Water mass characteristics in the western North Pacific based on a streamfunction projection

Abstract

A streamfunction projection method called gravest empirical mode (GEM) is applied to the hydrographic section at 137°E to filter out eddy noises in the western North Pacific and derive quantitative ensemble-average water mass properties in the North Equatorial Current region. The GEM fields capture more than 80% of total property variances in the thermocline layer. The core layer structures of key water masses, including the North Pacific Tropical Water (NPTW) and the North Pacific Intermediate Water (NPIW), are examined with a definition of water mass boundary based on property gradient. It shows that a tongue of maximal root-mean-square (RMS) residual exists in the upper half of NPIW for all water properties. These subsurface RMS tongues appear to be close to sharp property gradients. It is the first time a GEM diagnosis is applied to nutrient data, which reveals a drastic difference of N/P reaction rate ratio above and below the maximal-nutrient core at 1250 m. Additionally, a GEM velocity reconstruction successfully produces the North Equatorial Undercurrent (NEUC), demonstrating the stable thermal-wind nature of this newly-discovered current.