10:00 AM - 10:15 AM
[AOS16-05] Revisiting turbulent mixing hotspots over rough seafloor topography
Keywords:Turbulent mixing parameterization, Internal lee waves, Tidal flow, Tidal excursion parameter, Rough seafloor topography, Steepness parameter
We revisit the turbulent mixing hotspots in the vicinity of rough seafloor topography, so-called near-field mixing hotspots. Although the tidal excursion parameter Te = kHU0/ω >> 1 (kH : horizontal wavenumber of the seafloor topography, U0 : amplitude of the tidal current, ω : tidal frequency) indicating expected internal lee wave generation, the near-field mixing hotspots have been misinterpreted as being formed by breaking of high wavenumber internal tidal waves through the nonlinear interaction with the background Garrett-Munk (GM) internal wave field. In order to investigate the characteristics of near-field mixing hotspots, we conduct numerical experiments assuming a semidiurnal tidal flow interacting with finite-amplitude boundless sinusoidal seafloor topography and the background GM internal wave field in a vertical two-dimensional plane.
It is found that, as the amplitude of the seafloor topography increases (steepness parameter s = Nh/U0 > 0.3, N : buoyancy frequency, h : amplitude of seafloor topography,), the near-field mixing hotspot tends to be confined near the seafloor topography. This is because the near-inertial flow is generated just above the seafloor and then develops with time while promoting the breaking of bottom-generated upward propagating internal lee waves. It is suggested that the development of the near-inertial flow is due to the supply of part of the energy of the internal lee waves breaking in the critical layer; the amplified near-inertial flow further promotes the breaking of bottom-generated upward propagating internal lee waves. This provides a new scenario explaining the near-field mixing hotspots, which is consistent with the results of microstructure observations.
In contrast, as the amplitude of the seafloor topography decreases (steepness parameter s = Nh/U0 < 0.3), such an amplification of near-inertial flow never occurs so that the bottom-generated internal lee waves continue to propagate upward while interacting with the background GM internal wave field to create mixing hotspots extending higher up off the seafloor. When reaching the thermocline depth, such a near-field mixing hotspot is expected to significantly impact the meridional overturning circulation dynamics. In order to resolve the long-standing “missing mixing” problem in the future, microstructure observations as well as numerical experiments should be focused on mixing hotspots, such as tops of prominent marine ridges and/or seamounts where both strong tidal currents and relatively small amplitude rough seafloor topography can be expected.
It is found that, as the amplitude of the seafloor topography increases (steepness parameter s = Nh/U0 > 0.3, N : buoyancy frequency, h : amplitude of seafloor topography,), the near-field mixing hotspot tends to be confined near the seafloor topography. This is because the near-inertial flow is generated just above the seafloor and then develops with time while promoting the breaking of bottom-generated upward propagating internal lee waves. It is suggested that the development of the near-inertial flow is due to the supply of part of the energy of the internal lee waves breaking in the critical layer; the amplified near-inertial flow further promotes the breaking of bottom-generated upward propagating internal lee waves. This provides a new scenario explaining the near-field mixing hotspots, which is consistent with the results of microstructure observations.
In contrast, as the amplitude of the seafloor topography decreases (steepness parameter s = Nh/U0 < 0.3), such an amplification of near-inertial flow never occurs so that the bottom-generated internal lee waves continue to propagate upward while interacting with the background GM internal wave field to create mixing hotspots extending higher up off the seafloor. When reaching the thermocline depth, such a near-field mixing hotspot is expected to significantly impact the meridional overturning circulation dynamics. In order to resolve the long-standing “missing mixing” problem in the future, microstructure observations as well as numerical experiments should be focused on mixing hotspots, such as tops of prominent marine ridges and/or seamounts where both strong tidal currents and relatively small amplitude rough seafloor topography can be expected.