Cross-shelf structure of coastal upwelling : a two - dimensional extension of Ekman's theory and a mechanism for inner shelf upwelling shut down
Cross-shelf structure of coastal upwelling : a two - dimensional extension of Ekman's theory and a mechanism for inner shelf upwelling shut down
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DOI：
10.1357/002224008787536790
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发表时间：
2008-09
影响因子：
0.5
通讯作者：
Estrade Philippe;Marchesiello Patrick;De Verdière Alain Colin;Roy Claude
中科院分区：
文献类型：
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作者：
Estrade Philippe;Marchesiello Patrick;De Verdière Alain Colin;Roy Claude
非洲西北部沿海上升流区域的海面温度图像显示，上升流的核心有时位于远离海岸的地方。有三个区域的情况证明了这一点，它们有一个共同的特点，即大陆架宽而浅。这种上升流特征在加那利海流系统的生态学中起着关键作用。它创造了一个内锋，在高生产力的近岸环境中为生物材料，例如鱼卵和幼虫提供保留。一个分析模型已经开发的基础上的二维扩展的Ekman的解决方案。均匀海洋对上升流有利风的线性和稳定响应提供了上升流与海岸分离的机制。表层和底层Ekman层的合并导致了非常弱的越岸环流和Ekman输送辐散的“运动障碍”。在沿岸风的情况下，障碍物位于等深线h 0.4D附近，其中D是埃克曼层的厚度。这就产生了一个上升流单元，它基本上集中在0.5D < h < 1.25D的区域，上升流优先发生在等深线h <0.6D附近。结果表明，上升流的越岸宽度与Ekman水深与海底地形坡度的比值D/S成正比。这种解决方案的应用，真实的测深剖面合理化，不仅在西北非洲的海上上升流观测，但也对风驱动的沿海上升流的跨大陆架结构的地形的影响。该模型还量化了横岸风分量的影响，显示了它如何驱动近岸压力梯度调整以及它如何影响上升流。一个线性数值实验重现理论稳定的解决方案，从而允许调查的瞬态制度。松弛的假设在数值模型验证了线性假设的理论，然后允许调查的敏感性摩擦参数化和分层的影响。后者导致的“振荡”的上升流细胞与海迁移驱动的露头和水柱，和，海岸入侵驱动的“边界层分裂”的过程所造成的向岸平流的等密度圆顶和分层的内架。
Sea-surface temperature images of the coastal upwelling regions off Northwest Africa show that the core of upwelling is sometimes located far from the coast. This has been documented in three regions that share a common feature, namely a wide and shallow continental shelf. This upwelling feature plays a key role in the ecology of the Canary Current System. It creates an innerfront which provides retention for biological material, e.g. fish eggs and larvae, in the highly productive nearshore environment. An analytical model has been developed based on a two dimensional extension of Ekman's solution. The linear and steady response of a homogeneous ocean forced by an upwelling-favorable wind provides a mechanism for the upwelling separation from the coast. The merging of the surface and bottom Ekman layers induces a very weak cross-shore circulation and a “kinematic barrier” for the Ekman transport divergence. In the case of an alongshore wind, the barrier is located near the isobath h ≈ 0.4D, where D is the thickness of Ekman layers. This yields an upwelling cell which is essentially concentrated in the region 0.5D < h < 1.25D, with upwelling occurring preferentially near the isobath h ≈ 0.6D. It turns out that the cross-shore width of upwelling scales with D/S, the ratio of Ekman depth to bottom topographic slope. The application of this solution to real bathymetric profiles rationalizes, not only the offshore upwelling observations in Northwest Africa, but also the influence of topography on the cross-shelf structure of a wind-driven coastal upwelling. The model also quantifies the effect of the cross-shore wind component showing how it drives the nearshore pressure gradient adjustment and how it affects the upwelling. A linear numerical experiment reproduces the theoretical steady solution, thereby allowing investigation of the transient regime. Relaxation of the hypothesis in the numerical model validates the linear assumption of the theory and then allows investigation of the sensitivity to friction parameterizations and the influence of stratification. The latter leads to an “oscillation” of the upwelling cell with seaward migration driven by outcropping and homogeneization of the water column, and, coastal incursion driven by a “boundary layers splitting” process caused by shoreward advection of the isopycnal dome and stratification of the inner shelf.