Convective Building of a Pycnocline: Laboratory Experiments

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  • 1 School of Oceanography, University of Washington, Seattle, Washington
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Abstract

The convective building of a pycnocline is examined using a laboratory model forced by surface fluxes of saline water at one end and fresh water at the other. A deep recirculation evolves in the tank, which homogenizes the interior fluid by repeated passes through the dense, descending plume. A thin, fresh surface layer develops and modifies the effective buoyancy flux into the dense plume, causing the interior velocities to fall to an intermediate-time minimum. Adding bottom topography under the dense source greatly reduces the amount of entrainment that the descending plume undergoes. In this case, the tank fills with a deep, heavy layer, which causes the plume to “lift off” the bottom of the tank and detrain at successively higher depths in the water column. A simple numerical “plume” model shows that this cannot be a steady state, as it is not in diffusive balance; the plume must eventually return to the bottom of the tank and ventilate the interior waters. Adding rotation increases the surface mixing, thickens the halocline, and increases the observed variability in the salinity field.

Abstract

The convective building of a pycnocline is examined using a laboratory model forced by surface fluxes of saline water at one end and fresh water at the other. A deep recirculation evolves in the tank, which homogenizes the interior fluid by repeated passes through the dense, descending plume. A thin, fresh surface layer develops and modifies the effective buoyancy flux into the dense plume, causing the interior velocities to fall to an intermediate-time minimum. Adding bottom topography under the dense source greatly reduces the amount of entrainment that the descending plume undergoes. In this case, the tank fills with a deep, heavy layer, which causes the plume to “lift off” the bottom of the tank and detrain at successively higher depths in the water column. A simple numerical “plume” model shows that this cannot be a steady state, as it is not in diffusive balance; the plume must eventually return to the bottom of the tank and ventilate the interior waters. Adding rotation increases the surface mixing, thickens the halocline, and increases the observed variability in the salinity field.

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