TROPICAL CYCLONES

# TROPICAL CYCLONE

What is tropical cyclone?

A tropical cyclone is a rotating, organized system of clouds and thunderstorms that originates over tropical or subtropical waters and has a closed low-level circulation. Tropical cyclones rotate counterclockwise in the Northern Hemisphere. They are classified as follows:

• Tropical Depression: A tropical cyclone with maximum sustained winds of 38 mph (33 knots) or less.
• Tropical Storm: A tropical cyclone with maximum sustained winds of 39 to 73 mph (34 to 63 knots).
• Hurricane: A tropical cyclone with maximum sustained winds of 74 mph (64 knots) or higher. In the western North Pacific, hurricanes are called typhoons; similar storms in the Indian Ocean and South Pacific Ocean are called cyclones.
• Major Hurricane: A tropical cyclone with maximum sustained winds of 111 mph (96 knots) or higher, corresponding to a Category 3, 4 or 5 on the Saffir-Simpson Hurricane Wind Scale.

What causes Cyclones?

• The main source of energy for tropical cyclones is the warm oceans in the tropical regions.
• To initiate a tropical cyclone the sea-surface temperature generally needs to be above 26.5°C. However, existing cyclones often persist as they move over cooler waters.
• The development of a tropical cyclone also relies on favourable broad-scale wind regimes and can persist for several days with many following quite erratic paths. They lose their source of energy when they move over land or colder oceans causing them to dissipate.
• The point at which they cross land is called LANDFALL region.Weakening may also occur if the cyclone moves into an unfavourable wind regime which disrupts the structure of the system.
• Sometimes a decaying tropical cyclone may interact with a weather system in higher latitudes to cause impacts far from the tropics.

CHARACTERISTICS OF TROPICAL CYCLONE

The most common features are destructive winds and heavy rainfall that can lead to flooding. Storm surge, or coastal inundation by seawater, is a lesser known phenomenon but can be the most dangerous element of a cyclone. Though rare in Australia, tornadoes have been reported during cyclone events.

Severe wind

•  wind gusts in excess of 90-360 km/h can be expected around their centre in eyewall. 
• strongest winds are near the eye, damaging winds can extend hundreds of kilometres from the centre.
• The eye can have quite calm winds and cloud-free skies, but this lull is temporary and is followed by destructive winds from another direction.
• This is because, from above, the winds spiral around the eye in a clockwise direction (in the Southern Hemisphere). The effect of this on the ground is that winds on opposite sides of the eye blow in different directions.
• Wind damage is mostly caused by the maximum gusts in the cyclone. For this reason, the well-known tropical cyclone severity categories used by the Bureau of Meteorology to communicate warnings are based on maximum gust strengths.

Heavy rainfall

•  heavy rainfall over extensive areas.
• Rain can create severe impacts by causing floods and landslides.

Storm surge

• Storm surge is a rising of the sea as a result of wind and atmospheric pressure changes associated with a storm.
• It is caused by a combination of strong winds driving water onshore and the lower atmospheric pressure in a tropical cyclone. In the southern hemisphere the onshore winds occur to the left of the tropical cyclone's path.
• In Australia, this is the east side on the north west and north coasts and the south side on the east coast.
• The largest surge usually extends between 30 and 60 kilometres from the crossing point of the tropical cyclone centre, or eye. Its influence also depends on the local topography of the seafloor and the angle at which the cyclone crosses the coast.
• If the surge occurs at the same time as a high astronomical tide the area inundated can be extensive, particularly along low-lying coastlines.

DISTRUBUTION OVER THE GLOBE

•  north Atlantic Ocean
• the eastern parts of the northern Pacific Ocean.
• western parts of the northern Pacific Ocean.
• the southwestern Pacific.
• the southwestern  Indian Oceans.
•  southeastern Indian Oceans,.
•  northern Indian Ocean (Arabian Sea and Bay of Bengal).

• The six tropical cyclone Regional Specialized Meteorological Centres (RSMCs) together with six Tropical Cyclone Warning Centres (TCWCs) having regional responsibility, provide advisories and bulletins with up-to-date first level basic meteorological information on all tropical cyclones, hurricanes, typhoons everywhere in the world.
  
  
  
• The western Pacific is the most active and the north Indian the least active. An average of 86 tropical cyclones of tropical storm intensity form annually worldwide, with 47 reaching hurricane/typhoon strength, and 20 becoming intense tropical cyclones .

INDIAN SCENARIO REGARDING TROPICAL CYCLONE

North Indian Ocean

• This basin is divided into two areas: the Bay of Bengal and the Arabian Sea, with the Bay of Bengal dominating (5 to 6 times more activity). Still, this basin is the most inactive worldwide, with only 4 to 6 storms per year.
• This basin's season has a double peak: one in April and May, before the onset of the monsoon, and another in October and November, just after
•  Although it is an inactive basin, the deadliest tropical cyclones in the world have formed here, including the 1970 Bhola cyclone, which killed 500,000 people.
• Nations affected include India,Bangladesh, Sri Lanka, Thailand, Myanmar, and Pakistan. Rarely do tropical cyclones that form in this basin affect the Arabian Peninsula orSomalia; however, Cyclone Gonu caused heavy damage in Oman on the peninsula in 2007.

South-West Indian Ocean

• The South-West Indian Ocean is located within the Southern Hemisphere between the Africa's east coast and 90°E and is primarily monitored by the Meteo France's La Reunion RSMC, while the Mauritian, Australian Indonesian, and Madagascan weather services also monitor parts of it.
•  Until the start of the 1985-86 tropical cyclone season the basin only extended to 80°E, with the 10 degrees between 80 and 90E considered to be a part of the Australian region.
•  On average about 9 cyclones per develop into tropical storms, while 5 of those go on to become tropical cyclones that are equivalent to a hurricane or a typhoon.
•  The tropical cyclones that form in this area can affect some of the various Indian Ocean island nations and or various countries along Africa's east coast.

  Cyclonic storm Hudhud

Cyclonic storm Hudhud is building over the Bay of Bengal and is likely to bring torrential rains and strong wind to the region over the next few days. The name Hudhud has been given by Oman, and it is derived from the name of an Afro-Eurasian bird.

       Preparations for Hudhud

     The cyclone is scheduled to hit the Odisha coast by October 12. It is also expected to hit the Andhra             Pradesh coastline. However, it will not directly hitWest Bengal, though the state is expected to feel the         impacts of the cyclone. The states in the Bay of Bengal region have issued warnings asking fishermen not to venture deep into the sea.

The intensity of Hudhud is expected to be on the lines of that of cyclone Phailin which hit Odisha last year.

Evolution of Hudhud

Hudhud first appeared as a low pressure are over the South Myanmar coast adjoining Andaman Sea. This evolved in to a depression around 250 km east of Port Blair on 6 October. This depression is expected to intensify into a cyclonic circulation by 8 October. The immediate effect of the cyclone formation will obviously be felt on the Andaman & Nicobar Island where heavy rains and minor damage to loose and unsecured structures have been predicted. After crossing the Andamans, the cyclonic circulation is expected to south-west into the Bay of Bengal and take shape of a cyclonic storm.

The chief reason behind it is that Andhra Pradesh falls in the 12,000-km coastline within South Asia, which is vulnerable to cyclones. Over 95 per cent of the major cyclonic disasters experienced in the world have taken place in the region. And the incidence of cyclones is more in the Bay of Bengal when compared to the Arabian Sea. While the recurring disasters of different sorts have had a severe impact on the State’s economy and policies, the dilly dallying of government machinery played havoc with payment of compensation due to farmers. The glaring example of this can be attributed to the latest cyclone Phailin and the related compensation which is unpaid. The government officials could, with great difficulty, reach the cyclone-hit districts several months after its occurrence. And the farmers are still waiting for the damages incurred in the Phailin cyclone that hit crops in 2013.

JAPAN - Tropical cyclone VONGFONG

• After crossing Okinawa, on 12 October VONGFONG turned north-east and made landfall in southern Kagoshima prefecture on Kyushu island, overnight 12-13 October; then it moved over Shikoku Island in the morning of 13 October. Strong winds, heavy rain and storm surge affected parts of south-western Japan.

• As of 13 October, media reported one person missing and 68 injured (23 of them in Okinawa).

• In the next 24 h it is forecast to move north-east over Honshu island, weakening further. As of 13 October (JMA), there are warnings in effect for heavy rain, floods, storms and high waves for parts of Okinawa, Kyushu, Shikoku and Honshu.