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Keywords:

  • mid-depth freshening;
  • NPIW;
  • global warming

[1] We report a freshening at mid-depth in the North Pacific subtropical gyre by using long-term repeat hydrographic data along the 137°E section and one-time hydrographic data along the World Ocean Circulation Experiment Hydrographic Program (WHP) P2 and P3 sections. North of 15°N along the 137°E section, we estimated a linear freshening trend of 0.0015/yr between the main thermocline and the salinity minimum layer of the North Pacific Intermediate Water, mainly caused by isopycnal surface deepening due to warming, and by westward shifts of the salinity-minimum tongue due to strengthening of the subtropical gyre. Furthermore, along the WHP-P2 section, the linear freshening trend could be classified into several groups according to longitude. Such spatial differences in the freshening trend seem to reflect differences in the formation processes and mid-depth pathways of the salinity minimum waters.