Surface and intermediate water mass exchanges between the North Pacific subtropics and tropics and its temporal changes

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The purpose of the present study was to clarify circulation and mixing, and their temporal changes of major water masses in the subtropical/tropical boundary region, especially aiming to see inflow of anomalous subtropical water into the tropical gyre. We analyzed i) the repeat hydrographic section data in the western North Pacific, ii) the data from our own synoptic survey in the western tropical North Pacific, and iii) other available hydrography including WOCE/WHP. The main results are as follows.1. Isopycnal analysis of the data (i) and (iii) captured the low-salinity water carried by the North Equatorial Countercurrent (NECC) at about 5ィイD1oィエD1N, 200-300m and 26.5σィイD2θィエD2. This water probably originates in the northeastern subtropics and reaches to the tropics via the western boundary region. North Pacific Tropical Intermediate Water (NPTIW) was also captured as the high-salinity and low-oxygen water originated in the eastern tropics carried to the west, suffering considerable mixing with the adjacent subtropical water along its advection path.2. The CTD and ADCP data (ii) provided a "snapshot" of the flow field and the water mass distribution, which supports the view that the low-salinity water of the subtropical origin is carried into the NECC via the western boundary region. Comparison between the result #1 above and this snapshot suggests that NPTIW has little temporal variability while the low-salinity water varies associated with the NECC variability.3. The repeat hydrography (i) revealed the remarkable decadal change in North Pacific Tropical Water, which is originated in the central subtropics and carried by the North Equatorial Current. The change was caused not only by the thermohaline-forcing change in the formation region but also the dynamic-forcing change, that is the change in the wind field, resulting in the increased formation rate and the intensified gyre.