基金项目:国家重点研发计划(2018YFC1406302); 国家自然科学基金青年基金(42106005)
通信作者:hujy@xmu.edu.cn
Objective: The seasonal distributions of water masses and the temporal and spatial variations of temperature-salinity structure have important influences on the hydrology, climate and fishery sites of offshore waters. Previous researches mainly used classical methods to analyze the water masses in the western East China Sea in summer and winter. To reveal the less understood structure of water masses in the western East China Sea in spring and autumn, we collect conductivity-temperature-depth (CTD) data during spring and autumn cruises in the western East China Sea and analyze the spatial distributions of temperature and salinity, from which five water masses are classified using the fuzzy clustering method and T-S similarity method.
Methods: Water masses are preliminarily classified using the fuzzy clustering method. In order to present a more realistic distribution of water masses, the T-S similarity method is applied to further quantify the water sample by clearly specifying the boundaries between water masses. Given that fluid generally moves along the isopycnal surface, the T-S similarity method defines a T-S similarity number (TSSN) by calculating the distance between water samples and typical water mass curves on the isopycnal surface. TSSN ranges between 0 and 1. The closer the TSSN value is to 0, the higher the similarity of the water sample is to the typical water mass.
Results: The western East China Sea is classified into five water masses: the Min-Zhe coastal water, the mixed water, the Taiwan Warm Current water, the Kuroshio surface water and the Kuroshio subsurface water. The Min-Zhe coastal water is distributed in a zonal pattern along the western coast, forming a long and narrow strong front of temperature and salinity when it intersects with the mixed water in spring. The Kuroshio surface water occupies the offshore area shallower than 75 m in spring, with two high salinity water tongues extending to nearshore at the 50 m layer. In autumn, the Kuroshio surface water is only distributed northeast of the investigated region and is replaced by the Taiwan Warm Current water in the southeast. The Kuroshio subsurface water upwells across the continental shelf in both spring and autumn, and it upwells to 50 m in autumn, cutting off the Taiwan Warm Current water into two parts.
Conclusion: According to the temperature-salinity structure and classification of water masses derived from the observation results of the two cruises in April 2019 and November 2018, it is found that the distribution of temperature and salinity in the western East China Sea has spatial differences in horizontal and vertical directions, and the distribution of water masses also shows clear seasonal variation. Future measurement data in summer and winter in this region, as well as other parameters except for temperature and salinity may support the in-depth discussion of intact seasonal variation of water masses in the western East China Sea.