The Impact of Representations of Realistic Topography on Parameterized Oceanic Lee Wave Energy Flux

Abstract

Oceanic lee waves are generated when currents and eddies interact with sea-floor topography, and are important for causing turbulent mixing in the deep ocean when they break. Representing their effect in global models that cannot resolve them is challenging because estimates of wave generation depend on the sea-floor topography, which is not known at sufficient resolution globally, and is often too tall for standard theories to apply. Here, we employ a novel method that better represents (compared to existing methods) the role of high resolution, tall, topography in inference of lee wave energy flux. Our study highlights the need for continuing efforts toward high-resolution mapping of the sea floor and is a step forward toward representing lee waves in coarse-resolution climate models.

Energy Sinks for Lee Waves in the Northern South China Sea

“…Here we use the low-passed bathymetry as our reference bathymetry (black dashed line in Figure 4a) and regard the height above/below the low-passed bathymetry (i.e., HAB >0 or HAB <0) as the "crest"/"trough" of the small-scale topography. This composite method is more meaningful, since the "crest" is a better representation of the "effective topography" for lee wave generation (Baker & Mashayek, 2022). The 𝐴𝐴 𝐴𝐴 𝐾𝐾 IW is strongly bottom-enhanced especially in the shallow half of the slope region.…”

Large-scale impacts of small-scale ocean topography

“…These issues impact the drag and mixing representation adversely (Klymak et al. 2021; Baker & Mashayek 2022; Polzin & McDougall 2022). The quasi-geostrophic framework employed in Radko (2023) systematically prohibits accounting for many ageostrophic submesoscale and radiating processes.…”

“…These issues impact the drag and mixing representation adversely (Klymak et al. 2021; Baker & Mashayek 2022; Polzin & McDougall 2022).…”

“…Figure 1(a) shows the topography at the model's heart (hence the term 'sandpaper'). Such bathymetric representation, built upon the observationally derived spectrum of seafloor roughness (Goff & Jordan 1988), mimics the statistics of abyssal hills and has been widely used to study topographically induced internal waves and boundary turbulence (Baker & Mashayek 2022;Polzin & McDougall 2022). Such statistical representation allows for describing the topography in terms of Fourier modes.…”

“…Such turbulence consists of bottom-enhanced boundary turbulence (Polzin & McDougall 2022) and interior mixing generated by topographically induced internal waves that can transfer energy and momentum upward all the way to the surface mixed layer (Shakespeare & Hogg 2019;de Lavergne et al 2020;Baker & Mashayek 2021). While the energy associated with mixing is three orders of magnitude smaller than the total energy associated with the ocean meridional overturning circulation, mixing is essential for the overturning circulation as, in its absence, the deep ocean would remain filled (and stuck) with old dense waters of polar origin (Wunsch & Ferrari 2004;Ferrari 2014).…”