Japan's rainy season has begun in its southern regions up to nine days earlier than average, prompting speculation that Korea's monsoon may also arrive earlier than usual this year. However, whether Korea's rainy season will start early remains uncertain.
The Japan Meteorological Agency announced Friday (local time) that the Amami region in southern Japan appears to have entered its rainy season on June 3. This is nine days earlier than the average (June 12) and two days earlier than last year.
The Okinawa region also entered its rainy season on June 4, six days earlier than the average (June 10) and one day earlier than last year.
Both Amami and Okinawa are island regions in southwestern Japan, but they are classified as separate zones in rainy season statistics. Amami refers to the island region belonging to Kagoshima Prefecture, located between southern Kyushu and northern Okinawa, while Okinawa refers to the area of Okinawa Prefecture farther south. Unusually, this year the more northern Amami region entered the rainy season one day before Okinawa.
The Japan Meteorological Agency explained that cloudy and rainy weather has continued due to a stationary front and the inflow of humid air from the south. The agency forecast that cloudy and rainy days will continue for more than a week.
The expansion of the North Pacific High is cited as the background for the early rainy season in southern Japan. A seasonal rain front typically forms when warm, humid air from the south meets relatively cold air from the north. This year, the North Pacific High, carrying warm air, expanded its influence earlier than usual, pushing the frontal zone northward, according to analysts. If the upper-level jet stream also shifts farther north than average, the monsoon front could activate even earlier.
However, experts say it is difficult to conclude that Japan's early rainy season will directly lead to an early monsoon in Korea. The Korean Peninsula's rainy season depends not only on how much the North Pacific High expands after June, but also on the strength of cold air masses north of the peninsula and the position of the stationary front.
Experts say attention should be paid not to when Japan's southern rainy season begins, but to how quickly the southern frontal zone becomes active compared with previous years. How rapidly the monsoon front moves northward through Japan's Kyushu and main islands to the southern waters of the Korean Peninsula could become a key variable for Korea's rainy season, they said.
Meanwhile, Korea's average rainy season start dates are June 19 for Jeju, June 23 for the southern regions, and June 25 for the central regions.
According to the Korea Meteorological Administration's three-month outlook (May to July), there is a 50% probability — the highest — that this summer's monsoon rainfall will be similar to the average. The average precipitation compiled by the agency is about 148.6 mm in June and 280.7 mm in July.
