tropical cyclones

tropical cyclones Tropical cyclones are the single most damaging weather events anywhere in the world, with, on average, between 4000 and 5000 deaths attributable per tropical cyclone event. They devastate wide areas, sometimes with extreme loss of life and considerable damage to property. Winds can exceed 75 metres per second (m s⁻¹) (250 kilometres per hour); rain is torrential, perhaps yielding more than 250 mm in less than a day; and sea level is locally elevated by low atmospheric pressure and driving wind. These effects are commonly sustained for many hours, and the typical lifetime and track of such systems means that many communities may be affected within a few days by the same system.

The nature of the impact, split between loss of life and injury on the one hand and damage to property on the other, depends broadly on the community afflicted. The technological expertise, degree of preparedness, and ability to recover in countries such as Australia or the USA ensure that loss of life and injury are limited by effective warning, evacuation, and protection measures. On average fewer than a hundred deaths occur per tropical cyclone event in either of these countries. On the other hand, the cost of damage to property is frequently extreme. Hurricane ‘Hugo’ in September 1989 caused nearly US$10 billion damage in the USA and Caribbean, a large proportion of which was in the USA. In poorer countries, such as Bangladesh, it is loss of life which is more significant: during the Bangladesh cyclone of November 1970 more than 200 000 people lost their lives; in May 1990 a cyclone hit Andra Pradesh in India, killing 514 people, but directly affecting a further 6.5 million people in one way or another; hurricane ‘Hugo’ left 90 per cent of the population of Montserrat homeless. Death rates are commonly measured in thousands or tens of thousands per event.

Tropical cyclones are known by different names in different parts of the world (Fig. 1): hurricanes in the tropical North Atlantic and Caribbean; typhoons in the north-eastern Pacific; and simply as tropical cyclones in the south-western Pacific and the Indian Ocean. In some areas local names apply: for example, papagallos in the eastern North Pacific, baguios in the Philippines, and travados near Madagascar. The nature of tropical cyclones, however, is the same in all areas, and individuals are identified by name, alternating between female and male: Annie, Bob, etc. In a typical year the north-western Pacific area experiences the largest number, typically about 25 to 30 (Fig. 1), although the year-to-year variation is high, as in all areas of occurrence. Such variation has been linked to the occurrence of the ENSO (El Niño, Southern Oscillation) phenomenon. The north-eastern and south-western Pacific (Australian) areas each experience between ten and fifteen a year; the Caribbean commonly suffers less than ten. They also occur in the Indian Ocean: in the south-west, near Madagascar and southern Africa; and in the north, west of the Indian subcontinent and in the Bay of Bengal. In all Indian Ocean areas the annual incidence is generally much less than ten. Tropical cyclones also exhibit a strong seasonality; most of them occur during the late summer months of the respective hemisphere: July to October in the northern hemisphere; January to March in the southern.

There has until recently been comparatively little understanding of they origins, occurrence, and behaviour of tropical cyclones. Significant advances in our knowledge of them, and improvements in forecasting, have been made only since the deployment of weather satellites in the 1960s. Tropical cyclone systems develop over parts of the tropical oceans, drawing on heat and moisture from the ocean surface. Observation is difficult in these areas without satellite surveillance. Their effects are most noticed, however, once they reach land, by which time they may usually have reached their maximum severity. This fact emphasizes the need for constant satellite monitoring of their birth, development, and track.

Tropical cyclones form from pre-existing disturbances. Their life history is particularly well known for the tropical north Atlantic, where much research effort has been concentrated. In this region, many develop from so-called ‘easterly waves’, some of which also derive from pre-existing squall-lines over West Africa. In any part of the tropics subject to their occurrence, any pre-existing organized mass of cloud yielding precipitation is carefully monitored at the appropriate time of year. A tropical cyclone is born when the sustained wind speed exceeds hurricane force (33 m s⁻¹) and there is a definite cyclonic circulation about a central core, with a central pressure of less than 990 millibars or hectopascals (hPa). At lesser sustained speeds such organized systems are termed ‘tropical storms’ or ‘tropical depressions’. Central pressures can reach well below 950 mbar, similar to some of worst temperate depressions, though the pressure gradient is confined within a much smaller radius: rarely greater than about 300 km. Over the open ocean such low atmospheric pressures elevate the ocean surface to perhaps four metres above normal, and the wind will easily whip up waves well over 10 m high. This means that a tropical cyclone reaching land will, as well as bringing with it destructive winds and flood-inducing rains, commonly produce serious marine flooding in low-lying coastal areas.

The seasonal peaks in their occurrence, together with the location of the major areas of development, provide clues to the origin and development of cyclones as weather phenomena. The incidence of tropical cyclones is high when ocean surface temperatures exceed 27 °C: they draw their energy from the heat of the ocean surface. This same ocean, of course, ensures a continual supply of moisture. An active cyclonic (rotating) circulation can, however, develop only where there is sufficient rotational energy derived from the Earth's surface. Tropical cyclones are unable to form within about five degrees latitude of the Equator, where the Coriolis effect is very small: their main source areas lie between about 5° and 15° latitude. In either hemisphere these features possess an inward spiral of strong winds at the surface (counter-clockwise in the northern hemisphere; clockwise in the southern hemisphere), introducing heat and moisture to the system, rising within the cyclone to feed an outward spiral, linked to an upper anticyclone aloft. Both horizontal and vertical aspects of the circulation are important to its maintenance as an active weather feature. Should there be a pronounced pre-existing flow in the region, its ability to form will, however, be severely restricted. Such circumstances commonly prevail near the Indian subcontinent at the height of the monsoon, so that, for example, very few cyclones occur in the Bay of Bengal during July and August.

Both heat and moisture are important to the sustenance of a tropical cyclone. Once taken into the system, moisture plays an important part in maintaining the cyclone's continued development. The hot and very moist air fed in near the surface, once uplifted, is cooled, forcing condensation of excess moisture and adding further to cloud volume. In the process, massive quantities of latent heat are released, adding still further to the heat input to the system as a whole, reinforcing the upper outflow by expanding the air, and drawing in more air at the surface by enhancing uplift, so that the system is well on the way to becoming self-perpetuating. This also ensures that tropical cyclones, in contrast to other organized tropical disturbances, possess a ‘warm core’, with oppressively high temperatures and humidities, near the centre.

The strongest winds and the heaviest rain occur close to this central zone. Strong winds and heavy rainfall are also associated with organized spirals of cloud, frequently containing clusters of severe thunderstorms, often with localized tornado development. The spiral bands appear to migrate outwards through the system, decreasing in intensity and spreading horizontally as they move, commonly ending up well away from the main system as pre-cyclone squall-lines. The source for the spirals appears to be near the centre of the system, near to the ‘eye’. The eye is one of the main tell-tale signs that a system has achieved cyclone intensity. It is an almost totally cloud-free area at the centre of the vortex, with near-calm winds and brilliant sunshine. The eye may be from one to forty kilometres in diameter, and is surrounded by a wall of towering cumulonimbus clouds. Although tropical cyclones are dangerous simply because of the associated severe weather, the occurrence of the eye itself, ironically, has often compounded cyclone impact. The appearance of bright sunshine and the arrival of calm conditions may sometimes be seen as the end of the storm rather than a mere lull. Anyone venturing outside in the eye would, however, do well to consider that once it has passed, torrential rain and violent winds will return—from the opposite direction. Any debris loosened during the arrival of the storm will then be carried back in the opposite direction by winds on the other side of the system.

Tropical cyclones rarely outlast a week, although they may have travelled thousands of kilometres during that time (Fig. 1), initially moving broadly with the prevailing westward flow that occurs in the tropics, but subsequently gradually ‘recurving’ polewards around the subtropical anticyclones, perhaps into temperate latitudes. Subsequently their track might take them back eastwards across the temperate ocean, now as severe examples of temperate depressions, towards, for example, Europe. A former Caribbean hurricane (‘Charlie’) wrought havoc in western Britain in August 1986, though it is important to stress that it was not a hurricane by this stage: it had lost all the characteristics by which we define such a feature. The tracks of many individual tropical cyclones are, however, notoriously haphazard and unpredictable. They may remain nearly stationary for some days, move round in a circle, and may often reverse their former track. Once they make landfall, however, they generally begin to lose their identity, since the source of moisture over the ocean surface is removed. They can nevertheless still take torrential rainfall many hundreds of kilometres inland. Tropical cyclone ‘Bobby’ carried copious amounts of rainfall south-eastwards across the desert areas of Western Australia in February 1995. A cyclone can also track along a coastline, drawing on the adjacent ocean. The eastern seaboards of North America, Australia, and Asia can be particularly vulnerable if this occurs. In June 1972, hurricane ‘Agnes’ tracked up the eastern seaboard of the USA and retained its tropical identity as far north as the New England states, carrying torrential rain and strong winds over many populated parts of the country as it did so.

Graham Sumner

Bibliography

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Simpson, R. H. and and Riehl, H. (1981) The hurricane and its impact. Basil Blackwell, Oxford.