Presenter: Kayhan Momeni (University of Toronto)

Description:
Recent analyses of downscaled Internal Wave (IW) fields from an internal-wave-admitting global simulation into higher-resolution regional configurations have demonstrated a much-improved fit of the simulated IW spectra to the empirical Garrett-Munk spectrum. In particular, the properties of the continuum are much improved [Nelson et al., 2020, Pan et al., 2020]. The global simulation, known as LLC4320, is based on the Massachusetts Institute of Technology general circulation model (MITgcm) forced by both astronomical tidal potential and surface atmospheric processes. The Pan et al. [2020] analysis has demonstrated that, although the first five internal-gravity-wave modes of the ocean waveguide are all represented in the high-resolution regional domain, none of the expected higher-order modes are similarly-well defined. By employing a Galerkin method to solve the associated Sturm-Liouville problem under stratification conditions characteristic of the North Pacific at the time of the McLane Moored Profiler (MMP) observations and model predictions, we demonstrate both that these lower-order modes are accurately predicted and that the higher-order modes are so closely packed in frequency-wavenumber space that their breaking through nonlinear interactions would be guaranteed. Because the version of the global MITgcm employed a version of the KPP diapycnal mixing parameterization from which the normal background component of this parameterization had been essentially eliminated, we have been able to address the issue as to whether the remaining components of the KPP parameterization, in particular, those associated with internal wave shear, might be able to explain the physical origins of the background depth dependence of diapycnal diffusivity that would normally be employed in the ocean component of a modern coupled climate model. We explore the extent to which a KPP-like parameterization of mixing due to internal-wave shear-induced mixing might resolve this issue.