Typhoon No. 9 Rekima is heading north. The typhoon 'Rekima', which is heading northeast of Taiwan's southeast as of August, landed near Fuzhou in southern China on Saturday, 10, and then near Shanghai on the 11th, then back to the west sea on Monday, 12th. The Meteorological Agency is looking to advance. The path is still in flux, but is expected to pass around Qingdao on Tuesday (see figure below). If we continue to head northwest as forecast, our country is likely to be directly or indirectly affected by the typhoon 'Rekima'.
If Typhoon 'Rekima' actually affects the Korean Peninsula directly or indirectly, the eighth typhoon 'Francisco', which was weakened by the tropical low pressure zone after landing in Busan on the 6th, and north of the sea near Jindo on the 20th of last month, It is the third typhoon to affect the Korean peninsula this year, following the weakened fifth typhoon 'Danas'. In particular, the typhoon 'Rekima' is not a weak small typhoon like the typhoon Danas or Francisco. The typhoon 'Rekima' has developed into a very strong typhoon with a strong wind of about 170 kilometers per hour in the center. The Meteorological Agency predicts that the typhoon will remain as medium-sized in strength even after the typhoon has advanced to the west sea near Shanghai, China.

● Back Pacific High Pressure

If it's the same as the previous year, why is the typhoon heading toward the Korean peninsula after the heat wave starts? First of all, consider the North Pacific high pressure which has the greatest influence on the hurricane's path. The North Pacific high pressure is a barometer that is much larger than the typhoon, which usually travels along the edge of the North Pacific high pressure rather than pushing or traversing through it.

If the North Pacific High Pressure covers the Korean Peninsula broadly, the typhoon will not immediately fly north to the Korean Peninsula, but it is more likely to turn around the North Pacific High Pressure to China or Japan. On the contrary, if the North Pacific High Pressure has been withdrawn to Japan, the typhoon is likely to rise north of the Korean Peninsula along the edge of the North Pacific High Pressure. The road to the typhoon will open.

For example, on August 7, the Geopotential height, which is 5.5 km (500 hectopascals) at the top, shows that the North Pacific high pressure has receded toward Japan (see figure below). Normally, the 5880 isoline at an altitude of 500 hetopascals is viewed as the edge of the North Pacific high pressure, which can be seen receding toward the East Sea and Japan. The eighth typhoon 'Francisco' landed in Busan along the edge of the North Pacific high pressure. The typhoon was heading north along the path that the North Pacific High Pressure backed down.
In early August, when the heat wave is on the rise, it is common for the North Pacific high pressure to extend not only to the Korean peninsula but also to northeast China. Last year, the North Pacific high pressure developed wide and strong, resulting in record heat waves instead of typhoons on the Korean peninsula. As the North Pacific high pressure is relatively weaker than last year and backed off earlier this year, typhoons are coming up instead of the record heat wave like last year. In the same year, the heat wave appears from the end of August when the heat is receding, but this year, it appears in early August. Therefore, if the current situation continues for some time, it is likely that another typhoon will wind up near the Korean peninsula. Of course, the forces of the North Pacific high pressure are constantly changing, so if you expand a little more than now, it is likely that you will push the northbound typhoon a little more toward China.

● What is the possibility of Fujiwara effect?

There is a typhoon that must be seen together with the 9th typhoon 'Rekima'. Typhoon No. 10 is 'Crossa'. The typhoon 'Crossa' is expected to go north to Japan. However, it is not yet clear whether the typhoon 'Crossa' will move northward to Japan and move eastward along the archipelago or through the Japanese archipelago and affect the North Korean Sea peninsula to the East Sea. In particular, it is impossible to completely rule out the possibility of taking a route that was not thought of by the interaction between typhoon 'Rekima' and typhoon 'Crossa'. The so-called Fujiwara effect.
The Fujiwara effect refers to a phenomenon in which two typhoons of similar size affect each other within 1,000 ~ 1,400km, resulting in abnormal movement speeds or paths. A typical abnormal path that can be caused by the Fujiwara effect is a form in which two typhoons face each other and rotate counterclockwise.

The distance between the 9th typhoon Rekima and the 10th typhoon Crosa is about 1,500 km. Strictly applying the standard, it is true that the Fujiwara effect will not be significant at present. However, it can be considered that the distance between the two typhoons is a little closer than now, when the typhoon Rekima is north of Shanghai or the West Sea, and the typhoon 'Crossa' is north of the Japanese archipelago. It means that the possibility of interaction between two typhoons cannot be completely excluded in the future. The interaction between typhoon 'Crossa' and typhoon 'Rekima' may be a variable in predicting the movement path and speed of the two typhoons.

Although it is a scenario that I do not want to assume, the possibility of 'Rekima' heading north on the West Sea and the typhoon 'Crossa' going north on the East Sea after penetrating Japan. There is still a lot of uncertainty, but a forecast model from the ECMWF suggests a scenario in which the typhoon 'Rekima' is north of the West Sea and the typhoon 'Crossa' is north of the East Sea. It also means that the possibility of typhoons affecting the Korean Peninsula in the east and west can not be ruled out.

The average number of typhoons affecting the Korean Peninsula during the decade from 2001 to 2010 is 2.5 annually. The most common times of typhoons are from late August to early September. This is the time when the sultry North Pacific high pressure that normally covers the Korean Peninsula retreats and opens the way for typhoons. This year, it is likely that the road to the typhoon was opened earlier than usual. As the typhoon opens earlier, heatwaves will be less than last year, but the risk of typhoons increases.

In particular, the temperature of the sea level in the Northwest Pacific, the region where typhoons occur, is around 30 degrees Celsius, which is 1 degree higher than normal. This means that there is enough energy to supply typhoons. Differences in wind direction and intensity [wind shear, wind shear] between the upper and lower layers of the typhoon region are also good conditions for strong typhoons. Typhoon tends to weaken around the Korean peninsula due to lower sea temperatures than normal, but this year also requires preparation for strong typhoons.