UC San Diego

The topography of an environment is critical for influencing the hydrodynamics of a region and allowing diverse ecosystems to exist in areas with, otherwise, low productivity. We investigate the flow response associated with interactions of large-scale waves with bathymetry of two distinct spatial scales: coral reefs (O(1 km)) and seamounts (O(100 km)).

Momentum balances over coral reefs have mostly been estimated for cross-reef flow in wavy environments. In contrast, we examine the effect of coral reef roughness on alongshore tidal momentum; this is accomplished using month-long pressure, temperature, and velocity observations gathered over a 2-km stretch of a Hawaiian fringing reef system. We find that temporal acceleration plays an important role in the momentum balance. The two dominant forces affecting the alongshore depth-averaged flow acceleration are the tidal pressure gradient and the drag exerted by the coral reef bottom. We estimate drag coefficients and discuss the importance of coral reef drag throughout a tidal cycle.

The interaction between flow and bathymetry on a larger scale is studied using, primarily, satellite data and numerical model output in tropical regions of the Indian Ocean. Satellite observations are employed to characterize the generation of quasi-biweekly Yanai waves, oscillations with a wavelength estimated around 4,000 km, roughly 63% of the width of the equatorial Indian Ocean. These observations provide the first full picture of a quasi-biweekly Yanai wave in the Indian Ocean. Careful analysis suggests that these waves are generated in the western region of the basin, near the Seychelles Plateau, a relatively shallow (60-100m), large (~200 km), steep submarine platform. We examine the connection between Yanai waves and the Seychelles Plateau using model output from the global Los Alamos Parallel Ocean Program Model 2 (POP2), a theoretical stratified seamount-trapped wave model, and limited mooring data. POP2 output suggests that Yanai wave energy enters the Plateau system near the surface, at the western edge, inducing a resonant trapped wave response. The trapped wave redistributes equatorial energy to smaller scales, vertically in the water column, and anticyclonically along the Plateau circumference.