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on the afternoon of september 6, this year's no. 11 typhoon "makar" made landfall in wenchang, hainan and zhanjiang, guangdong at super typhoon level, becoming the strongest typhoon to land in my country this year.the central meteorological observatory issued red typhoon warnings and orange rainstorm warnings in succession, and extremely strong winds and rains occurred in many places including hainan, guangdong and guangxi.

typhoon makar's path probability forecast for the next 60 hours (photo source: central meteorological observatory typhoon network)

in meteorology, typhoons are clearly divided into summer typhoons (formed between june and august) and autumn typhoons (formed between september and november) based on the month of their formation.typhoon "makar" was formed at 14:00 on september 1 and officially became one of this year's autumn typhoons.it is not only the strongest autumn typhoon to hit my country in history, but also the strongest typhoon to hit my country since typhoon no. 9 "wilmason" in 2014.

image source: china meteorological administration

it is worth noting that although the peak of typhoon activity in my country is concentrated in july and august, that is, the period of frequent summer typhoons, this does not mean that autumn typhoons lack influence or strength. the average intensity of autumn typhoons that land in september exceeds that of summer typhoons.this is the month with the strongest typhoons in the year.。

september is the month with the strongest typhoons landing in my country (image source: china weather)

in addition to typhoon makar, some extremely destructive super typhoons are also autumn typhoons, such as typhoon meranti no. 14 in 2016 and typhoon mangkhut no. 22 in 2018, both of which landed in my country in september and were removed from the list because they caused significant economic losses to my country.

why do so many "tough guys" emerge in autumn typhoons? let's start with the principle of typhoon formation.

how are typhoons formed?

the scientific name of a typhoon is a tropical cyclone, which is an atmospheric vortex formed on the tropical or subtropical ocean surface. it rotates counterclockwise in the northern hemisphere and clockwise in the southern hemisphere.。

many people wonder how the air currents in the atmosphere rotate?

simply put, uneven temperature distribution will cause the air pressure to fluctuate. just like "water flows to low places", air flows from relatively high pressure areas to relatively low pressure areas. in this process, the direction of air flow will be deflected by the coriolis force (to the right of the original direction in the northern hemisphere and to the left in the southern hemisphere) (click here to learn about the coriolis force). when large-scale air flow is constantly deflected, it may rotate.

schematic diagram of air flow around low pressure systems in the northern hemisphere. low indicates low pressure, black arrows indicate the direction of pressure gradient force (high pressure points to low pressure), and red arrows indicate the direction of air flow under the influence of the coriolis force (image source: okiofuge)

for a large atmospheric vortex like a typhoon to appear over the ocean, the following conditions are required：

1.the sea surface temperature must be high enough, requiring to reach above 26-27°cthe warm shallow seawater contains a lot of water vapor and energy, which is the source of typhoons.primary energy。

2.there are suitable disturbances in the movement of the middle and lower atmosphere, which can trigger low-level airflow to converge and rise, which is the cause of typhoon generation.startup mechanism。

3.the coriolis force must be large enough, so that the convergent airflow can gradually form a counterclockwise rotating horizontal vortex.therefore, atmospheric disturbances usually need to be located more than 5 latitudes away from the equator to develop into typhoons.。

4.the horizontal wind speed difference between the upper and lower troposphere should not be too large.when water vapor condenses in the middle and upper layers, it releases a large amount of heat to heat the middle and upper layers of the atmosphere. when the difference in wind speed between the upper and lower layers is small, this heat can stably and efficiently heat the atmosphere above the low-level disturbance.the "warm heart" structure thus formed can ensure the continuous reduction of low-level air pressure, eventually forming a typhoon.。

schematic diagram of the typhoon formation process (image source: aristo, translated by the author)

why do typhoons always appear at certain times and places?

the "typhoon season" in the northern hemisphere is from june to november each year, when typhoons are mostly generated between 5° and 25° north latitude.why does the formation of typhoons have such temporal and spatial distribution characteristics?

typhoon best path dataset distribution map of global typhoon generation locations from 1979 to 2020 (image source: provided by the author)

the vast ocean needs the sun's heating to become a breeding ground for typhoons. after the spring equinox, the sun's direct point moves northward from the equator, gradually heating the tropical and subtropical oceans beyond the equator, which allows the surface sea temperature to reach the temperature required to generate typhoons.

a fat man is not made in one bite, and a typhoon also develops from "embryos" one by one. these "embryos" are atmospheric disturbances with a tendency to rotate in the lower atmosphere. in the tropical and subtropical regions, there is an atmospheric convergence zone that almost circles the earth.equatorial convergence zonethe convergence of air currents in the convergence zone can easily "roll up" the rotation of air currents, thus brewing one atmospheric disturbance after another.about 85% of typhoon "embryos" are born in the equatorial convergence zone.。

the intertropical convergence zone in the satellite cloud image is characterized by a cloudy distribution. the green line shows the intertropical convergence zone (itcz), and the blue arrows are the northeast and southeast trade winds (image source: nasa)

the position of the equatorial convergence zone moves north and south as the position of the sun's direct point changes. it is located in the northern hemisphere in summer and early autumn.as a result, the "embryos" generated in the equatorial convergence zone also move away from the equator. on the warm ocean surface of the northern hemisphere, these "embryos" can develop into typhoons when the time is right under the influence of the greater coriolis force.

schematic diagram of the three-circle circulation of the earth's atmosphere under the action of the coriolis force. the equatorial convergence zone is formed by the convergence of the hadley circulation in the northern and southern hemispheres in the three-circle circulation at the southeast trade wind and northeast trade wind in the lower layer. (image source: the atmosphere, translated by the author)

why are autumn typhoons generally stronger?

now that we understand the basic principles of typhoon formation, we can explore why autumn typhoons are often more ferocious than summer typhoons.

the reason why autumn typhoons are so powerful is mainly because the ocean temperatures are higher in autumn.after the summer solstice, the sun's direct point moves southward from the tropic of cancer, and the subtropical ocean surface is heated for the second time, reaching the highest sea surface temperature of the year. with sufficient nutrients provided by the warm ocean, typhoons generated at this time generally grow strong and powerful, and their huge size is particularly eye-catching on satellite cloud maps.

global sea surface temperature changes over time from 2005 to 2015 (image source: wikimedia)

after entering september, the summer monsoon in the south china sea weakens and retreats southward. at this time, the autumn typhoons generated in the northwest pacificit is less affected by the south china sea monsoon, and its structure is more symmetrical, which is more conducive to the typhoon's own rotation and enhancement.

historical data show thatin the northwest pacific region, although september does not have the most typhoons, it is the month with the highest proportion of super typhoons and strong typhoons.。

(photo source: china weather)

what impact does global warming have on typhoon activity?

typhoons are often associated with the heat of summer and autumn, and many people believe that global warming means that typhoons will become more frequent. however, the facts are contrary to this.

statistical studies show that the average annual number of typhoons worldwide has shown a significant downward trend since the 20th century.。

since the beginning of the 20th century, the average annual number of typhoons generated in (a) the world, (b) the southern hemisphere, and (c) the northern hemisphere has decreased significantly. (image source: chand et al., 2022)

on the one hand,the warm ocean raises the threshold for atmospheric disturbances to develop into typhoons, because the increase in temperature allows the air to hold more water vapor, which means that more water vapor is needed to saturate the air and condense to release energy.。

on the other hand, under global warming, the arctic region has warmed more than the global average temperature (a phenomenon known as the "arctic amplification effect"), which has led to a decrease in the temperature difference between the arctic and the equator and a weakening of the hadley circulation driven by thermal differences. this means thatthe convergent upward movement of the equatorial convergence zone weakens, and its ability to lift water vapor weakens。

equatorial convergence zone (image source: nasa, translated by the author)

similar to the characteristics of autumn typhoon activity, although the frequency of typhoon generation decreases as the sea temperature rises,the overall rise in sea temperature means that once a typhoon is formed, the possibility of it developing into a strong typhoon or even a super typhoon will become increasingly high. the more water vapor in the air also means that the rainfall brought by future typhoons will be more torrential.。

the autumn typhoon "makar" that landed this time has become the "king of winds" in the western pacific and the south china sea this year, and is extremely destructive. the hospital reminds friends in the typhoon-affected areas to take protective measures, minimize outdoor activities, and make adequate preparations to resist the typhoon!