Mathematics – Logic
Scientific paper
Apr 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....8702s&link_type=abstract
EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #8702
Mathematics
Logic
Scientific paper
It is studied how baroclinic long Rossby waves are generated from an ocean ridge associated with barotropic Rossby waves incident to the ridge, using a two-layer semi-analytical model with planetary geostrophic approximation, a two-layer primitive numerical model, and oceanic general circulation model (GCM), which is Miami Isopycnic Coordinate Ocean model, for the North Pacific. The semi-analytical model and two-layer numerical model are used to study for the ridge that varies only zonal direction, but realistic bottom topography is used for the GCM. The semi-analytical model is governed by two-layer, linear, inviscid planetary geostrophic equations with rigid-lid and beta-plane approximations. Semi-analytical solutions are derived using the sinusoidal form in the meridional direction and in time with the barotropic wave input at the eastern end of the model domain. The barotropic waves interact with the ridge, whose shape is given by a Gaussian function in the middle of the model domain, generating the baroclinic Rossby waves. The baroclinic waves are generated by upwelling (downwelling) forcings caused by the lower layer flows, which can be primarily produced by the barotropic currents, toward (away from) the ridge. For a fixed ridge height and thickness of the upper and lower layers, the amplitudes of the baroclinic Rossby waves have their maximum, when the half of the wavelength of the baroclinic Rossby waves is comparable with the double of the e-folding scale of the Gaussian function determining the ridge shape. The occurrence of the maximal amplitudes is because the baroclinic waves are resonantly generated over both the eastern and western slopes of the ridge. That is, baroclinic waves forced by upwellings (downwellings) in the eastern side of the ridge associated with the westward (eastward) lower-layer flows are enhanced by the upwellings (downwellings) in the western side of the ridge associated with the eastward (westward) flows. This phase relationship occurs when the wave propagation takes the half of the oscillation period between the eastern and western side of the ridge. This means that the half-wavelength, which is the distance wave propagates in the half of the oscillation period, corresponds the distance between the eastern and western slopes of the ridge, i.e., the double of the e-folding scale. This result of the semi-analytical model is confirmed by the two-layer numerical model. When the ridge width and forcing period are fixed, the amplitudes of the baroclinic Rossby waves increase with the ridge height increases. This is because the intensity of the upwelling/downwelling is primarily proportional to the product of the gradient of the ridge and incident velocity. Using the GCM for the North Pacific, the resonant wave generation is found around Hawaiian Ridge. The period at which the first baroclinic Rossby wave has maximum amplitude is estimated as 0.6 years from the baroclinic Rossby wavelength, the ridge width, and the climatological mean stratifications. The variability of 0.6-year period around Hawaiian Ridge in the numerical simulation shows that the baroclinic Rossby waves are generated resonantly on both the eastern and western sides of the ridge in a manner identified in the semi-analytical and two-layer numerical models.
Minobe Shoshiro
Sakamoto Taka
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