When examining and analysing atmospheric phenomena, it is important to keep in mind that they are constantly in a state of flux. Meteorologists have identified the structure, characteristics, and distribution of cyclones that we previously indicated after thorough examination and research. A thorough review of the tropical cyclones’ impacts on the Indian Ocean region is important to note before we conclude this discussion.
Tropical cyclone frequency and intensity have significantly increased over the Indian subcontinent. While the Bay of Bengal has emerged as a source of high energy for tropical cyclones, the Arabian Sea has developed into a breeding ground for them. 5–6 cyclones arise each year, with 1-2 developing very severe storms. Therefore, we will emphasise the Indian Ocean’s dynamic nature in today’s discussion before outlining the rising patterns on both seas. We will also be able to clearly identify the reason for this escalation over the Indian Ocean. Last but not least, there will be a brief decadal analysis of tropical cyclones, as well as increased emphasis on cyclone detection and monitoring across the Indian Ocean.
Dynamic nature in Indian Sub-continent
In tropical and subtropical latitudes, tropical cyclones are one of the most frequent hydro-meteorological disasters, wreaking havoc on coastal and sea-based communities. Threats brought on by tropical cyclones include wind, storm surges, and heavy rainfall occurrences. With hundreds of fatalities and billions of dollars in property damage, these have been the most devastating natural disasters to hit the North Indian Ocean’s coastline. Risky coastlines include those of Bangladesh, Sri Lanka, and Myanmar, as well as the eastern coast of India. The majority of the world’s population—40 percent—lives less than 100 kilometres from the coast, making it extremely susceptible to tropical cyclones. India’s 7516-kilometer coastline, which it shares with the North Indian Ocean, is a part of the coastal zone. India has been sensitive to these conditions because of its limited continental shelf, low coastal terrain, high population density, geographic position, and physiological characteristics.
Recently, researchers looked into the asymmetry between the Arabian Sea and the Bay of Bengal, concentrating on years where the Arabian Sea or the Bay of Bengal had the majority of tropical cyclone activity in the North Indian Ocean. The disparity between years when the Arabian Sea develops significantly more storms compared to those when the Bay of Bengal does, and to identify the environmental elements that contribute to this asymmetry. Their findings demonstrated that greater moisture was available over the Arabian Sea during years when tropical cyclone activity was higher.
Cyclones develop in the Bay of Bengal as a result of the enormous low pressure that warm ocean water creates. According to meteorologists, the Bay where intense cyclones develop is concave or shallow, and when strong winds push the water, it condenses into a storm; hence the trough-like shape makes storm production easier. The Bay’s location is further complicated by the high sea surface temperature, which heightens storm fury. The Bay of Bengal also receives more rainfall due to the slow-moving winds and warm air currents that surround it, which helps to maintain relatively stable temperatures throughout the year. The continuous warm water input from perineal rivers like the Ganga and Brahmaputra makes it difficult for the heated water to mix with the cooler water below. Lack of air movement from north-western India to the Bay of Bengal during the post-monsoon season is another element that could cause cyclones in the Bay of Bengal.
The Arabian Sea, on the other hand, is calmer because of the stronger winds that help to dissipate the heat and because there isn’t always fresh water present. As a result, heated water can mix with cool water below the surface to lower the surface temperature. Winds from the Pacific Ocean are weaker and rarely reach the Arabian Sea because of the Western Ghats and the Himalayas.
Few climatologists think that the Bay of Bengal experiences more tropical cyclone activity because it has a higher sea surface temperature (SST) and moisture content than the Arabian Sea. For the increased activity in the Bay of Bengal, higher SST and the presence of disturbances from the Western North Pacific are to blame. Also, El Nino and La Nina episodes have a significant impact on cyclogenesis in the Bay of Bengal. The conflicting patterns in the Sea Surface Temperature cooling composite are caused by the fluctuation in the number of cyclones between El Nino and La Nina. Similarly, during El Nino, La Nina, and normal years, the Bay of Bengal’s sea surface temperature is higher than that of the Arabian Sea, and tropical cyclone activity responds to variations in sea surface temperature by intensifying or deterring.
According to the data, on average, 83 percent of all tropical cyclones that make landfall migrate westward and settle on India’s eastern coast during El Nino years. On the east coasts of India, Bangladesh, and Myanmar, 73% of all tropical cyclones that made landfall during La Nina years curled northwest and northward, making landfall north of 17°N latitude. When opposed to El Nino years, even the most severe cyclonic conditions are more common during La Nina years. The cyclones that occur more frequently during La Nina years may be the leftovers of tropical cyclones that developed in the North West Pacific. Therefore, cyclones created during the La Nina period travel farther and typically grow before making landfall, whereas the opposite situation is seen in El Nino years.
Increasing frequency and intensification in the Indian Ocean
The deadliest weather-related natural hazard, in terms of loss of life, property, and infrastructure, continues to be tropical cyclones. There has already been a discernible rise in the frequency and strength of cyclones with greater intensities due to global warming. Forecasting cyclones is becoming more difficult due to rapid intensification, and more powerful cyclones with rapid intensification are anticipated.
A maximum sustained wind speed increase of at least 55 km/h during a 24-hour period is considered a rapid intensification. Only a sharp reduction in pressure within the cyclone’s eye can cause such acceleration. India has made major strides in cyclone forecasting and dramatically decreased the number of cyclone-related fatalities by developing effective evacuation plans and other hazard mitigation measures in response to forecasts.
Previous research has demonstrated that the tropical Indian Ocean is warming at a quicker rate than the rest of the world’s oceans, owing to global warming. High sea surface temperatures are more likely to produce Marine Heat waves (MHWs) which are extreme temperature conditions that last for days to months. Due to MHW, the ocean is intensely warming, which has serious socioeconomic effects, including fish death and coral bleaching. It also has the capacity to interact with and affect other extreme occurrences like tropical cyclones. Extreme occurrences like MHWs and tropical cyclones can form and intensify because of the anthropogenic warming of the oceans and atmosphere. Tropical cyclones and marine heat waves are two examples of extreme phenomena that can occur when the water and atmosphere are connected.
The Bay of Bengal experiences consistently warm sea surface temperatures (about 28°C) and are more vulnerable to tropical storms. The North Indian Ocean is most susceptible to deaths worldwide since the Bay of Bengal is home to around 5-7 percent of the total number of tropical cyclones that occur globally each year. Now let’s consider that Cyclone Amphan, which intensified from category 1 (cyclonic storm) to category 5 (super cyclone) in less than 24 hours, was the first super cyclone in the Bay of Bengal in the previous 21 years. Amphan was also the most expensive tropical storm in the history of the North Indian Ocean, with 129 fatalities in India and Bangladesh and reported economic damages of almost $14 billion in India, according to the World Meteorological Organization. Amphan was the primary cause of displacement in 2020, accounting for 2.4 million displacements in India alone, of which about 8 00,000 were caused by authorities’ pre-emptive evacuations, according to the most recent IPCC assessment (AR6).
Now, the question arises, why did such unusual and unprecedented rapid intensification of cyclone Amphan cause to form a devastating super cyclonic storm? It occurred because there was a powerful MHW beneath the cyclone’s path, which corresponded with the cyclone’s track and allowed for the cyclone’s rapid intensification. This MHW had an extraordinarily high anomalous sea surface temperature of more than 2.5°C. The super storm Amphan followed a path that was remarkably similar to that of the previously exceptionally severe cyclone Fani in May 2019. As can be seen, Amphan had a shorter life over the ocean than Fani, which lasted seven days, however, Fani did not intensify into a super cyclone like Amphan did. The existence of MHW in Amphan, which was absent in Fani, was the primary distinction between these two cyclones. It can be inferred that Amphan evolved into a super cyclone, principally driven by a powerful MHW on its way, despite its brief duration and unfavourable meteorological conditions in comparison to Fani. It also demonstrates that ocean stratification and warming below the surface, in addition to surface warming, have a significant influence in these phenomena of compound extreme occurrences.
With this, not only the Bay of Bengal but also the Arabian Sea is becoming into the ideal ocean basin for the development of cyclones, where a spike of almost 52% was noted since 2001. Furthermore, it has been observed that cyclones in the Arabian Sea move more slowly, absorbing all available energy while at sea and ultimately intensifying when they reach the coast. Cyclone Tauktae originated near Lakshadweep in May 2021 and moved north before making landfall on the Gujarat coast. Because of how powerful the storm was, it continued to be active for 24 hours after making landfall and dumped rain in several areas of Rajasthan, Delhi, and Uttar Pradesh. On land, cyclones typically lose their strength. Storms develop and persist based on the energy made available by moisture and ocean heat load. The study also noted that the Arabian Sea’s accumulated cyclone energy, or overall wind energy over the course of a storm, had roughly tripled, indicating the potential magnitude of recent warming.
With this, we can say Southwest Indian Ocean territory would be colder earlier, but the sea surface temperatures across the Arabian Sea have changed more than those over the Bay of Bengal, though. Additionally, the Arabian Sea has significantly higher moisture availability than the Bay of Bengal, which appears to be becoming moisture-deficient. As a result, the study and research highlight these compound or individual extreme events that would become more common owing to global warming in the future, with the Indian Ocean experiencing a rise in both their strength and frequency.
A brief decadal analysis of the changing nature of cyclones over the Indian Ocean
Tropical cyclones are formed by severe disturbances that develop swiftly as a result of long-term changes in climatic factors across a specific region (a few days to a 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 have 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.
Monitoring and tracking cyclones over the Indian Ocean
Tropical cyclone forecasting has proven to be a difficult task. In order to precisely estimate the position of the cyclone and deliver the necessary warnings for disaster management, numerous techniques and models have been created. Storm surge is the tropical cyclones most catastrophic effect, especially for the Indian coastal districts. Due to the Indian region’s very variable bathymetry, even a small discrepancy in the predicted time of landfall can result in a completely different storm surge height. Also, many statistical models are developed to predict cyclone intensity. The upper-ocean heat storage, which is typically represented in the oceanic eddies and dynamic topography, is a crucial component that improves our understanding of how cyclones intensify in addition to atmospheric and sea surface temperature parameters. Radar altimeters’ anomalies in sea surface height can be used to determine this parameter.
When making an intensity or track forecast, new technologies have been used to take each of these aspects into account. The Space Applications Centre in Ahmadabad developed two-dimensional axisymmetric cyclone models as one of the first attempts to anticipate tropical cyclones. These models offered a fundamental understanding of the physical processes within a tropical cyclone and assisted in the interpretation of the satellite measurements, even if they had little to no applicability for real-time forecasting needs. Tropical cyclone research was aided by the development of next-generation satellite observations, particularly the wind scatter meter onboard the first European Remote Sensing (ERS-1) satellite.
With the arrival of the most significant international satellite missions like the Tropical Rainfall Measuring Mission, which offers eagerly expected insight into the tropical storm structure, the late 1990s were a very critical time for the advancement of tropical cyclone studies. Additionally, more powerful computational resources are now available, and more advanced mesoscale numerical weather prediction models have also been developed and made available. The primary consideration for all modelling activities has always been the active utilisation of satellite observations. The employment of a synthetic vortex in the model’s initial circumstances, where the tropical cyclones were repositioned in model fields in accordance with the satellite measurements, was one of the earliest applications in tropical cyclone forecasting experiments based on mesoscale models.
Later, more advanced methods of data assimilation, such as 3-D variational (3D-VAR) approaches, were used to best assimilate the satellite-derived data in order to enhance tropical storm prediction. The assimilation of winds and radiances from image sensors, as well as cloud motion winds produced from INSAT, is the most recent advancement in data assimilation research. At the moment, in the Satellite Meteorological Division, meteorological data from the INSAT-3D and INSAT3-DR satellites are operationally received and processed using the Multi-Mission Meteorological Data Receiving and Processing System (MMDRPS). Currently, the Dvorak approach is applied manually. Automation of this method has recently been the focus of efforts. The Satellite Application Unit, Satellite Meteorology Division, is now operating the Automated Dvorak technique version 8.2.1 in experimental mode. Subsequently, in order to locate tropical systems, Satellite Application Unit additionally employs operational microwave imagery from NOAA and Metop’s DMSP satellites.
A visualisation tool called Real-time Analysis of Product and Information Dissemination (RAPID) was created by IMD and ISRO in order to track and analyse satellite images and products from INSAT 3D and INSAT3-DR. RAPID also has a new cast tool that is reliant on satellites for its prediction. In order to help forecasters produce more objective now casts, RAPID, which is a geo-reference platform, delivers real-time information on genesis, growth, and decay along with its location and other geophysical features.
Additionally, India’s weather radar network currently covers the entire nation with 33 Doppler weather radars (DWRs), including radars from ISRO. It includes two sites with C-band and Polarimetric DWRs five in X-band. IMD radars are used to track cyclonic storms as well as to detect hail, rain, and thunderstorms. The forecasters use a variety of meteorological and hydrological products that are produced from radar data using software algorithms to determine the storm’s centre, path of motion, structure, and severity. Additionally, the current radars have been networked to deliver data to supercomputers in almost real-time for incorporation into numerical weather prediction (NWP) models for short-range forecasting.
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