Tajiri's Dramatic Autumn Crash: August 15.5m to October 6.4m
2026-03-16
Tajiri in Iwami, Tottori Prefecture is a premier diving site on the Sea of Japan coast. From 1,392 real observations, we discovered a dramatic seasonal crash: visibility plummets 9.1m in just 2 months, from August's 15.5m to October's 6.4m. We explain this crash through Sea of Japan-specific oceanic processes.
15.5m
Aug (peak)
6.4m
Oct (bottom)
−9.1m
2-month drop
1,392 obs
Real observations
Tajiri Monthly Visibility Data
| Month | Vis (m) | vs Prev |
|---|---|---|
| Jan | 7.8m | - |
| Feb | 8.2m | +0.4m |
| Mar | 8.5m | +0.3m |
| Apr | 8.1m | -0.4m |
| May | 7.9m | -0.2m |
| Jun | 8.8m | +0.9m |
| Jul | 9.7m | +0.9m |
| Aug | 15.5m | +5.8m |
| Sep | 9.4m | -6.1m |
| Oct | 6.4m | -3.0m |
| Nov | 7.1m | +0.7m |
| Dec | 7.5m | +0.4m |
August to September drops 6.1m, September to October drops another 3.0m — a rapid autumn decline.
Cause of the Crash: Sea of Japan Autumn Processes
Cause 1: Autumn Vertical Mixing
During summer, Sea of Japan waters stratify by temperature (warm surface, cold deep). This layered structure aids light transmission, achieving 15.5m visibility in August. In autumn, cooling air temperatures chill the surface water, making it denser and triggering mixing with deep water (vertical mixing). This mixing brings deep nutrients and suspended matter to the surface, rapidly reducing visibility.
Reference: Japan Meteorological Agency 'Sea of Japan Ocean Structure'
Cause 2: Plankton Redistribution
When vertical mixing supplies deep nutrients, autumn phytoplankton blooms occur. The Sea of Japan is known for particularly prominent 'autumn blooms' with rising chlorophyll in September-October. This directly translates to the visibility decline divers experience.
Cause 3: Winter Monsoon Arrival
From October onward, northwest winter monsoon winds strengthen over the Sea of Japan. These winds create rough seas, stir up bottom sediment through wave action, and alter coastal currents, further reducing visibility. Shallow bays like Tajiri are especially affected, and the 6.4m October value includes this monsoon influence.
Pacific Side Comparison: Why the Opposite Pattern?
Many Pacific-side sites (IOP, Futo, etc.) peak in autumn-winter — the exact opposite of Tajiri. On the Pacific side, summer turbidity (from plankton and rain) subsides in autumn, and Kuroshio brings clear water through winter. The Sea of Japan side, however, gets clear from summer stratification and turbid from autumn mixing — a completely reversed cycle.
Sea of Japan (Tajiri)
Best: Aug (15.5m)
Worst: Oct (6.4m)
Pacific (Izu)
Best: Dec–Feb (15m+)
Worst: Apr–Jun (~8m)
Planning a Dive at Tajiri
For visibility: target August
August is the only month exceeding 15m. It's crowded during Obon holidays, but visibility is outstanding.
After October: set expectations
From October, expect 6-8m range. Macro photography and nudibranch watching suit the conditions better.
July is surprisingly good too
July's 9.7m is the year's second-best. Timing it just after the rainy season ends, when stratification is developing, can yield good visibility.
About the Data
Tajiri data collected from Blue Line dive shop blog (1,392 observations). Monthly averages use all observations per month. Visibility uses reported daily values (mean of min/max).
🌊 Check Visibility Forecasts
View AI-powered 7-day visibility forecasts for 30+ dive sites across Japan.
Open Forecast App →