Tropical cyclones

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Tropical Cyclones

Tropical cyclones, known as hurricanes in the Atlantic and eastern and central Pacific, typhoons in the western North Pacific, and simply tropical cyclones elsewhere, are some of the most potentially destructive storms of our atmosphere. They are the strongest of sequence of rotating tropical stroms of progressively increasing wind strength. A tropical depression is the weakest, and defined as a low pressure system with a closed surface wind circulation with speeds less than 27 konts (50kph). With mean wind speeds above 27 konts (50kph) the system becomes known as a tropical cyclone. A tropical storm is a cyclone with mean wind speed between 34 konts (63kph) and 63 knots (117kph)whilst the hurricane or typhoon is a tropical cyclone with speeds of 64 konts and above (118kph).

In recent years hurricanes such as Gloria (1985), Hugo (1989) and Andrew (1992) have caused tremendous damage along the eastern seaboard of the United States but with relatively little loss of life. However, storms of similar intensivity such as david (1979) and Joan (1988), caused major loss of life in the Dominican respublic and Nicaragua. This differential impact on developed and developing countries in a clear reflection of variations in vulnerability.

Although storms are most obviously associated with high wind speed and even tornadoes the main cause of damage and loss of life is actually due to the heavy rainfall, flooding and, in coastal areas, the tidal surgues which often accompany the storms. The storm surge is the rapid rise of the sea surface which occurs as a storm approaches the coastline. It is caused by the drop in atmospheric pressure- a 100 mb fall in surface pressure results on sea-surface rise of about one metre- the piling up of water along the coast generated by strong anshore winds, and the decreasing water depth encountered by storm as it nears the coast. Surges can be considerable.

Major cyclones are not new. They

are a characteristic feature of certain areas of the tropics where suitable conditions of high seasurface temperature, high atmospheric humidity and atmospheric wind structure allow their development (Fig. 1). Most cyclones form during the period late sping through to late autumn for each hemisphere. Not all formation areas are equally productive in spawning storms. The western North Pacific produces the greatest number of cyclones,with about 30 per cent of the global total. The Indian ocean is also important, with the Atlantic and East Pacific storms being slighty less frequent. Interestingly, no storms develop in the tropical South Atlantic nor the eastern South Pacific where surface water temperatures are usually too cool. Unfortunately we still do not have a clear idea of how cyclone frequencies vary for each are, as they mostly accur over tropical oceans. Historically, surface observations were dependment upon island locations or shipping routes. In the area of Atlantic Ocean an Caribbeanafected by hurricanes there are many possible observing sites. However over the East Pacific region especially, the number of sites is small. Many storms could develop, mature and decay without being observed by a surface station. It is only with the development of geostationary satellites that it has become possible to produce a more accurate climatology of the frequency of tropical storms in each of these areas. A consequence of the improved observations is that the number of eastern Pacific storms is greater than had previously been believed but within the Atlantic region the number of storms has not changed.

This does not mean the number of storms in any years or decade is constant. The long- term average (1885-1990) for tropical storms and hurricanes in the Atlantic region is about 8 per year.

Research in the United States has indicated a strong relationship between tropical cyclone frequency and intensity and drought in the western Sahel. It has been shown that above- average rainfall during the previous year along the Gulf of Guinea region of West Africa, in combination with above- average rainfall in the western Sahel during June and July, is linked to hurricane- generated destruction along the east coast of the United states occurring after August 1 st. Damage is 10-20 times greater is such years as compared with those years when pre-August 1 st. precipitation for these regions is below average.

Although tropical storms affect many parts of world, knowledge of their impact varies. In developed countries, such as the united states, Japan and Australia, loss of life and estimated direct and indirect costs of a storms are soon known. In developing countries, however, the national infrastructure is such that similar information either cannot be produced or takes takes much longer to be determined. Many damaged properties will not have been insured and even the number of people living in an area may nor be known precisely. The amount of the damage for any given storm will be influenced by its severity and by the number of people living along its track. Equally important are the living standarts of the affected area- there will be much greater potential for financial loss if a storm passes over Miami compared to that for one which crosses Havana. The level of precautions and anticipatory measures adopted are also important in determining the level of losses.

With data from geostationary satellites, the formative stages of major hurricanes can be followed. Improved computer- modeling techniques of hurricane development have allowed more precision as to the areas likely to be affected. With improved communications, such forecasts can be convertes into broadcast warnings and the affected population evacuated.