Tropical cyclone
A tropical cyclone is a rapidly rotating storm system with a low-pressure area, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. This definition applies globally, though the name changes based on location and strength. A hurricane occurs in the Atlantic Ocean or northeastern Pacific Ocean. A typhoon happens in the northwestern Pacific Ocean. In the Indian Ocean and South Pacific, comparable storms are referred to as tropical cyclones. These systems generally have a well-defined center which is surrounded by deep atmospheric convection and a closed wind circulation at the surface. A tropical cyclone is generally deemed to have formed once mean surface winds in excess of 39 miles per hour are observed. It is assumed at this stage that a tropical cyclone has become self-sustaining and can continue to intensify without any help from its environment. Tropical cyclones typically form over large bodies of relatively warm water. They derive their energy through the evaporation of water from the ocean surface, which ultimately condenses into clouds and rain when moist air rises and cools to saturation. This energy source differs from that of mid-latitude cyclonic storms, such as nor'easters and European windstorms, which are powered primarily by horizontal temperature contrasts. Tropical cyclones are typically between 100 and 500 miles in diameter. The strong rotating winds of a tropical cyclone are a result of the conservation of angular momentum imparted by the Earth's rotation as air flows inwards toward the axis of rotation. As a result, cyclones rarely form within 5 degrees of the equator. South Atlantic tropical cyclones are very rare due to consistently strong wind shear and a weak Intertropical Convergence Zone. In contrast, the African easterly jet and areas of atmospheric instability give rise to cyclones in the Atlantic Ocean and Caribbean Sea. Heat energy from the ocean acts as the accelerator for tropical cyclones.
Tropical cyclones tend to develop during the summer, but have been noted in nearly every month in most tropical cyclone basins. Tropical cyclones on either side of the Equator generally have their origins in the Intertropical Convergence Zone (ITCZ), where winds blow from either the northeast or southeast. Within this broad area of low-pressure, air is heated over the warm tropical ocean and rises in discrete parcels, which causes towering thunderstorms to form. These showers dissipate quite quickly; however, they can group together into large clusters of thunderstorms. This creates a flow of warm, moist, rapidly rising air, which starts to rotate cyclonically as it interacts with the rotation of the earth. Several factors are required for these thunderstorms to develop further, including sea surface temperatures of around 80 degrees Fahrenheit and low vertical wind shear surrounding the system, atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to develop a low-pressure center, and a pre-existing low-level focus or disturbance. There is a limit on tropical cyclone intensity which is strongly related to the water temperatures along its path and upper-level divergence. An average of 86 tropical cyclones of tropical storm intensity form annually worldwide. Of those cyclones, 47 reach strengths higher than 74 miles per hour, and 20 become intense tropical cyclones, of at least Category 3 intensity on the Saffir, Simpson scale. Climate oscillations such as El Niño, Southern Oscillation (ENSO) and the Madden, Julian oscillation modulate the timing and frequency of tropical cyclone development. Rossby waves can aid in the formation of a new tropical cyclone by disseminating the energy of an existing, mature storm. Kelvin waves can contribute to tropical cyclone formation by regulating the development of the westerlies. Cyclone formation is usually reduced 3 days prior to the wave's crest and increased during the 3 days after.
The majority of tropical cyclones each year form in one of seven tropical cyclone basins, which are monitored by a variety of meteorological services and warning centers. Ten of these warning centers worldwide are designated as either a Regional Specialized Meteorological Centre or a Tropical Cyclone Warning Centre by the World Meteorological Organization's (WMO) tropical cyclone programme. These warning centers issue advisories which provide basic information and cover a systems present, forecast position, movement and intensity, in their designated areas of responsibility. Meteorological services around the world are generally responsible for issuing warnings for their own country. There are exceptions, as the United States National Hurricane Center and Fiji Meteorological Service issue alerts, watches and warnings for various island nations in their areas of responsibility. The United States Joint Typhoon Warning Center and Fleet Weather Center also publicly issue warnings about tropical cyclones on behalf of the United States Government. The Brazilian Navy Hydrographic Center names South Atlantic tropical cyclones, however the South Atlantic is not a major basin, and not an official basin according to the WMO. In the Northern Hemisphere, the North Atlantic is monitored by the United States National Hurricane Center based in Miami. The Eastern Pacific falls under the United States Central Pacific Hurricane Center in Honolulu. The Western Pacific is watched over by the Japan Meteorological Agency. The North Indian Ocean is the responsibility of the India Meteorological Department. In the Southern Hemisphere, the South-West Indian Ocean is managed by Météo-France Reunion. The Australian region involves multiple agencies including the Indonesian Meteorology, Climatology, and Geophysical Agency (BMKG), Papua New Guinea National Weather Service, Australian Bureau of Meteorology, Fiji Meteorological Service, and the Meteorological Service of New Zealand.
Scientists have found that climate change can exacerbate the impact of tropical cyclones by increasing their duration, occurrence, and intensity due to the warming of ocean waters and intensification of the water cycle. Tropical cyclones draw in air from a large area and concentrate the water content of that air into precipitation over a much smaller area. This replenishing of moisture-bearing air after rain may cause multi-hour or multi-day extremely heavy rain up to 30 inches from the coastline, far beyond the amount of water that the local atmosphere holds at any one time. This in turn can lead to river flooding, overland flooding, and a general overwhelming of local water control structures across a large area. According to the IPCC Sixth Assessment Report, major tropical storms likely became more frequent in the last 40 years. We can say with high confidence that climate change increased rainfall during tropical cyclones. We can say with high confidence that a 1.5 degree warming lead to increased proportion of and peak wind speeds of intense tropical cyclones. Between 1979 and 2017, there was a global increase in the proportion of tropical cyclones of Category 3 and higher on the Saffir, Simpson scale. The trend was most clear in the North Atlantic and in the Southern Indian Ocean. In the North Pacific, tropical cyclones have been moving poleward into colder waters and there was no increase in intensity over this period. With warming, a greater percentage plus 13% of tropical cyclones are expected to reach Category 4 and 5 strength. A 2019 study indicates that climate change has been driving the observed trend of rapid intensification of tropical cyclones in the Atlantic basin. Rapidly intensifying cyclones are hard to forecast and therefore pose additional risk to coastal communities.
Before satellite imagery became available during the 20th century, many of these systems went undetected unless it impacted land or a ship encountered it by chance. In the 1940s, routine aircraft reconnaissance started in both the Atlantic and Western Pacific basin in the mid-1940s, which provided ground truth data. Early flights were only made once or twice a day. In 1960, Polar-orbiting weather satellites were first launched by the United States National Aeronautics and Space Administration, but were not declared operational until 1965. It took several years for some of the warning centers to take advantage of this new viewing platform and develop the expertise to associate satellite signatures with storm position and intensity. Tropical cyclones are tracked by weather satellites capturing visible and infrared images from space, usually at half-hour to quarter-hour intervals. As a storm approaches land, it can be observed by land-based Doppler weather radar. Radar plays a crucial role around landfall by showing a storm's location and intensity every several minutes. Other satellites provide information from the perturbations of GPS signals, providing thousands of snapshots per day and capturing atmospheric temperature, pressure, and moisture content. In situ measurements, in real-time, can be taken by sending specially equipped reconnaissance flights into the cyclone. In the Atlantic basin, these flights are regularly flown by United States government hurricane hunters. These aircraft fly directly into the cyclone and take direct and remote-sensing measurements. The aircraft launch GPS dropsondes inside the cyclone. These sondes measure temperature, humidity, pressure, and especially winds between flight level and the ocean's surface. A new era in hurricane observation began when a remotely piloted Aerosonde, a small drone aircraft, was flown through Tropical Storm Ophelia as it passed Virginia's eastern shore during the 2005 hurricane season.
Although cyclones take an enormous toll in lives and personal property, they may be important factors in the precipitation regimes of places they affect, as they may bring much-needed precipitation to otherwise dry regions. Their precipitation may also alleviate drought conditions by restoring soil moisture, though one study focused on the Southeastern United States suggested tropical cyclones did not offer significant drought recovery. Tropical cyclones also help maintain the global heat balance by moving warm, moist tropical air to the middle latitudes and polar regions, and by regulating the thermohaline circulation through upwelling. Research on Pacific cyclones has demonstrated that deeper layers of the ocean receive a heat transfer from these powerful storms. The storm surge and winds of hurricanes may be destructive to human-made structures, but they also stir up the waters of coastal estuaries, which are typically important fish breeding locales. Ecosystems, such as saltmarshes and Mangrove forests, can be severely damaged or destroyed by tropical cyclones, which erode land and destroy vegetation. Tropical cyclones can cause harmful algae blooms to form in bodies of water by increasing the amount of nutrients available. Insect populations can decrease in both quantity and diversity after the passage of storms. Strong winds associated with tropical cyclones and their remnants are capable of felling thousands of trees, causing damage to forests. When hurricanes surge upon shore from the ocean, salt is introduced to many freshwater areas and raises the salinity levels too high for some habitats to withstand. Some species of plants and vegetation die due to the excess salt. Hurricanes can carry toxins and acids onshore
when they make landfall. The floodwater can pick up the toxins from different spills and contaminate the land that it passes over. These toxins are harmful to the people and animals in the area, as well as the environment around them.
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Common questions
What is a tropical cyclone and how does it form?
A tropical cyclone is a rapidly rotating storm system with a low-pressure area, strong winds, and heavy rain that forms over warm ocean waters. It derives energy from the evaporation of water which condenses into clouds when moist air rises and cools to saturation.
When do tropical cyclones typically occur and what are the seasonal patterns?
Tropical cyclones tend to develop during the summer but have been noted in nearly every month in most tropical cyclone basins. An average of 86 tropical cyclones of tropical storm intensity form annually worldwide with 47 reaching strengths higher than 74 miles per hour.
Where are tropical cyclones named differently based on location and strength?
Hurricanes occur in the Atlantic Ocean or northeastern Pacific Ocean while typhoons happen in the northwestern Pacific Ocean. In the Indian Ocean and South Pacific comparable storms are referred to as tropical cyclones and South Atlantic tropical cyclones are very rare due to consistently strong wind shear.
How has climate change affected the frequency and intensity of tropical cyclones since 1979?
Between 1979 and 2017 there was a global increase in the proportion of tropical cyclones of Category 3 and higher on the Saffir Simpson scale. A 2019 study indicates that climate change has been driving the observed trend of rapid intensification of tropical cyclones in the Atlantic basin.
When did satellite imagery become available for tracking tropical cyclones globally?
Polar-orbiting weather satellites were first launched by the United States National Aeronautics and Space Administration in 1960 but were not declared operational until 1965. Routine aircraft reconnaissance started in both the Atlantic and Western Pacific basin in the mid-1940s which provided ground truth data before satellite technology.