The region of the Seto Inland Sea is meteorologically called as "Seto Inland Climate Zone". In this region, the annual average temperature and precipitation are 15℃ and 1,000-1,600 mm respectively. It means that this region is mild compared to that of the rest region in Japan. On the other hand, the mountain areas around the Seto Inland Sea have large precipitation with annual precipitation of 2,000-3,000 mm.
The distribution of annual precipitation in the Seto Inland Sea region is shown in Figure 1-2.
The amount of water flow into the Seto Inland Sea from 669 river systems
is as large as 50,000,000,000 km
Through all seasons, the wind direction in the coastal areas is usually vertical against the shoreline. On the other hand, the wind direction in land areas largely depends on the geographical features. The wind pattern in the Seto Inland Sea is characterized by its regularity. The seasonal change of water temperature and salinity of the Seto Inland Sea is larger than that of Pacific Ocean because the sea condition in the Seto Inland Sea is generally affected by the weather conditions. Figure 1-3 shows the distribution of salinity in upper layer of the sea water column in summer.
Almost all bays and open seas in the Seto Inland Sea are shallow and less than 60 m in depth except Beppu Bay, Iyo Open Sea, Bungo Channel, Kii Channel and Hibiki Open Sea.
Hibiki Open Sea is less than 50 m in depth. Subou Open Sea is less than 50 m in depth and most of the sea area is occupied the flat plane 10-20 m in depth. Though almost all area in Iyo Open Sea is less than 70 m in depth, sea pot with maximum depth of 454m is located in the north side (Houyo Strait), which is the deepest area in the Seto Inland Sea.
In Beppu Bay, there is the geological hollow in the center of the bay.
Almost all area in Bungo Channel is less than 100 m in depth and the flat plane in this channel is 70-100 m in depth. Its depth tends to be deeper southward.
Hiroshima Bay is less than 40 m in depth and the flat plane with depth of 20-30 m develops. Almost all area of Aki Open Sea is less than 50 m in depth. Almost all area of Bungo and Hiuchi Open Seas are less than 40 m in depth and the flat plane with depth of 10-30 m develops.
Though the depth of Bisanseto Strait is expressed to be less than 20 m, there are many geographical hollows.
Harima Open Sea is less than 50 m in depth and the flat plane with depth of 20-40 m develops.
Osaka Bay is less than 60 m in depth and the flat plane with depth of 10-20 m develops. Especially, its east half part is less than 30 m in depth.
Kii Channel is less than 80 m in depth and the geographical groove develops from south to north in the east part from the center of this channel.
Through Kii Channel, Bungo Channel and Kanmon Strait, water exchanges between the Pacific Ocean and the Seto Inland Sea. Salinity of the surface layer is less than 32‰ in Osaka Bay, about30-32‰ in Bisan Strait, less than 31‰ in Hiroshima Bay and about 32‰ in Kii channel. These salinity data are lower than that of about 33‰ in Bungo Channel.
Tidal current in narrow straits, such as Naruto and Hayasuinoseto Straits, is fast and complex and reaches to 5-10 knots. the complex geographical features affect unique characteristics of the tidal current.
Note:made by Association for the Environmental Conservation of the Seto Inland Sea
Figure 1-2 Distribution of Annual Precipitation in the Seto Inland Sea Area (mm)
Note:made by the Association for the Environmental Conservation of the Seto Inland Sea
Figure 1-3 Salinity of Surface Layer in Summer in the Seto Inland Sea