Mesoscale Eddy Dissipation by a "Zoo" of Submesoscale Processes at a Western Boundary
Mesoscale Eddy Dissipation by a "Zoo" of Submesoscale Processes at a Western Boundary
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DOI：
10.1029/2020jc016246
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发表时间：
2020-11-01
影响因子：
3.6
通讯作者：
Forryan, Alexander
中科院分区：
文献类型：
--
作者：
Evans, Dafydd Gwyn;Frajka-Williams, Eleanor;Forryan, Alexander
中尺度涡旋是一种普遍存在的动力学特征，它倾向于沿海洋西边界向西传播和沿着消失。使用多尺度观测研究，我们评估在何种程度上涡消散通过直接级联的能量在西部边界。我们分析了基于船的微观结构和速度调查的数据，以及18个月的系泊部署，记录了在北大西洋巴哈马群岛以东的地形斜坡上撞击的反气旋和气旋性涡旋的能量耗散。这些观测结果揭示了高水平的湍流，陡峭和粗糙的地形坡度改变了与反气旋涡旋相关的向北流动。在近海底和中间深度（200-800米）都观察到消散增加。近底湍流发生在一个突出的悬崖，弗劳德数升高，建议水力控制的背风面。能量还以向上传播的内波的形式辐射。高架耗散在中间深度发生在强烈的垂直切变，地形坡度修改的垂直结构向北的涡流的区域。在这里，低理查森数和局部变化的等密度梯度的位涡（PV）表明，高架耗散与水平剪切不稳定。高架中层深度耗散也引起地形转向的流量。这导致了大的反气旋涡度和负PV附近的地形坡度，这表明离心不稳定性支撑的本地增强耗散。我们的研究结果提供了一个机械基准的现实代表性的涡流耗散海洋模式。
Mesoscale eddies are ubiquitous dynamical features that tend to propagate westward and disappear along ocean western boundaries. Using a multiscale observational study, we assess the extent to which eddies dissipate via a direct cascade of energy at a western boundary. We analyze data from a ship-based microstructure and velocity survey, and an 18-month mooring deployment, to document the dissipation of energy in anticyclonic and cyclonic eddies impinging on the topographic slope east of the Bahamas, in the North Atlantic Ocean. These observations reveal high levels of turbulence where the steep and rough topographic slope modified the intensified northward flow associated with, in particular, anticyclonic eddies. Elevated dissipation was observed both near-bottom and at mid depths (200-800 m). Near-bottom turbulence occurred in the lee of a protruding escarpment, where elevated Froude numbers suggest hydraulic control. Energy was also radiated in the form of upward-propagating internal waves. Elevated dissipation at mid depths occurred in regions of strong vertical shear, where the topographic slope modified the vertical structure of the northward eddy flow. Here, low Richardson numbers and a local change in the isopycnal gradient of potential vorticity (PV) suggest that the elevated dissipation was associated with horizontal shear instability. Elevated mid-depth dissipation was also induced by topographic steering of the flow. This led to large anticyclonic vorticity and negative PV adjacent to the topographic slope, suggesting that centrifugal instability underpinned the local enhancement in dissipation. Our results provide a mechanistic benchmark for the realistic representation of eddy dissipation in ocean models.