Omijima Visibility Analysis — 2,000 Days of Sea of Japan Diving Data

The only Sea of Japan site where our site-specific AI model beats the global one — Omijima's 2,095 days of data hide patterns found nowhere else.

We collected and analyzed 2,095 days of actual visibility observations from the Sea Again blog (Hatena Blog), spanning from 2007 to the present. This dataset is the second largest among Sea of Japan dive sites in our database, after Echizen (2,652 records), making it an invaluable resource for quantitatively understanding underwater conditions in the Sea of Japan.

Omijima presents a fascinating case for our AI prediction model. Normally, our visibility forecasts rely on a "general AI model" trained on data from all sites. However, Omijima is one of only four sites where the site-specific AI model (trained exclusively on that site's data) outperforms the general AI model. The general AI achieves AI accuracy 16%, while the site-specific AI reaches AI accuracy 20%. This suggests that Omijima's oceanographic dynamics are qualitatively different from the majority of Japanese dive sites.

Monthly Visibility Patterns: A Different Rhythm from the Pacific

The monthly visibility data for Omijima reveals seasonal patterns distinctly different from Pacific-side dive sites. Visibility peaks during summer, reaching approximately 10 meters in August and September. In contrast, winter months (December through February) see limited diving opportunities due to the harsh weather characteristic of the Sea of Japan, resulting in sparse data.

This "summer is best" pattern is the exact opposite of many Izu Peninsula sites, where winter delivers peak visibility. On the Pacific side, transparent Kuroshio-derived offshore water approaches the coast in winter, while summer brings plankton blooms and freshwater runoff that reduce visibility. On the Sea of Japan side, the mechanism is reversed: winter's fierce northwesterly monsoon winds churn up seafloor sediment, and spring snowmelt delivers massive nutrient loads and turbidity. As summer arrives, these disturbance factors subside, and the Tsushima Warm Current brings warmer, relatively clear water to the forefront.

While Omijima's annual average visibility is somewhat lower than the Izu area, summer conditions reliably provide around 10 meters of visibility -- entirely adequate for enjoyable diving. The true appeal of Omijima lies beyond the numbers, in its unique underwater landscape and biological diversity.

Why the Site-Specific AI Outperforms the General AI

Our AI prediction system builds two types of models: a general AI trained on integrated data from all sites, and site-specific AIs trained exclusively on each individual site's data. The system automatically selects whichever achieves higher accuracy. Out of more than 30 sites, the site-specific AI wins in only four cases, including Omijima.

The general AI learns generalized patterns from diverse site data, but the vast majority of its training data comes from Pacific-side sites. The Izu Peninsula alone contributes over 10,000 records, causing the model to optimize for Pacific-side dynamics (Kuroshio influence, high winter visibility).

At Sea of Japan sites like Omijima, the dominant mechanisms are reversed: Tsushima Current influence, winter monsoon-driven mixing, and summer visibility peaks. For the general AI, these patterns represent a "minority class" that is insufficiently learned. The site-specific AI, trained solely on Omijima's data, captures these unique dynamics more accurately.

That said, the site-specific AI's accuracy of 20% is low in absolute terms, indicating substantial room for improvement. Incorporating the complex oceanographic dynamics of the Sea of Japan -- such as Tsushima Current meanders, monsoon intensity variations, and coastal upwelling -- into the model could significantly enhance future prediction accuracy.

Comparison with Echizen: Sea of Japan Sister Sites

Comparing Omijima (Yamaguchi Prefecture) and Echizen (Fukui Prefecture) as the two primary Sea of Japan dive sites yields valuable insights into the region's diving environment. Both sites are influenced by the Tsushima Warm Current and share the fundamental "summer is best" pattern, but several differences emerge.

Echizen's effective diving season is restricted to April through October, with extremely sparse winter data (only 23 observations for November through February combined). Omijima, positioned further south (latitude 34.4 degrees N vs. Echizen's 36.0 degrees N), experiences somewhat mitigated winter harshness, allowing for a slightly longer data collection season.

Echizen's summer peak (August: 10.3m) and Omijima's summer peak fall within a similar range, suggesting that visibility ceilings at Sea of Japan dive sites may be governed by the water quality characteristics of the Tsushima Warm Current. This contrasts sharply with the Pacific side, where the Kuroshio delivers winter visibility exceeding 20 meters.

From an AI modeling perspective, Echizen favors the general AI while Omijima favors the site-specific AI. This difference may reflect Echizen's larger dataset (2,652 records) contributing more substantially to the general AI's training, whereas Omijima's 2,095 records may be insufficient for the Sea of Japan pattern to be adequately represented in the general AI.

Yearly Visibility Trends: 18 Years of Records

The approximately 18-year dataset spanning 2007 to 2026 constitutes a valuable long-term record for tracking changes in Omijima's underwater environment. While year-to-year variation in annual average visibility is apparent, no significant deterioration trend is observed, suggesting that the Sea of Japan's coastal environment has remained relatively stable.

Factors contributing to annual variation likely include fluctuations in Tsushima Warm Current flow volume, precipitation during the rainy season (tsuyu), and typhoon passage frequency. The Tsushima Current, a warm current flowing from the East China Sea into the Sea of Japan through the Korea Strait, varies considerably in volume from year to year. Years with stronger current flow bring higher water temperatures and increased warm-water species, though the impact on visibility is not necessarily unidirectional.

Shizuura and Funakoshi: Two Seas on One Island

Diving at Omijima takes place primarily at two sites: Shizuura and Funakoshi. Despite being on the same island, these two sites offer radically different underwater experiences.

Shizuura is located on the sheltered inner bay side of the island, characterized by calm surface conditions and a sandy-mud bottom. Relatively protected from wave action, it serves as a reliable dive site even during winter or rough weather. The sandy-mud substrate hosts abundant gobies and nudibranchs, making it an excellent field for macro photography. While visibility tends to be lower than the outer sea side, this actually creates favorable conditions for macro observation.

Funakoshi faces the open sea and features dynamic topography with rich biological communities. Rocky reef zones harbor diverse sessile organisms, and pelagic fish occasionally pass through. Visibility tends to be higher than at Shizuura, but the site is more susceptible to swell and current, and conditions can be unstable.

The ability to choose between these two sites is one of the great advantages of diving at Omijima. Depending on the day's sea conditions, divers can opt for patient macro photography at tranquil Shizuura or wide-angle underwater scenery at Funakoshi when conditions cooperate. Our visibility data reflects averaged values across both sites, though actual conditions can differ by several meters between them on any given day.

Omijima as a Nudibranch Paradise

Omijima is highly regarded among macro divers and underwater photographers as one of Japan's premier nudibranch observation sites. The sandy-mud environment at Shizuura is particularly conducive to hosting diverse nudibranch species, and encountering more than ten species in a single dive is not uncommon.

Seasonal variations in water temperature and the biotic shifts driven by the Tsushima Warm Current create changing nudibranch species compositions throughout the year. Aeolid nudibranchs increase in spring, while warm-water species join the mix from summer through autumn. This biological richness represents a major attraction that cannot be captured by visibility statistics alone.

Even on days with relatively low visibility, divers armed with macro lenses never lack for subjects. In fact, lower visibility can produce better background bokeh for macro photography, creating more favorable shooting conditions. From this perspective, Omijima's moderate visibility is not a disadvantage but can actually work in the photographer's favor.

Practical Advice

Best Season

• For visibility: August through September records the highest annual visibility. Water temperature is comfortable, and a wetsuit is perfectly adequate.
• For nudibranch watching: Spring (March through May) brings increased species diversity and abundance. Visibility may be slightly lower, but it is the peak season for macro enthusiasts.
• Times to avoid: December through February often sees diving cancellations due to rough Sea of Japan weather. Check the dive service's winter schedule in advance.

Access

Nagato City in Yamaguchi Prefecture is the gateway to Omijima. The nearest airport is Yamaguchi Ube Airport, approximately 1.5 hours by car. Nagato Station on the JR San-in Main Line provides bus or taxi connections. From the Hiroshima direction, the drive takes approximately 3 hours via the Chugoku Expressway.

Dive Planning Tips

At Omijima, the choice between diving at Shizuura or Funakoshi is typically determined by the day's sea conditions. Beyond weather forecasts, check wind direction (pay particular attention to north and northwest winds) and swell information. The most reliable approach is to confirm with the local dive service the evening before or morning of your dive.