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Alternate Title

Slope and Roughness Statistics of the Northern Gulf of Mexico Seafloor With Some Oceanographic Implications

Abstract

We analyzed 11 cross-slope and six along-slope bathymetric profiles over the continental slope of the northern Gulf of Mexico using statistical and time series techniques. Linear regressions account for over 93% of the water depth variability in nine north-south profiles; the remaining profiles follow quadratic polynomials accounting for over 92% of the variability. Seafloor gradients from the linear fits are generally ≤1°, but local gradients can reach ≅16° near the Sigsbee Escarpment (SE), which is smaller than previously documented. Seafloor roughness elements reach 13-300 m, with most <100m. Such rough bottoms could affect waves with wavelengths of tens of kilometers but not waves of hundreds of kilometers. Water depth power spectra are red (having the most energy at scales ≤10 km) and exhibit a k-2 dependence. Power spectra of short-scale gradients are near constant at scales >0.02 cpkm, implying a white noise process, and overall, these spectra exhibit an exponential dependence. Oceanographically, the slopes are large enough for topographic β-effects to dominate over the planetary β-effect, which allows approximating the topographic Rossby waves (TRWs) dispersion in terms of the Brunt-Vaisala frequency and bottom gradients. The steep SE can sustain minimum periods of ~18 d, which agrees with observed periods. Bottom trapping caused by stratification should be effective only for short waves, but observations suggest that bottom trapping is independent of wavelength. This discrepancy can be explained by the fact that the Gulf of Mexico can be approximated as a two-layer ocean, and TRWs are bottom trapped regardless of wavelength. The critical frequency and slope show that only diurnal and inertial frequencies (at this latitude) could be inducing strong vertical mixing on the study area. The initial conjecture that cyclonic eddies with diameters of 40-150 km are generated by flow-topography interaction was not upheld because the resonance conditions are not met. Finally, the analysis reveals that fluids inside basins cannot escape.

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