Evidence for Seafloor-Intensified Mixing by Surface-Generated Equatorial Waves

Turbulence-enhanced mixing in the abyssal ocean is thought to derive its energy from tidal and wind forcing in roughly equal proportions. While much work has focused on mixing generated by tidal flow over rough topography, less is known about wind-driven mixing in the abyss. In addition, most observational studies have focused on the mid- and high-latitudes, while inverse models suggest that most of the zonally-integrated diapycnal transport in the abyssal overturning cell occurs in the tropical oceans. We address these issues by analyzing full-depth microstructure turbulence profiles obtained near 110°W, 0°N in the Eastern Pacific. Mixing was intensified over the bottom 700m where the diffusivity κ_T reached values of 10^-3m²s^-1, of similar intensity and structure to that found elsewhere over rough topography. Here, in contrast, a patch of seafloor-intensified mixing characterized by turbulent kinetic energy dissipation rates ε reaching 10^-8Wkg^-1 was found over smooth topography. The strong turbulence was associated with flow unstable to both vertical shear instability and lateral inertial instability, as quantified using the Richardson number and absolute momentum. We suggest that these instabilities and the subsequent mixing were driven by a surface-generated equatorial wave, opening a unique energy pathway through which wind-generated wave energy directly drives seafloor-intensified mixing at low latitudes, with implications for the abyssal overturning circulation.