2019 Volume 83 Issue 3 Pages 151-163
Hydrographic data off the Pacific coast of Tohoku and off the Boso Peninsula were analyzed using newly developed software that conducts a statistical cluster analysis to reveal the spatial distribution and temporal variation of occurrence frequency of water-masses in surface and subsurface layers. The interannual variation of water-mass occurrence frequency off the coast of Tohoku in summer was most associated with the interannual meridional shift of the Oyashio first branch, and the cold, fresh (warm, saline) clusters were more frequently observed when the Oyashio first branch shifted southward (northward). This tendency was also detected at a 10-m depth, where the previous water-mass classification method was difficult to apply. At a 100-m depth, multiple clusters were classified as Tsugaru Warm Current water, but those that could not be formed by isopycnal mixing between Kuroshio and Oyashio waters were limitedly distributed along the coast of Tohoku. It was suggested these clusters originated from the Tsugaru Strait. Off the coast of the Boso Peninsula in spring, the occurrence frequency of cold water-mass tended to increase when the Kuroshio axis departed from the coast. To relate the water-mass variations to fisheries catch variations, the number of clusters may need to be determined by considering the physiological characteristics of each fish species, such as optimum temperature range. By expanding the analysis to different regions and fish species and accumulating knowledge, the software is expected to provide a useful tool for extracting characteristic water-mass variations and clarifying the relationship between water-mass and fisheries catch variations.