Pacific vs Sea of Japan: The Seasonal Visibility Flip
2026-03-16
Japan's dive sites show a complete seasonal reversal between Pacific and Sea of Japan coasts. Pacific: clear winter, murky summer. Sea of Japan: clear summer, murky winter. We quantified this well-known pattern with real data.
Monthly Visibility Comparison
| Coast | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Pacific | 15.5 | 14.5 | 12 | 10.2 | 10.7 | 11.2 | 10.5 | 11.1 | 11.2 | 12.2 | 12.3 | 14.7 |
| Sea of Japan | 11.5 | 9.6 | 8.8 | 8.2 | 8.5 | 9.1 | 9.7 | 10.9 | 9.8 | 8.1 | 8.6 | 10 |
| Gap | +4.0 | +4.9 | +3.2 | +2.0 | +2.2 | +2.1 | +0.8 | +0.2 | +1.4 | +4.1 | +3.7 | +4.7 |
Pacific avg (IOP, Futo, Kushimoto, Ito, Hirasawa). Sea of Japan avg (Echizen, Tajiri, Omijima, Sado). January gap: +4.0m. August gap: nearly zero (+0.2m).
Why the Reversal Happens
Pacific Coast
- Winter: less plankton + NW wind brings offshore water → clear (15.5m)
- Spring: plankton bloom → crash (10.2m)
- Summer: warm surface promotes plankton → low (10.5m)
Sea of Japan
- Winter: continental nutrient input + vertical mixing → turbid (8.8m)
- Summer: stratification traps plankton at surface, lower layers clear (10.9m)
- Autumn: cooling restarts mixing → declines again (8.1m)
Practical: When to Go Where
- Dec–Feb → Pacific coast only (14.5–15.5m vs Sea of Japan 9.6–11.5m)
- Jul–Aug → Sea of Japan becomes viable (10.9m vs Pacific 10.5–11.1m. Nearly equal)
- April → Both coasts at annual low. Okinawa is the answer
- October → Pacific recovering (12.2m). Sea of Japan at annual worst (8.1m). Beware
About the Data
Pacific (5 sites: IOP, Futo, Kushimoto, Ito, Hirasawa) vs Sea of Japan (4 sites: Echizen, Tajiri, Omijima, Sado). ~24,000 total observations.
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