Long-term thermohaline variability in the North Pacific subtropical gyre for 1996–2018 was investigated by repeat hydrography along 165°E conducted by the Japan Meteorological Agency. Potential temperature (θ) and salinity (S) in North Pacific Tropical Water (NPTW), characterized by the salinity maximum, exhibit an interannual or longer-timescale variation with significant warming and salinification. The θ–S of NPTW originates from mixed layer temperature (MLT) and salinity (MLS) in the isopycnal outcrop region. In the NPTW formation region, the MLS determines surface density and controls the meridional position of the outcrop region. High (low) MLS and the associated southward (northward) migration of the outcrop region increase (decrease) θ–S anomalies in NPTW. The θ–S in the main thermocline/halocline associated with subtropical mode water (STMW) shows a decadal-scale variation, with a significant cooling and freshening. These properties also derive from MLT and MLS in the isopycnal outcrop region. In the central North Pacific, including the eastern part of the STMW formation region, the MLT controls meridional migration of the outcrop region; during high (low) MLT, the outcrop region migrates northward (southward), and cold and fresh (warm and salty) STMW is formed. The signals are passed into the main thermocline/halocline through subduction of STMW. Consideration of the mechanism that generates θ–S anomalies via migration of the outcrop regions leads us to suggest surface warming and salinification in the subtropical gyre associated with global warming cause a cooling and freshening in the main thermocline/halocline and warming and salinification in NPTW, respectively.
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