Tropical cyclones are generated when climatic characteristics over the ocean vary over time, most notably the sea surface temperature and wind speed pattern. Potential vorticity, vertical wind shear, and pressure all play a role in these processes. Over the previous four decades, the frequency of stronger Tropical Cyclones (category 3–5) has increased, according to the IPCC’s sixth assessment report. Tropical cyclones have wreaked havoc on Oman, UAE, Iran, Muscat, and India’s west coast in recent years, and the Arabian Sea has become a breeding ground for them. The Bay of Bengal is home to over 80% of all cyclones that originate in the north Indian Ocean. Every year, 5-6 cyclones form, with 1-2 reaching a severe stage. Hence, this article aims to better understand tropical cyclone activity over the Indian Ocean by analysing the frequency and explaining observed higher intensity using various parameters.
A Susceptible Nature of Indian Ocean
Tropical cyclones are one of the most common hydro-meteorological disasters in tropical and subtropical latitudes, inflicting havoc on coastal and sea-based communities. Wind, storm surges, and widespread rainfall events are all threats that tropical cyclones bring with them. These have been the most devastating natural catastrophes around the rim of the North Indian Ocean, killing thousands of people and causing billions of dollars in property damage. The eastern coast of India, as well as the coasts of Bangladesh, Sri Lanka, and Myanmar, are at risk. Around 40% of the world’s population lives within 100 kilometres of the coast, making it very vulnerable to tropical cyclones. India’s coastline, which stretches for 7516 kilometres and is shared with the North Indian Ocean, is part of the coastal zone. Due to its small continental shelf, low coastal landscape, large population density, geographic location, and physiological traits, India has become vulnerable to such conditions.
Tropical cyclones are formed by severe disturbances that develop swiftly as a result of long-term changes in climatic factors across a specific region (few days to few weeks). These consequences are mostly determined by changes in sea surface temperature and regional wind speed patterns. Variations in vertical wind shear and potential vorticity are controlled by changes in wind speed. Changes in sea surface temperature cause low-pressure areas to emerge in the oceans, while changes in wind speed patterns tend to intensify cyclone formation. Over the previous 15 years, the severity of tropical cyclones in the North Indian Ocean has grown. Since 1980, the amount of cyclone energy accumulated in the Bay of Bengal has increased dramatically. With rising sea surface temperature and upper ocean heat content, the intensity of post-monsoon Tropical Cyclones in the Bay of Bengal has increased. Researchers have uncovered unexpected tropical cyclone trajectories over the Arabian Sea, as well as rapid intensification as a result of the warm ocean.
From 1982 until 2020, all tropical cyclones were divided into five separate storm types: Cyclonic Storm (CS), Severe Cyclonic Storm (SCS), Very Severe Cyclonic Storm (VSCS), Extremely Severe Cyclonic Storm (ESCS), and Super Cyclonic Storm (SuCS). During that time, 45 tropical cyclones formed in the Arabian Sea. The Arabian Sea is active most years, with at least one Tropical Cyclone and up to five Tropical Cyclones per year; however, it was not active in 1983, 1984, 1986–1991, 1997, 2000, 2005, 2008, 2013, 2016, and 2017. In the North Indian Ocean basins, the Bay of Bengal is noted for having the largest number of Tropical Cyclones. However, in recent years, Arabian Sea Tropical Cyclones had outnumbered Bay of Bengal Tropical Cyclones (2001, 2004, 2014, 2015, and 2019), as well as years when both the Arabian Sea and Bay of Bengal Tropical Cyclones had an equal share of North Indian Ocean Tropical Cyclones (1993, 1994, 1998, 2007, 2011, and 2012). During this time, 17 (38%) of the 45 Tropical Cyclones that formed are CS, 10 (22%) are SCS, 7 (16%) are VSCS, 9 (20%) are ESCS, and 2% are SuCS.
Arabian Sea vs Bay of Bengal
Researchers recently examined the asymmetry between the Arabian Sea and the Bay of Bengal, focusing on years when either the Arabian Sea or the Bay of Bengal experienced the majority of Tropical Cyclone activity in the North Indian Ocean. In order to determine which environmental factors account for the asymmetry that occurs between years when the Arabian Sea develops significantly more storms vs those when the Bay of Bengal does. Their findings showed that during years when the Arabian Sea experienced more Tropical Cyclone activity, more moisture was accessible over the Arabian Sea.
Few climatologists believe that the Bay of Bengal has a higher sea surface temperature (SST) and higher moisture content than the Arabian Sea, resulting in more climatological Tropical Cyclone activity. Higher SST and the presence of disturbances from the Western North Pacific are attributed to the increased activity in the Bay of Bengal.
The frequency of CS is decreasing in both basins, with the Bay of Bengal seeing a steep decline. In the case of the frequency of SCS, the frequency in the Bay of Bengal has been steadily declining. Even the SCS of the Arabian Sea is likewise declining. Surprisingly, the incidence of VSCS in the Bay of Bengal appears to be declining, while it appears to be growing in the Arabian Sea. The VSCS slope in the Arabian Sea has increased significantly since 2010. Simultaneously, the frequency of ESCS in the Arabian Sea is increasing, although Tropical Cyclones in the Bay of Bengal are not. Super cyclones were limited to the Bay of Bengal; the Arabian Sea did not see any super cyclones until after 2007. Five ESCS and four VSCS happened in the Arabian Sea between 2014 and 2020, out of a total of nine ESCS and seven VSCS in the previous 39 years. During this time, two SuCS hit the Arabian Sea, turning it into a hotspot for cyclonic activity. The decreasing trend in CS and SCS in the Bay of Bengal and the Arabian Sea shows a higher inclination for Tropical Cyclones to intensify, increasing the frequency of VSCS, ESCS, and SuCS.
Influence of ENSO on the Indian Ocean
The climate of various regions throughout the world is influenced by El Nino Southern Oscillation (ENSO) events. The current research examines the impact of ENSO events on Arabian Sea Tropical Cyclones. The analysis found 13 El Nino years, 13 La Nina years and 13 neutral years over the course of 39 years. During this time period, 17 (38%) of the 45 Tropical Cyclones formed during El Nino years, 16 (35%) during neutral years, and 12 (27%) during La Nina years. These incidents have no clear pattern. The presence of active Tropical Cyclones in the Arabian Sea cannot be attributed simply to large-scale climate variability, according to an examination of ENSO. Tropical Cyclones also have a random distribution. For example, the most active Tropical Cyclones year in the Arabian Sea was 2019, which was a neutral year with five Tropical Cyclones; 2018 was another active year in the Arabian Sea with three Tropical Cyclones, which was an El-Nino year; and 1998 was a La-Nina year in the Arabian Sea with three Tropical Cyclones, which was also a neutral year.
Whereas, in the Bay of Bengal, El Nino and La Nina events have considerable effects on cyclogenesis. The variation in the number of cyclones between El Nino and La Nina is responsible for the opposing features in the Sea Surface Temperature cooling composite. In general, in La Nina years, the number of cyclones is higher than in El Nino years. Similarly, during El Nino, La Nina, and normal years, the Sea Surface Temperature in the Bay of Bengal is greater than in the Arabian Sea, and Tropical Cyclone activity is increased or inhibited in response to changes in Sea Surface Temperature.
As per the analysis, during the El Nino years, about 83 percent of total landfalling Tropical Cyclones moved westward and made landfall on India’s eastern coast. During the La Nina years, 73 percent of all land-falling Tropical Cyclones curved northwest and northward, making landfall north of 17°N latitude on India’s, Bangladesh’s, and Myanmar’s east coasts. Even intense cyclonic conditions are higher in the La Nina years as compared to El Nino years. The frequent cyclones in the La Nina years might be the resultant remnants of Tropical Cyclones formed in the North West Pacific. Hence, cyclones formed during the La Nina period travels a longer distance and usually mature before their landfall, while an opposite scenario is observed in El Nino years.
Global Warming: A Catalyst for Intensification of Tropical Cyclones
Higher relative humidity, especially at mid-level, weak vertical wind shear, and a warm sea surface temperature all contributed to the growing intensity of severe cyclonic storms with huge socioeconomic consequences. This demonstrates the importance of global warming in causing this upward tendency. Climate change’s impact on extreme weather events, such as the frequency and intensity of tropical cyclones that occur over global ocean basins, is a source of concern. In the North Indian Ocean, high-intensity cyclones have become increasingly common, putting coastal areas at risk.
According to the newest assessment by the Intergovernmental Panel on Climate Change, the Indian Ocean is warming faster than other oceans, and experts are warning that as a result of climate change, India may experience increasing marine heat waves and flooding. Aerosol emissions hide some of the increase in marine heat waves in countries like India; therefore, reducing them is crucial for air quality. Increased heavy rain events and glacier melting will all have an influence on countries like India, as will more compound occurrences from sea-level rise, which may result in floods when tropical cyclones strike. The Indian Ocean region is warming faster than the rest of the world, which means the region’s relative sea level could rise. As a result, coastal regions will suffer sea level rise in the 21st century, contributing to more frequent and severe coastal flooding in low-lying areas, as well as coastal erosion.
Impact of these meteorological characteristics on future conditions
The research shows how meteorological parameters have risen in recent years, perhaps putting the world in a vulnerable situation in the future. However, experts and climate reports have given solutions for mitigating the impact. Yet, the aforementioned scenario will worsen India’s vulnerability, as well as that of other Indian Ocean coastal nations. Only anthropogenic activities can be dealt with, and they are the sole cause of these changes in climate parameters. Extremities are increasing in the Indian Ocean as a result of human effects, as evidenced by our findings. Climate change is primarily caused by greenhouse gas emissions from the subcontinent. The Indian Ocean is becoming more vulnerable to tropical disturbances and instability as a result of climate change. Hence, this region is becoming a hotspot for more severe cyclones and will continue to do so in the future.
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