Buoyancy Forcing by Turbulence above Rough Topography in the Abyssal Brazil Basin

Abstract

Observations of turbulent dissipation above rough bathymetry in the abyssal Brazil Basin are presented. Relative to regions with smooth bathymetry, dissipation is markedly enhanced above rough topography of the Mid-Atlantic Ridge with levels above bathymetric slopes exceeding levels observed over crests and canyon floors. Furthermore, mixing levels in rough areas are modulated by the spring?neap tidal cycle. Internal waves generated by barotropic tidal flow over topography are the likely mechanism for supplying the energy needed to support the observed turbulent dissipation. A model of the spatial and temporal patterns in the turbulent dissipation rate is used to constrain the diapycnal advection in an inverse calculation for the circulation in an area of rough bathymetry. This inverse model uses both beta-spiral and integrated forms of the advective budgets for heat, mass, and vorticity, and contains sufficient information to resolve the full three-dimensional flow. The inverse model solution reveals the presence of a bouyancy forced circulation driven by mixing in abyssal canyons. On isopycnals above the level of fracture-zone crests near the Mid-Atlantic Ridge, the flow is westward and fluid is downwelled toward greater density. Along deeper isopycnals, fluid is carried eastward and upwelled in canyons. The divergence of diapycnal mass flux is a significant forcing mechanism for this circulation. These results suggest that mixing in abyssal canyons plays an important role in the circulation of abyssal waters.