【作者】 乔璐璐；

【导师】 吴德星； 鲍献文； 王小华；

【作者基本信息】 中国海洋大学 ， 物理海洋学， 2008， 博士

【摘要】 冬季大风降温过程是渤黄海冬季常见而重要的天气现象,其对环流发展、泥沙输运有重要的影响。然而目前却鲜有在高时空分辨率的海表面风场、热通量场及波浪场作用下,对渤黄海环流和泥沙输运过程对大风降温事件响应的研究。论文以ECOMSED数值模型为基础,耦合SWAN浅海浪模式,以数值模拟为手段,采用6小时一次ERA40风场及海表热通量数据,对1999年~2000年冬季12月19日~21日的一次大风降温事件对渤黄海环流和泥沙输运过程的影响进行数值模拟。在这种高频、灾害性天气条件下,辽东湾及北黄海水位较冬季平均风时要降低超过50cm,北黄海水位降低的最大幅度达1m,在Ekman输运作用下,中国苏北沿岸出现水位的堆积;剧烈海表降温使得渤海以及黄海的大部分海域温度降低2～4oC,而在黄海东南海域温度有上升趋势,而底层升温区域甚至一直向西北扩展至山东半岛的东南,温度升高近1oC,升温范围与黄海暖流路径相一致,此现象预示了随风力加强的黄海暖流的热平流作用;大风事件的海底淤积结果可以用来解释山东半岛东南海域的淤积带:大风事件使得山东半岛东北部泥沙被再悬浮并受鲁北沿岸流的输送南移,并在山东半岛东南海域岬角环流的弱流区沉积,一次大风事件可导致山东半岛东南海域中心厚度0.5cm的带状淤积泥沙;并且一次大风事件可使得从渤海海峡向黄海和从北黄海向南黄海的净泥沙通量增大为平均风时的3~4倍。大风事件中,水位及流场的变化表现出分别滞后于最大风力0.5天和1天的位相差,南、北黄海的3个站位的流场观测结果,亦显示流速(尤其是黄海暖流)峰值与风力峰值有12~48小时的位相差,悬浮泥沙更是在风力减弱后数天才得以沉积。风驱动环流数值试验表明,冬季偏北风作用下水位的调整,是大风事件中黄海暖流的增强滞后于风力最大值的原因,正压梯度力控制下的逆风北向流流速极值要待风力松弛后滞后于风力极值约1天。除了极值出现时间的位相差,在风场建立和消失时,海面的调整和消失亦分别需要10天和20天的时间,加之黄海暖流微弱的北向流速,是海表温度数据显示的季节变化中黄海暖舌滞后于偏北风一个月的原因;另外,科氏力和底地形对黄海暖流的西偏起决定性作用。黄海暖流和海表热通量在冬季渤黄海热收支问题上的不同作用,一直存有争议。利用气候态卫星海表温度数据计算渤、黄海冬季热含量变化率,结果为-106 Wm-2,而NOC1.1a、ERA40、OAFlux+ISCCP和NCEPR四种海表热通量数据显示,冬季气候态渤黄海海域平均海表净热通量为-150 Wm-2,这就意味着海表热通量在渤黄海冬季热含量变化中起主要作用,而超过29%的正的热含量变化,则主要是黄海暖流热平流的作用。1999-2000年冬季一次大风事件,使得渤黄海海域平均热含量变化为-241Wm-2,海表净热通量作用为-281Wm-2,因此黄海暖流的热平流输送在此次大风事件中可超过40 Wm-2,是正常天气条件下的2倍。潮动力学对泥沙的再悬浮、输移等有重要的作用,同时,高浓度的泥沙又会使得底边界层层化,弱化底应力而反过来影响潮动力学,后者往往是研究者所忽略的。因为论文以黄河口潮动力过程生成的黄河口切变锋出发,讨论了切变锋的生成机制和对河口泥沙分布的影响。黄河口切变锋是以潮流流向相反或者流速显著不同而形成的切变带,黄河口外陡坡地形上的最大潮流等时线存在最大梯度,且潮流椭圆主轴与最大潮流等时线平行分布,是其主要生成机制;底摩擦、径流只能改变切变锋的强度,岸线变化对切变锋影响不大。黄河口外的高泥沙浓度主要是再悬浮产生,而与切变锋的作用关系不大。再谈回潮流作用下的渤、黄海泥沙分布,在苏北浅滩、朝鲜沿岸以及渤海海峡、辽东湾存在的较大泥沙浓度,亦主要是泥沙再悬浮产生;在这些悬沙高浓度区,会导致底边界层的层化,减弱底应力,抑制泥沙再悬浮,在泥沙浓度高的苏北浅滩、北黄海朝鲜沿岸,潮汐振幅和迟角被明显增大。总体来看,论文讨论了大风降温事件对水位、流场及泥沙输运过程的影响,从动力学上解释了山东半岛东南海域带状淤积区的形成,并初步分析了黄海暖流在短时间尺度和季节变化上与冬季风的位相差以及西偏于黄海深槽的原因;结合卫星资料和数值模式结果,对冬季平均及大风降温事件下的渤黄海冬季热收支进行了分析,给出海表热通量和黄海暖流所起的不同作用;论文从河口潮动力学与泥沙分布关系出发,首次给出黄河口切变锋的生成机制,并探讨了切变锋对黄河口泥沙分布的影响,在此基础上,进一步讨论了渤、黄海潮动力学与泥沙分布的相互关系,并给出泥沙浓度对潮动力学的反作用。更多还原

【Abstract】 The winter storm event, a familiar synoptic phenomenon in winter of the Bohai Sea and Yellow Sea, has the important effect on the circulation and sediment transport. However, few works has been carried on this subject by high resolution wind stress, heat flux and wave. Thus in this study, a 3-D numerical model, ECOMSED, coupled with shallow sea wave model, SWAN was used to study the effect of winter storm event on the circulation and sediment transport, taking the event on 19 to 21 December in the winter of 1999~2000 as an example. The model was driven by 6-h wind stress and surface heat flux in the Bohai Sea and Yellow Sea. Due to the Ekman transport, water was accumulated along the Subei and water level decreased over 50 cm to 100 cm in the Liaodong Bay and north Yellow Sea. Almost all the area the water temperature decreased 2 to 4oC, while the water temperature in the southeastern Yellow Sea increased 1oC at the bottom which indicated the effect of heat advection due to the strengthen Yellow Sea Warm Current. The mud ridge to the southeast of Shandong Peninsula can be described as result of winter storm event. High concentration sediment was transported from the northeast of the Shandong Peninsula to the southeast and deposited in the area of weak circulation. 0.5cm mud accumulation can be found to the southeast of Shandong Peninsula due to one storm event. And the net sediment flux is 3 and 4 times more than the effect of climitological wind, respectively from the Bohai Strait to Yellow Sea and from north Yellow Sea to the south.During the winter storm event, the phase of water level and circulation are 0.5 and 1 day lagged to the wind, respectively. Furthermore, field observation in the Yellow Sea also showed the 12 to 48 hours phase difference between wind and northern currents. The numerical experiment indicated that the positive pressure gradient, due to water accumulation in the south Yellow Sea, was the main force to drive the northward flow. Water adjustment made the maximum northward current lag behind the maximum northerly wind by 1day. And it took almost 10 and 20 days respectively for the surface slope to adjust to a stable state as the wind started and stopped. The combination effect of weak speed and phase lag of the wind driven compensation current can be used to explain the one month phase delay between the beginning/ending of winter northwesterly wind and the emergence/disappearance of the warm tongue in the Yellow Sea detected from the satellite SST data. And the sensitivity experiments indicated that the Coriolis force and topography played major roles in the westward shift path of the Yellow Sea Warm Current in the Yellow Sea.Different effect of heat advection and surface heat flux on the Yellow Sea heat budget in winter was still in heat debate. There was heat loss all over the Yellow Sea during winter and the winter averaged heat content change decreased with an area averaged rate of - 106 W m-2. Comparing with the similar horizontal distribution and winter averaged value of surface heat flux of -150 W m-2 averaged from four data sources, we conclude that surface heat flux played a dominant role in the YS heat content change during winter, while the effect of positive heat advection of the YSWC can account for between 17 % and 41% of the YS heat content change, calculated by four different heat flux data sets. During a winter storm event in winter of 1999 to 2000, the heat content change is -241Wm-2, while the effect of surface net heat flux is -281Wm-2, thus the heat advection transported 40Wm-2 heat into the Yellow Sea, which is two times more than the climatologic effect.On the one hand, sediments can be resuspended and transported by tidal current. On the other hand, tidal current can be affected by high sediment concentration by stratified bottom boundary layer. Tidal shear front off the Yellow River mouth is the result of tidal dynamics. Thus the mechanism of this front and its effect on the sediment distribution off the river mouth were studied. The sensitivity numerical experiments showed that the topography with a strong slope off the Yellow River mouth was a determining factor for the front generation, and a parallel orientation between the major axes of ellipses and co-tidal lines of maximum tidal current was a necessary condition. While the bottom friction and the river runoff had no effect on the front location but affected the front intensity, the front generation was not sensitive to the coastline variation. The study concluded that the bottom slope off the river mouth induces a strong variation in the bottom stress in a cross-shore direction, which produces both maximum phase gradient and sediment concentration variability across the tidal shear front. As for the sediment distribution in the Bohai Sea and Yellow Sea, it is high sediment concentration in the shallow water along Subei, Korea, in Bohai Strait and Liaodong Bay, which are resuspended by strong tidal currents. The high sediment concentration can stratify the bottom boundary layer, reduce the bottom shear stress then damp the sediment resuspension. Therefore, the sediment concentration was significantly reduced in above places, and the tidal amplitude and phase were increased and delayed.Therefore, based on the simulation of the circulation and sediment transport during the winter storm event, the formation of mud ridge to the southeast of Shandong Peninsula was described in dynamic method. The phase lag of Yellow Sea Warm Current to the northerly wind and westward shift path were discussed. Based on the satellite data and numerical simulation, the heat budge of Yellow Sea and Bohai Sea in winter was studied by discussing the effect of heat advection and surface heat flux. The generation mechanism of tidal shear front off the Yellow River mouth was first studied and its effect on the sediment distribution off the river mouth was also described. Although the effect of high sediment concentration on the tidal dynamics was studied in this work, it still needs further detailed study.更多还原