of a cyclone from the MRI-AGCM and d4PDF cyclone track data. A doubly nested tide-surge interaction model is developed to investigate the impact of islands on the destructive power and energy dissipation of a storm along the coast of Bangladesh. A vertically integrated shallow water model equations are solved by conditionally stable semi-implicit finite difference manner on an Arakawa C staggered grid system. The developed model is then applied to simulate the water level, PDI and cyclone energy (CE) associated with the catastrophic deadly cyclone 1991 and a recent cyclone MORA. The activity represents the increasing trend of destructive power and accumulated cyclone energy. The model result shows that there was no significant effect of the island on PDI and CE, but has a significant effect on the water level elevation.

INTRODUCTION

Bangladesh is a riverine country in South Asia, each year there are some few smaller and larger (4 to 7) cyclones strike this area (Paul et al., 2014), causing the huge economic damages. In the geological and geographical factors of Bangladesh play an important role in these storms, among of the appropriate factors such as the funneling shape of the coast, seasonal wind effect, high density population and the destruction of trees. It has been found from a research (Alam et al., 2003) and information from the Bangladesh Meteorological Department, the last 1974 to 1999 years there are 8.62% landfall occurred along the Myanmar coast, 23.56% in Bangladesh coast, 66.67% and 1.15% along the Indian coast and Sri-Lanka coast respectively. But, considering the coastal length, Bangladesh is more likely to suffer from cyclone than other countries, whereas the coastal length of Bangladesh is 580 km, Myanmar 1930 km, India (only east coast) 2513 km and Sri-Lanka 1340 km. The cyclone formation rate in this region is somewhat lesser than the Atlantic and Pacific, but the storms are very destructive with high intense (Islam and Peterson, 2009; Lin et al., 2009; McPhaden et al., 2009; Webster, 2008). The increasing trend of intensity depends on the climatic condition like Sea Surface Temperature (SST) and some other factors. The intensity is not only dependent on the SST, but the subsurface ocean thermal structure also plays an important role (Wada et al., (2012); Lin et al. (2008); Wada and Usui, 2007; Goni and Trinanes, 2003). Increasing intensity or economic losses whatever we say, all these factors depend on the occurrence of cyclone events along the Bay of Bengal. That's why, reviewing the cyclone occurrence, activity analysis under the present and future climate condition along the Bay of Bengal is an important issue. Some notable work has been conducted to investigate the atmospheric and oceanic conditions on the modulation of Tropical Cyclone (TC) activity in the BoB. The authors (Liebmann et al., 1994; Goswami et al., 2003; Ali et al., 2007; Sengupta et al., 2007; Kikuchi et al., 2009; Lin et al., 2009; Goswami et al., 2003), were explained the cyclone vorticity and modulation of meridional shear of zonal winds. Ali et al. (2007) and Sengupta et al. (2007) highlighted the importance of hydrography of the Bay of Bengal (BoB) on (TC) intensity. All these previous studies focused on the roles of different oceanic and atmospheric components to understand the tropical cyclone intensity along the BoB. Rao (1981) investigated the formation of disturbance along the BoB and he found that the storm formation is almost same in the pre-monsoon and post-monsoon seasons. But it is totally contradictory to the events happening in the current climate condition. In his further studies (subbaramayya and Rao, 1984), he analyzed the monthly occurrence relation between pre-monsoon and post-monsoon seasons. Alam et al. (2003) also investigated the storms and depression crossing the different coastal zone and his analysis was based on Bangladesh Meteorological Department (1974–1999) data set. In the present study, we have investigated the tropical cyclone activity along the BoB under the present and future climate condition, where the data were taken from the Meteorological Research Institute-Atmospheric Global Circulation Model (MRI-AGCM) (Mizuta et al., 2006), The database for policy decision making for future climate change (d4PDF) and Bangladesh Meteorological Department (BMD). Another notable aspect of this research is that the greatest discharge from the combination of the Ganges-Brahmaputra-Meghna river, and the discharge of the fresh water in all the rivers of the world is third (Milliman, 1991). Due to this sediment discharge, there are many floodplain sediment islands occurred along the Bay of Bengal. This small and big islands may have some influence on the surge elevation and cyclone energy. So for this, it is important to investigate the influence of the islands on cyclone energy. In this study, a doubly nested tide-surge interaction model is developed to investigate the impact of islands on the cyclone energy.

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