The Importance of Remote Forcing for Regional Modeling of Internal Waves

Abstract

Regional ocean general circulation models are generally forced at the boundaries by mesoscale ocean dynamics and barotropic tides. In this work we provide evidence that remotely forced internal waves can be a significant source of energy for the dynamics. We compare global and regional model solutions within the California Current System. Both models have similar inputs, forcings, and identical grids and numerics. The global model has a steric height power spectrum consistent with mooring observations at superinertial frequencies, while the regional model spectrum is weaker. The regional model also has less sea surface height variance at high wavenumber than the global model. The vertical velocity variance is significantly larger in the global model, except in the sheltered Southern California Bight. While the regional model has roughly equal high‐pass baroclinic and barotropic kinetic energy levels, the global model high‐pass baroclinic kinetic energy is 28% (0.39 PJ) greater than the barotropic energy. An internal wave energy flux analysis reveals that the regional model domain boundaries act as a sink of 183 MW, while in the global model the analysis domain boundaries act as a source of 539 MW. This 722 MW difference can account for the relative increase of 0.39 PJ high‐pass baroclinic energy in the global model, assuming a baroclinic kinetic energy dissipation time in the domain of approximately 6.3 days. The results here imply that most regional ocean models will need to account for internal wave boundary fluxes in order to reproduce the observed internal wave continuum spectrum.

Separating Energetic Internal Gravity Waves and Small‐Scale Frontal Dynamics

“…The CCS simulation has horizontal grid spacing of 500 m and 90 vertical levels. As described by Mazloff et al (2020), the regional CCS simulation was forced by hourly LLC4320 fields at the lateral boundaries, which includes forcing by external and internal tides, near-inertial waves, and higher-frequency IGWS. These lateral boundary conditions produce a realistic IGW spectrum (Mazloff et al, 2020).…”

“…This scale separation is used to effectively separate these two types of motion. The approach of filtering large vertical scales is successfully tested in a simulation of the CCS, a region west of the California coast where IGWs are energetic (Kim et al., 2011; Mazloff et al., 2020). Results indicate that the proposed approach effectively removes the contamination of horizontal and vertical motions by IGWs and yet retains SBM frontal dynamics at time scales shorter than 1 day.…”

Reconstructing Fine‐Scale Ocean Variability via Data Assimilation of the SWOT Pre‐Launch In Situ Observing System

“…Superimposed on the advective current motions is an internal wave field that comprises energetic M2 mode-1 and -2 baroclinic tides propagating southward from their generation site at the Mendocino Escarpment (Zhao, 2018;Zhao et al, 2019). A recent modeling study suggests that internal wave energy flux from outside the region may also play a role in the CCS (Mazloff et al, 2020). The transition scale-the wavelength at which the energy of geostrophically balanced flow is matched by submesoscale turbulence and waves-has recently been quantified using shipboard-ADCP observations in the southern CCS by Chereskin et al (2019).…”

Impact of Vertical Mixing Parameterizations on Internal Gravity Wave Spectra in Regional Ocean Models

“…These global models fall short of representing the real ocean due to a lack of resolution and/or insufficient parameterization of unresolved sub‐grid scale physical processes such as IW breaking. Regional ocean models have been shown to display improved IW spectra over those in the global models when run at higher horizontal and vertical resolutions, as long as the lateral boundary forcing includes remotely‐generated IWs from a global IW model (Mazloff et al., 2020; Nelson et al., 2020). Such remotely‐forced regional models run over short periods are relatively affordable computationally and can be used to study the sensitivity of the IW continuum due to changes in model parameters.…”