Sea Temperatures and Storm Intensity
Sea Temperatures and Storm Intensity
Temperature is a powerful driving force of storm activity. On land, for example, rapidly rising air on a summer day can trigger the development of thunderstorms and tornadoes. Similarly, in tropical regions of the globe, warm temperatures over the surface of oceans can provide the energy for the formation of hurricanes, typhoons, and cyclones, and can influence the development of rain-laden monsoon winds.
Since the 1950s, the world's oceans have absorbed almost 20 times as much energy as the atmosphere. The result has been the warming of ocean waters, not just within the upper sunlit 100 ft (30 m) of the surface but at depths of up to 1,500 ft (460 m). One consequence of this warming has been the expansion of the water. Since the ocean basins have a defined area, the extra volume has to go somewhere. The result has been increasing coastal sea levels. Combined with storm-related wind and wave action, the raised sea level can
produce a more powerfully destructive storm surge than would have occurred only a half-century before.
Another consequence of increasing sea temperatures is the increased temperature and humidity of the air over the water. The increased number, duration, and severity of tropical storms during the twentieth century may be related to increasing sea temperatures.
Historical Background and Scientific Foundations
Ocean storms that form over the Atlantic Ocean and eastern Pacific Ocean are called hurricanes. Storms forming over the northwestern Pacific are known as typhoons, while those forming over the Indian Ocean are dubbed cyclones. All are characterized by high winds and heavy rainfall and, if they come ashore, the potential for massive destruction and casualties.
These storms have been a part of human history for centuries. The earliest known report of a hurricane was made by Italian explorer Christopher Columbus (1451– 1506) during a voyage in 1495. Other notable hurricanes are the Great Hurricane of 1780, which killed more than 22,000 people in the Caribbean; the Galveston (Texas) hurricane of 1900, which killed up to 12,000 people; the Atlantic “Nor'easter” during Halloween of 1991 that was the basis of the film The Perfect Storm; Hurricane Mitch, which in 1998 killed an estimated 11,000 people throughout Central America; and Hurricane Katrina along the Gulf Coast in 2005.
The latter developed into a category five hurricane— the most severe category, characterized according to the Saffir-Simpson scale as a hurricane with winds exceeding 155 mph (250 km/h). The sixth-strongest Atlantic hurricane ever recorded as of 2007, it devastated the Gulf Coast of the United States. Downtown New Orleans, Louisiana, suffered extensive wind damage and 80% of the city was flooded when its levees failed to cope with the storm surge. In 2007, New Orleans had not fully recovered from the destruction, and more than 200,000 people who evacuated the city had yet to return.
Hurricane Katrina was part of a deadly 2005 hurricane season in the Atlantic. During that time, there were 11 tropical storms, of which five were hurricanes and two reached category five.
In the northern hemisphere, ocean storms occur most often near the equator during September and October. In the southern hemisphere, severe ocean storms are less frequent, but tend to develop between December and March.
WORDS TO KNOW
BUOYS: Tethered or free-floating devices that bear navigational aids, instruments, and in some cases radio equipment for automatically collecting and reporting data on oceanic and atmospheric conditions. Buoys may float on or beneath the surface, depending on their purpose.
CLIMATE MODELING: Mathematical representation of climate processes. Computer programs are written that describe the structure of Earth's land, ocean, atmospheric, and biological systems and the laws of nature that govern the behavior of those systems. Detail and accuracy are limited by scientific understanding of the climate system and computer power. Climate modeling is essential to understanding paleoclimate, present-day climate, and future climate.
EYEWALL: Ring of severe storms surrounding the eye of a cyclonic storm (e.g., hurricane).
GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.
MONSOON: An annual shift in the direction of the prevailing wind that brings on a rainy season and affects large parts of Asia and Africa.
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.
TROPICAL STORM: Tropical storms generally form in the eastern portion of tropical oceans and track westward. Hurricanes, typhoons, and willy-willies all start out as weak low pressure areas that form over warm tropical waters (e.g., surface water temperature of at least 80°F [27°C]). Initially, winds and cloud formations over the warm tropical waters are minimal. Both intensify with time. Formation of tropical storms also requires a significant Coriolis effect to induce proper spin in the wind formation. As the storm begins to organize itself into a coherent pattern, it will experience increased activity and intensity.
The genesis of ocean storms is temperature, specifically differences in temperature between one area and another. Because warm air rises and colder air falls, differences in air pressure are created. The resulting movement of air creates wind. If this pressure change is spread over a large area, a circular pattern of wind can develop, with higher pressure air being pulled into a central zone of lower pressure.
Warm ocean waters feed the storm. The highest winds develop in the eyewall, the region that surrounds the eye (the central portion) of the storm. The eye is often a region of light winds and clear sky, with the eyewall and regions farther removed from the center being areas of high winds and torrential rain.
The energy from the winds can push the ocean water. As a storm nears land, the result can be a storm surge, with waves that can reach 20 ft (6 m) or more in height. If the coastline is low, such as the case with the U.S. Gulf Coast that was battered by Hurricane Katrina, massive flooding can occur. The majority of deaths that occur during an ocean storm are due to the storm surge, rather than from the high winds.
Sea temperatures also influence the severity of monsoons, the rain-laden winds that change direction with the seasons, and which affect regions including India and portions of Asia. Computer-based modeling studies have indicated that changing ocean temperatures are altering the monsoon wind pattern, which is producing more intense monsoon rainfall than has historically been the norm.
Computer simulation programs and climate models are also being used by scientists at agencies—including the U.S. National Hurricane Centers in Miami, Florida, and Honolulu, Hawaii, as well as the Canadian Hurricane Center in Dartmouth, Nova Scotia—to anticipate when storms are apt to develop and to predict the path of an ongoing storm. Their calculations use data collected from orbiting satellites, weather balloons, buoys floating in the ocean, instruments on passenger planes, and specialized planes that can fly into the heart of the storm.
Impacts and Issues
Because severe ocean storms are driven by temperature differences, the influence of increasing atmosphere temperature on the frequency and severity of ocean storms has been studied. An analysis of the length and wind speed of every tropical cyclone that occurred during the 1970s, 1980s, and 1990s has shown that the severity of Atlantic and Pacific storms increased by about 70%. A 2007 study found that the number of category four or category five hurricanes increased during the same period.
These increases in storm severity coincided with rising sea temperatures in the regions that were most prone to storm activity. A 2006 study that utilized 22 climate computer models concluded that the rising sea temperatures were due to human activity, primarily the production of greenhouse gases.
The fact that these studies were conducted using data obtained from storms, and not from computer simulations, makes the concluded link between rising sea temperatures and storm activity compelling.
Although a great many climate researchers accept the link between sea temperature and storm frequency and severity, this view is not universal. A minority of scientists argue that the lack of data from storms in the nineteenth century makes it difficult to conclude that ocean storms have been becoming progressively more destructive.
In addition, even though the rise in ocean temperature was greatest during the 1990s, the total number of hurricanes and the length of the storms decreased. This ambiguity highlights the need for further research to clarify the complicated relationship between sea surface temperature and storm intensity, and to conclusively establish that the warming ocean is mainly a consequence of human activities.
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Brian D. Hoyle