Explainer: What are typhoons, and how are they linked to climate change?
Hong Kong Free Press
By Martina Igini
Tropical cyclones are powerful storm systems characterised 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 kilometres per hour 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 either as hurricanes or as typhoons. The former term is particularly used in the US as it comprises cyclones that originate in the Atlantic Ocean or northeastern Pacific Ocean.
Tropical cyclones that form 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 kilometres per hour. In southern China, Hong Kong and Taiwan, the minimum speed must be 118 kilometres per hour.
Typhoons are then classified further as their speed increases. Hong Kong, for example, refers to them simply as Typhoons when the wind speed does not exceed 149 kilometres per hour, after which they become Severe Typhoons. When the speed is 185 kilometres per hour or above they are known as Super Typhoons.
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 of the territory and poses a threat of deteriorating conditions.
The warning can increase to a T3, indicating strong winds; a T8, indicating gale or storm force winds; a T9, which means that gale or storm force winds are increasing; and the maximum T10, indicating that hurricane force winds with sustained speeds reaching 118 kilometres per hour of above are forecast.
How do tropical cyclones form?
To form, tropical cyclones need a minimum sea surface temperatures of 26 degrees Celsius. This 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 favourable wind conditions all contribute to fuelling the cyclone and making it more powerful.
Viewed from space, tropical cyclones have spiral cloud bands and a visible “eye,” the cyclone’s centre. 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 kilometres per hour and is typically 32 to 64 kilometres 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 cyclone-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. They can cause 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. Damage was estimated at US$186.3 billion.
In the Pacific, the most expensive tropical cyclone to date was Typhoon Doksuri in 2023, which caused over US$28 billion in damages, mainly in China. It is followed by Japan’s Typhoon Hagibis in 2019, which resulted in damages of US$18 billion.
Hong Kong’s costliest typhoon to date is Super Typhoon Mangkhut, which hit the territory in 2018 and resulted in HK$4.6 billion in direct economic losses.
How are typhoons linked to climate change?
Though tropical cyclones are fairly common, 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 last month reached 20.88 degrees Celsius, the second-highest value on record and just 0.01 degree shy of the value recorded in July 2023.
This capped 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, they are becoming more destructive – generating heavier rain and a higher storm surge.
“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 kilometres per hour. According to the attribution analysis, the winds were about 14 kilometres per hour, or 7 per cent, 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.
Gaemi 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, the WWA concluded that the rainfall that hit Taiwan and China’s Hunan province was made about 14 per cent and 9 per cent 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 three-hourly storm rainfall rates by 10 per cent and extreme three-day accumulated rainfall amounts by five per cent.
Adapting to the ‘new normal’
As climate change rewrites the rules on 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 increase in winds.
According to the authors, the fact that the scale is open-ended – meaning that anything beyond 252 kilometres per hour 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 257 kilometres per hours, like 2022’s Hurricane Ian in the US, or 346 kilometres per hour, like Mexico’s 2015 Hurricane Patricia.
For this reason, they suggest adding a hypothetical new category – Category 6 – to the scale. This, they say, would reflect the wind speed already reached in a number of storms 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 kilometres per hour, increased by 8 per cent per decade.
In 2023, the Intergovernmental Panel on Climate Change’s Sixth Assessment Report confirmed these observations, arguing that the proportion of Category 3 to 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.
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