The cyclonic disturbances over the North Indian Ocean, which includes the Arabian Sea, the Bay of Bengal and the land area in between, have gone through many significant changes in recent decades.
First, the number of cyclonic disturbances in the North Indian Ocean has declined considerably over the past four decades.
Second, this decline was mostly due to a rapid decrease in the number of cyclonic disturbances originating from the Bay of Bengal, the source of most past cyclones to have made landfall in south-east and west Asia. In contrast, the number of such disturbances originating from the Arabian Sea has been steadily increasing in recent years, though the count is still relatively low. More importantly, while the frequency of disturbances has decreased, the intensity of cyclones has increased in recent decades.
Chart 1 | The chart shows the 10-year rolling average of cyclonic disturbances between 1891 and 2022.
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For instance, in 2022, the 10-year average was 10.27, that is, in the previous 10 years (2013-2022), on average, there were 10 cyclonic disturbances per year. As the chart shows, the average was about 15 disturbances in the 1940s and 1950s and later in the 1970s. It decreased to below 10 in the 1990s, 2000s and 2010s. After a gap of almost three decades, the average went slightly above 10 cyclonic disturbances in 2023.
Chart 2 | The chart shows the 10-year rolling average of cyclonic disturbances between 1891 and 2022 originating from the Arabian Sea and the Bay of Bengal.
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The number of cyclonic disturbances originating from the Bay of Bengal has rapidly declined since the 1970s. In contrast, those originating from the Arabian Sea have considerably increased. From 1891 to 1962, on average, less than two disturbances per year were recorded from the Arabian Sea. However, since then, the average has increased, with over three cyclones being recorded in the 2020s (similar to the peak in the 1979-1983 period).
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A story published in The Hindu on Saturday reported that global warming is causing the Arabian Sea to heat up, which is resulting in more, stronger cyclones. Data show that about 34% of all disturbances that originated in the Arabian Sea have become severe cyclonic storms (a top speed of >48 knots) compared to the 19% of disturbances which originated from the Bay of Bengal and become severe cyclonic storms. The report further points out that the Indian Meteorological Department finds it tougher to forecast cyclones originating from the Arabian Sea as the models have to be adjusted given the oceanographic differences it has with the Bay of Bengal.
Chart 3 | The chart shows the peak intensity recorded by all cyclonic disturbances between 1994 and 2023.
The chart only plots the peak surface wind (in knots) recorded by a cyclone during its journey. For instance, Biparjoy reached a peak intensity of 90 knots, three days before its landfall on June 15 in Gujarat. As can be observed from the chart, between 2019 and 2023 (the latest five years), the share of severe cyclonic storms (>48 knots) in all cyclonic disturbances increased to 32%, the highest for any five-year period since 2004. In the last five years, 55% and 17% of the cyclonic disturbances were depressions (<33 knots) and cyclonic storms (34-47 knots), respectively. The 1999-2003 period too recorded a 34% share of severe cyclonic storms, with the 1994-1998 period recording 52%. So, the intensity of storms originating in the North Indian Ocean is once again on an upward cycle.
vignesh.r@thehindu.co.in
Source: Regional Specialised Meteorological Centre for Tropical Cyclones over the North Indian Ocean (RSMC,New Delhi).
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