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On the geographic variability of oceanic mesoscale motions

University of British Columbia

Quasi-synoptic expendable bathythermograph data were acquired, from the Canadian Armed Forces, the United States Navy and the United States National Oceanographic Data Center, for the Pacific and Atlantic Oceans. On the basis of these data and the results of previous studies using climatological data, six geographic regions were defined: the high-energy regions of the Northwest Atlantic and Northwest Pacific, and the low-energy regions of the Northeast Atlantic, Northeast Pacific, South Atlantic and South Pacific. Spatial series of two variables, representative of the upper layer (400 m) mesoscale variability, were obtained for each section - the mid- thermocline temperature and the geopotential anomaly (0 - 4000 kPa). The central moments and the wavenumber spectra of each variable were estimated for the six geographic regions, the combined high-energy areas and the combined low-energy areas. In the high-energy regions and the Northeast Atlantic, it was found that the temperature between 350 and 400 m is representative of the temperature variability due to the baroclinic eddy field, whereas, the temperature between 150 and 200 m is more representative of the eddy variability in most of the low-energy regions. The standard deviations of temperature, in the high- and low-energy regions, are 1.40 and 0.54°C, respectively. The standard deviations of the geopotential anomaly are 0.67 and 0.26 m²/s², respectively. The high-energy regions have dominant spectral wavelengths in the geopotential anomaly fields of 300 and 155 km, with corresponding baroclinic surface velocity scales of 9.6 and 17.5 cm/s. The low-energy regions have dominant wavelengths of 300 and 170 km with velocity scales of 4.5 and 5.5 cm/s, respectively. In general, the high-energy regions have a greater portion of their spectral variance concentrated in the higher wavenumbers (i.e. 280 to 100 km wavelengths), than the low-energy regions. The eddy kinetic energies per unit mass for the high- and low-energy regions were estimated at 250 and 36 cm²/s², respectively. The geographic variability of the governing dynamics was inferred by evaluating the quasigeostrophic scaling parameters (i.e. the Rossby number (Ro), the Burger number (B) and the sphericity parameter (β*)) and the Rossby wave steepness parameter (M). Also, the properties of free linear dispersive Rossby waves were calculated with the observed wavelengths and the spectral power-laws of the temperature spectra were compared with several models of nonlinear geophysical turbulence. It was found that Ro<<1, B = 0(1) and β*<<1, which is consistent with the scaling for quasigeostrophy. The dynamics inferred from these analyses exhibit a distinct geographic variability. Motions with wavelengths greater than 200 km in all regions are consistent with linear/nonlinear Rossby wave theory. Mesoscale perturbations in the high-energy regions are, of course, more nonlinear than the corresponding length scales in the low-energy regions. Motions, with wavelengths less than 200 km in the high-energy regions, are consistent with quasigeostrophic turbulence theory, more specifically, with Charney's (1971) model of three-dimensional quasigeostrophic turbulence. Motions with wavelengths less than 200 km in the low-energy regions have dynamics that are intermediate between linear/nonlinear Rossby wave theory, and quasigeostrophic turbulence theory.

Text

2010-08-08

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http://hdl.handle.net/2429/27209

Oceanography

University of British Columbia

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