Relative dispersion at the surface of the Gulf of Mexico
Relative dispersion at the surface of the Gulf of Mexico
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DOI：
10.1357/002224003322201205
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发表时间：
2003-05-01
影响因子：
0.5
通讯作者：
Ohlmann, C
中科院分区：
文献类型：
--
作者：
LaCasce, JH;Ohlmann, C
我们研究了墨西哥湾表面漂流物对和三对的相对运动。均方对间距随时间呈指数增长，从最小解析尺度（1 km）到 40-50 km，电子折叠时间为 2-3 天。此后，色散表现出对时间的幂律依赖性，指数在 2 到 3 之间（取决于所使用的测量方法），直至数百公里的尺度。应变大部分是各向同性的，只有微弱的区域变化。但有迹象表明西部盆地存在各向异性，可能是由于边界流平流造成的。在早期阶段和晚期部分阶段，这对速度是相关的。初始调整后，早期阶段的相对位移分布是非高斯分布且近似恒定，表明存在局部应变。三重结果同样表明两个生长阶段。在早期阶段，平均面积和最长三角形边随时间呈指数增长，后者的增长速度与双粒子结果一致。大多数三角形都是在这段时间绘制出来的。在后期，三角形不断增大，纵横比系统性减小，表明向等边形状的演变。尽管表面散度会影响这些统计数据，但它们仍然与二维湍流中发现的情况非常相似。如果是这样，我们将推断在变形半径（40-50 公里）以下的尺度上存在熵级联，这可能是光谱局部的。后者意味着颗粒分离的增长来自于与分离尺寸相同的流动特征。也可能存在逆向能量级联，其尺度大于变形半径，这可能是由斜压不稳定性驱动的。然而，后期统计数据也可能反映大规模剪切造成的色散。我们没有解决后期幂律增长的上限（即我们没有观察到最终的扩散阶段）。这可能反映剪切分散。但它也可能源于表面收敛，这可能导致长时间的粒子相关性，正如最近对限制内部湍流的表面上的粒子进行的数值模拟所示。
We examine the relative motion of pairs and triplets of surface drifters in the Gulf of Mexico. The mean square pair separations grow exponentially in time from the smallest resolved scale (1 km) to 40-50 km, with an e-folding time of 2-3 days. Thereafter, the dispersion exhibits a power law dependence on time with an exponent of between 2 and 3 (depending on the measure used) up to scales of several hundred kilometers. The straining is for the most part isotropic, with only weak regional variations. But there are suggestions of anisotropy in the western basin, probably due to boundary current advection.The pair velocities are correlated during the early phase and a portion of the late phase. The relative displacement distributions during the early phase are, after an initial adjustment, non-Gaussian and approximately constant, suggestive of local straining.The triplet results likewise suggest two growth phases. During the early phase, the mean area and the longest triangle leg grow exponentially in time, the latter with a rate consistent with the two-particle results. Most triangles are drawn out during this time. During the late period, the triangles grow and their aspect ratios systematically decrease, suggesting an evolution to an equilateral shape.Although surface divergences should affect these statistics, they nevertheless strongly resemble those found with two-dimensional turbulent flows. If so, we would infer an enstrophy cascade at scales below the deformation radius (40-50 km) which is probably spectrally local. The latter implies that growth in particle separations comes from flow features the same size as the separations. It is also possible there is an inverse energy cascade to scales larger than the deformation radius, driven possibly by baroclinic instability. However, the late period statistics may also reflect dispersion by a large scale shear.We do not resolve an upper bound on the late time power law growth (i.e. we do not observe an ultimate diffusive stage). This may reflect shear dispersion. But it may also stem from surface convergences which can cause long time particle correlations, as seen in recent numerical simulations of particles on a surface bounding an interior turbulent flow.