Spatial Variation of Diapycnal Diffusivity Estimated From Seismic Imaging of Internal Wave Field, Gulf of Mexico

06 Feb 2018

Bright reflections are observed within the upper 1000~m of the water column along a seismic reflection profile that traverses the northern margin of the Gulf of Mexico. Independent hydrographic calibration demonstrates that these reflections are primarily caused by temperature changes associated with different water masses that are entrained into the Gulf along the Loop Current. The internal wave field is analyzed by automatically tracking 1171 reflections, each of which is greater than 2~km in length. Power spectra of the horizontal gradient of isopycnal displacement, $\phi_{\xi_x}$, are calculated from these tracked reflections. At low horizontal wavenumbers ($k_x < 10^{-2}$ cpm), $\phi_{\xi_x} \propto k_x^{-0.2 \pm 0.6}$, in agreement with hydrographic observations of the internal wave field. The turbulent spectral subrange is rarely observed. Diapycnal diffusivity, $K$, is estimated from the observed internal wave spectral subrange of each tracked reflection using a fine-scale parametrization of turbulent mixing. Calculated values of $K$ vary between $10^{-8}$ and $10^{-4}$~m$^{2}$~s$^{-1}$ with a mean value of $K \sim 4 \times 10^{-6}$~m$^{2}$~s$^{-1}$. The spatial distribution of turbulent mixing shows that $K \sim 10^{-7}$~m$^{2}$~s$^{-1}$ away from the shelf edge in the upper 300~m where stratification is strong. Mixing is enhanced by up to four orders of magnitude adjacent to the shoaling bathymetry of the continental slope. This overall pattern matches that determined by analyzing nearby suites of CTD casts. However, the range of values recovered by spectral analysis of the seismic image is greater as a consequence of significantly better horizontal resolution.