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Ocean Surface Topography from Space
SCIENCE
Internal Tides from Space: High Resolution Mapping, Regional Characterization, and Developing a Correction for Wide-Swath Altimetry


Author:

James Girton - (University of Washington)


Co-Investigator(s):
  Matthew Alford
Zhongxiang Zhao
(University of Washington, Applied Physics Laboratory)
(University of Washington)

Abstract:


Internal tides (internal waves at tidal frequencies) have inherently smaller scales and more complex interference patterns than the barotropic tides that have been so successfully characterized by satellite altimetry. Nevertheless, the ability of low-mode internal tides to propagate over large distances makes them observable by altimetry and a significant source of sea-surface height (SSH) variability on O(200 km) scales and smaller. Thus, internal tides are both vital components of the ocean's energy-redistribution system and significant contributors of error to measurements of lower-frequency circulation processes. The upcoming Surface Water and Ocean Topography (SWOT) wide-swath altimeter will target small-scale (i.e., submesoscale) oceanic processes, requiring less than 1 cm RMS noise in a 1 km SSH measurement. Thus, internal tides (with SSH amplitudes of up to 2 cm and larger) will be a significant source of noise for these studies and must be corrected with great precision.

In recent years, we (Zhao, Alford, and Girton) have conducted internal tide studies using moorings, ship-based studies, and satellite altimetry and have made important strides in accounting for the differences among these platforms. Most notably, we have developed improved harmonic analysis and plane-wave fitting techniques to iteratively separate multiple internal tide waves in a given region using altimetry alone. The growing database of altimeter records on 3 distinct track patterns (20 years of combined T/P-Jason-1/2, 8 years of GFO, and 15.5 years of ERS (ERS-2+Envisat)) now allows for even higher resolution mapping of these waves.

We propose to further refine these techniques and extend their application over the global ice-free oceans, developing uncertainty estimates to rigorously quantify (a) how many beams are significant in a given region, (b) what are the accuracy of the magnitude and direction of the inferred energy flux in each beam (c) what is the region of influence of each beam, and (d) how much of the internal tide SSH variance can be reliably removed from observations at any arbitrary point in space and time. In addition, these refinements can be extended to the time-variability of the internal tide--both in the repeatable seasonal cycle and over multi-year subsets of the growing T/P-Jason altimeter record. By identifying both the robust features of the coherent internal tide field and the seasonally-varying aspects we will be able to develop an empirical model of the global internal tide that can be removed from the SWOT data for the purposes of low-frequency studies. Looking forward, the detailed internal tide knowledge and enhanced wavefitting techniques so developed will also be applicable to the SWOT measurements once a sufficient timeseries (approximately 3 years) is available, enabling even higher resolution mapping.

We will continue to include in-situ observations in both the mapping and time-variability efforts. We have collected a database of historical moorings around the globe for the study of internal tides. The database will provide ground truth for our multisatellite altimetric estimates. Additional mooring deployments by our group in coming years will also be relevant, particularly for the estimation of incoherent constituents that are missed by altimetry. Through ongoing collaborations with ocean modelers working on internal tides we plan to continue the two-way comparison efforts to improve both the tide-resolving models and our satellite processing techniques.



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