High-Resolution Observations of Internal Wave Turbulence in the Deep Ocean

An overview is presented of high-resolution temperature observations above underwater topography in the deep, generally stably stratified ocean. The Eulerian mooring technique is used by typically distributing 100 sensors over lines between 40 and 400 m long. The independent sensors sample at a rate of 1 Hz for up to one year with a precision better than 0.1 mK. This precision and sampling rate are sufficient to resolve all of the internal waves and their breaking including the large, energy containing turbulent eddies above underwater topography. Under conditions of a tight temperature-density relationship, the data are used to quantify turbulent overturns. The turbulent diapycnal mixing is important for the redistribution of nutrients, heat (to maintain the stable stratification) and the resuspension of sediment. The detailed observations show two distinctive turbulence processes that are associated with different phases of large-scale carriers (which are mainly tidal but also inertial, internal gravity waves or a sub-inertial sloshing motion): (i) highly nonlinear turbulent frontal bores during the upslope propagating phase, and (ii) Kelvin-Helmholtz billows, at some distance above the slope, during the downslope phase. While the former may be associated in part with convective turbulent overturning following Rayleigh-Taylor instabilities preceding and sharpening the bores, the latter are mainly related to shear-induced instabilities. Under weaker stratified conditions, away from boundaries, free convective mixing appears more often, but a clear inertial subrange in temperature spectra is indicative of dominant shear-induced turbulence. Turbulence is seen to increase in dissipation rate and diffusivity all the way to the bottom while stratification remains constant, which challenges the idea of a homogeneous ‘well-mixed bottom boundary layer’. With a newly developed five-lines mooring the transition is demonstrated from isotropy (full turbulence) to anisotropy (stratified turbulence/internal waves).