斯里兰卡以东海域涡旋偶极子的生成与维持机制

斯里兰卡岛以东海域在西南季风期间常会出现一对低频涡旋偶极子（即斯里兰卡穹顶气旋涡和反气旋涡）,对当地的海洋生态系统及气候有着重要影响。基于（1/12）°分辨率的混合坐标海洋模式（Hybrid Coordinate Oceanic Model,HYCOM）再分析资料以及多尺度子空间变换（MWT）和基于MWT的正则传输理论等方法,分析了它们的生成发展机制。结果发现这2个涡旋发展所需能量最终都来源于海表风应力做功,但具体可取3条完全不同的路径：①风应力直接驱动涡旋;②风应力驱动背景场的西南季风流,其后西南季风流发生正压不稳定以提供涡旋所需的涡动能;③西南季风流通过风应力做功获得的动能转换为有效位能,其后通过斜压不稳定给涡旋提供位能。涡旋最终衰亡的主要原因包括风应力对涡旋做功的减少、西南季风流正压不稳定的减弱、涡旋的能量频散以及低频涡向更高频扰动的正向能量级串。     

Three-dimensional modeling of tidal circulation and mixing over the Java Sea

A combination of a three-dimensional hydrodynamic model and in-situ measurements provides the structures of barotropic tides, tidal circulation and their relationship with turbulent mixing in the Java Sea, which allow us to understand the impact of the tides on material distribution. The model retains high horizontal and vertical resolutions and is forced by the boundary conditions taken from a global model. The measurements are composed of the sea level at coastal stations and currents at moorings embedded in Seawatch buoys, in addition to hydrographic data. The simulated tidal elevations are in good agreement with the data for the K₁ and M₂ constituents. The K₁ tide clearly shows the lowest mode resonance in the Java Sea with intensification around the nodal point in the central region. The M₂ tide is secondary and propagates westward from the eastern open boundary, along with a counterclockwise amphidromic point in the western part. The K₁ tide produces a major component of tidal energy, which flows westward and dissipates through the node region near the Karimata Strait. Meanwhile, the M₂ tide dissipates in the entire Java Sea. However, the residual currents are mainly induced by the M₂ tide, which flows westward following the M₂ tidal wave propagation. The tidal mixing is mainly caused by K₁ tide which peaks at the central region and is consistent with the uniform temperature and salinity along the vertical dimension. This mixing is expected to play an important role in the vertical exchange of nutrients and control of biological productivity.     

A three-dimensional coastal barotropic model in generalized curvilinear grids

INTRODUCTIONThemethodofautomaticallygeneratinggeneralizedcurvilinearmeshesisaneffectivetoolforobtainingnumericalsolutionsofpartialdifferentialequationsinaregionwitharbitraryboundaries.Thegrids,calledadaptivemesh ,canbeadaptedtotheshapeoftheboundaryortothespatialdis tributionofthesimulatedfield .Insteadofthetraditionalequidistantgrids,automaticallyproducedcurvilineargrids,whichcanbeadaptedtotheequationsolutions’featuresandtotheirregularshapeofthelateralboundary,areused .Thenumericalcalcul…     

莫桑比克海峡及其邻近海区正压潮流数值模拟与能量收支分析

莫桑比克海峡及其邻近海区是全球海洋潮流和潮能耗散最强的海区之一。文章利用高分辨率通用环流模式对该海区的正压潮流进行模拟, 并对该海区潮能通量和潮能耗散特征进行分析。结果表明, 莫桑比克海峡及其邻近海区的潮波主要是半日分潮占主导地位, 全日分潮可忽略不计, M₂分潮形成1个左旋潮波系统和1个右旋潮波系统, S₂分潮形成1个左旋潮波系统。莫桑比克海峡和马达加斯加岛南部等绝大数区域的M₂和S₂半日潮流是逆时针旋转, 在马达加斯加岛顶部等局部区域是顺时针旋转, 而且在海峡通道等复杂地形处潮流流速量级较大。潮能通量矢量主要来自东边界, 大部分潮能通量沿马达加斯岛北部传入莫桑比克海峡区域, 其中经过马达加斯加岛北部和进入莫桑比克海峡的M₂ (S₂)分潮的潮能通量分别为156.86GW (40.53GW)和148.07GW (36.05GW), S₂分潮潮能通量的量级大约为M₂分潮的1/5~1/4。底摩擦耗散主要发生莫桑比克海峡和马达加斯加岛南北部, 其中莫桑比克海峡M₂ (S₂)分潮的底摩擦耗散为1.762GW (0.460GW), 占其底部总耗散的43.74% (39.72%)。     

南亚高压季节变化中的正斜压环流转换特征

利用１９５８～１９９７ 年的 Ｎ Ｃ Ｅ Ｐ／ Ｎ Ｃ Ａ Ｒ 再分析资料,采用将大气环流在垂直方向分解为正压和斜压环流两个分量的方法,讨论了南亚高压季节演变中的正斜压环流的转换特征。指出（１）夏季,南亚高压以斜压性为主,其斜压分量约占 ７０ ％ 的比重,冬季以正压性为主,其正压分量约占 ７０ ％ 的比重;（２）由冬季的正压性高压向夏季的斜压性高压的季节演变中,南亚高压是在其斜压分量环流的引导下移动的,即其斜压分量环流的变化超前于其自身的变化;（３）由夏季的斜压性高压向冬季的正压性高压的季节演变则相反,南亚高压是在其正压分量环流的引导下移动的。     

Localized multi-scale energy and vorticity analysis: II. Finite-amplitude instability theory and validation

A novel localized finite-amplitude hydrodynamic stability analysis is established in a unified treatment for the study of real oceanic and atmospheric processes, which are in general highly nonlinear, and intermittent in space and time. We first re-state the classical definition using the multi-scale energy and vorticity analysis (MS-EVA) developed in Liang and Robinson [Liang, X.S., Robinson, A.R., 2005. Localized multiscale energy and vorticity analysis. I. Fundamentals. Dyn. Atmos. Oceans 38, 195–230], and then manipulate certain global operators to achieve the temporal and spatial localization. The key of the spatial localization is transfer-transport separation, which is made precise with the concept of perfect transfer, while relaxation of marginalization leads to the localization of time. In doing so the information of transfer lost in the averages is retrieved and an easy-to-use instability metric is obtained. The resulting metric is field-like (Eulerian), conceptually generalizing the classical formalism, a bulk notion over the whole system. In this framework, an instability has a structure, which is of particular use for open flow processes. We check the structure of baroclinic instability with the benchmark Eady model solution, and the Iceland–Faeroe Frontal (IFF) intrusion, a highly localized and nonlinear process occurring frequently in the region between Iceland and Faeroe Islands. A clear isolated baroclinic instability is identified around the intrusion, which is further found to be characterized by the transition from a spatially growing mode to a temporally growing mode. We also check the consistency of the MS-EVA dynamics with the barotropic Kuo model. An observation is that a local perturbation burst does not necessarily imply an instability: the perturbation energy could be transported from other processes occurring elsewhere. We find that our analysis yields a Kuo theorem-consistent mean–eddy interaction, which is not seen in a conventional Reynolds stress framework. Using the techniques of marginalization and localization, this work sets up an example for the generalization of certain geophysical fluid dynamics theories for more generic purposes.     

非均匀风场与急流强迫的水体涡旋动力特征模拟

通过数值模拟有限区域水气界面由强迫作用驱动形成的水体涡旋及环流动力结构特征,分析非均匀风场、水体急流、两者叠加以及环境边界和地转偏向力等因子的综合影响,探讨此类水体涡旋结构和动力特征。风应力驱动的水体涡旋尺度大,相对深厚,正涡旋具有下凹表面,负涡旋具有上凸表面。水体急流驱动的涡旋形成在急流两侧,对应急流所在深度及厚度尺度相对较小,也较浅,但流速与强度均大于风场驱动的涡旋环流。地形阻挡起着引导涡旋环流走向的作用;同时在北半球地转偏向力对急流侧向负涡旋形成和强度增强更为有利。此外正涡旋对应的辐合辐散势函数强于负涡旋,有利于正涡旋区垂直上升运动强于负涡旋中垂直下沉运动。非均匀风场及水体急流两种强迫叠加作用下,涡旋数量增加、尺度减小,底层的流场形态及强度与表层差异增大。形成的水体涡旋结构呈现多种形态：深厚的整层一致;浅薄的仅维持在上层,或上下层环流相反等。风应力驱动的涡旋以正压性为主,水体急流驱动的涡旋因急流的垂直强切变而具有强的斜压性,在正斜压动能的转换中,正压性涡旋区有斜压动能向正压动能转换,斜压性涡旋区有正压动能向斜压动能转换,均有利于这两个区域正负涡旋的维持。     

ON THE BAROTROPIC INSTABILITY OF LARGE SCALE VORTICES

By using the linearized barotropic vorticity equation in polar coordinates the stability of pertur-bations on a large scale circular basic flow is transformed into a generalized eigenvalue problem,yielding the relationship between the growth rate of the amplitude of perturbations and the az-imuthal wave number. Then, numerical experiments whose integration time is 60 model hours areperformed in terms of a quasi-geostrophic barotropic model in Cartesian coordinates using the per-turbation stream function field of unstable mode superimposed on a strong and weak circular basicflows as the initial fields. The experimental results reveal that the amplitudes of the initial pertur-bations in the model atmosphere grow with time. The amplitude of the perturbations superimposedon the strong circular basic flow grows quicker and forms a spiral-band-like structure.     

一个正压和斜压不稳定的充要条件

提出一个新的李雅普诺夫函数，首次得到了正压和斜压不稳定的充要条件，大大改善了热知的Ｒａｙｌｅｉｇｈ－郭晓岚定理和Ｆｊｏｒｔｏｆｔ定理，解决了Ｒａｙｌｅｉｇｈ（１８８０）提出的无粘切变流正压稳定性问题。     

Study on the Dynamic Process of the Onset of South China Sea Summer Monsoon

By using the NCEP/NCAR reanalysis data from 1958 to 1997, we first looked into the atmospheric flow conditions in the one month immediately prior to the onset of the South China Sea summer monsoon （SCSSM） each year. A monthly-averaged zonal basic flow of 40-yr composite was then calculated. The stability of Rossby wave in the basic flow was studied based on the spherical barotropic vorticity equation. Furthermore, the spectral function expansion method was adopted to define and compute the evolvement of a developing wave packet. The results indicate that there exists barotropic instability of spherical Rossby wave in the climatically-averaged flow field before the SCSSM onset. The instability is triggered by the westerly jet stream in the Southern Hemisphere, and the strongest instable perturbation lies to the south of the westerly jet stream. The peak of the developing spherical Rossby wave packet propagates from mid and high latitudes to low latitudes, though not crossing the equator, spurring the cumulus convection in the tropical zones. The eruption of the cumulus convection and its spread to monsoon regions help to speed up the adjustment of the general circulation and the SCSSM onset. It is concluded that elements that contribute to the SCSSM onset are on global scale, albeit the onset itself looks like a local phenomenon.