The Analyses Of The Subsurface Water Masses And Intermediate Water Masses Of The Tropical Western Pacific And Their Decadal Variation

Tropical Western Pacific (TWP) is the region with complex distribution of current system and water masses. Warm pool in the western pacific is one of the largest heat sources in the world, whose variation is the source of ENSO cycle, and has significant influence on global climatological variability. The Low-Latitude Western Boundary Currents (LLWBCs), are important components of the tropical and subtropical circulation system, and make great contribution to mass, heat and salinity transport between hemisphere and gyres. The Indonesian Throughflow (ITF) flows from in the Pacific to the Indian Ocean, and plays a vital role in global thermohaline circulation.The TWP is a crossroad of water masses which form in other sea areas. Many subsurface and intermediate water masses originated from middle or high latitude enter and pass through the TWP carried by different current systems. Distribution and variation of water masses in the TWP have strong tie with large scale ocean circulation on various time scales, and with global water cycle. The deeper the potential density level of the water mass, the longer time scale of climatological variation. Therefore, research on the distribution, diffusion and variation of subsurface and intermediate water masses in the TWP has great significance in ocean circulation dynamics and climate dynamics.The well-known water masses in the TWP includes subsurface North Pacific Tropical Water (NPTW) and South Pacific Tropical Water (SPTW) which form in the central parts of the North and South Pacific, respectively, and North Pacific Intermediate Water (NPIW) and Antarctic Intermediate Water (AAIW) originated from high latitude. Their fates in the TWP have not been fully identified. It is also an interesting question whether the TWP can find other subsurface and intermediate water. Meanwhile, it is much concerned but still controversial with regard to the source of ITF which enters the Indian Ocean from the Pacific. Accordingly, in order to understand the features of the ITF which plays important part in the global thermohaline circulation, it is crucial to make certain which subsurface and intermediate water masses enter and how enter from the Pacific to the Indonesian Sea. In this paper, ARGO delayed mode data provided by the Chinese ARGO data center and high-resolution CTD data in the WOD01 dataset provided by NODC of the USA are used for research. Distribution, source and fate of the subsurface and intermediate water in the TWP are studied through the distribution and variation of the core position of water masses which is determined by salinity extremum method.Features of diffusion of the NPIW, AAIW, NPTW and SPTW are discussed, and the results are more accurate than previous studies. The characteristics, distribution, cause of formation and the source of Western Pacific Tropical Water (WPTW) and Western Pacific Tropical Intermediate Water (WPTIW) which are newly found are systemically analyzed. Based on these, decadal variation of the subsurface and intermediate waters mentioned above since the 1980s is analyzed, and the source and the path of the ITF at different levels are discussed. Main conclusions are reached as follows:(1)In the subsurface layer the SPTW spans the equator west of 137°E. SPTW does not reach 5°N east of 131°E, but can locally influence 6°N west of 131°E. The transport of the Mindanao Eddy (ME) and Halmahera Eddy (HE) may play a main role in this local process. NPTW mainly lies in between 10°N and 20°N, extends to the Philippine coast from east to west carried by North Equatorial Current (NEC), and bifurcate into two branches. The south branch of NPTW extends southward to about 2°N along Mindanao coast in the west of 130°E, with part of it flows westward into the Sulawesi Sea, and part of it has eastward extension trend after it come across SPTW.(2) In the intermediate layer AAIW intrudes northward to 12°N-13°N and reaches about 13°N near 125°E. The main body of NPIW distributes north of 10°N, east of 122°E, and has the trend of extending from northeast to southwest. It can reach near 4°N between west of 132°E and Mindanao coast.(3) A subsurface water mass is found between the known NPTW and SPTW, which has never been reported before and is called Western Pacific Tropical Water (WPTW). WPTW exists between 3°N and 12°N with the core salinity about 34.8psu. Its potential density is about 23.7σθ-24.7σθ. WPTW has the same characteristics of high salinity as NPTW and SPTW, but its salinity maximum is less than that of NPTW and SPTW. WPTW originates the region where evaporation is greater than precipitation in northern Pacific, but the region is further east and north(east of 140°W, 10-25°N). It is carried westward to the western boundary by south part of the NEC, part of which is carried eastward by North Equatorial Counter Current (NECC) and is confined to a narrow region between the NEC and the NECC. WPTW joins in recirculation of tropical gyres more, but NPTW joins in recirculation of subtropical gyres more. The effect on the ENSO circulation of WPTW is worth being paid attention, because its source and path is close with ITCZ.(4) An intermediate water mass is located between 1°N and 10°N, which has the uniform salinity and vertical salinity minimum characteristic. It has not been paid attention before and is called WPTIW in this paper. WPTIW lies between 1°N and 10°N and its core potential density is about 26.0σθ-26.5σθwest of 180°. The range of WPTIW reduces to 3°N to 8°N and its core potential density extends to 25.3σθ-26.4σθeast of 180°. West of 180°WPTIW is formed mainly by mixing of the lower part of NPIW and the transitional water between AAIW and SPTW.Because of the strong shear and mixing of westward NEC, SEC and eastward NECC, it is independent relatively and its salinity is uniform. East of 180°the core potential density of WPTIW is lower. Besides the mixing of the lower part of NPIW , SSM and the transitional water between AAIW and SPTW, it is formed also because of the intrusion of tropical Eastern Pacific intermediate high salinity water. Different from NPIW and AAIW, which originate from subpolar region, WPTIW is the production of tropical recirculation. (5) In the 1980s-1990s and after 2000, the patterns of the studied subsurface and intermediate water are basically consistent, which both extend in the TWP and interweave near the western boundary. In the two periods, the westward extension degree of SPTW varies little, but the northward extension degree increases. It spreads to 5°N in the former period and then 6°N-7°N at present. The southward extension degree of NPTW decreases near the western boundary. It only spreads to 4°N between 2000 and 2005; while it could spread to 2°N in the former period. The northward extension degree of AAIW increases near the western boundary. It spread to 13°N between 2000 and 2005; however it spread to 11°N in the former period. The southward extension degree of NPIW decreases near the western boundary.(6) In the different layers, the paths of the South Pacific water and North Pacific water entering ITF are different. The origin of subsurface ITF is basically from North Pacific. The origin of intermediate ITF is from both North Pacific and South Pacific. The subsurface and intermediate water of north Pacific flow through Sulawesi Sea, Makassar Strait and reach Flores Sea., and the trend gets more obvious with the deeper isopycnal layer. The subsurface water of south Pacific does not enter Sulawesi Sea and Maluku Sea. AAIW obviously enters Banda Sea through Halmahera Strait and Maluku Strait.