On turbulence and normal modes in a basin
Abstract:The problem of forced, geostrophic turbulence in a basin is revisited. The primary focus is the time dependent field, which is shown to be approximately isotropic (in contrast to the strongly zonally anisotropic fields seen in periodic domains). It is also approximately homogeneous, away from the boundaries. Phenomenological arguments suggest the isotropy occurs because the inverse cascade of energy is arrested by basin normal modes rather than by free Rossby waves. Peaks in the velocity spectra at modal frequencies are consistent with basin modes, as has been noted previously. We discuss which modes would be excited and whether dissipation or the mean flow would be expected to alter the modes and their frequencies.
A relatively novel feature is the use of Eulerian velocity statistics to quantify the wave and turbulence characteristics. These measures are more suitable to this environment than measures like wavenumber spectra, given the inhomogeneities associated with the boundaries.
With regards to the mean, we observe a linear 〈q〉 - 〈〉 relation in the region of the mean gyres (at the northern and southern boundaries), consistent with previous theories. This is of interest because our numerical advection scheme has implicit rather than explicit small scale dissipation, and requires no boundary conditions on the vorticity. The gyre structure is however somewhat different than in an (inviscid) Fofonoff-type solution, suggesting dissipation cannot be neglected.
Document Type: Regular Paper
Publication date: May 1, 2002
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