Unusual Loss of the Rainy Season: Japan Observed with "Zero Fronts" in Mid-June—Where Are the Four Seasons Headed?

1. What Happened—The Anomaly of "Zero Rainy Season Skies in June"

According to the weather chart published by the Japan Weather Association on the 18th, the line of the rainy season front covering the Japanese archipelago had completely disappeared. Normally, the front would be stagnant near Honshu, but it was nowhere to be found either north or south, resulting in a "mid-summer pattern" with the Pacific high-pressure system extending as if it were midsummer. tenki.jpnofia.net.

When meteorologist Yuji Sugie examined the past 10 years (11th to 20th of June at 9 a.m. observations, 100 days in total), he found that there was only one day during this period without a front—the occurrence probability was about 1%. nofia.net.

Why is "Zero Front" a Problem?

• Insufficient Dam Water Storage: The rainy season plays a role in supplementing water resources for the summer. In years when the front does not function, there have been instances of water intake restrictions in systems like the Tone River.
  
  

• Impact on Agriculture: If there is not enough rain during the rice transplanting period, it can cause delays in growth and a decline in quality.
  
  

• Increased Risk of Heatstroke: If high pressure remains instead of the front, sunlight and temperature rise sharply, leading to the peak heat period without adequate heat acclimatization.
  
  

2. What is the Rainy Season Front?

The rainy season front (East Asian summer monsoon front) is a stationary front that occurs near Japan due to the clash between the warm, humid air of the Pacific high-pressure system and the cold, dry air of the Okhotsk high-pressure system.

From early June to mid-July each year, it meanders and stagnates near Honshu, bringing prolonged rain and heavy downpours. For readers from abroad, it is known as the "Rainy Season in Japan (Tsuyu)" and is a significant weather factor that influences travel plans.

3. The Basis of "1%"—A Rare Case Seen in Past Data

This statistic is a preliminary value based on Sugie's manual analysis, but it uses the actual weather charts published by the JMA, making it highly reliable. While future scrutiny is necessary, it can be said to be an unusual event occurring "once in several decades."nofia.net

4. Is it just a coincidence this year? ―― Three meteorological factors

4-1 Unusual expansion of the Pacific High

• The sea surface temperature (SST) anomaly reached +1.0℃ in the western North Pacific, activating convection

• The high-pressure center shifted eastward more than usual, covering the skies over Honshu
  
  
  

4-2 Meandering of the westerlies (jet stream)

• Due to the blocking high around the Tibetan Plateau, the westerlies meandered significantly northward

• The front was "pushed up" and retreated to the vicinity of the Sea of Japan to Sakhalin
  
  
  

4-3 Weakening of the Siberian residual cold air mass

• The heating inside Eurasia accelerated after spring, and the strength of the Okhotsk Sea high-pressure was 15hPa below the average

• The supply of cool air was insufficient, weakening the temperature difference necessary for front formation
  
  

5. Will the four seasons disappear? ―― Global warming and the "two-season" debate

Meta-analysis by research institutions reports that since the 1950s, summer in the Northern Hemisphere has extended from an average of 78 days to 95 days, and winter has shortened from 76 days to 73 daysjircas.go.jp. In Japan, too, there were consecutive heatwaves and heavy rains in 2024, with domestic researchers warning of the "two-season" phenomenonnippon.com.

5-1 "Narrowing" of spring and autumn

• Cherry blossoms are blooming an average of seven days earlier than 30 years ago, and the peak of autumn foliage is delayed by two weeks

• The "optimal planting period" for crops is compressed, increasing heat stress during the harvest period
  
  
  

5-2 Transformation of the rainy season and heavy rain risk

• In years when the rainy season ends early, linear precipitation zones are more likely to appear in late July (bimodal pattern)

• The polarization of "short rainy season + localized heavy rain" simultaneously causes urban water damage and agricultural drought
  
  

6. Impact and adaptation ―― Actions for visitors and residents in Japan

1. Heatstroke measures

   • There is a possibility of continuous temperatures exceeding 35℃.Check the WBGT (Wet Bulb Globe Temperature) indoors.

   • Be mindful of the differences in air conditioning settings on public transportation and wear clothes that are easy to put on and take off.
     
     

2. Water and energy conservation actions

   • There is a risk of early implementation of water intake restrictions as early as June. Accommodation facilities distribute water-saving sheets.
     
     

3. Redesign travel plans

   • Shopping customers expecting a “Rainy Season Sale” should consider regional differences such as in Northern Japan and Okinawa.
     
     

4. Utilize disaster prevention apps

   • Even with a dominant high-pressure system, sudden heavy rains are possible when fronts revive or typhoons approach.
     
     

7. Global Perspective—The Worldwide Progress of “Seasonal Reorganization”

• Europe: The phenomenon known as "the disappearance of spring" is increasing due to the influence of the Atlantic multi-jet stream.

• North America: The frequency of "Flash Drought" occurrences has doubled in the Plains region.

• Southeast Asia: Observations of a "shoulder season shift" due to delayed monsoon arrival
  
  
  

Japan's current case is merely a "part" of the seasonal transformations reported worldwide. Rather, the geographical condition of being an "archipelago prone to drastic climate changes despite its latitude" may be amplifying the changes.

8. Conclusion—How Will Japan, the “Country of Four Seasons,” Survive?

The temporary disappearance of the rainy season front is likely to repeat in the future. For Japan, where the four seasons have been the foundation of culture, economy, and tourism,a shift in thinking from "preserving the four seasons" to "adapting to changing seasons"is essential.

• Tourism: From seasonal products to "experience design incorporating weather risks"

• Agriculture: Transition to high-yield, heat-resistant varieties, smart irrigation

• Administration: Develop scenarios for water resources and power demand one month earlier than usual in the event of front disappearance
  
  
  

Japan's four seasons will not be completely lost. However, qualitative changes such as the "shrinkage of spring and autumn," "bimodal rainy season," and "prolonged summer"are progressing. Balancing tradition and adaptation is the key to becoming a "new seasonal tourism nation" in the era of climate crisis.

Reference Article List

• Yuji Sugie "Unprecedented: The Disappearance of the Rainy Season Front in Mid-June, Only a 1% Probability" Yahoo! News (2025/06/17) nofia.net

• Tomomi Yoshida "Will the Rainy Season Return? The Disappearance of the Rainy Season Front" tenki.jp (2025/06/18) tenki.jp

• nippon.com "Is Global Warming Targeting Japan? The Era of Extreme Heat and Heavy Rain, 'From Four Seasons to Two'" (2024/12/26) nippon.com

• JIRCAS "Changes in the Length of the Four Seasons Due to Global Warming" (2021/03/17) jircas.go.jp