Sack Lunch Seminar (SLS)

High-resolution observations of internal wave induced 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 to monitor temperature variations by typically 100 sensors distributed 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 the large, energy containing turbulent eddies and all of the internal waves and their breaking above underwater topography. Such underwater wave breaking is the key mechanism for the redistribution of nutrients and heat (to maintain the ocean stably stratified), and the resuspension of sediment.

Under conditions of tight temperature-density relationship, the temperature data are used to quantify turbulent overturns. These observations show two distinctive turbulence processes that are associated with different phases of a large-scale, mainly tidal, internal gravity wave: i) highly nonlinear turbulent 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, 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. With stratification, turbulence is seen to increase in dissipation rate and diffusivity all the way to the bottom, which challenges the idea of a homogeneous bottom boundary layer. With a newly developed five-lines mooring, the transition from isotropy (full turbulence) to anisotropy (stratified turbulence/internal waves) is revealed.