CoAuthors

J. Thomas Farrar (Woods Hole Oceanographic Institution, United States); Steven R. Jayne (Woods Hole Oceanographic Institution, United States); Theodore S. Durland (Oregon State University, United States)

A useful descriptor of oceanic variability is the frequency-wavenumber spectrum, which is becoming increasingly valuable as the altimetric time series continues to extend. In this work we present a global analysis of the zonal wavenumber-frequency spectrum of sea surface height (SSH) derived from the AVISO/Duacs merged gridded altimeter product. We show that the global ocean can be characterized by a small set of frequency-wavenumber spectra with distinct characteristics. In low latitude regions, as shown by previous studies, the spectrum is dominated by the first two Rossby waves and the first baroclinic Kelvin mode. The mixed planetary-gravity mode and higher modes are outside the range of scales resolved by the gridded altimetry product. Mid-latitude areas, characterized by a relatively weak westward flow and eddy kinetic energy, present a spectrum with a well-defined ‘ridge’ of enhanced SSH variance. These non-dispersive spectra are characteristic of mid-latitude areas on all ocean basins. Lastly, regions of strong eastward flow, like the Antarctic Circumpolar Current, can be characterized by a spectrum with significant SSH variance associated with both westward and eastward propagation. These spectra show little or no trace of an elevated spectral ridge and are more dispersive in nature.
We attempt to explain the main features seen in the frequency-wavenumber spectra derived from altimetry by using simple analytical and numerical models. In particular, we show that the main features observed in the spectra of strong eastward flows can be captured by the dispersion relation derived from the 2-layer Phillips model with the appropriate vertical shear. Mid-latitude spectra, mainly defined by the non-dispersive ridge of elevated variance, have been extensively study in the literature. In particular the source of the non-dispersive line has been a subject of debate. Previous attempts to reproduce it ranged from modified versions of standard Rossby wave theory, that take into account rough bottom topography or vertical shear, to a fully non-linear quasi-geostrophic regime. Here we explore the nature of the non-dispersive line by seeding a 2-layer quasi-geostrophic model with mesoscale perturbations. A discussion of the non-dispersive ridge captured by the model under different regimes is presented.