On the Asymmetry between Cyclonic and Anticyclonic Flow in Basins with Sloping Boundaries
On the Asymmetry between Cyclonic and Anticyclonic Flow in Basins with Sloping Boundaries
复制标题
倾斜边界盆地旋旋流与反气旋流的不对称性研究
DOI：
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发表时间：
2008
期刊：
影响因子：
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通讯作者：
J. Nycander
中科院分区：
文献类型：
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作者：
O. Nøst;J. Nilsson;J. Nycander
本文给出了由地应力强迫的倾斜边界盆地正压环流的实验室实验和数值模拟结果。重点放在具有明显小于单位数的大尺度Rossby数的流动上。室内实验和数值模拟结果表明，气旋环流遵循等深线，其流型与强迫强度无关。对于反气旋环流，流型随强迫强度的变化而变化。对于弱强迫，它类似于气旋式地形导向型；对于强强迫，它发展出强横坡气流。线性动力学在气旋环流和反气旋环流之间是对称的，很好地描述了气旋环流和弱强迫反气旋环流。非线性动力学分析表明，地形操纵的气旋流都是无粘非线性方程的稳定和定常能量极小解。这意味着对于地形操纵的气旋流，非线性项(相对涡度平流)总是很小的。对于反气旋气流，情况则截然不同。没有反气旋的地形导向流可能永远不是定常无粘方程的解。如果这种稳定的反气旋气流确实存在，它很可能是不稳定的，因为它必须对应于能量的鞍点，而不是最小或最大值。当Rossby数大于Ekman数时，非线性项是重要的，这是强迫最强的反气旋实验的情况。在这些实验中，相对涡度的平流阻止了气流跟随地形，从而产生了相对涡度较强的位置和横坡水流。横坡水流的发展可以从地形不规则的盆地中的位涡守恒来理解。实验室实验和理论分析表明，反气旋气流与陡峭地形的分离与北卡罗来纳州哈特拉斯角的墨西哥湾流与大陆斜坡分离等特征是一致的。
The authors present results from laboratory experiments and numerical simulations of the barotropic circulation in a basin with sloping boundaries forced by a surface stress. Focus is placed on flows with large-scale Rossby numbers that are significantly smaller than unity. The results of the laboratory experiments and simulations show that cyclonic circulation follows the isobaths, the flow pattern being independent of the strength of the forcing. For anticyclonic circulation, the flow pattern changes with forcing strength. It is similar to the cyclonic topographically steered pattern for weak forcing, and it develops strong cross-slope flows for strong forcing. Linear dynamics are symmetric between cyclonic and anticyclonic circulations and give a good description of the cyclonic and weakly forced anticyclonic circulations. The analysis of the nonlinear dynamics shows that topographically steered cyclonic flows are all stable and steady energy-minimum solutions to the inviscid nonlinear equations. This implies that the nonlinear terms (advection of relative vorticity) are always small for the topographically steered cyclonic flow. For anticyclonic flow, the situation is very different. It is possible that no anticyclonic topographically steered flow is ever a solution to the steady inviscid equations. And if such a steady anticyclonic flow does exist, it is likely to be unstable, since it must correspond to a saddle point in energy rather than to a minimum or a maximum. The nonlinear terms are important when the Rossby number is larger than the Ekman number, which is the case for the anticyclonic experiments with strongest forcing. For these experiments, the advection of relative vorticity prevents the flow from following topography, creating locations with strong relative vorticity and cross-slope flow. The development of cross-slope flow can be understood from the conservation of potential vorticity in basins with irregular topography. The separation of anticyclonic flow from steep topography shown in the laboratory experiments and the theoretical analysis herein are in agreement with features like the Gulf Stream separation from the continental slope at Cape Hatteras, North Carolina.