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

通过野外观测和历史资料分析，研究了本戈海峡的一次海底入侵现象，即冷水从陆架斜坡区底部侵入该海峡。研究结果表明：(1)底侵主要发生在初夏至晚秋的小潮期间。这种春小潮周期性是本戈海峡底层入侵的独特特征。(2)由于反复的底层入侵，本戈海峡底层水温在初夏至盛夏期间呈下降趋势。盛夏至秋季，航道下层冷水退至陆架坡区，底层水温升高。因此，本戈海峡的底层水温在一年中有两次达到最低，分别在夏季和冬季。(3)入侵的水体中含有大量的营养物质。在Hayasui海峡和几何形状复杂的沿海地区，营养物质通过垂直混合提供给海峡的欢乐层，为夏季的生物生产提供了支持。结果表明，本戈海峡的叶绿素浓度在夏季暂时达到最大值。(4)海底入侵强度逐年变化，90年代呈下降趋势。这导致了本戈海峡生物产量的减少。(5)本戈海峡出现了一种来自太平洋的暖水入侵现象，称为巨藻。底部侵入体强度的年际变化与久节强弱有很强的正相关关系，即久节强弱时底部侵位较强。

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.