Laminar transport and turbulent cascades in downslope rotating gravity currents.

Abstract

We present experimental results from large-scale laboratory experiments of rotating downslope gravity currents intruding into a two-layer stratified ambient performed in the Coriolis Rotating Platform in Grenoble. By means of PIV velocity and conductivity data for the density measurement, we show that mixing occurs mostly on the slope area during the descent rather than once the current has penetrated the stratified ambient, where the Richardson number remains above the stability threshold of 1/4. Looking at the time evolution of the vertical density profile in the stratified receiving ambient, two distinct mixing regimes can be identified, the first issued by laminar transport through Ekman dynamics, the second by turbulent transport due to intermittent cascading events. Vertical density gradients reveal a linear piece-wise dependence on the density anomaly, highlighting an advection-diffusion process as proposed by the theoretical model of Munk & Wunsch (1998). If the gravity current flow is laminar on the slope, the structure shows a linear variation of the density with depth ; For the turbulent transport regime characterized by intermittent cascades, an exponential shape is rather observed. The shape of the density structure allows to estimate bulk mixing coefficients and entrainment velocities at the top and the bottom of the intruding gravity current, which can be further compared to oceanographic observational data.