FY 1994

A numerical study of local convection in the benthic ocean induced by episodic hydrothermal discharges

Lavelle, J.W., and E.T. Baker

J. Geophys. Res., 99(C8), 16,065–16,080, doi: 10.1029/94JC01203 (1994)

A nonhydrostatic primitive equation model is used to investigate motions and water column property distributions accompanying the rise of hot hydrothermal fluids into a rotating water column stratified in temperature and salinity. After preliminary model experiments used to fix the level of turbulent mixing, the model is applied to a megaplume event, the release of a large amount of hydrothermal heat into the benthic ocean over a period of several days. Events of this kind have produced water masses with anomalous temperature and salinity distributions ~20 km in diameter, ~800 m thick, centered vertically 700-1000 m off the seafloor. The model allows the study of the initial time development and maturation of the plume from such an event. Preliminary experiments demonstrate the sensitivity of plume rise to the level of turbulent mixing and to the ratio of turbulence viscosity to turbulence diffusivity, i.e., Prandtl number. Rise to maximum plume height occurs in 4-6 N⁻¹, where N is buoyancy frequency. Consequences of a megaplume-sized release of heat are examined over an initial 30-day period. Vertical circulation accompanying a hydrothermal event leads to a slight, negative salinity anomaly below the plume's temperature anomaly core. The plume's heat (potential temperature) anomaly is shown to be a composite of hydrothermal heat and ambient heat that has been redistributed by the same vertical circulation cell. Horizontal circulation is dominated by an anticyclonic flow vortex centered just above the temperature anomaly maximum at ~800 m above bottom and a cyclonic flow vortex at the seafloor. Azimuthal flow accounts for the largest fraction of kinetic energy within 4.5 hours (~1.7 f⁻¹) after the start of heat release; azimuthal velocities reach maximum values of ~70 cm s⁻¹, 90% of which is attained within the initial 10 hours. Even as much as 4 weeks past cessation of heat release maximum azimuthal velocities are of the order of ~10 cm s⁻¹. The balance of vertical forces is hydrostatic by 12 hours, save for near-bottom frictional effects. During plume formation, the radial force balance is primarily cyclostrophic near axis and geostrophic at a distance.