Hydrographic Diagnostic Analysis In The Western Tropical Pacific Based On A Streamfunction Projection Method

Western tropical Pacific is an area of great scientific interest due to its influence on global climate variability. However, the previous hydrographic studies based on geographic space were mostly descriptive because of observation data paucity and complicate dynamic processes in the western tropical Pacific. This study applied a new streamfunction projection method called gravest empirical mode(GEM) to quantitatively analyze the thermohaline structures in the North Equatorial Current(NEC) region and western equatorial Pacific region and obtained some new findings including water mass identification and nutrient vertical structures in the western tropical Pacific. By analyzing TRITON mooring observations and Argo data, we studied the vertical coherent structures and equatorial Rossby wave in the western equatorial Pacific and further construct a new GEM method which is suitable in the equatorial Pacific. Major findings are listed as follows:The GEM method is applied to the hydrographic section at 137°E(2000-2012) to derive quantitative ensemble-average water mass properties in the NEC 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.Based on long-period TRITON moored buoy measurements(2000-1013) we studied the performance of GEM method in the western equatorial Pacific. Results showed that the amplitude and phase of thermohaline structure vary with depth. Since there is no robust vertical coherent structure in the western equatorial Pacific, GEM method has bad performance on the subsurface thermohaline reconstruction. We construct a new equatorial-GEM(E-GEM) field parameterized by SSH, pressure, and month and significantly improve GEM estimation, especially in the deep ocean. It reduces the root-mean-square(RMS) residuals for temperature estimates by 20% near the thermocline and captures more than 50% of the total thermal variances below 500 dbar. Besides, the three-dimensional structure of equatorial annual Rossby wave was examined with annual harmonic analysis of Argo gridded profiles. Result shows that annual Rossby wave propagates equatorward with mean speed of 52 cm/s. Lag-correlation tracing process further shows that the sub-thermocline seasonal signal in the western equatorial Pacific originates from the eastern equatorial Pacific via downward and southwestward propagation of annual Rossby waves.