The Japan Meteorological Agency (JMA) has conducted oceanographic researches in the western North Pacific for more than 50 years, which include several repeat hydrographic sections across the North Pacific subtropical gyre and western boundary current system. As a result of the researches, freshening of the subsurface waters of the Kuroshio and the North Equatorial Current (NEC) has been recently observed.
The subsurface waters of the Kuroshio and its recirculation south of Honshu show rapid freshening over the last five years, salinity of which reached 34.9 in 1996, turned a decreasing trend after 1997, and then below 34.8 in 2001. Subsurface warming of the Kuroshio occurs by about 0.5°C as well as freshening and the both effects lead to high dynamic height south of the axis of the Kuroshio. The subsurface water of the Kuroshio recirculation corresponds substantially to the North Pacific Subtropical Mode Water (NPSTMW). Precipitation has recently increased over the Kuroshio extension region, which coincides with formation areas of NPSTMW, and this is a probable cause of the surface water freshening.
The recent freshening of the subsurface waters was observed to extend to the lower latitude regions. The North Pacific Tropical Water (NPTW) is characterized by salinity maximum in the subsurface of the NEC, and 1978-1997 averaged salinity of NPTW is 35.05 on 137°E meridian. In 1998 freshening (order of 0.1) of NPTW was found on 137°E meridian. Salinity of the southern part (12°-15°N) of NPTW recovered in 2000. On the other hand, the northern part (15°-20°N) remains low saline. The NPTW is formed in the areas along the Tropic of Cancer. The northern part of NPTW is supposed to be formed west of 165°E because no continuous freshening water is observed on 165°E meridian. Precipitation along the Tropic of Cancer and west of 165°E increased in 1998. The freshening of NPTW should be attributed to the increase of precipitation. NPTW is bifurcated at the western boundary off the Philippines and its northern part is transported northward as the main volume of the Kuroshio water. Therefore, the northern part of NPTW and the Kuroshio water probably have some characteristics in common.
In the regions south and east of Japan, the annual precipitation amount in the past a few years was as large as normal, while the subsurface water masses which originate in the areas were freshened. It is strongly suggested that this large precipitation contributed to the freshening of the Kuroshio and NEC subsurface waters and furthermore to the subduction processes of the surface water masses. |
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