Studies on the Interaction between North Pacific Intermediate Water and the Kuroshio

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We here hypothesize that variation of the cross-gyre transport from the subarctic to subtropical gyres producing NPIW induces long-term variations in the Kuroshio/Kuroshio Extension current system. In order to validate this hypothesis and detect the actually occurring formation processes of NPIW, we conducted data-analysis, numerical experiments and field observations; we successfully obtained extraordinary results. (1) Based on four surveys of water masses and ocean currents using CTD/LADCP observation, we revealed about 10Sv of cross-gyre Oyashio transport through the following two processes : direct cross-gyre transport along the western boundary and cross-subarctic front (SAF) process occurring along the offshore SAF due to isopycnal mixing. (2) The intrusion of western boundary currents across the wind-driven gyre boundary and related cross-gyre transport are realistically reproduced with a eddy-permitting three layer numerical models with realistic topography and annual mean wind … More stress, with special emphasis on the Oyashio southward intrusion and North Pacific intermediate Water (NPIW) formation. About 3Sv of the model diapycnal upwelling occurring due to strong tidal mixing along the Kurii islands induces a meridional-diapycnal overturning circulation. These results explain the observed features that about 4Sv of the cross-gyre Oyashio transport along the western boundary participate in the formation of NPIW. (3) High-resolution intensive observation using CTD/towed-CTD (MVP : Moving Vessel Profiler) and LADCP/ADCP were performed around the Kuroshio Extension. These observations revealed that intermediate-depth frontal waves with the wavelength of about 200km propagate in the downstream direction with the phase speed of 0.2m/s, amplifying the amplitude. The waves break down and mix the two water masses very efficiently in the downstream regions until 150E. A linear stability analysis reveals that the observed frontal waves are possibly the baroclinically unstable waves due to the coupling between the surface and intermediate-depth vorticity anomalies. The Oyashio water thus enhances intensive mixing and NPIW formation by entering into the Kuroshio Extension. Less