Why did Typhoon Kanu suddenly make a U-turn?

In recent days, news about "Typhoon Kanu may make a big turn and land in our country" has frequently become a hot search.

Image source: Sina Weibo

Typhoon Kanu has been in existence for 11 days since it was formed on July 28. Looking back at its path, it has undergone several major adjustments and is now about to make a sharp 90-degree turn, moving in a zigzag pattern.

Judging from the current situation, it may move northward to affect my country's northeast region in the later period, bringing significant rainfall.

Why did Typhoon Kanu have such a strange "movement"? What determines the path of a typhoon?

Let’s talk about it in detail today.

How are typhoons formed?

We know that typhoon refers to a tropical cyclone with sustained wind speeds of 8 or above in the northwest Pacific Ocean and the South China Sea. A typhoon in a narrow sense is a tropical cyclone of 12 to 13 levels.

The formation of a typhoon is a very complicated process, and humans still have not fully figured out the reasons for its formation.

But typhoons do not occur everywhere in the world. For example, we rarely hear of typhoons hitting places like Russia, Chile, and Argentina. This is because there are two necessary conditions for a typhoon to form .

First, where a typhoon occurs, the seawater temperature must be above 26°C, and the water temperature must reach this temperature within 60 meters of the water depth. In this way, the water vapor on the sea surface evaporates quickly, forming a low-pressure center, which is an important basis for the formation of a typhoon.

Secondly, there is the Coriolis force that we learned about in high school geography .

The formation of a typhoon requires a sufficiently large Coriolis force. After low pressure forms in the center of the typhoon, the surrounding air moves toward the center of the typhoon and begins to form an air mass that rotates counterclockwise (clockwise in the southern hemisphere) under the action of the Coriolis force.

Therefore, typhoons generally form not very close to the equator . Only a very small number of typhoons appear within 1.5 degrees of latitude from the equator.

Based on these two necessary conditions, it can be seen that typhoons are mainly produced in tropical waters and often appear in summer and autumn .

The picture below shows several areas around the world where typhoons are most likely to occur.

Typhoon-prone areas around the world, image source: wikimedia

As can be seen from the picture, there is a typhoon area in the northwest Pacific Ocean around our country - a typhoon-prone area. According to the data from the National Oceanic and Atmospheric Administration (NOAA), typhoons occurring here account for almost 1/3 of the world's typhoons.

Where will the formed typhoon go?

The typhoons in our country mainly come from the northwest Pacific Ocean. Typhoon No. 5 "Dusurui" this year and Typhoon No. 6 "Kanu" this time both originated here.

How do typhoons decide where to go after they form? Why don't they go east or south? Also, why do typhoons like Kanu appear?

There are too many factors that affect the movement trajectory of a typhoon. It is an extremely complex chaotic system, which brings great difficulties to the prediction of the typhoon path .

Today, although there are many models for predicting typhoon trajectories, no model can predict the typhoon's next movement trajectory with 100% accuracy . Here, we can only briefly talk about it from two aspects.

First of all, an important factor that affects the trajectory of typhoons is the "subtropical high pressure". Most typhoons in the Northern Hemisphere are generated in the tropical convergence zone on the south side of the subtropical high pressure. Typhoons will move forward under the guidance of the subtropical high pressure, just like an adult leading a child .

The high-pressure airflow in the northern hemisphere rotates clockwise, so when a typhoon is on the south side of the subtropical high pressure, it will move westward under the guidance of the easterly wind. When it reaches the west side of the subtropical high pressure, it will turn northward, and when it reaches the north side of the subtropical high pressure, it will turn northeastward.

If the subtropical high pressure is very strong and in a belt-like shape, extending from the sea to the inland of China, the typhoon will generally travel westward and land in China . If the subtropical high pressure is not in a belt-like shape and does not enter the inland of China, but is on the eastern ocean, the typhoon will follow the edge of the subtropical high pressure and take a parabolic path - traveling westward, northward, and then northeastward.

A single subtropical high pressure guides a typhoon, and the path is relatively simple. But if the subtropical high pressure breaks into multiple ones, and they compete with each other and guide in opposite directions, the path of the typhoon becomes very complicated, which is equivalent to parents disagreeing and the child not knowing who to follow.

This time, Kanu suddenly turned around the southeast coast of our country and southern Japan because the subtropical high pressure broke into three pieces. The high pressure on the land northwest of the typhoon and the high pressure on the south of the typhoon were fighting each other. After the high pressure on the north weakened, the high pressure on the south became slightly stronger, guiding the typhoon to move slowly eastward. After that, the high pressure on the east of the typhoon strengthened again and stretched westward, taking over the typhoon and guiding it northward.

Typhoon Kanu's path, Image source: Screenshot from Zhejiang Provincial Water Resources Department website

Another factor that affects typhoons is beta drift, which is related to the conservation of angular momentum and the Coriolis force, and it can also pull the typhoon toward the northwest.

In addition, if there are other typhoons around a typhoon and the two are close to each other (if the two typhoons have large circulations, the distance is within 1,500 kilometers; if the two typhoons have small circulations, the distance is within 1,000 kilometers), mutual rotation will occur, which is also called the Fujiwara effect .

Therefore, although the typhoon paths formed in the northwest Pacific vary greatly, they generally move northwestward, mainly along the following three paths:

Straight west: Typhoons along this path will mainly affect countries and regions such as the Philippines, southern China, Taiwan, China and Vietnam.

Movement towards the northwest arc: Typhoons along this path may affect the Philippines, Taiwan, China, the southeastern coastal areas of China, as well as Japan and South Korea.

Northward movement: Such a typhoon may mainly affect some islands in the Pacific Ocean.

Image source: wikimedia

This is not the first time that a typhoon like Kanu has been so erratic. In comparison, Kanu's path is not the most "strange" .

For example, the picture below shows the path of Typhoon Wayne in 1986. It circled the South China Sea several times, becoming one of the typhoons with the strangest path.

Typhoon Wayne's path, image source: wikimedia

There was also Typhoon Knight in 1991. After it formed, it headed westward, but suddenly made a 180-degree turn to the east. Just when everyone thought it was going to plunge into the Pacific Ocean, Knight made another 180-degree turn and swept across the southern part of Taiwan, China, causing very serious damage.

When it turned south and everyone thought it was leaving, Knight made another 180-degree turn and landed in Guangdong. Its trajectory was much more complicated than that of Kanu this time.

Typhoon Nate's path. Image source: wikimedia

The path of a typhoon is really difficult to predict. When facing a typhoon, we cannot take it lightly. We must be prepared and do everything possible to reduce the losses caused by the typhoon.

References

[1] China Meteorological Administration - "Conditions for the Formation of Typhoons"

[2] NOAA website:

[3] Hu G, Lu MH, Reynolds DR, et al. Long-term seasonal forecasting of a major migrant insect pest: thebrown planthopper in the Lower Yangtze River Valley[J]. Journal of PestScience, 2019, 92: 417-428.

Author: Scientific Scraps

Audit丨Xin Xin, Senior Engineer of China Meteorological Administration

Senior Engineer, Lhasa Beijing Meteorological Bureau