【作者】 孙群；

【导师】 管长龙；

【作者基本信息】 中国海洋大学 ， 物理海洋， 2003， 博士

【摘要】 海洋上混合层在大气和海洋之间的水汽、热量和动量的交换过程中有着重要的作用。海浪为存在于海气界面上的一种重要运动形态，研究其对混合层的动力学结构和热力学结构的影响具有重要科学意义。 本文综述了海洋上混合层研究的现状和存在问题，系统地研究了海浪破碎对混合层动力学结构的影响。 海洋上混合层的动力学和热力学结构特征主要受到海表风应力、净热通量和浮力通量的影响，而混合层的形成与维持依赖于混合层内的湍流生成和湍流垂向混合。文中简要描述了混合层内的湍流生成和湍流与其它形式能量相互转化的物理机制。 利用NODC提供的Levitus(1994)全球气候月平均混合层深度资料，分析了三种不同混合层深度定义下的混合层深度的空间分布特征和季节变化规律。分析表明，基于温度阶跃0.5℃得到的MLD较深，尤其在1～4月，这可能主要是因为没考虑MLD此时受到的盐度变化的影响；基于温差为0.5℃对应的密度定义的MLD最浅。尽管如此，三种定义下得到的MLD的空间分布特征及季节变化规律却是大体一致的。整体而言，夏季由于太阳辐射增强，风应力减小，导致MLD变浅，不超过50米；冬季相反，MLD较深，且在局部海域(如西北太平洋、北大西洋和南大洋)出现MLD显著加深的现象，约在100～300米之间，最深处在600米以上，具有明显的季节变化规律。而在10°S～10°N之间的赤道附近海域，因风应力和净热通量的季节变化不大，且该区降水量较大，浮力通量增加，MLD较浅，低于50米，且没有明显的季节变化。 以现有的理论研究为基础，通过分析海洋上混合层中考虑波浪运动的能量平衡方程，探讨了混合层中波—湍相互作用的物理机制，并对波浪破碎导致混合层内湍流混合加强和耗散增加的现象进行了分析。由于海浪理论和湍流理论都存在着一些目前无法解决的问题，所以文中对混合层中波—湍相互作用物理机制的探讨只是一个初步的尝试，远不是完善。同时，由于无法获得观测资料，本文所得的波浪破碎对海洋上混合层影响的数值研究结果没能与现场观测结果比较。 采用一维2.5阶湍封闭混合层模式，根据参数化方法对波浪破碎导致的能量耗散率进行了估计，通过改变湍动能方程的上边界条件引入波浪破碎对湍动能生成的影响，计算了不同风应力强迫下的混合层流场结构和湍能量收支(下述给出的结果是取风速为20m／s得到的)。模拟结果表明，当考虑波浪破碎的影响时，混合层深度比无波浪影响时的结果加深了约50cm；随着风应力作用的增加，波浪破碎加强了混合层中的湍流混合，促使加深幅度也增加。当混合层内温度分布均匀时，在混合层底部形成一温度跃层，并且考虑波浪破碎影响时，混合层内的温度比无波浪影响时均匀分布的温度降低了0.013℃。 一维模式的流场结果表明，混合层中的流场是由惯性波动与时间平均流两部分叠加而成，且通过时间平均消去惯性波动后得到平均流，其不同深度的流速矢量构成一Ekman螺旋，表层流速矢量相对风向右偏了30.3’。当考虑波浪破碎的影响时，湍流混合的加强，促使上层流场趋于均匀分布，混合层内速度切变减小，表层流速分量均有明显减小，流速分量U减小了19.8 cm/s，流速分量V减小3.2cm/s，表层流速矢量相对风向右偏了49.70，比无波浪影响时向右多偏了19.40，并且随着风速的增加，这种右偏角度的增加量也相应地增加，而取10m/s风速时，右偏增加的角度只有12’。 通过对一维模式湍动能方程中各项进行深度积分，可以分析混合层中的湍动能收支问题。结果表明，当不考虑波浪的影响时，混合层中的湍流生成和耗散间的局部平衡主要通过湍动能的剪切生成项、耗散项和浮力生成项来实现。当考虑波浪破碎对混合层中湍流生成的影响时，混合层上部的垂直扩散项和耗散项都有显著的增加，由于流速减小，剪切生成项也相应地减小。同时，湍动能方程中各项的垂向分布表明，波浪破碎在海表输入的湍动能通量主要在近海表2.5米以内的水层内被耗散掉，在该深度内，被耗散掉的湍动能占整个混合层内耗散总能量的92.0%，与无波浪影响的结果48.2%相比，增加了约1倍，而剪切生成的作用减小了3.5%。 为了更全面地研究波浪破碎对混合层的影响，将国际上盛行的主维海洋模式一POM模式和海浪模式一认叭M模式相结合，利用认认M模式计算的海浪破碎导致的能量耗散率，作为POM模式湍动能方程的上边界条件，引入波浪破碎在海表产生的一向下输入的湍能量通量。受垂向分辨率限制，三维联合模式试验只计算了风速为10m/s的情况。结果表明，当考虑波浪破碎对混合层的影响时，混合层深度加深了约50cm，并且在混合层加深过程中，混合层达到某一深度所需时间明显提前，即在同一时刻，考虑波浪破碎的混合层深度比无波浪影响的要深，例如当混合层深度为30米时，前者比后者提前46.9小时。同时，混合层内均匀分布的温度也降低了0.013℃。这也说明，在三维模式计算中，波浪破碎对上混合层的影响要比一维模式中明显，这可能是因为三维模式中全面地考虑了混合层中一些其它物理过程(如水平平流项的影响)。 当考虑波浪破?更多还原

【Abstract】 The ocean surface mixed layer plays an important role in the processes of the exchange of heat and momentum between the upper ocean and the atmosphere. The effect of wind waves on the dynamical and thermal structure of the ocean mixed layer is significant.Previous work on ocean mixed layer and about the effect of wave breaking on the mixed layer structure are first summarized. Then the effects are studied using numerical models.The understanding of the effects of atmospheric forcings, including mainly the surface wind stress, net heat flux and buoyant flux, on the dynamical and thermal structure of the ocean surface mixed layer necessitates a proper knowledge of the mechanisms and influences of the different turbulent mixing processes involved in the upper ocean. A brief description of the physical mechanism of the turbulence production and its transform with the other forms of the energy is given.The global climatological monthly mean data of the mixed layer depth (MLD) supplied by Levitus (1994) in NODC based on three different criteria, are used to analyze the space distribution and seasonal variability of MLD. It is found that MLD based on a temperature change from the ocean surface of 0.5 degree Celsius is deeper, especially from Jan. to Apr., while MLD based on a variable density change from the ocean surface with a temperature change of 0.5 degree Celsius is shallower. However, the space distribution and seasonal variability of MLD obtained from different criteria are consistent with each other. In general, MLD becomes shallower and less than 50 meters with solar radiation intensifying and wind stress weakening in summer, and becomes deeper in winter, especially in oceans (such as the northwest Pacific, the north Atlantic and the Southern Ocean), where the MLD’s are between 100 and 300 meters (the deepest depth being 600 m) and have evident seasonal variability. The MLD of the ocean near the Equator from 10?S to 10?N are less than 50 m and have no evident seasonal variability.We analyze the energy balance equations for mean flow, turbulence and wave motion in the ocean surface mixed layer, and discuss the wave-turbulence interaction involved. The observed phenomena, in which vertical turbulence mixing and turbulence dissipation are enhanced by the action of breaking wave, are explained.However, there are some inextricable problems in the theories of both wind waves and turbulence, so our theoretical analysis about the wave-turbulence interaction is rudimentary and incomplete. Furthermore, because of the scarcity of measurements and data, our work has to be limited to investigating the effect of wave breaking on the ocean surface mixed layer by the numerical models.A one-dimensional oceanic mixed layer model with the M-Y level-2.5 turbulence closure schemes is employed. The rate of energy loss by breaking waves is estimated by parameterization and incorporated into the model as a source of turbulence kinetic energy (TKE) by modifying the existing surface boundary condition of TKE equation. The velocity field and turbulence energy budget are calculated under different forcing conditions (The results given below are for a wind speed of 20 m/s.). When the effect of surface wave breaking is considered, MLD is 50 cm deeper than that obtained without wave breaking, and with the enhancement of wind stress forcing, the deepening of MLD is increased. When the temperature is uniform within the mixed layer, a thermocline appears at the bottom of this layer, and when the effect of surface wave breaking is considered, the temperature is 0.013 癈 lower than the result without wave breaking.On the other hand, it is indicated that the velocity field consists of a depth-independent inertial oscillation and a time-mean shear flow, which can be obtained by subtracting the inertial oscillations time mean in one inertial period. The hodograph of the velocity vector of time-mean flow at the different depths consists of a Ekman spiral, and the surface velocity is 30.3?to the right of the wind. When the更多还原