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IMPACT OF TIDES ON THE WAVENUMBER SPECTRA OF AN EDDYING GLOBAL OCEAN CIRCULATION MODEL
James Richman, Brian Arbic, Alan Wallcraft1, Joseph Metzger
Naval Research Laboratory
(Abstract received 12/20/2010 for session X)
ABSTRACT
Tidal potential forcing has been added to a realistic eddy-resolving (1/12.5° equatorial resolution) six year simulation, 2003-2009, of global Hybrid Coordinate Ocean Model (HYCOM). The model resolves the eddying general circulation and the barotropic and low mode baroclinic internal tides. The dynamics of the low-frequency upper ocean are thought to be quasigeostrophic, which suggests that the wavenumber spectrum of the sea surface height at scales shorter than the Rossby deformation radius should be very steep, varying with wavenumber as k-5. Altimeter observations in the regions of large kinetic energy suggest a less steep slope approaching k-11/3, which is consistent with surface quasigeostrophic theory. In the mesoscale-to-submesoscale wavenumber band between 300 km and 20 km, we find that the spectral slopes are strongly impacted by the presence of internal tides. For instance, in the North Pacific near Hawai’i where intense beams of internal tides are generated, the wavenumber spectrum shows peaks near the mode-1 internal tide wavelength and a much flatter spectrum at shorter wavelengths. If we separate the high-frequency internal tidal motions from the low-frequency motions, we find a low-frequency wavenumber spectrum consistent with the surface quasigeostrophic theory with a distinctly different wavenumber spectrum for the internal tides. In lower tidal energy regions such as the eastern South Pacific, the high and low frequency spectra remain distinct, but the internal tides do not dominate the wavenumber spectrum. This study has important implications for the planned wide-swath satellite altimeter, which will measure sea surface height at much higher horizontal resolution than current-generation satellite altimeters. In regions of high internal tide activity, tidal corrections will need to be accurate at short scales in order for wide-swath data to be adequate for the study of low-frequency motions.
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2011 LOM Workshop, Miami, Florida February 7 - 9, 2011