Professor Kevin E. Trombes of the National Center for Atmospheric Research has been tracking and investigating severe storms for many years. He said that since 2004, the activities of tropical storms have been obviously "rampant": four hurricanes hit Florida, unprecedented; 10 typhoons landed in Japan, exceeding the historical record by 4! In 2005, the number of summer hurricanes in the North Atlantic once again broke records, including destructive hurricanes Katrina and Rita. But by 2006, the total number of hurricanes and typhoons had been greatly reduced. Then the question is, if global warming really has an impact on the occurrence of typhoons, why did it happen so little in 2006?
Let's look at how typhoons are formed: the predecessor of typhoons is tropical atmospheric disturbance (the constant state of the atmosphere is destroyed or changed). When the disturbance developed into an organized thunderstorm system, it began to rotate.
If the wind speed exceeds 62 kilometers per hour, meteorologists will number it. If the maximum wind speed of the system exceeds 1 19 km per hour, it is called a tropical cyclone. But there are different names in different regions, such as the Atlantic Ocean and the Northeast Pacific Ocean, the Northwest Pacific Ocean and the Indian Ocean.
In recent years, meteorologists have calculated models to simulate the generation of typhoons according to the characteristics of different regions.
First of all, the formation of typhoons has a most basic condition-warm sea water, and their "birthplace" is mostly in tropical waters. Due to the strong direct sunlight, seawater absorbs a lot of heat and dissipates in the form of evaporation. The evaporated water vapor condenses to form rain, and finally releases heat into the atmosphere.
In winter, the wind will send heat to high latitudes and then radiate it into space; But in summer, heat stays at the equator and rises to the sky through convection. Convection can cause all kinds of weather phenomena-from cumulus clouds to thunderstorms. Under proper circumstances, thunderstorms can form a vortex, absorb a lot of heat energy from the ocean, and eventually form a typhoon.
The second key factor of typhoon formation is vortex, which comes from atmospheric disturbance. For example, the atmospheric disturbance in the North Atlantic usually comes from the west coast of Central Africa, because the temperature difference between the desert in the interior of Africa and the forest in the coastal mountainous area is very large, which is conducive to atmospheric disturbance.
Sea surface temperature must be higher than 26℃, water vapor should be sufficient, there is a low pressure area on the ocean surface, and wind shear between high and low latitudes (wind shear is an atmospheric phenomenon, which is a sudden change of wind speed in horizontal and vertical directions) is weak, which is also the condition for vortex formation.
In a word, temperature and vortex are the key factors of typhoon formation. In connection with the well-known global warming problem in recent years, is the frequent occurrence of typhoons related to this?
Articles published in the American magazines Nature and Science in 2005 affirmed the conclusion that global warming leads to frequent typhoons. Peter Webster, a meteorologist at Georgia Institute of Technology in the United States, and his colleagues wrote in Science magazine that according to the data survey in the past 35 years, the number of tropical cyclones with intensity exceeding 4 (equivalent to a strong typhoon) increased significantly, and after the mid-1980s, the number climbed by about 57%. Kerry immanuel of Massachusetts Institute of Technology also published a paper in Nature, arguing that global warming is inseparable because tropical cyclones need warm sea surface to provide energy.
Other climatologists also believe that the number of numbered North Atlantic storms and typhoons has gradually increased since 1994. It is worth noting that the SST in the North Atlantic at north latitude 10 ~ 20 degrees has also increased, which is consistent with the increase in the number of hurricanes.
However, some scientists believe that the evidence is still insufficient to prove that global warming has a direct impact on typhoons. For example, Trenberth of the National Center for Atmospheric Research believes that the trend of increasing typhoon intensity pointed out by these studies is still not convincing enough, because the time to analyze the data is not long enough, only more than 30 years.
In addition, there is another view that the increase of SST in the North Atlantic after 1994 is only a reflection of the impact of the Atlantic Oscillation (AMO). AMO is a natural cycle. After several decades of relatively low temperature, the SST in the North Atlantic will increase for several decades, and then gradually decrease (the maximum temperature difference is about 0.5℃).
However, neither those who support the impact of global warming on typhoons nor those who oppose it can reasonably explain the abnormal phenomenon in 2006.
It is only speculated that this may be related to the "La Nina phenomenon" (specifically, the sea surface temperature in the East Pacific is lower than normal by more than 0.5℃ for six consecutive months)-La Nina was formed from the end of 2005 to the beginning of 2006, and brought strong sea breeze in the North Atlantic, which lowered the sea surface temperature. The sea surface temperature in 2006 was lower than that in 2004 and 2005, so the typhoon in 2006 converged.