Abstract

Seismic oceanography is a novel technique of producing detailed acoustic images of ocean finestructure. Here we present a series of ocean seismic images from a region in the southwest Atlantic. Finestructure imaged in the data occurs dominantly along the Subantarctic Front and Polar Front and at the boundary layer between Antarctic Intermediate Water and underlying Upper Circumpolar Deep Water. The images correlate well with proximal hydrographic data. One particularly striking image captures a km scale overturning at the confluence between North Atlantic Deep Waters and circumpolar waters and illustrates how water mass fronts can act as both barriers and blenders to ocean mixing.

Turbulent mixing rates in Drake Passage and the southwest Atlantic are known to be remarkably intense and widespread. We have developed an automated technique to spectrally analyse internal wave undulations captured in the seismic images and deduce the spatial distribution of mixing across seismic transects. Resultant seismic reflection displacement spectra exhibit a continuum from internal wave type characteristics at low wavenumbers to more turbulent dominated motions at higher wavenumbers. Deduced diapycnal mixing rates, which although patchy, are typically 10-4 m2s-1. Higher energy levels are found at the sides of lenses of homogeneous waters, regions of internal wave breaking and along rougher regions of seabed. A new technique of extracting cross-track geostrophic current velocities from ocean seismic images will also be discussed. The method utilises the slopes of seismic reflections which act as proxies for those of isopycnal surfaces.