OS042-0020
Spatial Heterogeneity of Diapycnal Mixing within the South Atlantic Thermocline

Tuesday, 15 December 2020
Poster
Jingxuan Wei, Texas A&M University College Station, College Station, TX, United States, Robert Reece, Texas A&M University, Geology and Geophysics, College Station, TX, United States, Will Fortin, Lamont -Doherty Earth Observatory, Palisades, NY, United States, Kathryn Gunn, University of Miami, Rosenstiel School of Marine and Atmospheric Science, Miami, United States and Tanner Michael Michael Acquisto, Columbia University, Lamont-Doherty Earth Observatory, New York, NY, United States
Abstract:
Diapycnal mixing is the primary driving force of overturning circulation in the ocean. The canonical diapycnal diffusivity from global calculations suggests that an average diffusivity of O(10-4) m2 s-1 is required in the abyssal ocean to maintain the observed overturning circulation, while O(10-5) m2 s-1 is required in the main thermocline. Although enhanced diffusivity is found above rough topography, existing observations are on average roughly consistent with the calculated diffusivity. However, the distribution of mixing is spatially patchy and temporally intermittent, conventional oceanographic measurements are severely limited on sampling horizontal direction, particularly in the ~103-101 m range where nonlinear motions develop which lead to turbulence. The present observations of mixing are too sparse to reflect global mixing patterns, while the full geography of mixing is essential for global circulation model. Recent studies have shown that Seismic Oceanography (SO) is a suitable tool to investigate small scale processes, as it has the capability to sample water column with resolution of O(101) m in both vertical and lateral directions. In this work, we estimated diapycnal diffusivity from an integration of internal wave displacement spectra and seismic amplitude spectra. We present high resolution diffusivity maps of the main thermocline in the South Atlantic at 30o S from Rio Grande Rise to the Mid-Atlantic Ridge (MAR). Our results show that diapycnal mixing is patchy in the stratified thermocline. For locations above smooth topography, the averaged diffusivity is not always consistent with the canonical value of O(10-5) m2 s-1, some locations have increased average diffusivity of O(10-4) m2 s-1 with mixing hotspots of O(10-3) m2 s-1 that have length scale of ~5-10 km, which may be caused by energetic near-inertial motions or breaking of remotely generated internal waves. We observe enhanced diffusivity above MAR, and present the decaying pattern of mixing away from MAR. Our results demonstrate that mixing in the ocean interior is patchy on length scales that are typically unresolved by conventional oceanography observations, and SO is a robust tool to fill this gap, which will contribute to the understanding of global mixing pattern and assist the development of ocean circulation models.
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