潮岬灯台直下の潮波と黒潮

摘要

黒潮は直進路をとっている時，その北縁が，ほぼ潮岬に接する形で流れている。潮岬の先端近くの沖合には，浅瀬が存在しており，小さな島や岩礁が多数存在している。黒潮接岸時には，強い東向流が岸のすぐ近くにも現れ，この浅瀬や島にぶつかる形で白波が立つ。これを地元では潮波と呼んでいる。%We observed regularly the white-capped breakers which are produced by strong current in the sea just off the lighthouse of Cape Shionomisaki. The white-capped breakers are classified into seventeen ranks from - 8 through +8. Correlations of the ranks of white-capped breaker with several quantities such as the east-west component of current speed observed with ADCP at a fixed point, the sea level difference between Kushimoto and Uragami tide gauge stations, and the separation distance of the northern edge of the Kuroshio measured southwards from the tip of Cape Shionomisaki. The separation distance was determined from the position of the temperature front seen in satellite images. These correlations are very significant, and the observation of white-capped breaker is shown to be very useful to know oceanic conditions in the vicinity of Cape Shionomisaki. We defined that white-capped breakers are clearly seen above rank 4. This means that significant currents would exist in the sea just off the lighthouse at least for ranks above rank 4 through rank 8. If we define the northern edge of the Kuroshio with the northern limit of eastward current zone of the Kuroshio, the separation distance of the northern edge of the Kuroshio should be zero for these cases. It is hard to believe that currents are geostrophic in shallow waters just near coast. The current would be flowing under the balance between sea surface gradient and friction due to existence of coast or sea bottom. Maekawa et al. (2011) showed that the distribution of sea surface height in the sea just near the tip of the Cape Shionomisakai is produced by the temperature and salinity structures in shallow water less than 300 m, and that the sea level difference between Kushimoto and Uragami is produced by the surface Kuroshio water which had been brought into near coast region off the southwestern coast of the Kii Peninsula. The east-west sea level gradient produced just off Cape Shionomisaki would create an eastward current just along the coast. Besides, significant ranks (7 and 8) of white-capped breakers can be seen relatively often in summer season. This would be related to the seasonal variation of the sea level difference between Kushimoto and Uragami discussed by Nakamura et al. (2012).