A NUMERICAL STUDY OF LOCAL CONVECTION IN THE BENTHIC OCEAN INDUCED BYEPISODIC HYDROTHERMAL DISCHARGES

A nonhydrostatic primitive equation model is used to investigate motio ns and water column property distributions accompanying the rise of ho t hydrothermal fluids into a rotating water column stratified in tempe rature and salinity. After preliminary model experiments used to fix t he level of turbulent mixing, the model is applied to a megaplume even t, the release of a large amount of hydrothermal heat into the benthic ocean over a period of several days. Events of this kind have produce d water masses with anomalous temperature and salinity distributions a pproximately 20 km in diameter, approximately 800 m thick, centered ve rtically 700-1000 m off the seafloor. The model allows the study of th e initial time development and maturation of the plume from such an ev ent. Preliminary experiments demonstrate the sensitivity of plume rise to the level of turbulent mixing and to the ratio of turbulence visco sity to turbulence diffusivity, i.e., Prandtl number. Rise to maximum plume height occurs in 4-6 N-1, where N is buoyancy frequency. Consequ ences of a megaplume-sized release of heat are examined over an initia l 30-day period. Vertical circulation accompanying a hydrothermal even t leads to a slight, negative salinity anomaly below the plume's tempe rature 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. Horizont al circulation is dominated by an anticyclonic flow vortex centered ju st above the temperature anomaly maximum at approximately 800 m above bottom and a cyclonic flow vortex at the seafloor. Azimuthal flow acco unts for the largest fraction of kinetic energy within 4.5 hours (appr oximately 1.7 f-1) after the start of heat release; azimuthal velociti es reach maximum values of approximately 70 cm s-1, 90% of which is at tained within the initial 10 hours. Even as much as 4 weeks past cessa tion of heat release maximum azimuthal velocities are of the order of approximately 10 cm s-1. The balance of vertical forces is hydrostatic by 12 hours, save for near-bottom frictional effects. During plume fo rmation, the radial force balance is primarily cyclostrophic near axis and geostrophic at a distance.