The velocity and vorticity structure of the Agulhas Current at 32°S
The velocity and vorticity structure of the Agulhas Current at 32°S
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32°S 厄加勒斯洋流的速度和涡度结构
DOI：
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发表时间：
1999
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通讯作者：
H. Bryden
中科院分区：
文献类型：
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作者：
L. Beal;H. Bryden
本文用一台低置式声学多普勒海流剖面仪（LADCP）直接测量了西南印度洋西部边界流--厄古拉斯海流的全水深流速场。1995年2月至3月，作为世界海洋环流实验印度洋方案的一部分，在南非东海岸外32°S处的海流上建立了15个综合电导率-温度-深度（CTD）和LADCP站。深层的速度结构的阿古拉斯电流被发现是非常不同的，比以前所描述的使用地转估计。特别是，LADCP的结果揭示了一个V形图案的水平上没有运动的阿古拉斯电流和阿古拉斯暗流被观察到低于800米的深度，直接下方的极地流动的阿古拉斯电流的表面核心赤道流动。直接速度和地转速度的比较表明，厄古拉斯电流的速度结构基本上是地转低于约200米的深度，其中的差异一般小于估计的误差。在这些深度以上，LADCP测量的切变和地转切变表现出差异，速度可以发散。船载ADCP数据支持LADCP结果在表面附近，表明这两个仪器测量的是真实的信号。地转和LADCP测得的剪切是很好的匹配远离表面，和深度平均适合（低于200米），它们之间表现出小的标准偏差。参考LADCP，厄古拉斯海流的地转体积输送为73 Sv，与LADCP的直接输送估计值仅相差3%。最近文献中对阿古拉斯海流输送的全深度估计由Toole和Warren [1993]给出，他们估计地转体积输送为85 Sv。从他们的截面和这项工作的斜压速度结构是相似的，表明在运输估计的差异是由于选择的地转参考水平，并最终到以前未观察到的厄古拉斯暗流的存在。CTD/LADCP组合资料对位涡分析非常有用，因为速度和密度测量是同时和一致的。涡度结构的阿古拉斯电流表明存在一个“混合边界”在中层深度，类似于在墨西哥湾流中观察到的功能。这一特征似乎与等密度线的急剧上弯有关，与大陆坡脚重合。边界抑制横流混合的中间水团，并可能有助于解释外观的离散细丝的红海水近岸南极沃茨在同一密度层，尽管强烈的横流剪切。
The full depth velocity field of the Agulhas Current, the Western Boundary Current of the southwest Indian Ocean, has been directly measured using a lowered acoustic Doppler current profiler (LADCP). Fifteen combined conductivity-temperature-depth (CTD) and LADCP stations were occupied across the current at 32°S off the east coast of South Africa in February-March 1995, as part of the World Ocean Circulation Experiment Indian Ocean program. The deep velocity structure of the Agulhas Current was found to be very different than previously described using geostrophic estimates. In particular, LADCP results reveal a V-shaped pattern for the level of no motion across the Agulhas Current and an Agulhas Undercurrent is observed flowing equatorward below 800 m depth, directly beneath the surface core of the poleward flowing Agulhas Current. Comparisons of direct and geostrophic velocities suggest that the velocity structure of the Agulhas Current is essentially geostrophic below about 200 m depth, where differences are generally less than estimated errors. Above these depths the LADCP-measured shears and geostrophic shears exhibit differences and velocities can diverge. Shipboard ADCP data support LADCP results near the surface, indicating that both instruments are measuring real signals. The geostrophic and LADCP-measured shears are well matched away from the surface, and a depth-averaged fit (below 200 m) between them exhibits small standard deviations. The geostrophic volume transport of the Agulhas Current, as referenced to LADCP, is 73 Sv, and is just 3% different from the direct LADCP transport estimate. The most recent full depth estimate of Agulhas Current transport in the literature is given by Toole and Warren [1993], who estimate a geostrophic volume transport of 85 Sv. The baroclinic velocity structures from their section and from this work are similar, indicating that the difference in the transport estimates is due to the choice of geostrophic reference level and ultimately to the presence of the previously unobserved Agulhas Undercurrent. Combined CTD/LADCP data are very useful for potential vorticity analysis, since velocity and density measurements are simultaneous and coincident. The vorticity structure of the Agulhas Current suggests the presence of a “mixing boundary” at middepths, similar to features observed in the Gulf Stream. The feature appears to be associated with a sharp upturn in isopycnals, coincident with the foot of the continental slope. The boundary inhibits cross-stream mixing of intermediate water masses and may help explain the appearance of a discrete filament of Red Sea Water inshore of Antarctic Intermediate Waters in the same density layer, despite intense cross-stream shears.