Anna Savage (University of Michigan)
"Sea surface signatures of internal gravity waves
Sea-surface height (SSH) is a fundamental variable in physical oceanography and is one of few globally observable quantities. SSH is a complicated manifestation of many oceanic processes, and, as such, is difficult to predict over a wide range of space and time scales. To approach this challenge, high-resolution global general circulation ocean models are used to study internal tides, the internal gravity wave (IGW) continuum, and other contributions to sea- surface variability. Using these models, we estimate the SSH variance in low-, tidal-, and high-frequency phenomena by integrating over bands in frequency space. This frequency driven approach allows us to create global maps without having to quantify how these phenomena propagate in space. The frequency-spectral variance maps provide both results consistent with previous studies (in bands that have been well-documented) as well as global maps of the non-stationary component of the internal tides and the IGW continuum. As global general circulation ocean models have only begun to be able to resolve the IGW continuum, comparing a 1/12 degree resolution model with a 1/25 degree resolution model shows that the IGW variance estimates have not reached numerical convergence, and will continue to increase with increased horizontal and vertical resolution of ocean models. In addition to the frequency-spectrum approach, we also examine the spatio-temporal variability of two high-resolution ocean models by computing wavenumber-frequency (K-ω) spectra of SSH over a select number of boxes that we believe are representative of global ocean variability. These K-ω spectra show high energy along curves representing the linear dispersion relations predicted by the Sturm-Liouville problem for internal waves. The wavenumber spectral density of SSH, a quantity of fundamental importance in testing theories of geostrophic turbulence, is dominated by high frequency motions in high-wavenumbers. These results implicate the IGW continuum as an important contribution to small scale SSH variability. "