FY 1997

Buoyancy-driven plumes in rotating, stratified cross flows: Plume dependence on rotation, turbulent mixing, and cross-flow strength

Lavelle, J.W.

J. Geophys. Res., 102(C2), doi: 10.1029/96JC03601, 3405–3420 (1997)

A three-dimensional time-dependent convection model is used to describe circulation and property fields in rotating, stratified, and moving fluids near a point source of heat. The study context and, consequently, model scales are those of chronically discharging hydrothermal vent fields found at submarine ridge crests. Hydrothermal plumes having distinctive thermal and chemical anomalies have been observed to rise several hundreds of meters above the deep-sea floor before being advected away by background cross flows typically of magnitude 1-4 cm s. The model is used to study effects of rotation and indicate differences in plumes with respect to variation of subgrid-scale turbulence intensity and cross-flow strength. Counterrotating vorticity () couplets in all three coordinate directions develop in the lower plume stem at startup and follow the plume to the level of neutral buoyancy; for a nonrotational case ( = 0), patterns resemble those previously found for jets injected into homogenous cross flow. Ambient fluid entrainment into the convecting column is primarily from the upstream side, but deflection of background flow around both sides of the rising column is the root of the relative vorticity (_z) couplet in the lower plume. Turbulence intensity within the buoyant region of the plume and/or globally controls smoothness and temporal variability of distal nonbuoyant plume distributions, allowing or preventing oscillations of potential temperature, , for example, at background buoyancy frequency, N. Over the range of turbulent mixing studied, rise height of plumes did not change appreciably, but breadth of plumes, counterintuitively, increased for decreasing turbulent mixing strength. Increasing cross-flow strength, U, bends model plumes such that rise height U^-0.4. For the two largest values of cross flow, for which R, the ratio of maximum vertical velocity to U, took values of 2.8 and 1.0, plumes showed evidence of bifurcation.