Abstract
We investigate the impact of high-frequency variability in forcing on the dynamics of a density current using a high-order non-hydrostatic spectral element model, SEOM (Spectral Element Ocean Model) in the streamfunction-vorticity formulation. Turbulent structures and instabilities depend strongly on forcing and boundary conditions. We introduce time-dependent disturbances through forcing at the inlet boundary and through variation in background transport. Steady forcing and forcing at the inlet boundary at very short periods for experiments at Reynolds number, Re=15,000, result in a regime where the passage of the gravity current head with a strongly overturning tail gives way to a stable two-layer system with internal waves on the density interface. Time-dependent forcing at intermediate periods results in turbulent flow regimes with a wide range of time and length scales. At longer forcing periods, individual turbulent bore heads are observed propagating through the system. Forcing through variation in background transport rather than at the inlet boundary changes the distribution of density classes across the flow. Experiments at very high Reynolds number, Re=50,000, result in a highly non-linear flow regime, where the mixing is less affected by temporal variability in forcing.