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Departmental Colloquium

Title
Entrainment and horizontal cross flows and their affect on hydrothermal plume vertical velocity and temperature fluctuations  
Guest Speaker
Dr. Daniela Di Iorio  
Guest Affiliation
University of Georgia, Department of Marine Sciences  
Host
Dr. Steve Lewis  
When
Thursday, February 10, 2011 4:00 pm - 5:00 pm  
Location
Physics 202  
Details

Deep sea hydrothermal plumes are driven by gravitational buoyancy forces and may rise up to hundreds of meters above their orifice. Ambient ocean water is entrained into the plume during its ascent, which makes the plume diluted and cooled.   Long term measurements of physical properties of hydrothermal plumes are limited and are important for understanding heat flow from the Earth’s interior.  In Sept 2007 an acoustic scintillation system was used to specifically quantify the temporal variability of the vertical velocity and temperature fluctuations of the hydrothermal plume of Dante at 2155m depth. Six weeks of data was collected and using the space-time coherence of the acoustic amplitude signal, hourly vertical velocity measurements were obtained.  Theoretical developments comparing acoustic forward scattering from turbulence and from particles show that suspended particles within the plume produce negligible amplitude fluctuations compared to turbulence modeled by an isotropic and homogeneous Kolmogorov model for the temperature variability.

The vertical velocity and temperature fluctuations show a significant negative correlation with the horizontal flow.  The hydrothermal plume of Dante and its interaction with the horizontal flow within the Main Endeavour Field can be generalized as follows: 1) when the horizontal flow is weak (during the ebbing tide), less ambient ocean water is entrained into the plume. In such a case, the plume is faster and hotter and the temperature fluctuations increase within the plume; 2) when the horizontal flow is strong (during the flooding tide), more ambient ocean water is entrained into the plume. In such a case, the plume is slower
and cooler with reduced temperature fluctuations.   Results from an integral plume model based on the conservation equations of mass, momentum, density deficit and dissolved tracers and taking into account ambient stratification and horizontal cross flows are compared with observations showing consistent results.

 

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