North Indian Ocean tropical cyclone

Very severe cyclonic storms (Luban and Titli) over the Arabian Sea and the Bay of Bengal in October 2018

The North Indian Ocean is the least active 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.[1] 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.[2]

The basin is divided into two sub-basins  – the Bay of Bengal and the Arabian Sea.[3]

The Bay of Bengal, located in the northeast of the Indian Ocean. The basin is abbreviated BOB by the India Meteorological Department (IMD).[4] The United States's Joint Typhoon Warning Center unofficially designates as B to classify storms formed in the Bay of Bengal.[5] The Bay of Bengal's coast is shared among India, Bangladesh, Myanmar, Sri Lanka and western part of Thailand.[6] This sub-basin is the most active and produces some of the deadliest cyclones of all time.[7] The most intense cyclone in the bay was the 1999 Odisha cyclone.[8]

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).[4] The United States's Joint Typhoon Warning Center unofficially designates as A to classify storms formed in the Arabian Sea.[9] The Arabian Sea's coast is shared among India, Yemen, Oman, UAE, Iran, Pakistan, Sri Lanka, Maldives and Somalia.[10] 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 rare in the Arabian Sea, but the basin can produce strong tropical cyclones.[10] Cyclone Gonu was the strongest and the costliest recorded tropical cyclone in the basin.[11]

The systematic scientific studies of tropical systems in the Bay of Bengal and Arabian Sea was started during the 19th century by Henry Piddington.[12] 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.[12] These memoirs gave accounts and tracks of individual storms in the Bay of Bengal and the Arabian Sea.[12]

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.[13] 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 90 knots (165 km/h; 105 mph) and 120 knots (220 km/h; 140 mph) an extremely severe cyclonic storm.[14]

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.[15]

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.[24][25] Research has found that in recent decades the sea surface temperatures has risen up by 1.2–1.4 °C (2.2–2.5 °F) in the Arabian Sea.[25] During Cyclone Amphan’s rapid intensification, sea surface temperatures were as high as 33 °C (91 °F) in the Bay of Bengal, and parts of the Arabian Sea reached 32 °C (90 °F) before the formation of Cyclone Nisarga.[26] 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.[25] Higher temperatures caused the cyclones to become more powerful and lead to tropical cyclone formation faster. Rising sea level also caused higher storm surge.[26] 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.[26]

• The most intense tropical cyclone of the basin was the 1999 Odisha Cyclone. It was the strongest tropical cyclone to hit the Indian state of Odisha. The maximum recorded wind speed was 260 km/h (160 mph) with minimum barometric pressure of 912 mbar (26.93 inHg).[30]
• The costliest tropical cyclone was Cyclone Amphan which struck the West Bengal coast on May 20. The recorded damage cost was US$13.2 billion, beating the record of Cyclone Nargis which was US$4 billion.[31]
• The deadliest tropical cyclone was the 1970 Bhola Cyclone, which caused over 500,000 fatalities in the Ganges-Brahmaputra delta region alone. It made landfall in the Tazumuddin Upazila in East Pakistan (now known as Bangladesh).[32][33]
• The smallest tropical cyclone was Cyclone Ogni, which struck the Andhra Pradesh coast on October 30, 2008. The cyclone's diameter was only 100 km (62 mi).[34]
• The largest tropical cyclone was the 1999 Odisha Cyclone, which strike the coast of Odisha.[8][35]
• The wettest tropical cyclone was Severe Cyclonic Storm ARB 01 (2004). One of the weather stations recorded 184 cm (72 in) between May 5  – 7, 2004.[36]

• Tropical cyclone
• Atlantic hurricane season
• Pacific hurricane season
• Pacific typhoon season
• South-West Indian Ocean tropical cyclone
• Australian region tropical cyclone
• South Pacific tropical cyclone
• South Atlantic tropical cyclone
• Mediterranean tropical-like cyclone