Every summer in Kyushu,
moisture carried by winds
from distant oceans and lands
falls as days of rain.
It lands on your umbrella,
is absorbed into the soils of Fukuoka,
and lingers as mist over the mountains—
the humidity in the summer air.
Long before you felt it,
this moisture had already been on a journey:
some from the faraway Indian Ocean,
crossing lands, hovering over bays,
and climbing mountains;
some from the western Pacific Ocean,
moving slowly along warm currents.
These flows never arrive in Kyushu
the same way twice,
and each encounter brings
a different pattern of rain.
Some years unfold with gentle persistence;
others erupt into torrents
that overwhelm rivers and cities.
Are you ready to follow
the far-reaching paths of water
and discover the living, sensitive system
that shapes each year's rain?
🌧️ Meiyu-Baiu rainfall
Refers to the seasonal monsoon rains that occur from June to July across East Asia, bringing prolonged precipitation vital for water resources and agriculture — yet causing droughts when weak and floods when strong.
⛩️ Kyushu
The southwesternmost of Japan’s main islands, which receives much of its annual rainfall during this period. In July 2018 and 2020, record-breaking rains hit the region, triggering severe floods and landslides.
Where does the moisture that becomes rain come from?
A single rainfall event in Kyushu⚲ is a composite body, assembled from moisture that evaporated over the Indian Ocean⚲, the Pacific Ocean⚲, the East Asian continent⚲, and nearby seas such as the East China Sea and the Kuroshio region⚲.
These distant and local waters do not merely contribute traces; together, moisture transported from the Indian (47.7%) and the Pacific Ocean (26.2%) combines to account for nearly three-quarters of the total rainfall during extreme Meiyu-Baiu events.
🚠 From the Indian Ocean
A long journey crossing lands, hovering over bays, and climbing mountains
As the land over South Asia heats faster than the surrounding oceans, it pulls in winds from the sea. These southwesterlies sweep across the Indian Ocean, gathering moisture. In summer, this circulation acts as a massive "monsoon engine," driven by long-term heat storage and release.
While much of this rain falls over India⚲, multiple streams continue eastward toward the Bay of Bengal⚲. Here, deep convection repeatedly lifts air, releases rain, and recycles the vapor, preparing it for a longer journey.
Carried by monsoonal southwesterlies across the South China Sea⚲, the East Asian continent, and the East China Sea, this distant moisture provides nearly half (47.7%) of Kyushu's total rainfall during extreme Meiyu-Baiu events.
⛵ From the Pacific Ocean
Drifting slowly along warm currents
Under the steady influence of the North Pacific Subtropical High (NPSH), warm, humid air builds up over the Western Pacific and the Philippine Sea⚲. This moisture travels slowly and persistently at low atmospheric levels.
Guided by the high-pressure circulation, this flow glides over the Kuroshio Current and the East China Sea to reach eastern Kyushu. Averaging 28.6% of Meiyu-Baiu rainfall, it provides vital background humidity and fuel for sustained precipitation.
🌧️ The Meiyu-Baiu Front
Where the moisture meets
Over Kyushu, the two journeys intersect. Pacific moisture arrives first, gradually increasing at low levels. Then, often abruptly, Asian Monsoon (AM) moisture surges in from above, driven by a Baiu frontal depression.
During the peak of extreme events, these air masses merge vertically. Low-level Pacific moisture feeds convection from below, while mid-level Indian Ocean air enhances updrafts and condensation from above.
The rain is layered—built from moisture that has traveled thousands of kilometers, risen and fallen multiple times, and crossed continents and seas.
📊 Year-to-Year Variability
The Pulse of the Seasons
The interplay of these moisture sources creates a distinct rhythm across the Meiyu-Baiu seasons (June–July, 2004–2023).
An analysis of the last two decades reveals how remarkably the intensity can shift:
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💧 Light seasons: Years with significantly less rainfall, such as 2004 (344.6 mm) and 2005 (451.1 mm).
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💧💧 Normal seasons: Years that reflected the typical rhythm, such as 2019 (711.0 mm) and 2023 (693.7 mm).
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💧💧💧 Heavy seasons: Years like 2020 (1191.3 mm) and 2012 (1046.2 mm) saw massive accumulation. In these years, mid-level Asian Monsoon (AM) moisture contributed more than half of the total rainfall (51.4%), with the Indian Ocean providing more than 80% of that moisture.
During extreme rainfall events, this AM contribution becomes even more dominant (57.8%), with the Indian Ocean alone providing nearly half (48.4%). Transported northeastward to the Meiyu-Baiu front, this influx of warm, moist air directly feeds the condensation process, enhancing atmospheric instability in the middle levels. This facilitates deep convection reaching above 500 hPa, almost 1.5 times the height of Mount Fuji, driving the most extreme rainfall in Kyushu during heavy seasons.
A single catastrophic Meiyu-Baiu event, such as the downpour on July 6, 2018, which dumped 116.8 mm of rain, can account for 14.1% of an entire season's precipitation (827.4 mm) in just one day. Over a mere three days (July 5–7, 2018), this same system unleashed 253.7 mm, accounting for nearly a third (30.7%) of the entire season's rainfall, an event the Japan Meteorological Agency (JMA) described as "heavy rain at a level we've never experienced."
