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Ocean Surface Topography from Space
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High-wavenumber variability of sea surface height: Evaluating sub-100-km scales with altimetry, ADCP, and model output


Author:

Sarah Gille - (University Of California, San Diego)

Co-Investigator(s):
  Dr. Teresa Chereskin
University Of California, San Diego


Abstract:
The TOPEX/Poseidon and Jason series of altimeters provided pioneering observations of mesoscale sea surface height variability in the ocean. A new class of altimeters, including CryoSat-2, AltiKa, Sentinel-3, and Jason-CS will further refine our understanding of oceanic variability by providing a means to study submesoscale variability in sea surface height. Theoretical predictions of submesoscale variability originally hypothesized largely geostrophic motions on scales smaller than 50 to 60 km in most parts of the ocean. However, preliminary assessments suggest that when tides and internal wave generation mechanisms are enabled, submesoscale sea surface height has a signature of internal waves rather than geostrophic motions, at least on scales ranging from 10 to 100 km. We have carried out preliminary exploration of these issues in the Drake Passage, using altimetry from AltiKa, Acoustic Doppler Current Profiler (ADCP) data from the L. M. Gould, and numerical model results from the JPL 1/48 degree version of the MITgcm with tidal forcing and internal wave generation. The data products agree in showing high wavenumber spectral slopes that are more consistent with internal wave generated variability rather than geostrophically balanced motions. However, Drake Passage is a highly energetic regime with significant wind forcing and large internal tide generation. In the proposed research, we intend to extend our initial work from Drake Passage to a range of additional environments that we have selected both because of the availability of high-quality data from in situ and satellite sources, as well as model output, and also because they span a range of dynamical regimes.

We have targeted the following regions:

  1. The California Current, because of its long history of observations through the California Cooperative Oceanic Fisheries Investigations (CalCOFI) program, which now includes extensive ADCP and HF radar data, and also because it has been targeted as a key region for Surface Water Ocean Topography (SWOT) calibration/validation, so there is considerable effort to understand small-wavenumber variability in this region.
  2. The region south of the Kuroshio, because ADCP and in situ observations are available from the Kuroshio Extension System Study (KESS) and from ship transits through the region. In this region studies with the Ocean General Circulation Model for the Earth Simulator (OFES) have indicated that the seasonal variability of the region differs for mesoscale and submesoscale processes. We hypothesize that tidally generated internal waves might alter the relationships seen in the OFES model.
  3. The tropical Pacific, because mesoscale spectral slopes are distinctly flatter in the tropics than at mid-latitudes, and because the Cryosat-2 satellite has been run in SAR mode for this region, allowing us to examine sea surface height from multiple satellites.
  4. The Bay of Bengal, where recent ONR-funded observations have probed the structure of submesoscale motions and have found small scale fronts to show little evidence of geostrophic balance. Similar issues could also be explored in other regions, where recent field work has closely examined small-scale motions, but to our knowledge, an equivalent assessment of high-wavenumber altimetric variability has not been yet been carried out.

For each of these targeted regions, our objectives are to use spectral methods to identify the dominant physical properties governing oceanic variability at the submesoscale and to ask how these submesoscale motions contribute to oceanic transport of heat and freshwater and to the interactions of eddies and submesoscale structures with larger scale mean flow. Theoretical developments that articulate relationships between velocity spectra and potential energy spectra can be tested in locations where ADCP measurements and buoyancy observations are both available (for example, from thermosalinograph data).



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