Influence of Coriolis force upon bottom boundary layers in a large‐scale gravity current experiment: Implications for evolution of sinuous deep‐water channel systems
Résumé
Oceanic density currents in many deep‐water channels are strongly influenced by the Coriolis force. The dynamics of the bottom‐boundary layer in large geostrophic flows, and low Rossby number turbidity currents, are very important for determining the erosion and deposition of sediment in channelized contourite currents and many large‐scale turbidity currents. However, these bottom boundary layers are notoriously difficult to resolve with oceanic field measurements, or in previous small‐scale rotating laboratory experiments. We present results from a large, 13 m diameter, rotating laboratory platform that is able to achieve both stratified and highly turbulent flows in regimes where the rotation is sufficiently rapid that the Coriolis force can potentially dominate. By resolving the dynamics of the turbulent bottom boundary in straight and sinuous channel sections, we find that the Coriolis force can overcome centrifugal force to switch the direction of near‐bed flows in channel bends. This occurs for positive Rossby numbers less than +0.8, defined as Ror=U/Rf, where U is the depth‐averaged velocity,the Coriolis parameter. Density and velocity fields decoupled in channel bends, with the densest fluid of the gravity current being deflected to the outer‐bend of the channel by the centrifugal force, while the location of velocity maximum shifted with the Coriolis force, leading to asymmetries between left‐ and right‐ turning bends. These observations of Coriolis effects on gravity currents are synthesized into a model of how sedimentary structures might evolve in sinuous turbidity current channels at various latitudes.
Many of the largest currents in the oceans depths are dense, gravity‐driven, flows that pour down deep‐water channels. These large gravity currents include dense overflows, as well as sediment‐laden turbidity currents and contourite flows. The dynamics of these gravity currents can be strongly affected by the Coriolis force. This study examines the effect of the Coriolis force on the flow structure of oceanic gravity currents flowing through straight and sinuous channels. A set of 22 experiments were carried out on the world's largest rotating experimental facility, the LEGI Coriolis platform in Grenoble, France. Our detailed measurements of velocity and internal density structure imply that at higher latitudes Coriolis force dominates and changes the direction of the flow near the bed leading to differences between flows going around a left‐turning versus a right‐turning bend. Our observations are relevant to the large deep‐water channels formed on the ocean floor by successive turbidity currents. Near the Equator these channels tend to be noticeably sinuous, however recent studies have shown that this sinuosity decreases with latitude. One possibility is that latitudinal variations in the Coriolis force may influence the evolution of these channels through changing near‐bed patterns of erosion and deposition.