Assessing water mass mixing dynamics in the Yellow Sea and East China Sea using dissolved inorganic iodine species

Abstract

The Yellow Sea (YS) and East China Sea (ECS) are interconnected, characterized by a semi-enclosed continental distribution and seasonally varying current systems. Dissolved inorganic iodine species (DIIS) concentrations, which act as quasi-conservative tracers, were measured in the YS and ECS to analyze circulation patterns. In the YS, iodide concentrations were higher at the surface, whereas iodate dominated bottom waters. In the bottom waters ($>40$ m), both iodide and total inorganic iodine were lower in November than in July. This decrease was attributed to the stronger intrusion of the Kuroshio-origin Yellow Sea Warm Current (YSWC), with lower iodide concentrations. Based on iodide and salinity, the proportion of YSWC-derived water in the YS bottom water increased from 26% in July to 34% in November. DIIS were further applied in the optimum multi-parameter analysis (OMPA) method, together with tracers such as temperature, salinity, and humic-like fluorescent dissolved organic matter. OMPA effectively characterized the water mass distributions across the YS and northern ECS, revealing key features such as the surface intrusion of Changjiang Diluted Water and the majority ($>$ 70%) of Yellow Sea Bottom Cold Water at the bottom of the central YS. At the surface of the mid-YS ($>$35°N), iodide concentrations exceeded 200 nM, which could not be explained by known water masses in the YS or ECS. To account for this, we introduced Bohai Surface Water (BSW) into the OMPA, revealing a 25% contribution that elucidates a transport pathway in the Yellow Sea previously overlooked.