【作者】 夏长水；

【导师】 张庆华； 乔方利；

【作者基本信息】 中国海洋大学 ， 物理海洋学， 2005， 博士

【摘要】 海洋中温度的垂直结构大体可以分为三层:上混合层、温跃层和底层弱垂直温度梯度层。对于海洋环流模式来说,无论对于大尺度全球气候模拟还是小尺度的近海模拟,准确地再现上层海洋的垂直混合过程和模拟上混合层是很重要的。目前广泛应用Mellor和Yamada(M-Y)湍流混合模型在层化较强的流体(如夏季上层海洋)中对湍流混合系数计算效果不佳。模拟得到的夏季海洋表层温度(SST)偏高、上混合层偏浅且温跃层强度偏低是采用M-Y湍流混合模型的海洋环流数值模式所面临的普遍问题。 袁业立于1979年指出,造成表层海水混合的主要因素有三种:(1) 由非稳定层化造成的自由浮力对流;(2) 流动的剪切不稳定性;(3) 海浪的搅拌。在层化较强的上层海洋中,海浪的搅拌的作用可以超过前两种的作用的总合。M-Y湍流混合模型可以较好地描述和表达前两种机制造成的混合,但不包括海浪的搅拌造成的混合,这是采用M-Y湍流混合模型的海洋环流模式所模拟的夏季海洋上混合层偏浅的根本原因。如何准确地描述海浪引起的垂直涡动扩散系数并应用到实际的三维海洋模式中是一项具有挑战性的工作。 本文在袁业立和乔方利等提出的波浪运动混合的理论框架的基础上,基于MASNUM~1(前身为LAGFD-WAM)海浪波数谱数值模式和POM(普林斯顿海洋模式)环流模式建立了MASNUM浪-流耦合模式。耦合的方法是首先利用MASNUM海浪波数谱数值模式来计算波浪的方向谱,再利用袁业立、乔方利提出的理论计算浪致混合Bv和海浪对海流的动量转移项,并将其引入到环流模式中。 本文首先将MASNUM浪-流耦合模式应用到准全球大洋,所模拟的海浪有效波高同Topex/Poseidon高度计观测结果符合较好。由海浪模式得到的浪致混合Bv和由M-Y湍流模型得到垂向混合系数Kh相比,在夏季的中高纬度海区的上层,Bv比Kh要大;在低纬度热带海区上层Bv小于Kh。将浪-流耦合模式和采用M-Y湍流模型的原始POM模式模拟得到的夏季上层温度结构同Levitus资料比较表明:浪-流耦合模式的结果明显比原始POM的结果和Levitus资料相吻合,浪-流耦合模式模拟得到的夏季上混合层厚度比原始POM要深,和Levitus资料更多还原

【Abstract】 The vertical temperature structures of the ocean generally can be divided into three layers: the surface mixed layer, the thermocline and the layer with weak vertical temperature gradient. An accurate representation of the upper ocean mixing processes and thus the oceanic surface mixed layer (ML) is important for ocean circulation models, whether they are aimed at small-scale coastal simulations or for large-scale global climate simulations. The widely used Mellor-Yamada turbulence closure scheme often underestimates the vertical mixing in the upper ocean with strong stratification, and thus the sea surface temperatures (SST) is overestimated, ML is too shallow and the seasonal thermocline is underestimated, especially during the summer.Yuan Yeli (1979) suggested that there are three mechanisms responsible for the water mixing in the upper ocean: (1) the free buoyancy convection developed with unsteady stratification, (2) the turbulence mixing caused by shear of unsteady velocity field; (3) the wave stirring. In the upper ocean with strong stratification and weak flow shear mixing induced by wave stirring is often larger the former two mechanisms. The M-Y turbulence scheme can describe the mixing induced by the former two mechanisms well, but the wave-induced mixing is not included in it. That is the main reason why the surface mixing layer simulated by the circulation model using M-Y turbulence closure model is too shallow in summer. How to parameterize the wave-induced mixing and apply it into the three-dimensional ocean circulation model is a challenging job.Base on the theory of wave-induced mixing put forward by Yuan Yeli and Qiao Fangli this paper established the MASNUM wave-circulation coupled model, which incorporated the MASNUM wave model and the Princeton Ocean Model (POM). Thecoupling method is that first calculate the wave-number spectrum then compute the wave-induced mixing Bv and put it into the circulation model.The paper firstly applied the MASNUM wave-circulation coupled model into the quasi-global ocean. The simulated significant wave height agrees the Topex/Poseidon altimeter observation well. In the summer upper ocean in the middle and high latitude the wave-induced mixing Bv is larger than the vertical mixing derived from the M-Y turbulence closure model while in the low latitude Bv is smaller than Kh. Comparing the simulated summer upper ocean temperature structure by the wave-circulation coupled model and the original POM with the Levitus data, it suggests there is obvious difference between the result of the original POM and the the Levitus data and the result of the coupled model agrees the Levitus data well. The mixed layer depth in the coupled model result is much deeper than the result of the original POM and it fit the Levitus data well. This suggests that the wave-induced mixing play the key role in forming the summer surface mixed layer in the middle latitude, it is also significant to the upper mixed layer in the tropical area.On the other hand the Yellow Sea Cold Water Mass (YSCWM) as a unique ocean phenomena attracts the interests of many oceanographers, but controversy remains on the bottom layer circulation and the vertical circulation of the YSCWM. Based on the MASNUM wave-circulation coupled model the paper established the MASNUM wave-tide-circulation coupled model by introducing periodical the tidal current in the open boundary. The model is applied to study the circulation pattern of the summer Yellow Sea. The simulated tide, temperature and salinity agree the observation well, the simulated current filed is also confirmed by observation. A comprehensive three-dimensional circulation structure of the Yellow Sea in summer is put forward based in the model result and the observation data:(1) In the surface layer (0-4m), driven by wind the prevailing current direction is northeastward.(2) The upper layer (4-40m) is dominated by a basin scale anti-clockwise (cyclonic) gyre, diagnostic analysis of the momentum balances and sensitive study shows the cyclonic circulation in the upper layers is mainly aquasi-geotropic flow along tidal-induced temperature front, it is also strengthened by the Eulerian residual tidal currents. The stream function of the YS shows the net circulation of the YS is anti-clockwise (cyclonic). (3) In the bottom layer (below 40m) the water diverges from the YS trough towards seashore and the divergence velocity is strong along the temperature front area. The baroclinic pressure gradient force due to the strong tidal induced temperature front in the bottom layer drives the water to diverge. There also exists a weak southward current. Tidal residual current and the northward wind transport in the surface layer may contribute to the formation southward flow in the bottom layer.On the vertical circulation structure, the vertical circulation structures vary along different sections. The upwelling is front-scale instead of basin-scale. The upwelling exists along the bottom slope. The upwelling of the vertical circulation is mainly induced by two mechanisms: the baroclinic pressure gradient force due to the tidal induced temperature front and compensation upwelling of the Ekman transportation. A circulation cell is found in the lower layer of the front near the Korean coast with upwelling along the coastal side of the front and downwelling along the offshore side of the front.The MASNUM wave-circulation coupled model established in this paper successfully solve the problem of the surface mixing layer simulated by the circulation model using M-Y turbulence closure model is too shallow in summer. It is important for ocean circulation models, whether they are aimed at small-scale coastal simulations or for large-scale global climate simulations. The MASNUM wave-tide-circulation coupled model established in this paper also enhanced the ability of the original POM in simulating the circulation in the coastal areas.更多还原