The sea surface height spectrum of internal waves reconstructed from ADCP and altimetry
Event: 2023 Ocean Surface Topography Science Team Meeting
Session: Science III: Mesoscale and sub-mesoscale oceanography
Presentation type: Type Oral
Contribution: PDF file
Internal waves are highly energetic and therefore have posed a long-standing challenge for calculating geostrophic velocities from ocean observations. Their specific contribution to sea surface height (SSH) remains largely unknown, but internal waves are hypothesized to influence the submesoscale range of scales, particularly in the tropics. Here we use acoustic Doppler current profiler (ADCP) data in combination with 1-Hz along-track satellite altimetry to estimate the internal-wave component of the SSH wavenumber spectrum. Our approach uses the method by Bühler et al (2014), which partitions the KE measured by ADCPs into a vortical (geostrophic only) and a wave component. The method relies on a key assumption that the internal wave field is described by the Garrett-Munk spectral model (GM). Using frequency spectra from moored current observations and a modification of the decomposition theory, we demonstrate that GM is sufficiently accurate to enable the KE decomposition. Linear wave theory and GM are then used to convert the ADCP-inferred wave KE spectrum into a SSH spectrum. We compute estimates in the southeast tropical Pacific, a region previously identified with a strong internal wave field with characteristics consistent with GM, and where ship sampling is sufficient and velocity statistics are consistent with the decomposition requirements. The reconstructed internal wave spectrum consists of the continuum inferred from the ADCP data and the stationary internal tides, obtained from an empirical altimetric based model. Non-stationary internal tide energy is inferred from the residual between a reconstruction of the full SSH measured spectra, which includes modeled altimeter noise, and the direct altimetry-derived spectra. The key results are that i) the internal wave continuum SSH lies just below the original projections for the SWOT noise floor, and ii) internal tides at mode 4 may come into full view in this area. This estimate corroborates the only other known inference of the internal wave SSH spectrum. Although geostrophic variability at scales less than 200 km appear to be fully drowned out by internal waves, the results here suggest that the southeast tropical Pacific is an area where the details of the internal wave field could now be studied from space, if SWOT delivers only modestly more accurate measurements than specified by the instrument requirements. The capability to measure the waves could in turn allow improvements to existing algorithms or the development of new algorithms to remove internal waves, allowing the extraction of smaller-scale geostrophic currents from these new observations.