Breaking Internal Waves and Ocean Diapycnal Diffusivity in a High-Resolution Regional Ocean Model: Evidence of a Wave-turbulence Cascade

While the primary origin of ocean diapycnal diffusivity is commonly attributed to stratified turbulence induced by breaking internal waves (IWs), verifying diffusivity values in ocean circulation models within specific geographical regions remains challenging due to limited microstructure measurements. Recent analyses of a downscaled global ocean simulation into higher-resolution regional setups northeast of Hawaii, reveal a notably enhanced fit between simulated IW spectra and in-situ profiler measurements like the Garrett-Munk spectrum [Nelson et al. (2020), Pan et al. (2020), Thakur et al. (2022)]. In this study, we utilize this dynamically downscaled ocean simulation to scrutinize the dynamics of IW-breaking and the wave-turbulence cascade in this region explicitly. Employing a modified version of the Kappa Profile Parameterization (KPP), we infer the horizontally-averaged vertical profile of diapycnal diffusivity. Comparing this inferred profile to the background profile used in low-resolution coupled climate models—such as the Community Earth System Model (CESM) by the US National Center for Atmospheric Research (NCAR)—is a central aspect of our investigation. Our exploration reveals that the wavefield in the high-resolution regional domain is dominated by a well-resolved spectrum of low-mode IWs, predictable through appropriate eigenvalue computations for stratified flow. Finally, we propose a novel tentative approach to enhance the KPP parameterization. This approach holds promise for refining our understanding of diapycnal diffusivity, offering valuable insights for improving ocean circulation models.   References: AD Nelson, BK Arbic, D Menemenlis, WR Peltier, MH Alford, N Grisouard, and JM Klymak. Improved internal wave spectral continuum in a regional ocean model. Journal of Geophysical Research: Oceans, 125(5):e2019JC015974, 2020. Yulin Pan, Brian K Arbic, Arin D Nelson, Dimitris Menemenlis, WR Peltier, Wentao Xu, and Ye Li. Numerical investigation of mechanisms underlying oceanic internal gravity wave power-law spectra. Journal of Physical Oceanography, 50(9):2713–2733, 2020. Ritabrata Thakur, Brian K Arbic, Dimitris Menemenlis, Kayhan Momeni, Yulin Pan, W Richard Peltier, Joseph Skitka, Matthew H Alford, and Yuchen Ma. Impact of vertical mixing parameterizations on internal gravity wave spectra in regional ocean models. Geophysical Research Letters, 49(16): e2022GL099614, 2022.