IntroductionThe nearshore sea off the northeastern Shandong Peninsula is characterized by intensive mariculture, whose ecosystem is involved in the large marine ecosystem of the Yellow Sea. However, ocean currents in this area are poorly explored. Observations suggested overturning currents were robust phenomena in winter in this area.MethodsNumerical simulations and experiments were used to investigate the mechanisms of overturning currents.ResultsThere were two classes of wind-driven overturning currents. One consisted of surface southeastward currents, nearshore downwelling currents, and bottom northeastward currents. The other consisted of surface northeastward currents, nearshore upwelling currents, and bottom southwestward currents.DiscussionThe underlying dynamics involved local wind forcing and propagation of coastal trapped waves (CTWs). Northwesterly winds in the Bohai Sea and North Yellow Sea drove surface southward currents and converged water toward coastline off the northeastern Shandong Peninsula, generating nearshore sea level rising. The resultant southward sea level slope drove nearshore bottom northward currents. Meanwhile, high sea level in the southern part of Bohai Sea and North Yellow Sea also propagated as CTWs clockwise around the Shandong Peninsula, which further enhanced nearshore bottom northward currents and caused eastward currents in the entire water column off the northeastern Shandong Peninsula. Southwesterly winds in the Bohai Sea and North Yellow Sea drove surface northward currents, generating nearshore sea level dropping off the northeastern Shandong Peninsula. The resultant northward sea level slope caused bottom southward currents. Meanwhile, the southwesterly winds caused CTWs with low sea level in the south part of the Bohai Sea and Yellow Sea. The northward sea level slope of CTWs enhanced nearshore bottom southwestward currents. The current study emphasized winds in winter drove not only local currents but also propagation of CTWs in the Bohai Sea and North Yellow Sea. The sea level slope of CTWs regulated surface and bottom Ekman layers driven by local winds.

Traditionally, the Yellow Sea (YS) is believed to be well mixed throughout the water column or to experience significant temperature inversions in winter. Recently, we found a significant positive surface-bottom temperature difference (SBTD) as large as 3.7 degrees C in northeastern coastal waters of the Shandong Peninsula in the YS in winter. By analyzing 4 years (June 2016-September 2020) of online in situ observations, this study examined monthly and interannual variations in winter positive SBTD and the underlying dynamics. The winter positive SBTD generally formed in November, peaked in December/January, and disappeared in March. At the interannual time scale, this SBTD was strongest in 2018 and weakest in 2020. The underlying dynamics involved the combined effects of prevailing winter monsoon winds and resultant surface heat loss and current heat advection. When the surface northerly wind was strong, strengthened southward advection of cold air over the ocean cooled the water column intensely, whereas wind-enhanced southward advection of warm surface water partly offset the sea surface cooling and resulted in surface temperature cooling that was considerably slower than bottom temperature or even surface temperature warming. These processes together generated strong positive SBTDs. When the surface northerly wind was weak, all of the above processes became weak and resulted in weak positive SBTDs. The relationship with Yellow River runoff was also discussed. This study enriches our knowledge of winter thermal structure in the YS. More observations and coupled physical-biogeochemical simulations are needed to further clarify the impacts on climate and marine ecology.

Hypoxia has been observed worldwide and causes great damage to marine ecosystems. In the northeastern mariculture areas of the Shandong Peninsula, summer oxygen depletion and even hypoxia have been observed in the bottom layer, but the underlying dynamics are still poorly understood. In the current study, the physical controls of variations in bottom layer oxygen concentrations in summer and their interannual variations in the northeastern mariculture areas of the Shandong Peninsula were first investigated using in situ observations collected during 2016-2020. Over the past 5 years, the dissolved oxygen (DO) concentrations of the bottom layer have ranged from 0.8 to 13.0 mg L-1 and exhibited a robust annual cycle, with the lowest value occurring in August. Hypoxia events covering several days were observed intermittently in the summers of 2016 and 2017. Summer oxygen depletion was caused by the combined effects of ocean stratification and enhanced biogeochemical oxygen consumption. At the interannual time scale, stratification and bottom layer DO concentrations showed synchronous variations and both peaked in 2018. The underlying mechanism was related to bottom layer currents, which were created by southerly monsoon winds. Strong/weak southerly surface winds induced a strong/weak bottom shoreward advection of cold, oxygen-rich water and resulted in strong/weak stratification and high/low DO concentrations. The results provide a good reference for coastal mariculture management in the Shandong Province and enrich our understanding of the global distribution of coastal hypoxia.

为丰富山东半岛近岸海洋牧场海域水动力环境研究,本文利用2019年12月3日至2020年1月1日在山东半岛东北部4个海洋牧场获取的海流资料,应用功率谱分析、调和分析、余流主轴分析和相关分析,探讨冬季各海洋牧场的潮流、余流特征及其影响机制.结果表明:(1)各海洋牧场潮流由M2分潮潮流主导,受地形边界限制,各主要分潮潮流均为往复流,且潮流椭圆主轴平行岸线.(2)不同海洋牧场呈现不同的余流特征和影响机制.烟台安源海洋牧场余流大致垂直于岸线流向近岸,平均流速约为0.9～1.7 cm/s;日平均流以经向流为主,与经向风呈显著正相关,海水受北风强迫在近岸堆积.威海瑜泰海洋牧场余流大致垂直于岸线流向外海,平均流速约为1.4～1.7 cm/s;日平均流亦以经向流为主,与经向风呈显著负相关,表层海水受北风强迫向近岸堆积,在近岸产生下降流,海面以下存在北向的离岸流.威海西港海洋牧场余流为东南向,平均流速约为2.5～3.0cm/s,日平均流具有较为显著的正压性.荣成楮岛海洋牧场余流为东北向,平均流流速约为5.6～9.9cm/s,日平均流表明海水沿桑沟湾南岸流出海湾,推测桑沟湾海水在湾内逆时针流动.研究成果利于进一步研究山东半岛邻近海域动力环境的多尺度时空变化特征及其影响机制.