2024-09-21
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"prasang" brings heavy rain to shanghai (photo by chen long)
"bebejia" has just left and "prasang" has arrived. autumn typhoons come in groups every week.
halfway through september, my country was hit by two severe typhoons in succession. from september 6 to 8, typhoon makar, the 11th typhoon this year, with a maximum wind speed of 17, swept through the philippines, china, vietnam and other places with super typhoon-level winds, causing serious disasters; on the eve of the mid-autumn festival, typhoon bebejia landed in lingang new city with a peak wind speed of 14, and shanghai encountered the strongest typhoon in 75 years.
in recent years, news of strong typhoons seems to appear more and more frequently every summer and autumn. typhoons are formed over tropical oceans, and the warm ocean provides sufficient heat and water vapor for the occurrence and development of typhoons. global warming has caused the ocean temperature to rise, and a question that follows is, does this mean that typhoons are becoming more and more numerous and stronger?
the simpler the question, the harder it is to answer.
as far as typhoons are concerned, the first problem is observation data. typhoons spend most of their life cycle in the ocean. in the days before meteorological satellites, human observations of tropical cyclones around the world were very limited. on the vast ocean, many typhoons and hurricanes that rolled up huge waves may never be known from their appearance to their demise. at that time, only in coastal areas did humans have some observations of typhoons, but a lot of information was not available. with the widespread use of meteorological satellites and the improvement of observation levels, the number of typhoons observed by people has increased, but this does not represent whether the actual number of typhoons is increasing.
image source: pixabay
the relationship between global warming and typhoons has always been a hot topic in international research, with the earliest breakthrough coming in 2005. in that year, renowned typhoon expert emanuel and meteorologist webster published papers in nature and science magazines respectively, pointing out that due to global warming, the potential destructive power of tropical cyclones in the north atlantic and northwest pacific has increased by more than double and about 75% respectively over the past 30 years; in the 1970s, the world had an average of about 10 super typhoons (category 4 to 5 hurricanes) per year, and after the 1990s, the number reached 18 per year, almost doubling - the largest increase was in the north pacific, indian ocean and southwest pacific, and the smallest increase was in the north atlantic.
for more than 20 years, researchers have studied this issue from multiple perspectives, including observation, theory, and numerical simulation. the data used has become increasingly sophisticated, and the resolution of numerical models used to simulate typhoons has also become increasingly higher. many studies have reached a relatively consistent conclusion on the relationship between global warming and strong typhoons, namely that global warming may lead to an increase in the intensity of typhoons and hurricanes, and the intensity of tropical cyclones may increase, resulting in more super typhoons.
the number of typhoons has decreased, and typhoons have a "brake" effect on sea temperatures
however, there is controversy in the academic community over the impact of global warming on the number of typhoons and hurricanes. some studies have shown that global warming may lead to an increase in the frequency of tropical cyclones; other studies believe that global warming may lead to a decrease in the frequency of tropical cyclones. the core of the disagreement between the two is how to evaluate the "brake" effect of typhoons on sea temperatures.
the ocean is heated mainly by the sun's shortwave radiation. therefore, the ocean surface temperature is high, while the lower water temperature is low. when a typhoon passes by, it will stir up huge waves on the sea surface, causing the lower cold water to be rolled up to the surface. therefore, the sea surface temperature drops after the typhoon, just like stepping on the "brake" of the rising sea temperature.
the "brake" effect of a typhoon can be strong or weak: when a typhoon is stronger, it can roll up cold water with lower temperature, so the "brake" effect is stronger; when a typhoon is weaker, the cold water it rolls up is not that cold, so the "brake" effect is weaker.
that is to say, under the trend of global warming, typhoons are becoming more powerful, and when a super typhoon rages, the sea surface temperature is likely to cool down after experiencing the "brake" effect, thus affecting the formation and development of subsequent typhoons, thereby leading to a decrease in the number of typhoons.
autumn typhoons are active, and the tough ones are more destructive
since hurricanes and typhoons originate from tropical oceans, warmer sea surface temperatures mean that hurricanes will carry heavier rainfall and are more likely to bring extreme rainfall. numerical models show that by the end of this century, the average rainfall brought by tropical cyclones will increase by about 10%-15%.
typhoon bebejia brings heavy rainfall to shanghai (photo by zhang ting)
stronger hurricanes and typhoons also mean stronger winds, which will be more destructive, especially considering the continued rise in sea levels in the future. storm surges during hurricanes and typhoons will also be more destructive.
global warming leads to warming of the ocean, which means that as typhoons move toward mid-latitudes, they can continue to gain energy from the ocean and become stronger, so there will be more and more tropical cyclones that reach typhoon level before making landfall. this trend has been confirmed by observational data.
the researchers analyzed the data of typhoons that made landfall in my country from july to september between 1973 and 2017, and found that since 2004, about 9.7% of the typhoons that made landfall in southern my country have rapidly intensified within 24 hours before landing, while between 1973 and 1987, this proportion was only 1.6%, and between 1988 and 2003 it was 3.1%.
in addition, because warm sea temperatures last longer, typhoons are more active in autumn. compared with summer typhoons, there are more "tough guys" in autumn typhoons, and a high proportion of strong typhoons and above. and because the subtropical high pressure in the northwest pacific that affects the typhoon path weakens and retreats eastward in autumn, the typhoon path is more inclined to japan and south korea.
for example, from september to october 2018, japan was hit by super typhoons "jebi", "trami" and "konnie" one after another. among them, "trami" swept across japan, causing the closure of kansai international airport, 500,000 households in kyushu and okinawa, and about 560,000 households in the tokyo electric power company area. japan's high-speed trunk trains, buses, ferries, subways and suburban trains were completely interrupted, resulting in a total of 5 deaths, 1 missing, and more than 200 injuries.
strong northward expansion: "slow progress" brings stronger wind and rain
due to global warming, ocean temperatures have risen, and tropical regions have expanded toward the poles of the northern and southern hemispheres. the typhoon's impact range has also expanded accordingly, with its activity range in the northern hemisphere becoming more northerly. the number of typhoons "going north" has increased, and they have even advanced into areas where they have never appeared before.
in august 1975, typhoon nina drove straight westward to the western part of henan province, bringing heavy rains to a large mountainous and hilly area between the funiu mountains and the tongbai mountains in henan province, setting many historical records for heavy rains in my country. the heavy rains caused many reservoirs to burst, with an area of 12,000 square kilometers of waterlogging, and destroyed 102 kilometers of the beijing-guangzhou railway, causing traffic disruption for 16 days, affecting 10.155 million people, and causing direct economic losses of nearly 10 billion yuan.
typhoon bebejia knocked down trees on the roadside (photo by zhao lirong)
studies have shown that the speed of tropical cyclones is also slowing down. from 1949 to 2016, the speed of tropical cyclones decreased by 10%. in 1949, the average speed of tropical cyclones exceeded 19 kilometers per hour, and in 2016, the average speed dropped to about 17 kilometers per hour. this slowdown is more obvious over land, with tropical cyclones slowing down by 30% over the land in the northwest pacific region and 20% over the land near the north atlantic and australia.
when a typhoon moves more slowly, on the one hand, it can develop more fully in the tropical ocean and grow into a stronger typhoon; on the other hand, it rages on land for a longer time, thus bringing stronger wind and rain disasters. when a typhoon moves slowly, its circulation is easier to maintain stability, which is more conducive to the long-distance transport of water vapor. for example, before and after the heavy rainstorm disaster in zhengzhou in july 2021, the moving speed of typhoon "fireworks" far away in the east china sea was only 5-15 kilometers per hour, which made the easterly airflow on the north side of the typhoon stable for a long time, continuously transporting water vapor from the sea to the central plains.
text: wei ke (researcher at the institute of atmospheric physics, chinese academy of sciences)
photos: all images are from visual china unless otherwise noted
editor: liu qi
editor-in-charge: xu qimin
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