The Effects of Rotation and River Discharge on Net Mixing in Small-Mouth Kelvin Number Plumes

Kelly L. Cole Department of Oceanography, Texas A&M University, College Station, Texas

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Robert D. Hetland Department of Oceanography, Texas A&M University, College Station, Texas

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Abstract

Small-mouth Kelvin number plumes, or plumes with a source width smaller than the deformation radius, are characterized by near-field plume regions of rapid lateral expansion and strong vertical mixing. Net plume mixing, or the dilution of a plume by ocean water between the estuary mouth and the far-field plume, is examined using idealized numerical experiments with the Regional Ocean Modeling System (ROMS). The density anomaly of plume water entering the far field is determined from isohaline analysis of the modeled salinity field. The experiments indicate that when estuarine discharge increases, net plume mixing decreases in a rotating environment but increases in a nonrotating environment. Scaling analysis supports that this opposite trend in behavior is related to rotation turning the plume, limiting the lateral expansion and suppressing shear mixing. The results of this study explain different trends in net plume mixing reported in previous studies and compare favorably to observations of the Fraser River plume.

Current affiliation: Darling Marine Center, University of Maine, Walpole, Maine.

Corresponding author address: Kelly L. Cole, Darling Marine Center, University of Maine, 193 Clarks Cove Rd., Walpole, ME 04573. E-mail: kelly.cole@maine.edu

Abstract

Small-mouth Kelvin number plumes, or plumes with a source width smaller than the deformation radius, are characterized by near-field plume regions of rapid lateral expansion and strong vertical mixing. Net plume mixing, or the dilution of a plume by ocean water between the estuary mouth and the far-field plume, is examined using idealized numerical experiments with the Regional Ocean Modeling System (ROMS). The density anomaly of plume water entering the far field is determined from isohaline analysis of the modeled salinity field. The experiments indicate that when estuarine discharge increases, net plume mixing decreases in a rotating environment but increases in a nonrotating environment. Scaling analysis supports that this opposite trend in behavior is related to rotation turning the plume, limiting the lateral expansion and suppressing shear mixing. The results of this study explain different trends in net plume mixing reported in previous studies and compare favorably to observations of the Fraser River plume.

Current affiliation: Darling Marine Center, University of Maine, Walpole, Maine.

Corresponding author address: Kelly L. Cole, Darling Marine Center, University of Maine, 193 Clarks Cove Rd., Walpole, ME 04573. E-mail: kelly.cole@maine.edu
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