Xing, C.; Zhao, Q.; Hu, Z., and Liu, Y., 2018. Direct observations and modeling of summertime coastal circulation patterns in the western Bohai Sea.

Eutrophication of the coastal waters in the western Bohai Sea results in serious environmental problems. In summer, severe brown tide frequently happens in the Qinhuangdao coastal areas, causing enormous losses to the local fishery. A survey array with four sets of seabed-based current observation systems equipped with acoustic Doppler current profilers was deployed in September 2013 to investigate the hydrodynamic background of the brown tide. The observations suggest that the shore currents flow generally northward and offshore currents flow southward during the survey period. The coastal circulation system and its formation mechanisms are studied with a three-dimensional nested-grid ocean circulation model. The model results suggest that coastal currents form a clockwise eddy-shaped circulation system with a diameter of 65 km. Numerical sensitivity experiments imply that the tides and summer monsoons generate the observed coastal circulation system. The combined effects of the tidal residual currents and wind-induced currents transport water northward and shoreward, resulting in a higher water level in the north of Qinhuangdao. The barotropic pressure gradient then produces southward pressure compensation flows. Consequently, the northward shore currents and the offshore southward compensation flows form the clockwise eddy-shaped circulation. The summer monsoons are important driving forces to generate the observed coastal circulation system. Because the summer monsoons are periodically occurring meteorological processes of this region, the observed coastal circulation system is anticipated to appear every summer. The environmental problems in the Qinhuangdao coastal areas are mainly induced by the land-based pollution discharges. Combined with observations on the surrounding land-based pollution discharges, the information obtained through this study could be used to track the nutrient transport and eventually help to mitigate adverse phytoplankton blooms.