Authors: Prasad K. Bhaskaran
Energy Review, Vol 4. Issue 3. 2022
The topic of ‘tropical cyclones’ is undoubtedly a fascinating subject and concentrated efforts to understand its behavioural characteristics over the global ocean basins have attracted immense interest amongst the scientific community. It is considered as one amongst the most destructive extreme weather events that can lead to a trail of destruction during its landfall. Though considerable studies were reported on the behavioural aspects of tropical cyclones, there are hardly any studies, in particular for the Indian Ocean region, conducted to properly understand and quantify the cyclone track length and its residence time. The India Meteorological Department (IMD) is the nodal agency and operational weather forecasting centre in India that provides predictions of cyclone track, intensity, landfall location and time for cyclonic disturbances formed and developed over the North Indian Ocean region. Based on the intensity scale, tropical cyclones that form over this region can be classified into different categories, such as depression (maximum wind speed between 8.7 m/s and 17 m/s); cyclonic storm (maximum wind speed between 17.5 m/s and 24 m/s); severe cyclonic storm (maximum wind speed between 24.6 m/s and 32.4 m/s); very severe cyclonic storm (maximum wind speed between 33 and 46.3 m/s); extremely severe cyclonic storm (maximum wind speed between 47 m/s and 61 m/s); and super cyclonic storm (maximum wind speed greater than 62 m/s).
The Working Group on Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR6) that was released recently, covered an in-depth assessment on the extremes and irreversible changes in the ocean and cryosphere in a changing climate. The study has identified vulnerable regions, their cascading effects, and impacts on the human and the natural systems. The report also discussed sustainable and resilient risk management strategies. It highlights that climate change induced by anthropogenic effects have increased effects on the environmental parameters such as precipitation (in medium-confidence scale), wind field (in low-confidence scale), and extreme sea levels (in high-confidence scale). The cascading impacts associated with these events are also expressed in a high-confidence scale. Prior studies have also reported on the poleward migration of maximum tropical cyclone intensity. The IPCC-AR6 has reported with low-confidence that anthropogenic effects in the recent decades have led to tropical expansion. Also, it reported with a low-confidence, an increasing evidence that the annual global Category 4 and 5 tropical cyclones have increased in the recent decades.
This report also dealt with changes in the Arctic Sea ice, extreme wave heights and its contribution to extreme sea level, coastal erosion and associated flooding. With medium-confidence, it reported there was loss of Arctic Sea ice that has led to increased wave heights during 1992-2014. With respect to regions, in the Southern Ocean and North Atlantic Ocean, extreme wave heights have increased by around 1.0 cm/year and 0.8 cm/year respectively during 1985-2018 with a medium-confidence level. There are other emerging areas such as Marine Heat Waves and its impact on the marine ecosystem, having significant impact over the last two decades with a very high-confidence level. The scope of this article limits only on the tropical cyclone activity over the North Indian Ocean region in a changing climate.
A recent study conducted by our team at IIT Kharagpur had evaluated the track length, residence time and translation speed for tropical cyclones that formed over the North Indian Ocean. A comprehensive study was carried out examining the historical track archives of 185 tropical cyclonic events that formed during the past 37 years (1982-2018). The best-track information from India Meteorological Department available at 6-hourly intervals was used in this study. Relevant parameters in the best-track archive comprises the basin origin, location, date and occurrence time, T-number, central pressure, maximum sustained wind speed, pressure drop, category, etc. The study also analysed all the categories from stages of deep depression to super cyclonic storms. Interesting findings from the study clearly indicate that cyclone track length has decreased by about 13%, whereas the residence time for all categories have increased by 7.3% during the study period. Considering the high intense cyclone category, the study showed that both track length and residence time have increased that was statistically significant. The observed increase was about 10% and 43% for track length and residence time respectively. Further, the study also performed a segmented analysis, and it clearly revealed a significant upward trend in both track length and residence time during the period 2009-2018. The variation in translation speed on annual and inter-seasonal scales for cyclonic disturbances in the Bay of Bengal region showed deceleration at a rate of 2.5 km/h in the past three decades. Also, tropical cyclones normally intensify at a moderate rate of about 7.7 m s-1/ day, while some of them experience rapid intensification during their life history covering a period of 4 to 5 days in the North Indian Ocean basin. Rapid intensification/decay occurs when there is an increase/decrease in the maximum sustained wind speed by about 15.4 m/s during a 24-hour period. A proper understanding on the variability of tropical cyclone activity has a direct bearing on risk assessment and coastal vulnerability having wide socio-economic implications. The study is believed to have potential applications in context to coastal vulnerability in a changing climate.
(Dr. Prasad K. Bhaskaran is a professor at the Department of Ocean Engineering and Naval Architecture, Indian Institute of Technology Kharagpur, India.)■□■