Implications of Eddy Cancellation for Nutrient Distribution Within Subtropical Gyres
Implications of Eddy Cancellation for Nutrient Distribution Within Subtropical Gyres
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DOI：
10.1029/2018jc013842
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发表时间：
2018-09-01
影响因子：
3.6
通讯作者：
Marshall, David P.
中科院分区：
文献类型：
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作者：
Doddridge, Edward W.;Marshall, David P.
中尺度涡旋在副热带环流营养收支中的作用仍然知之甚少，也缺乏约束。我们探索了一种新的机制，中尺度涡可能有助于这些营养收支，即涡取消。涡抵消描述了中尺度涡的整流效应，以对抗欧拉平均埃克曼泵送。我们提出了一个理想化的轴对称两层模型的营养在风驱动的环流，并探讨该模型的敏感性，其参数值的变化。我们发现，剩余的埃克曼泵送速度有实质性的影响营养浓度，模式水厚度。这些结果表明，剩余的Ekman泵和模式水厚度的响应是非单调的：对于这些参数的小值的营养浓度随参数的增加而减少。然而，超过一个临界值，进一步增加Ekman泵或模式水厚度增加整个高度理想化的模型中的营养浓度。薄模式水层促进营养物质从深渊的垂直扩散，而较厚的模式水层通过减少参数化的颗粒通量通过温跃层增加生产力。模式水厚度的影响是调制的剩余Ekman泵的速度：强大的Ekman泵抑制模式水厚度对营养盐浓度的影响。我们使用卫星和现场测量，以评估模式水厚度对初级生产力的影响，并找到一个统计上显着的关系，较厚的模式水与较高的生产力。副热带环流是太平洋和大西洋上跨越数千公里的大型海洋环流，在那里几乎没有增长。浮游植物是构成海洋食物链基础的微型植物，需要营养，而营养供应短缺。然而，一些浮游植物确实生活在这些环流中，几十年来它们如何获得营养一直是个谜。教科书上说，风把表面的沃茨推到环流的中心，然后向下移动，从上层阳光照射的沃茨中带走营养物质，阻止浮游植物生长。然而，最近的一些研究表明，这可能是不正确的。我们建立了一个理想化的模型，考虑到这个新的结果。与我们的理想化模型，我们发现，浮游植物在表面的生长是由一层水的表面以下称为模式水的影响，较厚的模式沃茨有助于支持更多的浮游植物生长的表面附近。使用来自卫星，自主海洋机器人和船舶的数据，我们发现支持我们理想化模型的结果：较厚的模式水增强了浮游植物的生长。这是我们对营养物质如何供应到亚热带环流表面沃茨的理解的根本变化。
Plain Language Summary The role of mesoscale eddies within the nutrient budget of subtropical gyres remains poorly understood and poorly constrained. We explore a new mechanism by which mesoscale eddies may contribute to these nutrient budgets, namely eddy cancellation. Eddy cancellation describes the rectified effect of mesoscale eddies acting to oppose the Eulerian-mean Ekman pumping. We present an idealized axisymmetric two-layer model of a nutrient in a wind-driven gyre and explore the sensitivity of this model to variations in its parameter values. We find that the residual Ekman pumping velocity has a substantial impact on nutrient concentration, as does mode water thickness. These results suggest the response to both residual Ekman pumping and mode water thickness is nonmonotonic: for small values of these parameters the nutrient concentration decreases as the parameter increases. However, beyond a critical value, further increases in Ekman pumping or mode water thickness increase nutrient concentration throughout our highly idealized model. A thin mode water layer promotes vertical diffusion of nutrients from the abyss, while a thicker mode water layer increases productivity by reducing the parametrized particulate flux through the thermocline. The impact of mode water thickness is modulated by the residual Ekman pumping velocity: strong Ekman pumping suppresses the influence of mode water thickness on nutrient concentrations. We use satellite and in situ measurements to assess the influence of mode water thickness on primary productivity and find a statistically significant relationship; thicker mode water correlates with higher productivity. This result is consistent with a small residual Ekman pumping velocity.Subtropical gyres are large oceanic circulations that span thousands of kilometers in the Pacific and Atlantic oceans, where very little grows. Phytoplankton, the microscopic plants that form the basis of the oceanic food chain, need nutrients, which are in short supply. However, some phytoplankton do live in these gyres and how they get their nutrients has been a mystery for decades. The textbooks state that winds push the surface waters into the center of the gyres, and then downward, removing nutrients from the upper sunlit waters and preventing phytoplankton from thriving. However, some recent works showed that this might not be correct. We build an idealized model that takes account of this new result. With our idealized model we find that the growth of phytoplankton at the surface is affected by a layer of water well below the surface called mode water; thicker mode waters help to support more phytoplankton growth near the surface. Using data from satellites, autonomous ocean robots, and ships, we find support for the result from our idealized model: thicker mode water enhances phytoplankton growth. This is a fundamental change in our understanding of how nutrients are supplied to the surface waters of subtropical gyres.