Long-term variations of North Pacific Tropical Water along the 137°E repeat hydrographic section

The spatiotemporal variability of North Pacific Tropical Water (NPTW), characterized as a subsurface salinity maximum, is investigated using a half-century-long repeat hydrographic dataset of the Japan Meteorological Agency (JMA) along the 137°E meridian, historical objective analysis of monthly global temperature and salinity dataset, sea surface dynamic height dataset, and atmospheric dataset. The cross-sectional area of NPTW (S > 34.9) at 137°E shows decreasing trend with freshening after the mid-1970s. Significant decadal-scale variation is also revealed; this resulted from meridional excursion of its northern boundary, fluctuating between 15°N and 25°N. It was found that the northern boundary excursions at 137°E were induced by westward expansion of the NPTW formation region related to excess evaporation and increased Ekman pumping in the formation region. It is also inferred that the larger eddy activity around the northern boundary of the NPTW also possibly influences the northward movement of the NPTW northern boundary at 137°E through enhanced eddy mass transport.