A Case Study on the Variability of Summer Water Properties in the Southeastern Yellow Sea Based on the Hydrological Data from 1995 to 2019

The long-term variability of the summer water properties in the southeastern Yellow Sea is described using the hydrological data. The results indicate warming trend in the upper layer and cooling trend in the deeper layer resulting in a strengthen thermocline. While, the mean and the core temperatures in the southeastern Yellow Sea Cold Water Mass both tend to rise slowly, which are previously thought to be fallen. At the deep layer (below 30 m depth), the temperature cooling trend in summer is even stronger than that in winter, which contradict to the stronger surface heat flux in summer. This study proposes that the strengthen thermocline during warming seasons (spring and summer) prevents the heat transferring from surface to the deep layer across isopycnal lines. Furthermore, combined with the metrological data, the strengthen thermocline induces stronger southward tidal residual current at the deep layer, which facilitates the enhanced complementary wind-driven current induced by the southerly monsoon. Thus, the southward tidal induced residual current effectively contributes to the southward motion of the colder water from the northern area. The declined trending of salinity due to the increase of the Changjiang River discharge can be conducive to the strengthen summer thermocline.