Gravity currents are ubiquitous in polar regions and marginal seas and play a crucial role in the formation of deep waters in the ocean. They contribute to the vorticity and energy transfers towards the ocean interior. We present results from an experimental study on downslope intruding gravity currents into an initially two-layer stably stratified ambient on a rotating platform at high buoyancy Reynolds numbers.
A novel experimental design to produce the downslope gravity flow has been employed, using an axisymmetric configuration and a uniform flow injection that enabled to study the long term evolution of surface baroclinic vortices and of the gravity current, monitoring at the same time the evolution of the global circulation and the vorticity produced in the central deep area. The structure of the current, its relevant scales and the characteristics of the generated surface vortices fairly agree with previous results in the literature in smaller scale installations. Discrepancies are attributable to both the influence of topographic Rossby waves and viscous effects that are much reduced in the Coriolis Platform. Rotating intrusive gravity currents in a two-layer stratified ambient behave very differently from dense currents following the bottom slope. Substantial differences appear for the induced global circulation which depend on the nature of the intrusion. In particular, intruding gravity currents give rise to a strong turbulent environment at intermediate and bottom depths in the central area, with submesoscale vortices (i.e. with a typical size smaller than the Rossby deformation radius) and a large variety of scales. In contrast, when the dense current follows the bottom slope no significant vorticity production in the bottom and intermediate layers is reported. This clearly suggests that bottom boundary layers detaching from the boundary and propagating toward the ambient interior as in intrusive currents give an important contribution to the turbulence dynamics.

The shape of the vertical density profile in the stratified receiving ambient enables to identify the two distinct regimes: the first issued by the laminar transport through Ekman dynamics, and the second by turbulent transport due to the intermittent cascades. Cascades show to be intrinsic to rotating gravity currents, i.e. they arise without any external tuning, and the related transport does not exhibit any characteristic length scale suggesting self-organized criticality. Cascades reveal to be the main contributor to the vorticity and turbulence in the ocean interior. Two mechanisms for vorticity production are recognized: first, the spreading of the cascade into the interior, and second, the meandering and break up of the deep boundary current induced by the passage of the cascade. The turbulence in the receiving ambient reveals to be horizontally isotropic, non-stationary and non-homogeneous. Energy is injected  through the cascades at the penetration length scale and forces the turbulence in the intrusion area close to the slope. The central area far from the boundaries is characterised, instead, by freely evolving two-dimensional turbulence, forced at large scales. These results suggest a complementary way to interpret oceanic observations of gravity currents spreading in the ocean interior.

By means of velocity and density measurements, we show that no mixing occurs once the current has detached from the boundary. Vertical density gradients reveal a piece-wise linear dependence on the density anomaly for the turbulent transport, suggesting an advection-diffusion process. For this regime, the scale height is deduced and an analytical model based on the critical Froude number is proposed to predict its value. Results show that the total thickness of the intruding current is on average 2.5 times the scale height. For laminar intrusions the scale height diverges whereas the thickness of the intrusion is a few times the Ekman layer thickness. Comparing the intrusion scale height with its measured vertical extension has led to a criteria to distinguish between laminar and turbulent regimes, which is corroborated by two additional independent criteria, one based on the sign of the local vorticity and the other based on the local maxima of the vertical density gradient.

The derived model enables to connect directly laboratory experiments and deep sea observations and emphasizes the importance of laboratory experiments in understanding climate dynamics.

Eletta Negretti est responsable scientifique de la plateforme Coriolis au LEGI.

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