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Convective Building of a Pycnocline: A Two-Dimensional Nonhydrostatic Numerical Model

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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 two-dimensional nonhydrostatic numerical model forced by a balanced salinity dipole (source and sink). Although the forcing fields are steady, the model develops oscillations that renew the model’s analog of “deep waters” only intermittently. The oscillation cycle consists of a freshwater layer that advects along the surface, capping off the water column under the dense source and preventing sinking; after a time, continuing densification forms a plume that breaks through the salinity barrier and convects beneath the dense source, ventilating the deep water. Increasing the viscosity reduces but does not eliminate this cycle. When the hydrostatic assumption is added, the model evolves systematically different salinity distributions than the nonhydrostatic model due to the isolation of part of the tank by a persistent convective column. The deep flow is also different in this case because of differences between the entrainment/detrainment profile of a hydrostatic plume and one modeled explicitly. The model evolves a characteristically skewed distribution of densities that is similar to the distribution of temperature in the World Ocean. Rotation increases the range of this distribution due to the inhibition of meridional flow.

* Current affiliation: Scripps Institution of Oceanography, University of California, San Diego, San Diego, California.

Corresponding author address: Dr. David W. Pierce, Climate Research Division, 0224, Scripps Institute of Oceanography, La Jolla, CA 92093-0224.

Email: dpierce@uscd.edu

Abstract

The convective building of a pycnocline is examined using a two-dimensional nonhydrostatic numerical model forced by a balanced salinity dipole (source and sink). Although the forcing fields are steady, the model develops oscillations that renew the model’s analog of “deep waters” only intermittently. The oscillation cycle consists of a freshwater layer that advects along the surface, capping off the water column under the dense source and preventing sinking; after a time, continuing densification forms a plume that breaks through the salinity barrier and convects beneath the dense source, ventilating the deep water. Increasing the viscosity reduces but does not eliminate this cycle. When the hydrostatic assumption is added, the model evolves systematically different salinity distributions than the nonhydrostatic model due to the isolation of part of the tank by a persistent convective column. The deep flow is also different in this case because of differences between the entrainment/detrainment profile of a hydrostatic plume and one modeled explicitly. The model evolves a characteristically skewed distribution of densities that is similar to the distribution of temperature in the World Ocean. Rotation increases the range of this distribution due to the inhibition of meridional flow.

* Current affiliation: Scripps Institution of Oceanography, University of California, San Diego, San Diego, California.

Corresponding author address: Dr. David W. Pierce, Climate Research Division, 0224, Scripps Institute of Oceanography, La Jolla, CA 92093-0224.

Email: dpierce@uscd.edu

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