Collaborative Research: Impact of Bottom Boundary Layer Drag and Topographic Wave Drag on the Eddying General Circulation

近年来，由于有观点认为能量耗散驱动的混合对经向热量和碳输运有很强的控制，海洋能量源和汇的量化引起了很大的兴趣。本文研究了二次底边界层阻力和地形内波阻力对海洋环流的能量收支有重要贡献的假设，并对中尺度涡旋的动力学和统计有重要影响。将通过在理想模式和现实的全球海洋环流模式中采用不同强度的底部阻力系数，以及在现实模式中采用波浪阻力来检验这一假设。将模型中海洋表面涡旋动能的水平长度尺度和能级与卫星高度计观测数据进行比较，并将模型中涡旋动能的垂直结构与地下流计观测数据进行比较。根据研究人员先前使用理想化的两层准地转模型所做的工作，预计当底部阻力中等强度时，涡流将与观测结果最接近。假设底边界层阻力和波浪阻力对大气环流的能量收支都有重要贡献。智力优势：将在模型中研究底部阻力灵敏度，这些模型弥补了pi先前使用的两层准地转模型的几个缺陷；海洋分层截断为两层，缺少表面边界效应，底部平坦，水平均匀平均流。该项目将利用一种新的理想模式，同时解析多种内部准地转模态和地表模态，以及具有高垂直分辨率、非均匀强迫和粗糙地形的现实涡旋环流模式。研究人员将在以往经验的基础上，将地形内波阻力插入到海洋潮汐模型中，将波浪阻力插入到海洋环流模型中。研究人员手头有一个波阻力方案，适用于这里感兴趣的低频流。该方案来自大气界的一位合作者，他已经在一般环流模式中使用低频流的波浪阻力方案超过20年。波浪阻力计算也建立在与海洋地球物理学家合作的基础上，这导致了小尺度（1-10公里）地形粗糙度的合成数据集的构建。在最先进的全球水深数据集中，小规模的粗糙度没有得到充分的体现，然而，根据理论，它是低频流在粗糙地形上产生内波的原因。更广泛的影响：这里提出的工作将有助于讨论海洋能量耗散，通过研究两个可能的能量汇及其对涡旋环流动力学的影响。这里提出的工作也应该导致对涡流海洋模型的改进，这些模型对它们所采用的阻尼系数很敏感。随着计算机能力的不断增强，用于气候模拟的海洋模型将很快能够分辨涡旋。因此，对于气候研究以及其他应用来说，改进和测试涡旋解析海洋模型是很重要的。这里提出的工作将有助于实现这一目标。该项目将资助一名博士后科学家和一名研究生。博士后将使用现实模型，研究生将使用理想模型。该项目还为首席PI提供资金，使其能够继续与高中生和本科生暑期实习生一起工作，就像他从2006年开始的每个暑假一样。最后，该项目为首席研究员Arbic博士提供旅行资金，继续参加Jurgen Theiss博士每年举行的出发前会议，为学生参与nsf资助的桑给巴尔海峡项目做准备。

In recent years the quantification of oceanic energy sources and sinks has generated much interest, due to arguments that mixing driven by energy dissipation exerts a strong control on meridional heat and carbon transport. This project examines the hypothesis that both quadratic bottom boundary layer drag and topographic internal wave drag contribute significantly to the energy budget of the oceanic general circulation, and have a major impact on the dynamics and statistics of mesoscale eddies. The hypothesis will be tested by employing bottom drag coefficients of various strengths in both idealized models and realistic eddying global ocean general circulation models, and by employing wave drag in the realistic models. Horizontal length scales and energy levels of ocean surface eddy kinetic energy in the models will be compared to satellite altimeter observations, and the vertical structure of modeled eddy kinetic energy will be compared to a database of subsurface current meter observations. Based on previous work by the investigators with idealized two-layer quasi-geostrophic models, it is expected that eddies will compare most closely to observations when the bottom drag is moderately strong. It is also hypothesized that bottom boundary layer drag and wave drag both contribute significantly to the energy budget of the general circulation. Intellectual merit: Bottom drag sensitivity will be investigated in models which remedy several deficiencies of the two-layer quasi-geostrophic models previously employed by the PIs; the truncation of oceanic stratification to two layers, the missing surface boundary effects, the flat bottom, and the horizontally homogeneous mean flows. The project will utilize a new idealized model which simultaneously resolves multiple interior quasi-geostrophic modes and the surface mode, as well as realistic eddying general circulation models with high vertical resolution, inhomogeneous forcing, and rough topography.The investigators will build upon their previous experience inserting topographic internal wave drag into ocean tide models, to insert wave drag into ocean general circulation models. The investigators have on hand a wave drag scheme which is appropriate for the low-frequency flows of interest here. The scheme comes from a collaborator in the atmospheric community, which has been employing schemes for wave drag on low-frequency flows in general circulation models for over twenty years. The wave drag calculation also builds upon collaborations with marine geophysicists, which have resulted in the construction of synthetic datasets of small-scale (on order 1-10 km) topographic roughness. Small-scale roughness is insufficiently represented in state-of-the-art global bathymetric datasets, yet, according to theory, is responsible for the generation of internal waves by low-frequency flows over rough topography.Broader impacts: The work proposed here will contribute to the discussion of oceanic energy dissipation, through investigations of two plausible energy sinks and their impacts on the dynamics of the eddying circulation. The work proposed here should also lead to improvements in eddying ocean models, which are sensitive to the damping coefficients they employ. As computer power continues to increase, the ocean models used in climate simulations will soon become eddy-resolving. Thus it is important for climate studies as well as other applications for eddy-resolving ocean models to be improved and tested. The work proposed here will contribute to that goal. The project will support a post-doctoral scientist and a graduate student. The post-doc will work with the realistic models, and a graduate student will work with the idealized models. The project also provides funding for the lead PI to continue working with high school student and undergraduate summer interns, as he has every summer starting in 2006. Finally, the project provides travel funds for the lead investigator, Dr. Arbic, to continue to participate in the pre-departure meetings Dr. Jurgen Theiss will hold every year to prepare students for their involvement in the NSF-funded Zanzibar Channel Project.