Model Results: North Atlantic euphotic zone nitrate, the role of eddies, 1993-1997, 2007 (U.S. JGOFS Synthesis & Modeling Phase project results)

<p>Research topic: the role of eddies in basin-scale biogeochemical budgets of the North Atlantic</p> <p>Coupled physical-biogeochemical models provide a useful framework for testing our ideas about the underlying physical, biological and chemical controls on elemental cycling in the ocean. Up to now, efforts to do so on basin scales have produced mixed results.&nbsp; The seminal work of Sarmiento et al (1993) and Fasham et al (1993) demonstrated that the large scale distributions of chlorophyll in the surface waters of the North Atlantic could be simulated with a reasonable degree of accuracy using a simple planktonic ecosystem embedded in a coarse resolution general circulation model.&nbsp; While the general characteristics of these large scale patterns were consistent with observations, detailed comparisons with time series data at specific sites revealed that the underlying flux balances maintaining these distributions were not as satisfactory. In particular, the nutrient budget in the Sargasso Sea was dominated by horizontal advection.&nbsp; This result is quite different from recent regional modeling studies (McGillicuddy 1995; McGillicuddy and Robinson, 1997) which have suggested that upwelling in the interiors of cyclonic mesoscale eddies is the dominant mode of nutrient supply to the euphotic zone in the open ocean.</p> <p>During the last several years, the World Ocean Circulation Experiment Community Modeling Effort (CME) has made significant progress in the development of more realistic general circulation models. Areas of improvement particularly relevant to biogeochemical processes include (1) aspects of the time-mean flow, (2) treatment of the surface boundary layer, and (3) the resolution of mesoscale eddies. This next generation of GCMs represents a substantial step forward in the physical bases on which coupled biogeochemical simulations can be built. It is proposed herein to construct the first basin scale eddy-resolving biogeochemical simulations of the North Atlantic to examine the impact of mesoscale processes on large scale elemental budgets.&nbsp; A suite of experiments is planned which are based on 1 degree (coarse), 1/3 degree (eddy-permitting) and 1/6 degree (eddy-resolving) CME simulations.&nbsp; The coarse resolution runs will be used to benchmark the Sarmiento and Fasham results, and serve as a control run for the higher resolution calculations.&nbsp; Although the eddy-permitting case will most likely not be entirely sufficient for a comprehensive study of mesoscale processes, it does represent a meaningful intermediate step that will provide useful information without incurring the substantial computational cost of the 1/6 degree runs.&nbsp; This will set the stage for a relatively small number of fully eddy-resolving runs which have only recently become feasible from a computational point of view.&nbsp; Understanding gained from these experiments will be used to guide the development of parameterizations for mesoscale biogeochemical processes to permit their inclusion in larger scale (global) models.</p><p>研究主题：北大西洋海盆尺度（basin-scale）生物地球化学收支（biogeochemical budgets）中的涡旋作用</p> <p>耦合物理-生物地球化学模型（coupled physical-biogeochemical models）为检验我们关于海洋元素循环的潜在物理、生物及化学调控机制的构想提供了有效框架。迄今，在海盆尺度开展此类研究得到的结果不尽一致。萨米恩托（Sarmiento）等人（1993）与法沙姆（Fasham）等人（1993）的开创性研究表明，通过将简单浮游生态系统（planktonic ecosystem）嵌入低分辨率海洋环流模式（General Circulation Model, GCM），可较为精准地模拟北大西洋表层海水叶绿素的大尺度分布特征。尽管这些大尺度分布的总体特征与观测结果相符，但针对特定站位的时间序列观测数据进行的详细对比显示，维持这些分布的关键通量平衡并不尽如人意。具体而言，马尾藻海的营养盐收支（nutrient budget）以水平平流（horizontal advection）为主导。这一结果与近期的区域模拟研究（麦吉利卡迪，1995；麦吉利卡迪与罗宾逊，1997）结论截然不同，后者提出，开放海域真光层（euphotic zone）的营养盐供给主要依赖气旋式中尺度涡（mesoscale eddies）内部的上升流过程。</p> <p>近年来，世界大洋环流实验社区模拟工作组（Community Modeling Effort, CME）在构建更具真实性的海洋环流模式方面取得了显著进展。与生物地球化学过程密切相关的改进方向包括：（1）时间平均流场特征，（2）表面边界层处理方案，（3）中尺度涡的分辨率。新一代海洋环流模式为耦合生物地球化学模拟奠定了更为坚实的物理基础，实现了实质性突破。本研究拟开展首次北大西洋海盆尺度涡旋解析型生物地球化学模拟，以探究中尺度过程对大尺度元素收支的影响。本研究计划开展一系列基于CME模拟的试验，分辨率分别为1°（低分辨率）、1/3°（允许涡旋分辨率）与1/6°（涡旋解析分辨率）。低分辨率模拟将用于复现萨米恩托与法沙姆的研究结果，并作为高分辨率模拟的对照试验。尽管允许涡旋分辨率的模拟可能无法完全满足中尺度过程的全面研究需求，但它仍是极具意义的中间步骤，可在无需承担1/6°模拟所需高昂计算成本的前提下提供有效参考。这将为为数不多的全涡旋解析模拟铺平道路，而这类模拟直至近期才在计算层面具备可行性。本研究通过试验获得的认识，将用于指导中尺度生物地球化学过程参数化方案的开发，以便将其纳入更大尺度（全球）的海洋模式中。</p>