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

Oceanic lee waves are generated when quasi-steady flows interact with rough topography at the bottom of the ocean, providing an important sink of energy and momentum from the mean flow and a source of turbulent kinetic energy. Linear theory with a spectral representation of topography is typically used to inform parameterizations of lee wave generation. Here, we use a realistic wave resolving simulation of the Drake Passage, a hot-spot of lee wave generation, to investigate the utility of such parameterizations for areas of complex large scale topography. The flow is often blocked and split by large amplitude topographic features, creating an effective topography, and calling into question the spectral representation of small scale topography for lee wave generation. By comparing the resolved modeled wavefield to parameterizations employing various representations of topography, we show that spectral methods may not be appropriate in areas of rough topography. We develop a simple topographic representation consisting of an ensemble of topographic peaks, which allows physical treatment of flow blocking at finite amplitude topography. This method effectively predicts bottom vertical velocities and lee wave energy flux, whereas spectral methods overestimate the energy flux by approximately 4 times. Our results also imply that the nature of lee waves in such regions can be misrepresented by a spectral approach to topographic representation. This leads to both an overestimate of wave energy flux and an underestimate of wave nonlinearity, with implications for the mechanisms by which lee waves break and mix in the abyssal ocean.

Automated summary

This study investigates the characteristics of lee waves in complex large scale topography areas using a realistic wave resolving simulation. The results show that spectral methods may not be suitable for rough topography areas, and a new method is proposed to effectively predict the lee wave energy flux.