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

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

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.