Study on the intrusion of cold water from the shelf slope region ito the Bungo Channel

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A bottom intrusion in the Bungo Channel, which is a phenomenon of cold water intrusion from the bottom of the shelf slope region into the channel, was investigated through field observations and analyses of historical data. The results are summarized as follows :(1) The bottom intrusion occurs mainly in neap tidal periods from early summer through late autumn. This spring-neap periodicity is a unique feature of the bottom intrusion in the Bungo Channel.(2) The bottom water temperature in the Bungo Channel decreases from early summer to midsummer due to the repeated bottom intrusion. From midsummer to autumn, the cold water in the lower layer of the channel retreats to the shelf slope region, and the bottom water temperature increases. As a result, the bottom water temperature in the Bungo Channel reaches minimum twice in a year, in summer and winter.(3) The intruded water contains a lot of nutrients. The nutrients are supplied to the euphoric layer of the channel by the vertical mixing in the Hayasui Strait and the coastal area with complicated geometries, supporting the biological production in summer. As a result the chlorophyll concentration in the Bungo Channel tentatively reaches maximum in summer.(4) An intensity of the bottom intrusion varies year by year, and it is decreasing in 1990s. This results in The decrease of the biological production in the Bungo Channel.(5) A phenomenon of warm water intrusion from the Pacific Ocean occurs in the Bungo Channel, and it is called a kyucho. Year-to-year variation of the intensity of the bottom intrusion has a strong positive correlation with that of the kyucho, that is, the bottom intrusion is strong when the kyucho is strong.