What Are Tropical Cyclones? Hurricanes and Typhoons, And Their Link to Climate Change, Explained

Tropical cyclones, the more general term for hurricanes and tropical storms, are among the costliest weather disasters globally. These rather common weather phenomena are becoming more frequent and intense because of climate change. Earth.Org looks at what tropical cyclones are, how they form, and how global warming is fuelling them.

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Tropical cyclones are powerful storm systems characterized by low pressure, strong winds, and heavy rain. They form over warm ocean waters, mostly in tropical regions.

Once they sustain wind speeds exceeding 63 km/h (39 mph), they are considered a tropical storm and receive a name. Between 80 to 90 named tropical cyclones form every year around the world.

Depending on the location, named tropical cyclones are referred to as hurricanes or typhoons. The former term is particularly used in the US as it comprises cyclones that originate in the Atlantic Ocean or northeastern Pacific Ocean.

Hurricanes

The Saffir-Simpson wind scale is the most widely recognized risk assessment method for hurricanes. Developed in 1971 by civil engineer Herbert Saffir and meteorologist Robert Simpson and introduced to the general public in 1973, the scale classifies hurricanes into five categories based on their sustained winds. To be classified as a hurricane, a storm must have a one-minute-average maximum sustained winds of at least 74 mph or 119 km/h (Category 1). Currently, the highest classification is assigned to storms with winds blowing at a speed of at least 157 mph or 252 km/h (Category 5).

The scale also estimates the extent of potential damage to properties, infrastructure, and livelihoods, with Categories 3-5 hurricanes – also known as major hurricanes – expected to cause “devastating” to “catastrophic” damage and loss due to the strength of their winds. However, the scale does not take potentially deadly hazards such as storm surge, rainfall flooding, and tornadoes into account.

Typhoons

Tropical cyclones forming in the northwestern Pacific Ocean are known as typhoons. Depending on the area, the minimum wind speed needed for a cyclone to be classified as a typhoon varies. Vietnam and Japan classify it as such when winds reach a minimum speed of 98.2 km/h (61 mph). In southern China, Hong Kong and Taiwan, the minimum speed must be 118 km/h (73 mph).

Typhoons are then classified further as their speed increases. Hong Kong, for example, refers to them simply as “Typhoons” when wind speed does not exceed 149 km/h (92.6 mph), after which they become “Severe Typhoons” (TS). Only when the speed is 185 km/h or above (>115 mph), they take the name of “Super Typhoons” (SuperT).

Hong Kong also relies on a set of numeric warming signals to indicate the threat or effects of a typhoon, with the lowest level (T1 or “standby signal”) issued when a tropical cyclone approaches within 800 kilometres (497 miles) of the territory and poses a threat of deteriorating conditions. According to the strong winds range, the warning can increase to a T3 (Strong Wind), T8 (Gale or Storm), T9 (Increasing Gale or Storm) and T10 (Hurricane).

China, Japan and the Philippines rely on a similar, six-category classification with slight variations in wind speeds. Meanwhile India classifies any tropical cyclones that develop within the North Indian Ocean in seven categories according to their sustained wind speeds, with the lowest being a Depression (3-minute sustained wind speeds of between 31-50 km/h or 20-31 mph) and the highest being an Extremely Severe Cyclonic Storm (hurricane-force winds of 167-220 km/h or 104-137 mph).

How Do Tropical Cyclones Form?

To form, tropical cyclones require a minimum sea surface temperatures of 26C (79F). This temperature provides the necessary heat and moisture to fuel the storm, as opposed to cooler waters, which can weaken or dissipate it.

Hurricanes, or typhoons, form when warm ocean waters heat the air above them, causing the warm, moist air to rise. As this air ascends, it cools and condenses, forming clouds and creating a low-pressure zone beneath. This low pressure allows more air to rush in from surrounding areas. As the system continues to develop, it can lead to the formation of thunderstorms. If there are no strong winds to disrupt the process, the storm can intensify and evolve into a hurricane, or typhoon, depending on its location.

Warm ocean waters, atmospheric instability, and favorable wind conditions all contribute to fuelling the cyclone and make it more powerful.

Viewed from space, tropical cyclones have spiral cloud bands and a visible “eye,” the cyclone’s center. In some cases, the structure of the eye is visible as well. The eye is a calm, generally clear area of sinking air and light winds of maximum 24 km/h (15 mph) and is typically 32-64 km (20-40 miles) across.

The diameter of a tropical cyclone is usually around 200 to 500 kilometers (124-311 miles), but can reach up to 1,000 kilometers (621 miles).

Economic Impact

Tropical cyclones are the costliest weather events globally, particularly in prone regions such as the Caribbean, Southeast Asia, and southern US. The immense damage they can cause stems from high winds, heavy rainfall, flooding, and storm surges, which can devastate infrastructure, homes, and agriculture. The economic impact can be significant, leading to billions of dollars in damages, loss of life, and long-term recovery costs.

The world’s top-ten costliest tropical cyclones all occurred in the Atlantic Ocean. Topping the ranking is Hurricane Katrina, which in 2005 hit the US city of New Orleans and surrounding areas, killing 1,392. Damages were estimated at US$186.3 billion. Katrina is closely followed by Hurricane Harvey (2017), which led to economic losses of $157 billion, and Hurricane Maria (2017), which caused damage worth $112 billion.

Between 2017 and 2023, 137 separate billion-dollar disasters have cost the US more than $1 trillion, largely driven by landfalling Category 4 or 5 hurricanes in five of the last seven years, including Hurricanes Harvey, Irma, Maria, Michael, Laura, Ida, and Ian.

2022 was the nation’s third-costliest year ever for climate disasters, with a total of 18 major climate disasters collectively racking up $165 billion in damages. Category 5 Hurricane Ian alone cost the US about $112.9 billion.

In the Pacific, the most expensive tropical cyclone to date is the 2023 Typhoon Doksuri, which caused over $28 billion in damages, primarily in China. It is followed by Japan’s Typhoon Hagibis in 2019, which resulted in damages for $18 billion, and Typhoon Jebi in 2018, Japan’s costliest typhoon in terms of insured losses.

Hong Kong’s costliest typhoon to date is Super Typhoon Mangkhut, which hit the territory in 2018 and resulted in HK$4.6 billion (US$593 million) in direct economic losses.

Tropical Cyclones and Climate Change: What’s the Link?

Though tropical cyclones are a rather common weather phenomenon, there has been a significant increase in their intensity in recent decades, which scientific observations link to anthropogenic climate change. These abnormal trends are attributed largely to rising ocean temperatures.

The world’s seas have been exceptionally warm for more than a year. The average sea surface temperature in July reached 20.88C, the second-highest value on record and only 0.01C shy of the value recorded in July 2023. This put an end to a 15-month period of record-breaking sea surface temperatures. The latest temperature trends makes it “increasingly likely” that 2024 will be the warmest year yet, beating last year, according to the EU weather agency Copernicus.

While the number of typhoons is not necessarily increasing, those that do form are becoming more destructive – generating heavier rain and a higher storm surge. The likelihood of storms reaching major hurricane status (category three or higher) has also increased significantly over the past decades.

“Fossil fuel-driven warming is ushering in a new era of bigger, deadlier typhoons,” said Ben Clarke, Researcher at the London Imperial College’s Grantham Institute for Climate Change and the Environment. Clarke was part of a team of researchers at World Weather Attribution (WWA) that last month published a study on Typhoon Gaemi, which battered the Philippines, Taiwan and eastern China in July. They found that the typhoon, which left more than 100 people dead, was intensified by fossil fuel-driven global warming.

Gaemi saw sustained winds peak at 233 km/h (145 mph), the equivalent of a Category 4 hurricane. According to the attribution analysis, the winds were about 9 mph (14 km/h) or 7% more intense due to human-made climate change.

The typhoon brought huge amounts of rainfall, too. The Philippines, which was not in Gaemi’s path, saw its seasonal rains exacerbated by the typhoon’s influence, triggering devastating floods that killed 34 people. It made a second landfall in eastern China, leading to heavy flooding, mudflows, and landslides. While it could not determine how climate change influenced rainfall in the Philippines, WWA concluded that the rainfall that hit Taiwan and China’s Hunan province was made about 14% and 9% heavier, respectively.

Another attribution analysis revealed that climate change affected rain patterns and their intensity during the 2020 North Atlantic hurricane season. Researchers found that human-made climate change increased the extreme 3-hourly storm rainfall rates by 10% and extreme 3-day accumulated rainfall amounts by 5%.

Unprecedented Times

In May, the US National Oceanic and Atmospheric Administration (NOAA) predicted that this year’s Atlantic hurricane season will likely be “above-normal,” owing to a combination of near-record heat in the Atlantic Ocean and the development of La Niña conditions in the Pacific.

La Niña, a phenomenon that typically occurs every 3 to 5 years, is expected to develop between September and November this year. It is associated with the periodic cooling of ocean surface temperatures in the central and east-central equatorial Pacific, weaker Atlantic trade winds and less atmospheric stability, conditions that are conductive to Atlantic hurricane activity.

NOAA predicts a range of 17 to 25 named storms (wind speed of 39 mph or 65 km/h) this year, eight to 13 of which are forecast to become hurricanes (wind speed of 74 mph or 119 km/h). Of these, 4 to 7 are forecast to be major hurricanes. If true, the predictions would make this year the ninth consecutive to see an above-normal hurricane season, according to the World Meteorological Organization (WMO).

To some extent, the US has already had an unusual season this year. Hurricane Beryl, which roared across the Windward Islands in the Caribbean in May, marked an exceptionally early start to the Atlantic hurricane season, becoming the earliest Category-5 storm on record. This confirms a trend already observed by scientists, which found that the hurricane season is starting earlier and lasting longer, with conditions in June resembling what is typically expected in August or September.

“The precocity of Hurricane Beryl is mainly linked to the strong warm anomaly underway in the Atlantic Ocean, where temperatures above 29°C have been recorded,” explained Enrico Scoccimarro, a scientist at the Euro-Mediterranean Centre on Climate Change (CMCC). “We see a trend towards an extension of the warm period in the Atlantic Ocean and therefore the intensification of storms becomes more likely in these months.”

But besides longer seasons, tropical cyclones are becoming more unpredictable and destructive because of climate change. This is partly linked to the fact that these storms are intensifying more rapidly as warmer ocean waters provide more fuel, with winds increasing by 56 km/h (about 35 miles per hour) in a 24-hour period.

Rapidly intensifying storms leave less time for authorities to issue warning systems, putting coastal communities in great danger. In 2022, for example, Hurricane Ian devastated portions of Florida after it rapidly intensified, packing two days’ worth of rapid intensification into less than 36 hours.

Adapting to the New Normal

As climate change rewrite the rules of tropical cyclones, some experts have pointed out that existing warning systems might be revisited to reflect these changes.

A 2023 paper argued that the 1971 Saffir-Simpson hurricane wind scale is no longer accurate in measuring the climate change-driven exponential increase in winds.

According to the authors, the fact that the scale is open-ended – meaning that anything beyond 157 mph or 252 km/h is classified as Category 5 and assigned the same level of wind hazard – reflects a flaw in the system, no matter if it is blowing 160 mph (257 km/h), like 2022 Hurricane Ian in the US, or 215 mph (346 km/h), like Mexico’s 2015 Hurricane Patricia. For this reason, they suggested adding a hypothetical new category – Category 6 – to the scale. This, they say, would reflect the wind speed that has already been reached in a number of storms that happened in the last decade, including Typhoon Haiyan (2013), Typhoon Meranti (2016), Typhoon Goni (2020), and Typhoon Surigae (2021) in the Western Pacific and Hurricane Patricia (2015) in the Eastern Pacific.

A 2020 analysis of satellite records from 1979 to 2017 found that the likelihood of a storm reaching Category 3 or above, with sustained winds of 185 km/h, increased by 8% per decade. In 2023, the Intergovernmental Panel on Climate Change’s (IPCC) Sixth Assessment Report (AR6) confirmed these observations, arguing that the proportion of Category 3-5 tropical cyclones as well as the frequency of rapid intensification events have likely increased globally over the past four decades. And with our atmosphere and oceans set to continue warming in the coming years as the climate crisis intensifies, there is little doubt that wind speeds will also progressively strengthen.

Featured image: NASA Goddard Space Flight Center via Wikimedia Commons.

This article was originally published on September 4, 2024.