Origins and transports of the low-salinity coastal water in the southwestern Yellow Sea

In the southwestern Yellow Sea there is a low-salinity and turbid coastal water, the Subei Coastal Water (SCW). The origins of freshwater contents and thus the dissolved terrigenous nutrients in the SCW have been debated for decades. In this study, we used a well-validated numerical model to quantify the contributions of multiple rivers, i.e., the Changjiang River in the south and the multiple Subei local rivers (SLRs) in the north, in forming this year-round low-salinity coastal water. It is found that the freshwater contents in the SCW is dominated by the Changjiang River south of 33.5°N, by the SLRs north of 34.5°N, and by both sources in 33.5°-34.5°N. Overall, the Changjiang River contributes~70% in the dry season and~80% in the wet season of the total freshwater contents in the SCW, respectively. Dynamics driving the Changjiang River Plume to flow northward is the tidal residual current, which can even overwhelm the wind effects in winter seasons. The residual currents turn offshore near the Old Yellow River Delta (OYRD) by the collision of the two tidal wave systems, which transport the freshwater from both sources into the interior Yellow Sea. Water age experiments show that it takes 50-150 d for the Changjiang River Plume to reach the SCW in the spring and summer seasons, thus there is a 2-month lag between the maximum freshwater content in SCW and the peak Changjiang River discharge. In the winter and autumn seasons, the low salinity in inner SCW is the remnant Changjiang River diluted water arrived in the previous seasons.