SUBDUCTION PROCESSES IN THE NORTH PACIFIC BASED ON ANALYSES OF HIGH-QUALITY DATA

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The annual subduction rate of the North Pacific was calculated based on isopycnally averaged hydrographic climatology, high-resolution winter mixed-layer climatology, and various wind stress climatologies from ship reports, numerical weather prediction products, and satellite products. The subduction rate and the volume of the permanent pycnocline were compared in potential temperature-salinity (θ-S) space, revealing that the θ-S relation of the lower permanent pycnocline is different from that of the water subducted from winter mixed layer; the former is systematically warmer and saltier than the latter. This feature is not consistent with the classical view that the θ-S relation of the permanent pycnocline is directly maintained by the water subducted from the winter mixed layer.Conducting the research cruises and the profiling float observations, it was demonstrated that the frontal structure in the subtropical-subarctic transition region, from which the mixed layer water is subduct … More ed into the lower permanent pycnocline, plays an essential role in determining the θ-S relation of the permanent pycnocline as follows. We found that the deep mixed layer on the poleward side of the density-compensating temperature and salinity front forms vertically homogeneous water mass which has lower temperature and salinity than but almost the same density as Central Mode Water (CMW) and called this water mass Transition Region Mode Water (TRMW). In early spring, the warmer and saltier water on the equatorward side of the front spreads over TRMW, causing active salt fingering near the top of TRMW, which transform TRMW into CMW within a few to several months.This study presents an essential mechanism by which the θ-S relation of the permanent pycnocline is maintained, which will serve as the basis for detailed understanding of the structure and origin of the permanent pycnocline and development of ocean general circulation models and material cycle models for long-term climate prediction. Less