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EFFECTS OF CYCLONES ON FISHERIES AND AQUACULTURE

Milind B. Katare1, Harshavardhan D. Joshi2, Niti Sharma2, Madhuri Phathak2, Archana Chirome2 and Prashant Gangwar2


1College of fisheries, Kankanadi, Mangalore, Karnataka, INDIA

2Central Institute of Fisheries Education, Seven Bunglows, Versova, Andheri (w.),

Mumbai-400061, Maharashtra, INDIA


INTRODUCTION

Cyclonic-force winds may be encountered all over the globe, but it is only above the warm seas of the tropics that a ripple of instability in the air can become a genuine cyclone, the deepest of all low-pressure weather systems.(Natural Disasters, 1995)

What is a Cyclone?

Cyclones are huge revolving storms caused by winds blowing around a central area of low atmospheric pressure. In the northern hemisphere, cyclones are called hurricanes or typhoons and their winds blow in an anti-clockwise circle.In the southern hemisphere, these tropical storms are known as cyclones, whose winds blow in a clockwise circle.


How do Cyclones occur?

Cyclones develop over warm seas near the Equator. Air heated by the sun rises very swiftly, which creates areas of very low pressure. As the warm air rises, it becomes loaded with moisture which condenses into massive thunderclouds. Cool air rushes in to fill the void that is left, but because of the constant turning of the Earth on its axis, the air is bent inwards and then spirals upwards with great force. The swirling winds rotate faster and faster, forming a huge circle which can be up to 2,000 km across. At the centre of the storm is a calm, cloudless area called the eye, where there is no rain, and the winds are fairly light.Satellite view over a hurricane, with the eye at the center As the cyclone builds up it begins to move. It is sustained by a steady flow of warm, moist air. The strongest winds and heaviest rains are found in the towering clouds which merge into a wall about 20-30 km from the storm's centre. Winds around the eye can reach speeds of up to 200 km/h, and a fully developed cyclone pumps out about two million tonnes of air per second. This results in more rain being released in a day than falls in a year in a city like London.


When and where do Cyclones occur?

Cyclones begin in tropical regions, such as northern Australia, South-East Asia and many Pacific islands. They sometimes drift into the temperate coastal areas,threatening more heavily populated regions to the South. Northern Australia has about four or five tropical cyclones every year during the summertime wet season. For a cyclone to develop, the sea surface must have a temperature of at least 26°C.


Why do Cyclones occur?

When warm air rises from the seas and condenses into clouds, massive amounts of heat are released. The result of this mixture of heat and moisture is often a collection of thunderstorms, from which a tropical storm can develop.The trigger for most Atlantic hurricanes is an easterly wave, a band of low pressure moving westwards, which may have begun as an African thunderstorm. Vigorous thunderstorms and high winds combine to create a cluster of thunderstorms which can become the seedling for a tropical storm.Typhoons in the Far East and Cyclones in the Indian Ocean often develop from a thunderstorm in the equatorial trough. During the hurricane season,the Coriolis effect of the Earth's rotation starts the winds in the thunderstorm spinning in a circular motion.

Cyclone Danger

Cyclones create several dangers for people living around tropical areas. The most destructive force of a cyclone comes from the fierce winds. These winds are strong enough to easily topple fences, sheds, trees, power poles and caravans, while hurling helpless people through the air. Many people are killed when the cyclone's winds cause buildings to collapse and houses to completely blow away.


A cyclone typically churns up the sea, causing giant waves and surges of water known as storm surges. The water of a storm surge rushes inland with deadly power, flooding low-lying coastal areas. The rains from cyclones are also heavy enough to cause serious flooding, especially along river areas.



Storm surge damage

  • Storm Surge - simply water that is pushed toward the shore by the force of the winds swirling around the storm.

  • Advancing surge combines with normal tides to create the hurricane storm tide - can increase the average water level 15 feet or more.



Long after a cyclone has passed, road and rail transport can still be blocked by floodwaters. Safe lighting of homes and proper refrigeration of food may be impossible because of failing power supplies. Water often becomes contaminated from dead animals or rotting food, and people are threatened with diseases like gastroenteritis.



Tropical cyclones are classified into three main groups, based on intensity and speed of wind.

  • Tropical Depression = < 39 mph

  • Tropical Storm = 39 mph — 73 mph

  • Hurricane = > 74 mph

  • Major Hurricane = > 110 mph (Cat 3 or greater)



Tropical depression

A tropical depression is an organized system of clouds and thunderstorms with a defined, closed surface circulation and maximum sustained winds of less than 17 metres per second (33 kn) or 38 miles per hour (61 km/h). It has no eye and does not typically have the organization or the spiral shape of more powerful storms. However, it is already a low-pressure system, hence the name "depression".The practice of the Philippines is to name tropical depressions from their own naming convention when the depressions are within the Philippines' area of responsibility.

Tropical storm

A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 17 metres per second (33 kn) (39 miles per hour (63 km/h)) and 32 metres per second (62 kn) (73 miles per hour (117 km/h)). At this point, the distinctive cyclonic shape starts to develop, although an eye is not usually present. Government weather services, other than the Philippines, first assign names to systems that reach this intensity (thus the term named storm)

Hurricane or typhoon

A hurricane or typhoon (sometimes simply referred to as a tropical cyclone, as opposed to a depression or storm) is a system with sustained winds of at least 33 metres per second (64 kn) or 74 miles per hour (119 km/h). A cyclone of this intensity tends to develop an eye, an area of relative calm (and lowest atmospheric pressure) at the center of circulation. The eye is often visible in satellite images as a small, circular, cloud-free spot. Surrounding the eye is the eyewall, an area about 16 kilometres (9.9 mi) to 80 kilometres (50 mi) wide in which the strongest thunderstorms and winds circulate around the storm's center. Maximum sustained winds in the strongest tropical cyclones have been estimated at about 85 metres per second (165 kn) or 195 miles per hour (314 km/h)


FORECAST PROCESS


  • HURRICANE WATCH - A hurricane watch is issued for a specified coastal area for which a hurricane or a hurricane-related hazard is a possible threat within 36 hours.


  • HURRICANE WARNING - A hurricane warning is issued when a hurricane with sustained winds of 74 mph or higher is expected in a specified coastal area in 24 hours or less. A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continues, even though the winds may have subsided below hurricane intensity.


IMPACT ON FISHERIES AND AQUACULTURE

  • Loss of habitat

  • Loss of juveniles and brood fishes

  • Loss of livelihoods of farmers and fishermen

  • Spread of diseases

  • Loss of capital assets

  • Loss of pens and cages

  • Loss of biodiversity

  • Tropical cyclones becoming more intense

  • Increased peak wind speeds

  • Higher mean and peak rainfall

  • Heavy rains create inland flooding

  • Damage to infrastructure



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