Using ADCP data and altimetry to evaluate high-wavenumber variability in the California Current and the tropics
Event: 2017 Ocean Surface Topography Science Team Meeting
Session: Science III: Mesoscale and sub-mesoscale oceanography
Presentation type: Type Poster
At scales smaller than 50-100 km, observations from altimetry, Acoustic Doppler Current Profiler (ADCP) data, and numerical model output all suggest that oceanic variability is dominated by processes associated with internal waves and not in geostrophic balance. These unbalanced small-scale motions typically have relatively shallow spectral slopes (k<sup>-2</sup> for velocities), but they can be hard to evaluate in many parts of the world, because of the challenges associated with obtaining observations at high spatial resolution. Previous studies have focused on regions with strong baroclinic jets. Here we examine 2 contrasting regions: the California Current, where ADCP data have been collected for many years as part of regular surveys of the region, and the tropics, where regular research ship operations have produced a broad range of long transits that were previously unprocessed and unavailable for research. We carry out our updated assessment of ADCP observations in combination with an examination of new altimetry from the same regions to evaluate the properties of high-wavenumber variability and in particular the length-scales at which balanced geostrophic motions transition into unbalanced motions dominated by internal waves. This analysis is facilitated by three new altimeters: AltiKa, Sentinel-3, and Cryosat. Together these offer the possibility of evaluating sea surface height variability at smaller scales than can be resolved from the Ku-band Jason altimeters. Each of these altimeters, however, offers its own challenges: AltiKa's Ka-band altimeter shows a spectral bump at small scales that is not easily eliminated, although retracking may provide a means to minimize the bump. Cryosat's SAR mode altimetry is only available over geographically limited regions, and because the satellite orbit does not repeat, the signal associated with the time-invariant geoid is not easily removed. Sentinel-3, although on a new ground track with a geoid that is not well mapped, appears to have a smaller spectral bump than AltiKa but higher noise at high wavenumber. By jointly examining altimetry and ADCP data (along with model data when available), we can assess the commonalities in the physics that each resolves, as well as the potential sources of error.