Seasonal Water Masses distribution and Mixing through a Mixed-layer Optimal Multi-Parameter Analysis in the Changjiang Estuary and Its Adjacent Area

Abstract:

The water masses in the Changjiang Estuary and its adjacent area mainly consist of the fresh water input from Changjiang River Diluted Water (CDW), the Taiwan Strait Warm Water (TSWW) coming from the northward current through the Taiwan Strait, and the warm and salty Kuroshio Subsurface Water (KSSW) that upwells onto the shelf in the northeast of Taiwan. The quantitative knowledge of water mass proportions and their variations at different time scales are of great importance to understand the physical impact on the local ecosystem and biogeochemical cycle. In this study, we develop a modified Optimal Multi-Parameter method that is adapted for the mixed layer water analysis by including the atmospheric effects as a correction term. The mixed-layer OMP analysis (MLOMPA) is then employed to determine the water mass contribution in the river plume area using observed hydrographic data in different season. The results show that the water mass distributions from the MLOMPA are significantly different from the traditional OMP analysis, especially for the proportion of the TSWW and KSSW. In winter, the Kuroshio intrusion is dominated by the KSSW while the MLOMPA gives a non-neglectable portion of the TSWW east of 123E in the current case. In summer, both methods show that the upper mixed layer water is mostly originated from the TSWW and the lower layer from the KSSW. The percentage of the TSWW is only slightly smaller with the MLOMPA. A second experiment using the Kuroshio Surface Water (KSW) in replace of the KSSW finds out that the KSW intrusion is not evident in our region. During autumn, the TSWW accounts for a major portion of the intrusion, and the warm and salty KSSW is only observed at the bottom of the southeastern part of the study area. The sensitivity of these results to the relevant parameters (e.g. the properties of the Source Water types, the atmospheric heat flux etc.) is also evaluated to test the robustness of this new MLOMPA. It turns out that the heat flux has the most significant impacts on the percentage of the TSWW (10.4%) and KSSW (8.06%) in summer. In winter, the uncertainty is generally very small, only about 5% to the salinity of the CDW. In autumn, the transport time of the TSWW has more influence on the percentage of water masses. The uncertainty of KSSW-variable has the smallest impact (<1%).The values of uncertainty are almost less than 7%.Overall these sensitivity tests demonstrate the reliability and robustness of our results from the MLOMPA.