Diagnosis Of The Kuroshio In The East China Sea Along The PN-line With Geostrophic Empirical Mode Method

Based on the repeated hydrographic sections at the PN-line during 1987-2006, we apply a streamfunction projection method called geostrophic empirical mode (GEM) to diagnose the seasonal and interdecadal variation of the Kuroshio water masses in the northern part of Okinawa Trough. By projecting the historical data into a baroclinic streamfunction space, the GEM method can reduce the noise associated with the meso-scale variability of the Kuroshio. The generated GEM fields are parameterized by pressure and baroclinic streamfunction and reflect the basic structure of the Kuroshio. Analysis of the seasonal GEM fields after 1987 shows that the Kuroshio Intermediate Water attains salinity minimum in summer and maximum in spring. We also construct GEM fields from the Bottle data before 1987, and the comparison demonstrates the interdecadal trend of the Kuroshio, that is, the Kuroshio Surface Water becomes warmer and less saline, and salinity decreases in the Kuroshio Intermediate Water and increases in the Kuroshio Tropical Water. After discussing the possible origin of the intermediate water at the PN-line, we find that the core of the intermediate water always lies at the shore side of the Kuroshio because the North Pacific Intermediate Water intrudes between the Okinawa Island and Miyako Island and is advected to the western part of the Kuroshio.