Turbulence properties of a deep sea hydrothermal plume: field and model comparisons

A turbulent convection model for a hydrothermal fluid injected into a tidally modulated stratified crossflow is developed to characterize the finescale processes of the dissipation rates of turbulent kinetic energy (TKE) and thermal variance. Our implementation of the large eddy simulation, allows us to estimate the TKE dissipation rate from estimates of the TKE production from shear, buoyancy and from vertical advection and turbulent transport of TKE up to 30 m above the source area. Similarly, the dissipation rate for thermal variance is also estimated from the production from heat fluxes acting on thermal gradients and from the advection and turbulent transport of thermal variance. These dissipation rates are needed for model comparison with acoustical observations of the structure parameter for refractive index variations. Six different model experiments were run with varying heat flux output and areal extent, and with either isotropic or anisotropic Smagorinsky coefficients in the horizontal and vertical directions, in order to find the best fit to observations made at the Dante plume in the Main Endeavour vent field. Measurements were made at 20 m from the vent orifice and comparisons with the model are made 20 m from the virtual source calculated based on the characteristics of the vertical flux. The model run with a total heat flux of 50 MW over an area of 4x4.5 m2 with horizontal and vertical Smagorinsky coefficients of CSH=0.14 and CSV=0.04 respectively, compares favorably with observations of vertical velocity and the level of refractive index fluctuations. These observations provide a method to quantify the unobservable finescale contributions of the hydrothermal plume turbulence.