In the

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or approximately 20% of the water area of Earth#Surface, Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia (continent), ...

north of the equator,

tropical cyclone A tropical cyclone is a rapidly rotating storm system with a low-pressure area, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depending on its locat ...

s can form throughout the year on either side of the

Indian subcontinent The Indian subcontinent is a physiographic region of Asia below the Himalayas which projects into the Indian Ocean between the Bay of Bengal to the east and the Arabian Sea to the west. It is now divided between Bangladesh, India, and Pakista ...

, 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 A Regional Specialized Meteorological Centre (RSMC) is responsible for the distribution of information, advisories, and warnings regarding the specific program they have a part of, agreed by consensus at the World Meteorological Organization as ...

(RSMC) is the

India Meteorological Department India Meteorological Department (IMD) is an Indian agency of the Ministry of Earth Sciences of the Government of India. It is the principal agency responsible for meteorological observations, weather forecasting and seismology. IMD is headquar ...

(IMD) and it is responsible to monitor the basin, issues warning and name the storms.

Sub-basins

The basin is divided into two sub-basins the

Bay of Bengal The Bay of Bengal is the northeastern part of the Indian Ocean. Geographically it is positioned between the Indian subcontinent and the Mainland Southeast Asia, Indochinese peninsula, located below the Bengal region. Many South Asian and Southe ...

and the

Arabian Sea The Arabian Sea () is a region of sea in the northern Indian Ocean, bounded on the west by the Arabian Peninsula, Gulf of Aden and Guardafui Channel, on the northwest by Gulf of Oman and Iran, on the north by Pakistan, on the east by India, and ...

. The Bay of Bengal, located in the northeast of the Indian Ocean. The basin is abbreviated ''BOB'' by the

(IMD). The United States's

Joint Typhoon Warning Center The Joint Typhoon Warning Center (JTWC) is a joint United States Navy – United States Air Force command in Pearl Harbor, Hawaii. The JTWC is responsible for the issuing of tropical cyclone warnings in the North-West Pacific Ocean, South P ...

unofficially designates as ''B'' to classify storms formed in the Bay of Bengal. The Bay of Bengal's coast is shared among India,

Bangladesh Bangladesh, officially the People's Republic of Bangladesh, is a country in South Asia. It is the List of countries and dependencies by population, eighth-most populous country in the world and among the List of countries and dependencies by ...

Myanmar Myanmar, officially the Republic of the Union of Myanmar; and also referred to as Burma (the official English name until 1989), is a country in northwest Southeast Asia. It is the largest country by area in Mainland Southeast Asia and has ...

Sri Lanka Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka, also known historically as Ceylon, is an island country in South Asia. It lies in the Indian Ocean, southwest of the Bay of Bengal, separated from the Indian subcontinent, ...

and western part of

Thailand Thailand, officially the Kingdom of Thailand and historically known as Siam (the official name until 1939), is a country in Southeast Asia on the Mainland Southeast Asia, Indochinese Peninsula. With a population of almost 66 million, it spa ...

. 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 1999 Odisha cyclone ( IMD designation BOB 06, JTWC designation 05B) was the most intense recorded tropical cyclone in the North Indian Ocean and among the most destructive in the region. The 1999 Odisha cyclone organized into a tropi ...

. The Arabian Sea is a sea located in the northwest of the Indian Ocean. Tropical cyclones in the basin are abbreviated ''ARB'' by the

(IMD). The United States's

unofficially designates as ''A'' to classify storms formed in the Arabian Sea. The Arabian Sea's coast is shared among

Pakistan Pakistan, officially the Islamic Republic of Pakistan, is a country in South Asia. It is the List of countries and dependencies by population, fifth-most populous country, with a population of over 241.5 million, having the Islam by country# ...

, India,

Yemen Yemen, officially the Republic of Yemen, is a country in West Asia. Located in South Arabia, southern Arabia, it borders Saudi Arabia to Saudi Arabia–Yemen border, the north, Oman to Oman–Yemen border, the northeast, the south-eastern part ...

Oman Oman, officially the Sultanate of Oman, is a country located on the southeastern coast of the Arabian Peninsula in West Asia and the Middle East. It shares land borders with Saudi Arabia, the United Arab Emirates, and Yemen. Oman’s coastline ...

UAE The United Arab Emirates (UAE), or simply the Emirates, is a country in West Asia, in the Middle East, at the eastern end of the Arabian Peninsula. It is a federal elective monarchy made up of seven emirates, with Abu Dhabi serving as i ...

Iran Iran, officially the Islamic Republic of Iran (IRI) and also known as Persia, is a country in West Asia. It borders Iraq to the west, Turkey, Azerbaijan, and Armenia to the northwest, the Caspian Sea to the north, Turkmenistan to the nort ...

Maldives The Maldives, officially the Republic of Maldives, and historically known as the Maldive Islands, is an Archipelagic state, archipelagic country in South Asia located in the Indian Ocean. The Maldives is southwest of Sri Lanka and India, abou ...

and

Somalia Somalia, officially the Federal Republic of Somalia, is the easternmost country in continental Africa. The country is located in the Horn of Africa and is bordered by Ethiopia to the west, Djibouti to the northwest, Kenya to the southwest, th ...

Monsoons A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscil ...

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

. Cyclones are less frequent in the Arabian Sea, but the basin can produce strong tropical cyclones.

Cyclone Gonu Super Cyclonic Storm Gonu was an extremely powerful tropical cyclone that became the strongest cyclone on record in the Arabian Sea. The second named tropical cyclone of the 2007 North Indian Ocean cyclone season, Gonu developed from a pe ...

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

and

was started during the 19th century by

Henry Piddington Henry Piddington (7 January 1797 – 7 April 1858) was an English sea captain who sailed in East India and China and later settled in Bengal where he worked as a curator of a geological museum and worked on scientific problems, and is particula ...

. 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 Severe Cyclonic Storm Onil was the first tropical cyclone to be named in the northern Indian Ocean. Forming out of an area of convection several hundred kilometres southwest of India on October 1 2004, Cyclone Onil quickly attained its peak ...

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

. 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 The Andaman Sea (historically also known as the Burma Sea) is a marginal sea of the northeastern Indian Ocean bounded by the coastlines of Myanmar and Thailand along the Gulf of Martaban and the west side of the Malay Peninsula, and separated f ...

, or as a remnant of a

typhoon A typhoon is a tropical cyclone that develops between 180° and 100°E in the Northern Hemisphere and which produces sustained hurricane-force winds of at least . This region is referred to as the Northwestern Pacific Basin, accounting for a ...

from the

South China Sea The South China Sea is a marginal sea of the Western Pacific Ocean. It is bounded in the north by South China, in the west by the Indochinese Peninsula, in the east by the islands of Taiwan island, Taiwan and northwestern Philippines (mainly Luz ...

, and a few in the

. High sea surface temperatures and

humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...

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 Cyclogenesis is the development or strengthening of Cyclonic rotation, cyclonic circulation in the atmosphere (a low-pressure area). Cyclogenesis is an umbrella term for at least three different processes, all of which result in the development of ...

in the Bay, as it shares a fair portion of the increased quota of

ACE An ace is a playing card, die or domino with a single pip. In the standard French deck, an ace has a single suit symbol (a heart, diamond, spade, or a club) located in the middle of the card, sometimes large and decorated, especially in the ...

. 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 Sea surface temperature (or ocean surface temperature) is the ocean temperature, temperature of ocean water close to the surface. The exact meaning of ''surface'' varies in the literature and in practice. It is usually between and below the sea ...

and high

wind shear Wind shear (; also written windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical ...

. However a strong positive

Indian Ocean Dipole The Indian Ocean Dipole (IOD), is an irregular oscillation of sea surface temperatures in which the western Indian Ocean becomes alternately warmer (positive phase) and then colder (negative phase) than the eastern part of the ocean. Phenomen ...

may cause an increase of tropical cyclogenesis than usual which was seen in the

2019 season Nineteen or 19 may refer to: * 19 (number) * One of the years 19 BC, AD 19, 1919, 2019 Films * ''19'' (film), a 2001 Japanese film * ''Nineteen'' (1987 film), a 1987 science fiction film * '' 19-Nineteen'', a 2009 South Korean film * '' Dici ...

. Very few tropical cyclones develop in the months of June to September (

Monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in Atmosphere of Earth, atmospheric circulation and precipitation associated with annu ...

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 Odisha (), formerly Orissa (List of renamed places in India, the official name until 2011), is a States and union territories of India, state located in East India, Eastern India. It is the List of states and union territories of India by ar ...

West Bengal West Bengal (; Bengali language, Bengali: , , abbr. WB) is a States and union territories of India, state in the East India, eastern portion of India. It is situated along the Bay of Bengal, along with a population of over 91 million inhabi ...

. 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 The Arabian Peninsula (, , or , , ) or Arabia, is a peninsula in West Asia, situated north-east of Africa on the Arabian plate. At , comparable in size to India, the Arabian Peninsula is the largest peninsula in the world. Geographically, the ...

, however in some case these storm moves north-eastwards after crossing the 15°N

latitude In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...

and strikes the

Gujarat Gujarat () is a States of India, state along the Western India, western coast of India. Its coastline of about is the longest in the country, most of which lies on the Kathiawar peninsula. Gujarat is the List of states and union territories ...

i 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

and

Maharashtra Maharashtra () is a state in the western peninsular region of India occupying a substantial portion of the Deccan Plateau. It is bordered by the Arabian Sea to the west, the Indian states of Karnataka and Goa to the south, Telangana to th ...

. The remaining 11 percent makes landfall in either the

Horn of Africa The Horn of Africa (HoA), also known as the Somali Peninsula, is a large peninsula and geopolitical region in East Africa.Robert Stock, ''Africa South of the Sahara, Second Edition: A Geographical Interpretation'', (The Guilford Press; 2004), ...

or Pakistan. In rare cases, some storms make landfall in

, like

did in 2007. Other than Gonu, two storms:

Cyclone Yemyin Deep Depression BOB 03 and Cyclonic Storm Yemyin (JTWC designation: ''03B'') were a pair of deadly tropical cyclones that made landfalls on India and Pakistan in June 2007. The Pakistan Meteorological Department referred to both as Tropical Cyclo ...

and Kyarr made some or major impact in Iran. In the Bay of Bengal, most of the storms strikes either the Indian states of

and a significant number of storms hit the states of

Andhra Pradesh Andhra Pradesh (ISO 15919, ISO: , , AP) is a States and union territories of India, state on the East Coast of India, east coast of southern India. It is the List of states and union territories of India by area, seventh-largest state and th ...

and

Tamil Nadu Tamil Nadu (; , TN) is the southernmost States and union territories of India, state of India. The List of states and union territories of India by area, tenth largest Indian state by area and the List of states and union territories of Indi ...

. 30 percent of the cyclones strike the countries of

and

while the remaining 13 percent just dissipates off shore without making landfall.

Climate change

After a series of devastating cyclones in 2018 North Indian Ocean cyclone season, 2018, rising number of cyclones in the Arabian Sea in 2019 North Indian Ocean cyclone season, 2019 and a rising trend of rapid intensification in 2020 North Indian Ocean cyclone season, 2020 and 2021 North Indian Ocean cyclone season, 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 Ministry of Earth Sciences, 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

Before 1890

1890s

1900s

1910s

1920s

1930s

1940s

1950s

1960s

1970s

1980s

1990s

2000s

2010s

2020s

Records

*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

. The maximum recorded wind speed was with minimum barometric pressure of . *The costliest tropical cyclone was Cyclone Amphan, which struck the West Bengal coast on May 20, 2020. The recorded damage cost was 13.2 billion, beating the previous record of Cyclone Nargis which was 12 billion. *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

). *The smallest tropical cyclone was Cyclone Ogni, which struck the

coast on October 30, 2008. The cyclone's diameter was only . *The largest tropical cyclone was the 1999 Odisha Cyclone, which struck the coast of

. *The wettest tropical cyclone was 2004 North Indian Ocean cyclone season#Severe Cyclonic Storm ARB 01, Severe Cyclonic Storm ARB 01 (2004). One of the weather stations recorded between May 57, 2004.

Notes

References

{{DEFAULTSORT:North Indian Ocean Cyclone Seasons North Indian Ocean cyclone seasons,