Tropical cyclone

• 1. Introduction

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  A tropical cyclone is a rapidly rotating storm system, and the most intense one ever recorded carried a pressure of 870 hectopascals. That was Typhoon Tip, in the northwestern Pacific Ocean, in 1979. Each year, on average, somewhere between 80 and 90 named tropical cyclones spin up around the world. More than half of them grow winds of 65 knots or stronger. They go by a confusing tangle of names. A hurricane in the Atlantic is a typhoon across the International Dateline, and a cyclone in the Indian Ocean. The same storm can switch identities by crossing an imaginary line. What gives these storms their power? Why do they refuse to form within 5 degrees of the equator? And how does a process that begins with water evaporating off a warm sea end with a 13 meter wall of water drowning a coastline? This is the story of how the planet builds its largest engines of weather, how people learned to watch them, and what a warming world is doing to them.

• 2. Anatomy Of A Storm

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  At the center of a mature tropical cyclone, air sinks rather than rises. In a strong enough storm, that sinking suppresses cloud formation entirely, opening a clear eye where the weather turns calm even as the sea below stays violent. A typical eye spans 30 to 65 kilometers across, though observers have seen eyes as small as 3 kilometers and as large as 370. The cloudy rim around that calm is the eyewall, and it expands outward with height like the walls of an arena football stadium. Forecasters call this the stadium effect. The eyewall holds the fiercest winds, the fastest-rising air, the tallest clouds, and the heaviest rain. The worst wind damage on land happens precisely where a cyclone's eyewall comes ashore. The eye itself can reinvent itself through eyewall replacement cycles. An outer ring of thunderstorms organizes and creeps inward, robbing the original eyewall of moisture and angular momentum. When the inner wall weakens, the whole storm weakens with it, until the outer ring takes over and the cyclone can climb back to its earlier strength.

• 3. Birth Over Warm Seas

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  Sea surface temperatures of around 27 degrees Celsius are one of the ingredients a tropical cyclone needs to grow. Storms typically take shape during summer, drawing their origins from the Intertropical Convergence Zone, a broad band of low pressure where trade winds meet. There, warm ocean air rises in discrete parcels, builds towering thunderstorms, and gathers into clusters that begin to rotate as they feel the spin of the Earth. The rotation comes from conservation of angular momentum as air rushes inward, which is why cyclones rarely form within 5 degrees of the equator. The energy itself comes from evaporation. Water lifts off the ocean surface, rises, cools to saturation, and condenses into rain, releasing heat that feeds the storm. This sets tropical cyclones apart from mid-latitude systems like nor'easters and European windstorms, which run on horizontal temperature contrasts instead. A storm is generally considered to have formed once mean surface winds pass 35 knots. At that threshold it is assumed self-sustaining, able to keep intensifying on its own. On average, 86 tropical cyclones of at least tropical storm strength form worldwide each year, of which 20 reach Category 3 or higher on the Saffir-Simpson scale.

• 4. The Machinery Of Intensity

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  Water of 30 degrees Celsius, deep enough that waves cannot churn cooler water to the surface, is one of the conditions for rapid intensification, when maximum sustained winds jump by 30 knots or more in a single day. High ocean heat content, also called Tropical Cyclone Heat Potential, lets storms reach greater strength and offsets the cooling a passing cyclone leaves in its own wake. That wake is no small thing. As a storm crosses the ocean, wind-driven mixing pulls cold water up from the depths in a process called upwelling, and the resulting drop in surface temperature can feed back to weaken the very storm that caused it. Working against intensity is vertical wind shear, which displaces moisture and heat from a storm's core, and dry air entraining into that core, which dampens convection. Smaller cyclones intensify faster than larger ones. The Brown ocean effect can even let a storm hold its strength after landfall, drawing latent heat from saturated soil. Orographic lift can spike a storm's convection when its eye climbs over a mountain. The most extreme example of all this remains Hurricane Patricia in 2015, whose 185 knot winds made it the most intense cyclone ever recorded in the Western Hemisphere.

• 5. Tracks And Wandering Paths

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  Hurricane John traveled 13,280 kilometers during its 31-day life in 1994, the longest track of any Northern Hemisphere tropical cyclone and the second longest-lasting on record. A storm's path is usually approximated as two forces added together. The first is environmental steering, the prevailing winds that carry a cyclone along like leaves on a stream. The east-to-west trade winds on the equatorial side of the subtropical ridge push storms westward, guiding tropical easterly waves off the African coast toward the Caribbean and beyond. The second force is beta drift, a poleward and westward nudge that arises because the Coriolis force changes with latitude. Beta drift typically moves a storm between 1 and 3 meters per second and matters more for larger, more intense cyclones at higher latitudes. Sometimes two storms meet. When their centers come close, they begin orbiting one another, and the larger vortex dominates while the smaller one circles or spirals in to merge. This is the Fujiwhara effect, named after Sakuhei Fujiwhara. A storm may also recurve, swinging poleward and eastward as it interacts with the jet stream, as Typhoon Ioke did in 2006.

• 6. Watching From Sea And Sky

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  Sediment records in Western Australia suggest an intense tropical cyclone struck in the 4th millennium BC, long before anyone could name it. The historical record relies on proxy data such as overwash deposits, beach ridges, and old diaries. In the year 957, a powerful typhoon killed around 10,000 people in southern China. The south-west Indian Ocean record reaches back to 1848, and in 2003 the Atlantic hurricane reanalysis project pushed that basin's history back to 1851. Before satellite imagery, many storms went undetected unless they hit land or crossed a ship's path, which is why experts treat the earliest data as suspect. Routine aircraft reconnaissance began in the mid-1940s in the Atlantic and Western Pacific, providing ground truth. In 1960 the United States launched its first polar-orbiting weather satellites, declared operational in 1965. Today storms are tracked by satellites every quarter to half hour, by Doppler radar near landfall, and by hurricane hunter aircraft that fly straight through the eye, dropping GPS dropsondes to measure temperature, humidity, pressure, and wind. A remotely piloted Aerosonde drone first flew through Tropical Storm Ophelia off Virginia's eastern shore during the 2005 season. Forecasters now track position well, yet still struggle to predict intensity, hampered by the sheer complexity of these systems.

• 7. When The Storm Comes Ashore

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  Storm surge, the rise in sea level that a cyclone pushes ahead of itself, has historically caused 90 percent of tropical cyclone deaths. Cyclone Mahina produced the highest surge on record, 13 meters, at Bathurst Bay in Queensland, Australia, in March 1899. The deadliest tropical cyclone ever struck Bangladesh, then known as East Pakistan, in 1970, when a 20 foot surge killed at least 300,000 people. Tropical cyclones have caused roughly 2 million deaths since the 19th century. In Africa, Cyclone Idai hit central Mozambique in March 2019 with 1,302 fatalities and US$2.2 billion in damage, the deadliest on record for that continent. Réunion island holds the wettest records, where Cyclone Hyacinthe dropped 6,083 millimeters of rain over 15 days in January 1980. In the United States, Hurricane Katrina in 2005 and Hurricane Harvey in 2017 each ranks among the costliest natural disasters, with Harvey dropping 60.58 inches of rain over southeastern Texas. Storms also spawn tornadoes. Hurricane Ivan produced 120 of them, more than any other cyclone. Hurricane Catarina struck southeastern Brazil in March 2004 as the first hurricane on record in the South Atlantic Ocean.

• 8. A Warming World

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  When a hurricane's wind speed rises by 5 percent, its destructive power climbs by about 50 percent. The IPCC Sixth Assessment Report finds with high confidence that climate change has increased rainfall during tropical cyclones, and that 1.5 degrees of warming raises both the proportion and peak wind speeds of the most intense storms. The physics is direct. Warmer air holds more water vapor, and the Clausius-Clapeyron relation yields roughly a 7 percent increase per 1 degree Celsius of warming. Between 1979 and 2017 the share of Category 3 and stronger storms grew worldwide, most clearly in the North Atlantic and the Southern Indian Ocean. World Weather Attribution has tied specific rainfall increases to recent storms, from 4 percent for Hurricane Katrina to as much as 18 percent for Hurricane Ian. There is no consensus yet on overall frequency, with most models projecting a decrease even as intense storms grow more common. The reach of these storms may also be widening. The latitude of maximum intensity has expanded poleward, and a 2021 review concluded that range will probably keep expanding as the Hadley circulation warms. Researchers have even asked whether such storms could form elsewhere, and a 2013 study judged the tropics of Saturn's moon Titan unfavorable, though its hydrocarbon polar seas might one day hold enough thermal energy to power one.