The northern Indian Ocean accounts for 6% of global tropical cyclones each year. Despite the small fraction, cyclones over this basin resulted in, by far, the highest number of deaths compared to any other basin in the 20th century.
Some of the most devastating cyclonic storms have formed in this basin, causing extensive damage to life and property in surrounding countries. Super Cyclonic Storm Amphan, a system that made landfall in West Bengal in May 2020, was the costliest cyclone ever recorded in the northern Indian Ocean in terms of economic losses (at $14 billion), according to a report published by the World Meteorological Organization (WMO).
Unprecedented rapid intensifications of cyclones over the northern Indian Ocean
Cyclone activity in the northern Indian Ocean has increased significantly in recent years, with more pronounced changes seen in the Arabian Sea. Here, the frequency of cyclones has increased by 52% in recent decades compared to 1980-1999. Also, during the same period, the number of very violent cyclones increased by 150%.
Apart from the frequency, the intensity of these cyclones also increased by 40%, from 100 km/h to 140 km/h during the pre-monsoon season (April-early June); and by 20%, from 100 km/h to 120 km/h during the post-monsoon season (October-December).
The Arabian Sea cyclones also last longer in the ocean. Their average life cycle has increased by 80% compared to the 1980s. Due to the increase in their intensity and life cycles, the accumulated cyclonic energy of the cyclone has also almost doubled during the two hurricane seasons.
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All of these changes are related to the rapid warming of the Arabian Sea and a decrease in wind shear during hurricane seasons. In this basin, sea surface temperatures (SST) have increased by 1.4°C and 1.2°C during the pre- and post-monsoon seasons, respectively, over the past four decades, fueling thus intense and longer lasting cyclones in the basin. .
The rate of rapid cyclone intensification (wind speed increase of ≥ 55.6 km/h over a 24-hour period) in the northern Indian Ocean has also increased. In previous decades (1980–1999), one in four cyclones in the pre-monsoon season and one in six cyclones in the post-monsoon season intensified rapidly. But in recent decades (2000-2019), rapid intensification occurs with one in two cyclones in the pre-monsoon season and one in four cyclones in the post-monsoon season.
The percentage of cyclones undergoing rapid intensification in the northern Indian Ocean is higher even from the northwest Pacific Ocean, which is the most active cyclone basin on Earth.
Recent years have also seen unprecedented rapid intensifications of cyclones in the northern Indian Ocean. In May 2020, Cyclone Amphan in the Bay of Bengal intensified from Category 1 (129.6 km/h) to Category 5 (259.3 km/h) in just 24 hours. Similarly, in November 2020, Cyclone Gati in the Arabian Sea intensified from a tropical storm (wind speed 64.8 km/h) to a powerful Category 3 cyclone (wind speed 185 .2 km/h) in just 12 hours, making it the fastest intensification of any cyclone in this basin.
With the rapid warming of the ocean surface, an increase in the heat content of the ocean (the warming of the ocean basement) provides a large amount of heat reservoir for cyclones to intensify rapidly.
Challenges and way forward
Forecasts of the tracks of North Indian Ocean cyclones and their landfall positions have improved significantly over time due to improved weather models, leading to a sharp reduction in casualties.
However, forecasting the intensity of cyclones and their rapid intensification with long lead times remains a challenge. This is due to gaps in in-situ ocean observations that feed numerical weather prediction models, leading to an increase in the prediction error of these models.
Ocean surface and subsurface conditions play a crucial role in regulating the intensification of cyclones. Therefore, an increase in the number of in situ ocean observing networks such as Argos ocean moorings and buoys for real-time ocean profiling can give us more accurate estimates of ocean conditions.
Given the densely populated northern Indian Ocean coast and economic losses from cyclones exceeding $3 billion per year – which continue to increase even more with global warming – there is an urgent need to improve extended predictability. of these cyclones.
As the ocean becomes more conducive to intense cyclones, a large number (62%) of intense cyclones (wind speed 185 km/h) reach their maximum intensity less than 200 km from the coast. This means that we need high resolution data along the coastal zone, which is now scattered and restrictive due to regulations in exclusive economic zones.
Vineet Kumar Singh is currently pursuing her doctorate. from the Indian Institute of Tropical Meteorology, Pune. His research focuses on understanding the characteristics of cyclones in the northern Indian Ocean. His research interests focus on developing understanding of tropical cyclone mechanisms and creating improved techniques for forecasting future cyclones.
This article is a guest column reflecting the views of the author and does not necessarily represent the official views of The Weather Channel. The article has been partially edited for length and clarity.
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