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Ocean Surface Topography from Space
Multi-Scale Ocean Circulation in Satellite and In Situ Observations


Nikolai Maximenko - (University of Hawaii)

High-resolution global dataset, collected by satellite altimeters over almost 20 years, reveals complex organization of the ocean surface circulation on mesoscale. This organization is best seen in multi-year time average as a system of quasi-zonal jet-like features, covering all parts of the ocean. With all the complexity of the pattern, it is robust and withstands effects of stronger perturbations, associated with the interannual variability of large-scale flow and with mesoscale eddies, dominating the velocity field. Recent studies suggest that, in discord with the classical theory of freely evolving geophysical turbulence, these features are strongly controlled by the real-world boundaries of the oceans. Impact of the boundaries ranges from forcing through various localized vorticity sources to shaping the large-scale gyres, whose instability feeds open-ocean eddies. Proposed research will advance understanding of the dynamics of permanent mesoscale signatures of meandering fronts and organized eddies in the upper and intermediate depth ocean.

Surface circulation will be studied using improved mean dynamic topography. The improvement will be achieved through the use of new satellite missions (such as GOCE), expanded datasets of in situ observations (such as drifters), and refined assessments of the ageostrophic signal. Spatiotemporal statistics of eddies and other anomalies will be analyzed in western parts of the oceans to explain why such jets as the Kuroshio Extension and Gulf Stream remain narrow on long-time mean despite the strong velocity variability near their axes. In central and eastern parts of the oceans, preferred paths of eddies and statistics of eddy-genesis will be compared with the time-mean "striations" in the context of the dynamic of nonlinear beta-plume.

In addition to isolated quasi-zonal features, the project will characterize systems of branching jets, commonly observed in frontal zones. Synthesized together, the census of permanent mesoscale features will provide systematic description of a dense grid of ocean features that can be effectively used for validation and improvement of ocean models.

The tendency of mesoscale eddies to align along some (steady or moving) lines, that was recently detected in mid-latitude ocean regions free of strong currents, will be investigated by analyzing space correlations between individual eddies, derived from gridded altimetry products. The analysis will include statistical properties of eddies of different signs and 'ages.' The consistency of the internal eddy structure on optimally interpolated maps will be verified through comparison with along-track data, high-resolution SST, and model solutions. Effects of eddy shape and eddy alignment on large-scale eddy fluxes will be also evaluated. A new global dataset, characterizing mesoscale eddies, will be using trajectories, collected by the Global Drifter Program.

Search for "permanent" mesoscale anisotropic features will be extended into the intermediate depth ocean, using profiles of Argo floats and CTD's. Gridded datasets of ensemble-mean water properties, float velocities, and absolute dynamic height, referenced to absolute dynamic topography at the ocean surface, will be calculated for this purpose. A technique will be developed to correct errors in float velocity estimates at the 'parking' level, caused by the float ascent/descent through the currents, having strong vertical shear. The same profiles will be used to analyze the differences between vertical structures of large-scale flow and mesoscale eddies. Three-dimensional structure of time variability of the dynamic height will be compared with the structure of variability of the dynamic topography at the surface. Eddy properties will be also compared with the properties of the fastest growing instability mode, derived from a one-dimensional model, configured using hydrographic climatologies.

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