A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and/or squalls. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane (/ˈhʌrɪkən, -keɪn/), typhoon (/taɪˈfuːn/), tropical storm, cyclonic storm, tropical depression, or simply cyclone. A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; in the south Pacific or Indian Ocean, comparable storms are referred to simply as "tropical cyclones" or "severe cyclonic storms".
"Tropical" refers to the geographical origin of these systems, which form almost exclusively over tropical seas. "Cyclone" refers to their winds moving in a circle, whirling round their central clear eye, with their winds blowing counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The opposite direction of circulation is due to the Coriolis effect. 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 recondenses 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 fueled primarily by horizontal temperature contrasts. Tropical cyclones are typically between 100 and 2,000 km (62 and 1,243 mi) in diameter. Every year tropical cyclones impact various regions of the globe including the Gulf Coast of North America, Australia, India and Bangladesh.
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, they rarely form within 5° of the equator. Tropical cyclones are almost unknown in the South Atlantic due to a consistently strong wind shear and a weak Intertropical Convergence Zone. Conversely, the African easterly jet and areas of atmospheric instability give rise to cyclones in the Atlantic Ocean and Caribbean Sea, while cyclones near Australia owe their genesis to the Asian monsoon and Western Pacific Warm Pool.
The primary energy source for these storms is warm ocean waters. These storms are therefore typically strongest when over or near water, and weaken quite rapidly over land. This causes coastal regions to be particularly vulnerable to tropical cyclones, compared to inland regions. Coastal damage may be caused by strong winds and rain, high waves (due to winds), storm surges (due to wind and severe pressure changes), and the potential of spawning tornadoes. Tropical cyclones draw in air from a large area and concentrate the water content of that air (from atmospheric moisture and moisture evaporated from water) 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 40 kilometers (25 mi) 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. Although their effects on human populations can be devastating, tropical cyclones can relieve drought conditions. They also carry heat and energy away from the tropics and transport it towards temperate latitudes, which could play an important role in regulating global climate.
Background
A tropical cyclone is the generic term for a warm-cored, non-frontal synoptic-scale low-pressure system over tropical or subtropical waters around the world.[1][2] The systems generally have a well-defined center which is surrounded by deep atmospheric convection and a closed wind circulation at the surface.[1] Historically tropical cyclones have occurred around the world for thousands of years, with one of the earliest tropical cyclones on record estimated to have occurred in Western Australia in around 6000 BC.[3] However, 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.[4] These days, on average around 80 to 90 named tropical cyclones form each year around the world, of which over half of which develop hurricane-force winds of 65 kn (120 km/h; 75 mph) or more.[4] Around the world, a tropical cyclone is generally deemed to have formed, once mean surface winds in excess of 35 kn (65 km/h; 40 mph) are observed.[4] 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.[4]
Structure
Tropical cyclones on either side of the Equator generally have their origins in the Intertropical Convergence Zone, where winds blow from either the northeast or southeast.[5] Within this broad area of low-pressure air is heated over the warm tropical ocean and rises in discrete parcels which causes thundery showers to form.[5] These showers dissipate quite quickly, however, they can group together into large clusters of thunderstorms.[5] This creates a flow of warm, moist, rapidly rising air, which starts to rotate cyclonically as it interacts with the rotation of the earth.[5] There are several factors required for these thunderstorms to develop further, including sea surface temperatures of around 27 °C (81 °F) and low vertical windshear surrounding the system.[5]
Eye and center
At the center of a mature tropical cyclone, air sinks rather than rises. For a sufficiently strong storm, air may sink over a layer deep enough to suppress cloud formation, thereby creating a clear "eye". Weather in the eye is normally calm and free of convective clouds, although the sea may be extremely violent.[6] The eye is normally circular and is typically 30–65 km (19–40 mi) in diameter, though eyes as small as 3 km (1.9 mi) and as large as 370 km (230 mi) have been observed.[7][8]
The cloudy outer edge of the eye is called the "eyewall". The eyewall typically expands outward with height, resembling an arena football stadium; this phenomenon is sometimes referred to as the "stadium effect".[8] The eyewall is where the greatest wind speeds are found, air rises most rapidly, clouds reach their highest altitude, and precipitation is the heaviest. The heaviest wind damage occurs where a tropical cyclone's eyewall passes over land.[6]
In a weaker storm, the eye may be obscured by the central dense overcast, which is the upper-level cirrus shield that is associated with a concentrated area of strong thunderstorm activity near the center of a tropical cyclone.[9]
The eyewall may vary over time in the form of eyewall replacement cycles, particularly in intense tropical cyclones. Outer rainbands can organize into an outer ring of thunderstorms that slowly moves inward, which is believed to rob the primary eyewall of moisture and angular momentum. When the primary eyewall weakens, the tropical cyclone weakens temporarily. The outer eyewall eventually replaces the primary one at the end of the cycle, at which time the storm may return to its original intensity.[10]
Seasons
Worldwide, tropical cyclone activity peaks in late summer, when the difference between temperatures aloft and sea surface temperatures is the greatest. However, each particular basin has its own seasonal patterns. On a worldwide scale, May is the least active month, while September is the most active month. November is the only month in which all the tropical cyclone basins are in season.[11]
In the Northern Atlantic Ocean, a distinct cyclone season occurs from June 1 to November 30, sharply peaking from late August through September.[11] The statistical peak of the Atlantic hurricane season is September 10. The Northeast Pacific Ocean has a broader period of activity, but in a similar time frame to the Atlantic.[12] The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and March and a peak in early September.[11] In the North Indian basin, storms are most common from April to December, with peaks in May and November.[11] In the Southern Hemisphere, the tropical cyclone year begins on July 1 and runs all year-round encompassing the tropical cyclone seasons, which run from November 1 until the end of April, with peaks in mid-February to early March.[11][13]
Interaction with the upper ocean
The passage of a tropical cyclone over the ocean causes the upper layers of the ocean to cool substantially, which can influence subsequent cyclone development. This cooling is primarily caused by wind-driven mixing of cold water from deeper in the ocean with the warm surface waters. This effect results in a negative feedback process that can inhibit further development or lead to weakening. Additional cooling may come in the form of cold water from falling raindrops (this is because the atmosphere is cooler at higher altitudes). Cloud cover may also play a role in cooling the ocean, by shielding the ocean surface from direct sunlight before and slightly after the storm passage. All these effects can combine to produce a dramatic drop in sea surface temperature over a large area in just a few days.[19] Conversely, the mixing of the sea can result in heat being inserted in deeper waters, with potential effects on global climate.[20]
Movement
The movement of a tropical cyclone (i.e. its "track") is typically approximated as the sum of two terms: "steering" by the background environmental wind and "beta drift".[21]
Environmental steering
Environmental steering is the dominant term. Conceptually, it represents the movement of the storm due to prevailing winds and other wider environmental conditions, similar to "leaves carried along by a stream".[22] Physically, the winds, or flow field, in the vicinity of a tropical cyclone may be treated as having two parts: the flow associated with the storm itself, and the large-scale background flow of the environment in which the storm takes place. In this way, tropical cyclone motion may be represented to first-order simply as advection of the storm by the local environmental flow. This environmental flow is termed the "steering flow".[citation needed]
Climatologically, tropical cyclones are steered primarily westward by the east-to-west trade winds on the equatorial side of the subtropical ridge—a persistent high-pressure area over the world's subtropical oceans.[22] In the tropical North Atlantic and Northeast Pacific oceans, the trade winds steer tropical easterly waves westward from the African coast toward the Caribbean Sea, North America, and ultimately into the central Pacific Ocean before the waves dampen out.[23] These waves are the precursors to many tropical cyclones within this region.[24] In contrast, in the Indian Ocean and Western Pacific in both hemispheres, tropical cyclogenesis is influenced less by tropical easterly waves and more by the seasonal movement of the Inter-tropical Convergence Zone and the monsoon trough.[25] Additionally, tropical cyclone motion can be influenced by transient weather systems, such as extratropical cyclones.[citation needed]
Beta drift
In addition to environmental steering, a tropical cyclone will tend to drift slowly poleward and westward, a motion known as "beta drift". This motion is due to the superposition of a vortex, such as a tropical cyclone, onto an environment in which the Coriolis force varies with latitude, such as on a sphere or beta plane. It is induced indirectly by the storm itself, the result of a feedback between the cyclonic flow of the storm and its environment.[citation needed]
Physically, the cyclonic circulation of the storm advects environmental air poleward east of center and equatorial west of center. Because air must conserve its angular momentum, this flow configuration induces a cyclonic gyre equatorward and westward of the storm center and an anticyclonic gyre poleward and eastward of the storm center. The combined flow of these gyres acts to advect the storm slowly poleward and westward. This effect occurs even if there is zero environmental flow.[citation needed]
Multiple storm interaction
A third component of motion that occurs relatively infrequently involves the interaction of multiple tropical cyclones. When two cyclones approach one another, their centers will begin orbiting cyclonically about a point between the two systems. Depending on their separation distance and strength, the two vortices may simply orbit around one another or else may spiral into the center point and merge. When the two vortices are of unequal size, the larger vortex will tend to dominate the interaction, and the smaller vortex will orbit around it. This phenomenon is called the Fujiwhara effect, after Sakuhei Fujiwhara.[26]
Interaction with the mid-latitude westerlies
Though a tropical cyclone typically moves from east to west in the tropics, its track may shift poleward and eastward either as it moves west of the subtropical ridge axis or else if it interacts with the mid-latitude flow, such as the jet stream or an extratropical cyclone. This motion, termed "recurvature", commonly occurs near the western edge of the major ocean basins, where the jet stream typically has a poleward component and extratropical cyclones are common.[27] An example of tropical cyclone recurvature was Typhoon Ioke in 2006.[28]
Intensity
Size
There are a variety of metrics commonly used to measure storm size. The most common metrics include the radius of maximum wind, the radius of 34-knot wind (i.e. gale force), the radius of outermost closed isobar (ROCI), and the radius of vanishing wind.[30][31] An additional metric is the radius at which the cyclone's relative vorticity field decreases to 1×10−5 s−1.[8]
On Earth, tropical cyclones span a large range of sizes, from 100–2,000 kilometres (62–1,243 mi) as measured by the radius of vanishing wind. They are largest on average in the northwest Pacific Ocean basin and smallest in the northeastern Pacific Ocean basin.[32] If the radius of outermost closed isobar is less than two degrees of latitude (222 km (138 mi)), then the cyclone is "very small" or a "midget". A radius of 3–6 latitude degrees (333–670 km (207–416 mi)) is considered "average sized". "Very large" tropical cyclones have a radius of greater than 8 degrees (888 km (552 mi)).[29] Observations indicate that size is only weakly correlated to variables such as storm intensity (i.e. maximum wind speed), radius of maximum wind, latitude, and maximum potential intensity.[31][32]
Rapid intensification
On occasion, tropical cyclones may undergo a process known as rapid intensification, a period in which the maximum sustained winds of a tropical cyclone increase by 30 knots or more within 24 hours.[33] For rapid intensification to occur, several conditions must be in place. Water temperatures must be extremely high (near or above 30 °C, 86 °F), and water of this temperature must be sufficiently deep such that waves do not upwell cooler waters to the surface. On the other hand, Tropical Cyclone Heat Potential is one of such non-conventional subsurface oceanographic parameters influencing the cyclone intensity. Wind shear must be low; when wind shear is high, the convection and circulation in the cyclone will be disrupted. Usually, an anticyclone in the upper layers of the troposphere above the storm must be present as well—for extremely low surface pressures to develop, air must be rising very rapidly in the eyewall of the storm, and an upper-level anticyclone helps channel this air away from the cyclone efficiently.[34] However, some cyclones such as Hurricane Epsilon have rapidly intensified despite relatively unfavorable conditions.[35][36]
Dissipation
There are a number of ways a tropical cyclone can weaken, dissipate or lose its tropical characteristics, these include making landfall, moving over cooler water, encountering dry air or interacting with other weather systems. However, once a system has been declared dissipated or lost its tropical characteristics, there is a chance that a tropical cyclone could regenerate should it move back over open waters.[citation needed]
Landfall
Should a tropical cyclone make landfall or pass over an island, then its circulation could start to break down, especially if it is a mountainous island.[37] Should a system make landfall on a continent such as Africa, Australia, India or the United States, then it would be cut off from its supply of warm moist maritime air and would start to draw in dry continental air.[37] This combined with the increased friction over land areas, leads to the weakening and dissipation of the tropical cyclone.[37] Over a mountainous terrain, a system can quickly weaken, however, over flat areas, it may take two to three days for the circulation to break down and dissipate.[37]
Factors
A tropical cyclone can dissipate when it moves over waters significantly below 26.5 °C (79.7 °F). This will cause the storm to lose its tropical characteristics, such as a warm core with thunderstorms near the center, and become a remnant low-pressure area. These remnant systems may persist for up to several days before losing their identity. This dissipation mechanism is most common in the eastern North Pacific. Weakening or dissipation can occur if it experiences vertical wind shear, causing the convection and heat engine to move away from the center; this normally ceases development of a tropical cyclone.[38] In addition, its interaction with the main belt of the Westerlies, by means of merging with a nearby frontal zone, can cause tropical cyclones to evolve into extratropical cyclones. This transition can take 1–3 days.[39]
Artificial dissipation
Over the years, there have been a number of techniques considered to try and artificially modify tropical cyclones.[40] These techniques have included using nuclear weapons, cooling the ocean with icebergs, blowing the storm away from land with giant fans and seeding selected storms with dry ice or silver iodide.[40] However, these techniques have failed to appreciate the duration, intensity, power or size of tropical cyclones.[40]
Classification
Nomenclature and intensity classifications
Around the world, tropical cyclones are classified in different ways, based on the location, the structure of the system and its intensity. For example, within the Northern Atlantic and Eastern Pacific basins, a tropical cyclone with wind speeds of over 65 kn (75 mph; 120 km/h) is called a hurricane, while it is called a typhoon or a severe cyclonic storm within the Western Pacific or North Indian Oceans.[41][42][43] Within the Southern Hemisphere, it is either called a hurricane, tropical cyclone or a severe tropical cyclone, depending on if it is located within the South Atlantic, South-West Indian Ocean, Australian region or the South Pacific Ocean.[16][13]
Identification codes
Tropical cyclones that develop around the world are assigned an identification code consisting of a two-digit number and suffix letter by the warning centers that monitor them. These codes start at 01 every year and are assigned in order to systems, which have the potential to develop further, cause significant impact to life and property or when the warning centers start to write advisories on the system.[13][44]
Naming
The practice of using names to identify tropical cyclones goes back many years, with systems named after places or things they hit before the formal start of naming.[53][54] The system currently used provides positive identification of severe weather systems in a brief form, that is readily understood and recognized by the public.[53][54] The credit for the first usage of personal names for weather systems is generally given to the Queensland Government Meteorologist Clement Wragge who named systems between 1887 and 1907.[53][54] This system of naming weather systems subsequently fell into disuse for several years after Wragge retired, until it was revived in the latter part of World War II for the Western Pacific.[53][54] Formal naming schemes have subsequently been introduced for the North and South Atlantic, Eastern, Central, Western and Southern Pacific basins as well as the Australian region and Indian Ocean.[54]
At present, tropical cyclones are officially named by one of twelve meteorological services and retain their names throughout their lifetimes to provide ease of communication between forecasters and the general public regarding forecasts, watches, and warnings.[53] Since the systems can last a week or longer and more than one can be occurring in the same basin at the same time, the names are thought to reduce the confusion about what storm is being described.[53] Names are assigned in order from predetermined lists with one, three, or ten-minute sustained wind speeds of more than 65 km/h (40 mph) depending on which basin it originates.[41][43][16] However, standards vary from basin to basin with some tropical depressions named in the Western Pacific, while tropical cyclones have to have a significant amount of gale-force winds occurring around the center before they are named within the Southern Hemisphere.[16][13] The names of significant tropical cyclones in the North Atlantic Ocean, Pacific Ocean, and Australian region are retired from the naming lists and replaced with another name.[41][42][13]
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 centres.[4] Ten of these warning centres worldwide are designated as either a Regional Specialized Meteorological Centre or a Tropical Cyclone Warning Centre by the World Meteorological Organisation's tropical cyclone programme.[4] These warning centres issue advisories which provide basic information and cover a systems present, forecast position, movement and intensity, in their designated areas of responsibility.[4] Meteorological services around the world are generally responsible for issuing warnings for their own country, however, 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.[4][13] The United States Joint Typhoon Warning Center (JTWC) and Fleet Weather Center (FWC) also publicly issue warnings, about tropical cyclones on behalf of the United States Government.[4] 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.[55]
Preparations
Ahead of the formal season starting, people are urged to prepare for the effects of a tropical cyclone by politicians and weather forecasters amongst others. They prepare by determining their risk to the different types of weather, tropical cyclones cause, checking their insurance coverage and emergency supplies, as well as determining where to evacuate to if needed. When a tropical cyclone develops and is forecast to impact land each member nation of the World Meteorological Organization issues various watches and warnings to cover the expected impacts. However, there are some exceptions with the United States National Hurricane Center and Fiji Meteorological Service responsible for issuing warnings for other nations in their area of responsibility.[citation needed]
Impacts
Tropical cyclones out at sea cause large waves, heavy rain, flood and high winds, disrupting international shipping and, at times, causing shipwrecks.[56] Tropical cyclones stir up water, leaving a cool wake behind them, which causes the region to be less favorable for subsequent tropical cyclones.[19] On land, strong winds can damage or destroy vehicles, buildings, bridges, and other outside objects, turning loose debris into deadly flying projectiles. The storm surge, or the increase in sea level due to the cyclone, is typically the worst effect from landfalling tropical cyclones, historically resulting in 90% of tropical cyclone deaths.[57] The broad rotation of a landfalling tropical cyclone, and vertical wind shear at its periphery, spawns tornadoes. Tornadoes can also be spawned as a result of eyewall mesovortices, which persist until landfall.[58]
Over the past two centuries, tropical cyclones have been responsible for the deaths of about 1.9 million people worldwide. Large areas of standing water caused by flooding lead to infection, as well as contributing to mosquito-borne illnesses. Crowded evacuees in shelters increase the risk of disease propagation.[57] Tropical cyclones significantly interrupt infrastructure, leading to power outages, bridge destruction, and the hampering of reconstruction efforts.[57][59] On average, the Gulf and east coasts of the United States suffer approximately US$5 billion (1995 US $) in cyclone damage every year. The majority (83%) of tropical cyclone damage is caused by severe hurricanes, category 3 or greater. However, category 3 or greater hurricanes only account for about one-fifth of cyclones that make landfall every year.[60]
Although cyclones take an enormous toll in lives and personal property, they may be important factors in the precipitation regimes of places they impact, as they may bring much-needed precipitation to otherwise dry regions.[61] Tropical cyclones also help maintain the global heat balance by moving warm, moist tropical air to the middle latitudes and polar regions,[62] and by regulating the thermohaline circulation through upwelling.[63] 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. Tropical cyclone destruction spurs redevelopment, greatly increasing local property values.[64]
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 are able to cope with the salt and recycle it back into the ocean, but others can not release the extra surface water quickly enough or do not have a large enough freshwater source to replace it. Because of this, some species of plants and vegetation die due to the excess salt.[65] In addition, hurricanes can carry toxins and acids onto shore when they make landfall. The flood water can pick up the toxins from different spills and contaminate the land that it passes over. The toxins are very harmful to the people and animals in the area, as well as the environment around them. The flooding water can also spark many dangerous oil spills.[66]
Response
Hurricane response is the disaster response after a hurricane. Activities performed by hurricane responders include assessment, restoration, and demolition of buildings; removal of debris and waste; repairs to land-based and maritime infrastructure; and public health services including search and rescue operations.[67] Hurricane response requires coordination between federal, tribal, state, local, and private entities.[68] According to the National Voluntary Organizations Active in Disaster, potential response volunteers should affiliate with established organizations and should not self-deploy, so that proper training and support can be provided to mitigate the danger and stress of response work.[69]
Hurricane responders face many hazards. Hurricane responders may be exposed to chemical and biological contaminants including stored chemicals, sewage, human remains, and mold growth encouraged by flooding,[70][71][72] as well as asbestos and lead that may be present in older buildings.[71][73] Common injuries arise from falls from heights, such as from a ladder or from level surfaces; from electrocution in flooded areas, including from backfeed from portable generators; or from motor vehicle accidents.[70][73][74] Long and irregular shifts may lead to sleep deprivation and fatigue, increasing the risk of injuries, and workers may experience mental stress associated with a traumatic incident. Additionally, heat stress is a concern as workers are often exposed to hot and humid temperatures, wear protective clothing and equipment, and have physically difficult tasks.[70][73]
Climatology
Historically tropical cyclones have occurred around the world for thousands of years, with one of the earliest tropical cyclones on record estimated to have occurred in around 6000 B.C.[3] However, 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.[4] As a result, the ability of climatologists to make long-term analysis of tropical cyclones is limited by the amount of reliable historical data.[citation needed] However, reanalyses and research are being undertaken to extend the historical record, through the usage of poxy data such as overwash deposits, beach ridges and historical documents such as diaries.[3] During the 1940s, routine aircraft reconnaissance started in both the Atlantic and Western Pacific basin during the mid-1940s, which provided ground truth data, however, early flights were only made once or twice a day.[4] Polar-orbiting weather satellites were first launched by the United States National Aeronautics and Space Administration during 1960, but were not declared operational until 1965.[4] However, it took several years for some of the warning centres to take advantage of this new viewing platform and develop the expertise to associate satellite signatures with storm position and intensity.[4]
Influence of climate
Often in part because of the threat of hurricanes, many coastal regions had sparse population between major ports until the advent of automobile tourism; therefore, the most severe portions of hurricanes striking the coast may have gone unmeasured in some instances. The combined effects of ship destruction and remote landfall severely limit the number of intense hurricanes in the official record before the era of hurricane reconnaissance aircraft and satellite meteorology. Although the record shows a distinct increase in the number and strength of intense hurricanes, therefore, experts regard the early data as suspect.[75]
Paleoclimate and climate variability
Proxy records based on paleotempestological research have revealed that major hurricane activity along the Gulf of Mexico coast varies on timescales of centuries to millennia.[76][77] Few major hurricanes struck the Gulf coast during 3000–1400 BC and again during the most recent millennium. These quiescent intervals were separated by a hyperactive period during 1400 BC and 1000 AD, when the Gulf coast was struck frequently by catastrophic hurricanes and their landfall probabilities increased by 3–5 times. This millennial-scale variability has been attributed to long-term shifts in the position of the Azores High,[77] which may also be linked to changes in the strength of the North Atlantic oscillation.[78]
Of various modes of variability in the climate system, El Niño-Southern Oscillation has the largest impact on tropical cyclone activity.[79] Most tropical cyclones form on the side of the subtropical ridge closer to the equator, then move poleward past the ridge axis before recurving into the main belt of the Westerlies.[80] When the subtropical ridge position shifts due to El Niño, so will the preferred tropical cyclone tracks. Areas west of Japan and Korea tend to experience much fewer September–November tropical cyclone impacts during El Niño and neutral years.[81] During La Niña years, the formation of tropical cyclones, along with the subtropical ridge position, shifts westward across the western Pacific Ocean, which increases the landfall threat to China and much greater intensity in the Philippines.[81] The Atlantic Ocean experiences depressed activity due to increased vertical wind shear across the region during El Niño years.[82] Tropical cyclones are further influenced by the Atlantic Meridional Mode, the Quasi-biennial oscillation and the Madden–Julian oscillation.[79][83]
Climate change
Climate change can affect tropical cyclones in a variety of ways: an intensification of rainfall and wind speed, a decrease in overall frequency, an increase in frequency of very intense storms and a poleward extension of where the cyclones reach maximum intensity are among the possible consequences of human-induced climate change.[84]
Tropical cyclones use warm, moist air as their fuel. As climate change is warming ocean temperatures, there is potentially more of this fuel available.[85] 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.[86] With 2 °C warming, a greater percentage (+13%) of tropical cyclones are expected to reach Category 4 and 5 strength.[84] 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.[87]
There is currently no consensus on how climate change will affect the overall frequency of tropical cyclones.[84] A majority of climate models show a decreased frequency in future projections.[88] For instance, a 2020 paper comparing nine high-resolution climate models found robust decreases in frequency in the Southern Indian Ocean and the Southern Hemisphere more generally, while finding mixed signals for Northern Hemisphere tropical cyclones.[89] Observations have shown little change in the overall frequency of tropical cyclones worldwide,[90] with increased frequency in the North Atlantic and central Pacific, and significant decreases in the southern Indian Ocean and western North Pacific.[91]
There has been a poleward expansion of the latitude at which the maximum intensity of tropical cyclones occurs, which may be associated with climate change.[92] In the North Pacific, there may also have been an eastward expansion.[93] Between 1949 and 2016, there was a slowdown in tropical cyclone translation speeds. It is unclear still to what extent this can be attributed to climate change: climate models do not all show this feature.[88]
Warmer air can hold more water vapor: the theoretical maximum water vapor content is given by the Clausius–Clapeyron relation, which yields ≈7% increase in water vapor in the atmosphere per 1 °C warming.[94][95] All models that were assessed in a 2019 review paper show a future increase of rainfall rates.[84] Additional sea level rise will increase storm surge levels.[93][96] It is plausible that extreme wind waves see an increase as a consequence of changes in tropical cyclones, further exacerbating storm surge dangers to coastal communities.[88] The compounding effects from floods, storm surge, and terrestrial flooding (rivers) are projected to increase due to global warming.[96]
Observation and forecasting
Observation
Intense tropical cyclones pose a particular observation challenge, as they are a dangerous oceanic phenomenon, and weather stations, being relatively sparse, are rarely available on the site of the storm itself. In general, surface observations are available only if the storm is passing over an island or a coastal area, or if there is a nearby ship. Real-time measurements are usually taken in the periphery of the cyclone, where conditions are less catastrophic and its true strength cannot be evaluated. For this reason, there are teams of meteorologists that move into the path of tropical cyclones to help evaluate their strength at the point of landfall.[97]
Tropical cyclones far from land 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.[98]
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.[99] The aircraft used are WC-130 Hercules and WP-3D Orions, both four-engine turboprop cargo aircraft. These aircraft fly directly into the cyclone and take direct and remote-sensing measurements. The aircraft also 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. A similar mission was also completed successfully in the western Pacific Ocean. This demonstrated a new way to probe the storms at low altitudes that human pilots seldom dare.[100]
Forecasting
Because of the forces that affect tropical cyclone tracks, accurate track predictions depend on determining the position and strength of high- and low-pressure areas, and predicting how those areas will change during the life of a tropical system. The deep layer mean flow, or average wind through the depth of the troposphere, is considered the best tool in determining track direction and speed. If storms are significantly sheared, use of wind speed measurements at a lower altitude, such as at the 70 kPa pressure surface (3,000 metres or 9,800 feet above sea level) will produce better predictions. Tropical forecasters also consider smoothing out short-term wobbles of the storm as it allows them to determine a more accurate long-term trajectory.[101] High-speed computers and sophisticated simulation software allow forecasters to produce computer models that predict tropical cyclone tracks based on the future position and strength of high- and low-pressure systems. Combining forecast models with increased understanding of the forces that act on tropical cyclones, as well as with a wealth of data from Earth-orbiting satellites and other sensors, scientists have increased the accuracy of track forecasts over recent decades.[102] However, scientists are not as skillful at predicting the intensity of tropical cyclones.[103] The lack of improvement in intensity forecasting is attributed to the complexity of tropical systems and an incomplete understanding of factors that affect their development. New tropical cyclone position and forecast information is available at least every six hours from the various warning centers.[104][105][106][107][108]
Related cyclone types
In addition to tropical cyclones, there are two other classes of cyclones within the spectrum of cyclone types. These kinds of cyclones, known as extratropical cyclones and subtropical cyclones, can be stages a tropical cyclone passes through during its formation or dissipation.[109] An extratropical cyclone is a storm that derives energy from horizontal temperature differences, which are typical in higher latitudes. A tropical cyclone can become extratropical as it moves toward higher latitudes if its energy source changes from heat released by condensation to differences in temperature between air masses; although not as frequently, an extratropical cyclone can transform into a subtropical storm, and from there into a tropical cyclone.[110] From space, extratropical storms have a characteristic "comma-shaped" cloud pattern.[111] Extratropical cyclones can also be dangerous when their low-pressure centers cause powerful winds and high seas.[112]
A subtropical cyclone is a weather system that has some characteristics of a tropical cyclone and some characteristics of an extratropical cyclone. They can form in a wide band of latitudes, from the equator to 50°. Although subtropical storms rarely have hurricane-force winds, they may become tropical in nature as their cores warm.[113]
Notable tropical cyclones
Tropical cyclones that cause extreme destruction are rare, although when they occur, they can cause great amounts of damage or thousands of fatalities.
The 1970 Bhola cyclone is considered to be the deadliest tropical cyclone on record, which killed around 300,000 people, after striking the densely populated Ganges Delta region of Bangladesh on November 13, 1970.[114] Its powerful storm surge was responsible for the high death toll.[115] The North Indian cyclone basin has historically been the deadliest basin.[57][116] Elsewhere, Typhoon Nina killed nearly 100,000 in China in 1975 due to a 100-year flood that caused 62 dams including the Banqiao Dam to fail.[117] The Great Hurricane of 1780 is the deadliest North Atlantic hurricane on record, killing about 22,000 people in the Lesser Antilles.[118] A tropical cyclone does not need to be particularly strong to cause memorable damage, primarily if the deaths are from rainfall or mudslides. Tropical Storm Thelma in November 1991 killed thousands in the Philippines,[119] although the strongest typhoon to ever make landfall on record was Typhoon Haiyan in November 2013, causing widespread devastation in Eastern Visayas, and killing at least 6,300 people in the Philippines alone. In 1982, the unnamed tropical depression that eventually became Hurricane Paul killed around 1,000 people in Central America.[120]
Hurricane Harvey and Hurricane Katrina are estimated to be the costliest tropical cyclones to impact the United States mainland, each causing damage estimated at $125 billion.[121] Harvey killed at least 90 people in August 2017 after making landfall in Texas as a low-end Category 4 hurricane. Hurricane Katrina is estimated as the second-costliest tropical cyclone worldwide,[122] causing $81.2 billion in property damage (2008 USD) alone,[123] with overall damage estimates exceeding $100 billion (2005 USD).[122] Katrina killed at least 1,836 people after striking Louisiana and Mississippi as a major hurricane in August 2005.[123] Hurricane Maria is the third most destructive tropical cyclone in U.S. history, with damage totaling $91.61 billion (2017 USD), and with damage costs at $68.7 billion (2012 USD), Hurricane Sandy is the fourth most destructive tropical cyclone in U.S history. The Galveston Hurricane of 1900 is the deadliest natural disaster in the United States, killing an estimated 6,000 to 12,000 people in Galveston, Texas.[124] Hurricane Mitch caused more than 10,000 fatalities in Central America, making it the second deadliest Atlantic hurricane in history. Hurricane Iniki in 1992 was the most powerful storm to strike Hawaii in recorded history, hitting Kauai as a Category 4 hurricane, killing six people, and causing U.S. $3 billion in damage.[125] Other destructive Eastern Pacific hurricanes include Pauline and Kenna, both causing severe damage after striking Mexico as major hurricanes.[126][127] In March 2004, Cyclone Gafilo struck northeastern Madagascar as a powerful cyclone, killing 74, affecting more than 200,000 and becoming the worst cyclone to affect the nation for more than 20 years.[128]
The most intense storm on record is Typhoon Tip in the northwestern Pacific Ocean in 1979, which reached a minimum pressure of 870 hectopascals (25.69 inHg) and maximum sustained wind speeds of 165 knots (85 m/s) or 190 miles per hour (310 km/h).[129] The highest maximum sustained wind speed ever recorded was 185 knots (95 m/s) or 215 miles per hour (346 km/h) in Hurricane Patricia in 2015—the most intense cyclone ever recorded in the Western Hemisphere.[130] Typhoon Nancy in 1961 also had recorded wind speeds of 185 knots (95 m/s) or 215 miles per hour (346 km/h), but recent research indicates that wind speeds from the 1940s to the 1960s were gauged too high, and thus it is no longer considered the storm with the highest wind speed on record.[131] Likewise, a surface-level gust caused by Typhoon Paka on Guam in late 1997 was recorded at 205 knots (105 m/s) or 235 miles per hour (378 km/h). Had it been confirmed, it would be the strongest non-tornadic wind ever recorded on the Earth's surface, but the reading had to be discarded since the anemometer was damaged by the storm.[132] The World Meteorological Organization established Barrow Island (Queensland) as the location of the highest non-tornado related wind gust at 408 kilometres per hour (254 mph)[133] on April 10, 1996, during Severe Tropical Cyclone Olivia.[134]
In addition to being the most intense tropical cyclone on record based on pressure, Tip is the largest cyclone on record, with tropical storm-force winds 2,170 kilometres (1,350 mi) in diameter. The smallest storm on record, Tropical Storm Marco, formed during October 2008 and made landfall in Veracruz. Marco generated tropical storm-force winds only 37 kilometres (23 mi) in diameter. Marco broke the record of 1974's Cyclone Tracy.[135]
Hurricane John is the longest-lasting tropical cyclone on record, lasting 31 days in 1994. Before the advent of satellite imagery in 1961, however, many tropical cyclones were underestimated in their durations.[136] John is also the longest-tracked tropical cyclone in the Northern Hemisphere on record, with a path of 8,250 mi (13,280 km).[137] Cyclone Rewa of the 1993–94 South Pacific and Australian region cyclone seasons had one of the longest tracks observed within the Southern Hemisphere, traveling a distance of over 5,545 mi (8,920 km) during December 1993 and January 1994.[137]
Popular culture
In popular culture, tropical cyclones have made several appearances in different types of media, including films, books, television, music, and electronic games.[138] These media often portray tropical cyclones that are either entirely fictional or based on real events.[138] For example, George Rippey Stewart's Storm, a best-seller published in 1941, is thought to have influenced meteorologists on their decision to assign female names to Pacific tropical cyclones.[54] Another example is the hurricane in The Perfect Storm, which describes the sinking of the Andrea Gail by the 1991 Perfect Storm.[139] Hurricanes have been featured in parts of the plots of series such as The Simpsons, Invasion, Family Guy, Seinfeld, Dawson's Creek, Burn Notice and CSI: Miami.[138][140][141][142][143] The 2004 film The Day After Tomorrow includes several mentions of actual tropical cyclones and features fantastical "hurricane-like", albeit non-tropical, Arctic storms.[144][145]
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