In the Indian Ocean north of the equator, tropical cyclones can form throughout the year on either side of the Indian subcontinent, although most frequently between April and June, and between October and December.
The North Indian Ocean is the least active official basin, contributing only seven percent of the world's tropical cyclones. However the basin has produced some of the deadliest cyclones in the world, since they strike over very densely populated areas. The Regional Specialized Meteorological Centre (RSMC) is the India Meteorological Department (IMD) and it is responsible to monitor the basin, issues warning and name the storms.
Sub-basins
The basin is divided into two sub-basinsthe Bay of Bengal and the Arabian Sea.
The Bay of Bengal, located in the northeast of the Indian Ocean. The basin is abbreviated BOB by the India Meteorological Department (IMD). The United States's Joint Typhoon Warning Center unofficially designates as B to classify storms formed in the Bay of Bengal. The Bay of Bengal's coast is shared among India, Bangladesh, Myanmar, Sri Lanka and western part of Thailand. This sub-basin is the most active and produces some of the deadliest cyclones of all time. The most intense cyclone in the bay was the 1999 Odisha cyclone.
The Arabian Sea is a sea located in the northwest of the Indian Ocean. Tropical cyclones in the basin are abbreviated ARB by the India Meteorological Department (IMD). The United States's Joint Typhoon Warning Center unofficially designates as A to classify storms formed in the Arabian Sea. The Arabian Sea's coast is shared among Pakistan, India, Yemen, Oman, UAE, Iran, Sri Lanka, Maldives and Somalia. Monsoons are characteristic of the Arabian Sea and responsible for the yearly cycling of its waters. In summer, strong winds blow from the southwest to the northeast, bringing rain to the Indian subcontinent. Cyclones are less frequent in the Arabian Sea, but the basin can produce strong tropical cyclones. Cyclone Gonu was the strongest and the costliest recorded tropical cyclone in the basin.
History of the basin The systematic scientific studies of tropical systems in the Bay of Bengal and Arabian Sea was started during the 19th century by Henry Piddington. Piddington utilised meteorological logs of vessels that navigated the seas and published a series of memoirs, in the Journal of the Asiatic Society of Bengal between 1839 and 1858. These memoirs gave accounts and tracks of individual storms in the Bay of Bengal and the Arabian Sea.
During the 2004 post monsoon season the IMD started to name tropical cyclones within the basin, with the first one named Cyclone Onil during September 2004. During 2015 a modification to the intensity scale took place, with the IMD and WMO calling a system with 3-minute maximum sustained wind speeds between and an extremely severe cyclonic storm.
A study analysing the spring season of tropical cyclones in the Bay of Bengal found increases in both pre-monsoon precipitation and tropical cyclone intensity as a result of enhanced large-scale monsoon circulation after 1979. The deepened monsoon trough in the Bay of Bengal not only affects cyclone frequency and timing, but also acts to direct more cyclones towards Myanmar. Increased anthropogenic aerosols likely contributed to such a regional climate change.
Climatology Formation and frequency On average only five to six tropical cyclones form in the basin each year. Tropical cyclones form in the months of March to June and October to December, with peaks at May and November. Most of these storms form in the Bay of Bengal: either in the southeastern Bay of Bengal, in the Andaman Sea, or as a remnant of a typhoon from the South China Sea, and a few in the Arabian Sea. High sea surface temperatures and humidity makes the bays more favourable to tropical cyclone development. There are many tropical cyclones in the West Pacific; this may be another reason for increased tropical cyclogenesis in the Bay, as it shares a fair portion of the increased quota of ACE. Meanwhile, the storms in the Arabian Sea mostly form over south-eastern part of the Arabian Sea or a remnant of a tropical cyclone from the Bay of Bengal, however the frequency of cyclogenesis in the Arabian Sea is generally less, due to cooler sea surface temperature and high wind shear. However a strong positive Indian Ocean Dipole may cause an increase of tropical cyclogenesis than usual which was seen in the 2019 season. Very few tropical cyclones develop in the months of June to September (Monsoon months) because of high vertical wind shear. These storms form in the Bay of Bengal and peak as depressions or deep depressions before making landfall in Odisha or West Bengal. Another reason is the low life span in the sea which also avoids the intensification of these low-pressure systems.
Movement Most of the storms move in a north-westerly direction and starts curving either towards southwest or northeast. There's a higher frequency of recurving towards northeast rather going southwest. In the Arabian Sea these storms mostly move in north-westerly direction targeting the Arabian Peninsula, however in some case these storm moves north-eastwards after crossing the 15°N latitude and strikes the Gujarati coast. In the Bay of Bengal, storms generally moves north-westwards until reaching the east coast and then moves north-eastwards.
Intensification Intensification probability is maximum in the months of April, May and November in case of a depression becoming a cyclonic storm and severe cyclonic storm. More than half of the depressions intensify into a storm and a quarter intensify into a cyclone in these months.
Landfall In the Arabian Sea, most storms dissipate offshore without making landfall, but a significant number of tropical cyclones also impact the west coast, particularly the states of Gujarat and Maharashtra. The remaining 11 percent makes landfall in either the Arabian Peninsula, Horn of Africa or Pakistan. In rare cases, some storms make landfall in Iran, like Cyclone Gonu did in 2007. Other than Gonu, two storms: Cyclone Yemyin and Kyarr made some or major impact in Iran.
In the Bay of Bengal, most of the storms strikes either the Indian states of Odisha or West Bengal and a significant number of storms hit the states of Andhra Pradesh and Tamil Nadu. 30 percent of the cyclones strike the countries of Bangladesh, Sri Lanka and Myanmar while the remaining 13 percent just dissipates off shore without making landfall.
Climate change
After a series of devastating cyclones in 2018, rising number of cyclones in the Arabian Sea in 2019 and a rising trend of rapid intensification in 2020 and 2021, many climatologists agree that climate change have caused these activities. On average, five cyclonic storms occur every year in the Arabian Sea. However, in 2019 eight cyclonic storms formed, becoming the record highest number of tropical cyclones in the sub-basin, which was tied with the 1902 season. Research has found that in recent decades the sea surface temperatures has risen up by in the Arabian Sea. During Cyclone Amphan’s rapid intensification, sea surface temperatures were as high as in the Bay of Bengal, and parts of the Arabian Sea reached before the formation of Cyclone Nisarga. According to the Union Ministry of Earth and Science, the frequency of very severe cyclonic storms has risen up by one per decade in last two decades, despite the decrease of the overall tropical cyclone frequency in the same period. Higher temperatures caused the cyclones to become more powerful and lead to tropical cyclone formation faster. Rising sea level also caused higher storm surge. Researchers also predict that cyclones will be deadlier and stronger as the trend of warming sea surface temperatures continue. Rising sea levels also may cause severe flooding, strong storm surge and inundation of coastal towns.
Seasons +Historical storm formation by month between 1990 and 2020
Before 1890
1890s
Year Notes 1890 10 4 1 1891 13 4 3 Total includes 1 Land Severe Cyclonic Storm 1892 12 7 2 1893 12 10 4 1894 12 6 0 1895 11 5 4 1896 10 8 3 1897 12 6 8 1898 13 7 3 1899 7 3 0 Total1126028References
1900s
Year Strongeststorm Deaths Damage(USD) Notes 1900 1031 1901 632 1902 1375 1903 1482 1904 940 1905 1060 1906 1171 1907 1584 1908 961 1909 884Total1056020References
1910s
Year Strongeststorm Deaths Damage(USD) Notes 1910 652 1911 754 1912 962 1913 1062 1914 842 1915 960 1916 1485 1917 1031 1918 1150 1919 1163Total955421References
1920s
Year Strongeststorm Deaths Damage(USD) Notes 1920 950 1921 1041 1922 1366 1923 1643 1924 1360 1925 2073 1926 13103 1927 1872 1928 1370 1929 1560Total1406218References
1930s
Year Strongeststorm Deaths Damage(USD) Notes 1930 14101 1931 1151 1932 1462 1933 1683 1934 1650 1935 1562 1936 1863Two293$75,000 1937 19 6 2 1938 10 4 4 1939 19 7 3Total1526321References
1940s
Year Strongeststorm Deaths Damage(USD) Notes 1940 16 8 5 1941 19 8 4 1942 14 5 2 1943 14 7 1 1944 19 8 2 1945 15 3 2 1946 17 5 1 1947 18 4 2 1948 18 6 3 1949 12 1 1Total1625523References
1950s
Year Strongeststorm Deaths Damage(USD) Notes 1950 16 4 0 Sixteen Unknown Unknown 1951 15 4 2 Fifteen Unknown Unknown 1952 17 4 2 1953 10 1 1 1954 14 1 0 1955 13 6 2 1956 14 4 2 1957 7 4 2 1958 12 5 2 1959 16 6 3Total1343916UnknownUnknownReferences
1960s
Year Strongeststorm Deaths Damage(USD) Notes 1960 15 5 3 Ten > Vast majority of the fatalities resulted from two cyclones striking East Pakistan three weeks apart 1961 18 5 4 Winnie Three land depressions developed this season 1962 13 5 3 Twelve ≥ Deadliest storm, Harriet, crossed over from the Western Pacific 1963 17 6 4 Three 1964 16 7 5 > > 1965 14 6 4 1966 18 8 6 1967 15 6 4 1968 13 7 4 "Burma" 1969 14 6 1 Twelve Total1536138Three>47,000>$193.9 millionReferences
1970s Year Strongeststorm Deaths Damage(USD) Notes 1970 15 7 3 "Bhola" 300,000-500,000 86.4 million The Bhola cyclone is the deadliest tropical cyclone recorded worldwide 1971 15 7 6 "Odisha" 1972 18 7 6 09B 1973 16 6 3 14B 1974 12 7 3 06B 1975 20 7 4 02A 1976 14 10 7 02A 1977 18 5 5 "Andhra Pradesh" up to 50,000 $192 million Devastated Krishna Delta area in Andhra Pradesh 1978 14 5 3 "Sri Lanka" 1,000+ 1979 11 5 4 01B Total1536644"Andhra Pradesh">800,000>$278.4 millionReferences
1980s
Year Strongeststorm Deaths Damages(USD) Notes 1980 14 14 5 0 0 0 0 BOB 02 1981 12 12 5 3 3 0 0 BOB 03 1982 19 11 8 5 3 3 0 BOB 01 1983 7 4 2 1 1 1 0 Herbert 1984 7 7 4 3320 "Sriharikota" 430 1985 15 15 6 1100 "Bangladesh" 11,107 1986 8 3 1 0000 02B 11 1987 9 8 5 3100 01B 1988 9 5 5 3220 "Bangladesh" 6,740 $13 million 1989 10 5 3 2 1 1 1 Gay 1,785 $25.27 million Typhoon Gay crossed over from the West Pacific BasinTotal1108444211591Gay / Kavali>20,073>$38.27 billionReferences
1990s
Year Strongeststorm Deaths Damage(USD) Notes and References 1990 11 6 2 2 1 1 1 "Andhra Pradesh" 1991 9 4 3 1 1 1 1 "Bangladesh" 1992 13 11 7 2 1 1 0 Forrest Forrest crossed over from the West Pacific Basin 1993 5 4 2 2 2 0 0 BOB 02 1994 5 5 4 2 2 1 0 "Bangladesh" 1995 8 6 3 2 2 1 0 BOB 07 1996 10 8 6 4 2 0 0 "Andhra Pradesh" 1997 9 7 3 2 1 1 0 "Bangladesh" Unknown 1998 12 10 6 5 3 1 0 "Gujarat" 1999 10 8 5 3 3 2 1 "Odisha" The Odisha cyclone is the strongest cyclone recorded in the Northern Indian Ocean.Total936941251893"Odisha">168,923~$12.35 billionReferences
2000s
Year Strongeststorm Deaths Damage(USD) Notes 2000 7 6 5 2 2 2 0 "South India" 2001 6 5 4 1 1 1 0 "Gujarat" 2002 7 7 4 1 0 0 0 "West Bengal" 2003 7 5 3 3 1 0 0 "Sri Lanka" 2004 10 7 4 4 1 1 0 "Myanmar" 2005 12 7 3 0 0 0 0 Pyarr 2006 12 6 3 2 1 1 0 Mala 2007 11 8 4 2 2 2 1 Gonu 2008 10 7 4 1 1 1 0 Nargis The deadliest cyclone season since 1970Third-costliest cyclone season on record 2009 8 6 4 1 0 0 0 Aila Total90643817981Gonu>157,965$16.65 billionReferences
2010s