Tropical Cyclone

Introduction

A tropical cyclone can be fearsome. The high winds, violent thunderstorms, pounding rains, and wind-blown surges of water onto coastal areas that are associated with such a storm system can be destructive and deadly.

These punches are packed by a storm system that is visually similar to a hurricane (which is one type of tropical cyclone) in its spiral shape and, in some cases, by the presence of a zone of clear weather at the center called the eye. The system harbors thunderstorms, strong winds that blow from regions of higher air pressure farther out from the eye to the area of low pressure within the eye, and intense rainfall that can cause flooding when the storm moves ashore.

The term “tropical” refers to the typical location of the storm; they almost always develop in tropical equatorial regions, where rising warm air feeds the storm. The term “cyclone” describes the slowly revolving nature of the storm, which gives a storm the typical pinwheel pattern when viewed from high in the atmosphere.

Although some studies have proposed that climate change observed since the 1950s, specifically the increased warming of the atmosphere, is influencing the frequency

and severity of tropical cyclones, the scientific community remains divided over this suggestion.

Historical Background and Scientific Foundations

Tropical cyclones have likely been a fact of life ever since the modern atmosphere formed approximately 500 million years ago. Recorded descriptions of cyclones in the equatorial region of the Atlantic Ocean and Caribbean by European explorers date back to at least the seventeenth century.

Even then, the devastating power of tropical cyclones was recognized. Coastal areas bear the brunt of the cyclone's punch. Because a cyclone forms over the ocean, it still is fully potent when it makes landfall. Inland regions can be spared a storm's full fury, since the energy of the storm drops as it moves inland and away from the ocean. However, torrential rainfall can still occur inland, which can result in flooding.

At the coast, another danger of tropical cyclones is the wind-driven surge of water that can batter the land. A recent example is the devastating storm surge caused by Hurricane Katrina, which struck the U.S. Gulf Coast in August of 2005. This storm surge was huge; in Gulf-port, Mississippi, calculations revealed that at the shoreline the waves were 28 ft (8.5 m) high.

Similar to the classification system to rate the severity of hurricanes, a tropical cyclone is considered to be the most severe of three classes of tropical storms. According to the classification, a cyclone has sustained winds of at least 74 mph (119 km/h) per hour. Less severe is a tropical depression, with sustained winds of 38 mph (61 km/h), and a tropical storm, with sustained winds ranging from 39 to 73 mph (63 to 117 km/h).

A typical feature of a tropical cyclone is a central, funnel-shaped region. At the base of the funnel is an area of intense low pressure. Indeed, some of the lowest air pressures ever recorded have been in this part of a cyclone. The remainder of the funnel that tightly spirals upward results from the upward movement of warm moist air. At higher altitudes, the water vapor in the air condenses to form clouds that fan outward from the funnel. The interior of a cyclone is warmer than the surrounding air; when a cyclone passes over a region, temperatures climb.

Even though the airflow in the wall of the funnel is upward, the flow of the air in the center of the funnel can be downward. A strong downward flow can cause the base of the funnel to clear, producing a hurricane-like eye. Depending on the size of the cyclone, an eye can vary from several miles to over 200 mi (322 km) in diameter. As well, airflow moves downward in the areas farther away from the central funnel. Over the ocean, this cyclical pattern of the upward movement of warm air and downward flow of cooler air feeds the storm, and it can cause the storm to grow in size and intensity. Once over land, the water-derived heat energy that drives the storm is no longer there, and the cyclone will rapidly become less intense and fade within several days.

Another characteristic feature of tropical cyclones is banding, or the occurrence of periodic regions of rain and thunderstorms that are arranged in a spiral outward from the eye. Heavy rains and high winds, which are most intense nearer to the storm center, are separated by periods of calm weather that occur as the cyclone passes over a given area of land. Tornados can even form in the zones of rainfall.

The rotation of air within a cyclone differs with height. At the surface, the horizontal airflow is determined by the Coriolis effect (which results from the rotation of Earth). If the storm is in the Northern Hemisphere, the airflow will be counterclockwise, and is clockwise if the storm is in the Southern Hemisphere. At higher altitudes, however, the cyclonic flow weakens and can even reverse, so that the airflow at the top of a cyclone is opposite the surface pattern.

A cyclone can release a tremendous amount of energy. Scientists have calculated that the energy produced by a fully formed cyclone is equivalent to the detonation of a 10-megaton bomb every 20 minutes. To put this into perspective, the atomic bomb that obliterated the Japanese city of Hiroshima during World War II was approximately 0.01 megatons. So, a cyclone's energy can be equivalent to the explosion of more than 100 Hiroshima-type atomic bombs every 30 minutes.

Tropical cyclone formation tends to be most active in late summer, when water temperatures are highest. In the Atlantic Ocean north of the equator, tropical cyclones form most frequently from the beginning of June through the end of November; this period is known as the hurricane season. In contrast, in the Southern Hemisphere, the cyclone season in the Indian Ocean runs from April through December.

WORDS TO KNOW

CORIOLIS EFFECT: A pseudoforce describing the deflection of winds due to the rotation of Earth, which produce a clockwise or counterclockwise rotation of storm systems in the Southern and Northern Hemispheres, respectively.

INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE (IPCC): Panel of scientists established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) in 1988 to assess the science, technology, and socioeconomic information needed to understand the risk of human-induced climate change.

LANDFALL: When the center of a storm system formed over the ocean (e.g., hurricane) reaches land, the storm is said to have made landfall. Some hurricanes never make landfall but exhaust themselves over the ocean. Those that do make landfall quickly lose power and dissipate, since they draw their energy from warm ocean surface waters.

STORM SURGE: Local, temporary rise in sea level (above what would be expected due to tidal variation alone) as the result of winds and low pressures associated with a large storm system. Storm surges can cause coastal flooding, if severe.

WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. Although humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.

IN CONTEXT: FUTURE OF TROPICAL CYCLONES

“It is very likely that hot extremes, heat waves, and heavy precipitation events will continue to become more frequent.”

“Based on a range of models, it is likely that future tropical cyclones (typhoons and hurricanes) will become more intense, with larger peak wind speeds and more heavy precipitation associated with ongoing increases of tropical SSTs. There is less confidence in projections of a global decrease in numbers of tropical cyclones. The apparent increase in the proportion of very intense storms since 1970 in some regions is much larger than simulated by current models for that period.”

“Extra-tropical storm tracks are projected to move pole-ward, with consequent changes in wind, precipitation, and temperature patterns, continuing the broad pattern of observed trends over the last half-century.”

Statement of the Intergovernmental Panel on Climate Change (IPCC) as formally approved at the 10th Session of Working Group I of the IPCC in Paris, France, during February 2007.

SOURCE: Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Despite the devastation of a tropical cyclone, the tremendous rains associated with a storm can be beneficial to a region that is suffering through a drought. Additionally, a cyclone contributes to atmospheric circulation by moving warm tropical air to cooler latitudes north and south of the equator. This mass movement of air assists in maintaining a global atmospheric temperature that is fairly stable.

Impacts and Issues

During the past two centuries, tropical cyclones have directly killed at least two million people. Those who survive a cyclone are not necessarily free of danger. A cyclone can destroy infrastructure such as electricity, transportation, and running water, increasing the chances of the development of infectious diseases such as malaria, which can claim many more lives.

This devastation can occur even in developed countries. Some relief workers who aided victims of Hurricane Katrina in 2005 compared the conditions they encountered in New Orleans, Louisiana, as rivaling the most deplorable conditions they had encountered in the developing world.

Since the formation of cyclones depends on the ocean temperature, the documented warming of the oceans has been a suggested contributor to the increasing frequency and severity of tropical cyclones. Some scientists have gone even further, suggesting that global warming influences cyclone occurrence. In this scenario, the warming ocean provides more heat energy to fuel cyclone activity.

But, this view is not shared by the majority of scientists. In 2006, for example, the World Meteorological Organization (WMO) issued a statement that climate change could not be linked to any one cyclone event in the prior two years, during which cyclones were more severe and did more damage than had ever been recorded before. According to the WMO, data linking climate change and cyclones is still inconclusive.

However, two studies published in 2005 in the journals Nature and Science, which analyzed data on cyclones over the past half century, documented the increasing frequency, severity, and duration of cyclones in response to increasing ocean temperatures. Further-more, a 2007 report by the United Nations' Intergovernmental Panel on Climate Change (IPCC) described how this increased temperature has been greater than that predicted from a normal increase in global temperature (which has happened several times during Earth's existence). These studies imply that human activities do play a role in cyclone formation and ferocity; they highlight that more research is necessary before a firm conclusion can be made.

See Also El Ninño and La Ninña; Extreme Weather; Global Warming; Hurricanes; Sea Temperatures and Storm Intensity.

BIBLIOGRAPHY

Books

Bigg, Grant R. The Oceans and Climate. Cambridge, U.K.: Cambridge University Press, 2004.

Emanuel, Kerry. What We Know About Climate Change. Boston: MIT Press, 2007.

Periodicals

Emanuel, K. “Increasing Destructiveness of Tropical Cyclones over the Past 30 Years.” Nature 436 (2005): 686–688.

Shultz, J. M., J. Russell, and Z. Espinel. “Epidemiology of Tropical Cyclones: The Dynamics of Disaster, Disease, and Development.” Epidemiologic Reviews 27 (2005): 12–14.

Webster, P. J., G. J. Holland, J. A. Curry, and H.-R. Chang. “Changes in Tropical Cyclone Number, Duration, and Intensity in a Warming Environment.” Science 309 (2005): 1844–1846.

Web Sites

“Tropical Cyclone Structure.” National Weather Service, August 29, 2007. <http://www.srh.noaa.gov/jetstream/tropics/tc_structure.htm> (accessed November 10, 2007).

Brian D. Hoyle