Fig. 6
From: Self-sharpening induces jet-like structure in seafloor gravity currents
Sketched evolution of jet-sharpening in a gravity current, bounded by a lower seafloor interface and an upper flow—ambient fluid interface. Smooth flow boundaries result in predominantly parallel streamlines (a), with some internal variation due to turbulent fluid motion and coherent eddies shed into the flow, see Fig. 1. Deformation of the lower and upper flow boundaries (b), e.g. by bedforms or the onset of Holmboe (Ho) or Kelvin-Helmholtz (KH) instabilities, enhances vertical movement within the flow. Vertical fluid motion results in the formation of internal gravity waves due to the buoyancy restoring force arising through density stratification. Sheared by downstream flow, gravity waves generate up-velocity-gradient momentum transport. At a critical layer, on the gravity current flanks, shear results in gravity wave breaking (WB). Wave breaking results in the irreversible transfer of momentum into the flow, and mixing of flow density and potential vorticity (PV). Progressive momentum transfer accelerates the gravity current core (c), a process ultimately limited by viscous and turbulent dissipation. Wave breaking also results in the homogenous mixing of density and PV either side of the velocity maximum. The sharp PV gradient stabilises the velocity maximum, acting as a restoring mechanism i.e. gravity wave elasticity, and as a turbulent eddy barrier to mixing between layers. Thus, for flow over bedforms, the quasi-rigid layer containing the velocity maximum responds as a free-surface (c). In (a–c) subplots denote evolution of velocity, u, relative density, ρf/ρ − 1, and PV, based on vertical gradient of downstream velocity. Decreasing flow density is denoted by shading, from dark to light blue; streamlines are denoted by black arrows; red lines denote location of velocity maximum
