Of all natural disasters, tropical cyclones (TCs) are most deadly due to their recurrent presence which draw heavily on the resilience of the coastal communities. About 7 per cent of all global TCs are formed over the North Indian Ocean (NIO) basin (Neumann, 1993). This can vary between 5-7 per cent, if computed by considering the cyclonic storms that occurred until 2015. It is noticed that more cyclones form over the Bay of Bengal (BOB), as compared to the Arabian Sea (Fig. 1). TCs affect most coastal regions in south Asia and also parts of southeast Asia.
Cyclogenesis in the NIO region
Several factors play a significant role in generating more number of cyclones in BOB (McBride & Fraedrich, 1995) such as the low flat coastal terrain and funnel shape, shallow waters of BOB, monsoonal winds (troughs), higher sea surface temperature (SST) and more middle tropospheric moisture availability. Most monsoon troughs are generated due to re-intensification of westerly propagating disturbances or from in-situ depressions.
Kikuchi and Wang (2010) have shown that the Boreal Summer Intra-seasonal Oscillation (BSISO) modulates the tropical cyclogenesis over NIO, and genesis potential index is high during the active phase of the BSISO. In addition, Indian Ocean Dipole, El-Niño Southern Oscillation and Madden-Julian-Oscillation also play substantial role in modulating the frequency, intensity and track of cyclones (Girishkumar & Ravichandran, 2012; Sumesh & Kumar, 2013; Li et al., 2016). Due to the high wind shear over BOB, most ‘depressions’ formed during the pre-monsoon period don’t intensify into intense cyclones, though they cause heavy rainfall and affect the coastal areas. During the post-monsoon season, the chances of depressions transforming into intense cyclones is high, since the wind shear is very low over BOB.
The changing climate can have greater consequences in determining the frequency and intensity of cyclones and tropical cyclogenesis, probably triggering the formation of many more severe or intensified TCs over NIO (Webster et al., 2005; Mohanty et al., 2012). The probability of transformation of cyclonic disturbances to TCs or severe cyclonic storms is found to be increasing significantly due to decreased vertical wind shear over Arabian Sea and increased low level cyclonic vorticity over BOB (Mohapatra, 2015). Thus, more destruction in properties has been observed though the improved prediction has helped in saving more human lives in the recent past. In future, bigger and more intensified TCs are expected in the post-monsoon season as the warming period prevails in a changing climate scenario. (Mohanty et al., 2012).
Tropical cyclones over BOB
The India Meteorological Department (IMD) e-atlas data (1891-2015) suggests that the number of TCs (including depressions, cyclonic storms, and severe cyclonic storms) crossing coastal areas was 1012 in the BOB region, out of the 1108 that formed over the NIO. Nearly 77.56 per cent crossed Indian coasts, while 15.21 per cent and 11.16 per cent crossed into Bangladesh and Myanmar respectively. About 455 (45 per cent of the total formed over NIO) crossed into Odisha and West Bengal coasts, which accounts for 58 per cent of the 785 TCs that crossed into the eastern India coasts. Out of 208 severe cyclonic storms, 41 crossed into this region. The coasts of Bangladesh suffered the second highest number (34) of severe cyclonic storms. Thus, Odisha, West Bengal and Bangladesh are most vulnerable to BOB cyclones in South Asia. These areas are agriculturally fertile and thus densely populated. Because of high wind, torrential rain and storm surges, heavy loss of lives, crops, and property occur.
Since Numerical Weather Prediction (NWP) models are equipped with real time prediction capability, they are usually adopted for TC prediction over NIO. The improved capabilities of such numerical models help in improving the TC predictability. In recent times, the Weather Research and Forecasting (WRF) NWP model is being used for TC prediction over NIO (Mohanty et al., 2013). Being an advanced numerical mesoscale model, it has helped in improving the track and intensity prediction to a large extent. However, it is still important to further improve the numerical modelling framework by adopting advanced model physics, dynamics and data assimilation techniques. This is required for further improvement of TC tracking and intensity prediction, besides the spatio-temporal distribution and amount of rainfall predictability during these severe weather events. It is also essential to understand the changes in tropical cyclogenesis in the changing climate scenario and associated environmental features (Mohanty et al., 2012; Mohapatra et al., 2015) in order to enhance the TC prediction.
Notwithstanding the progress in predicting TCs, and the success in saving human lives, a lot more needs to be done to mitigate the losses of livelihood. With the number of severe cyclonic storms rising, there is a need for increasing awareness on cyclogenesis and prediction. This ought to be accompanied by mitigation policies involving the improvement of physical structures and shelters, and adopting early warning and improved evacuation procedures as well. Most significantly, vulnerability maps of each state suffering such damage should be prepared so that necessary steps could be taken to save precious lives and property.
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