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Idealized Models of Slantwise Convection in a Baroclinic Flow

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

Intermediate, or deep, convection in a baroclinic flow occurs along slanted paths parallel to the alongflow absolute momentum surfaces. These surfaces are principally tilted due to the vertical shear in velocity but can be further modified by a nonvertical axis of rotation. An inviscid Lagrangian parcel model, using realistic parameters, is utilized to illustrate, qualitatively, the different scenarios resulting from the combined action of inertial and gravitational forces acting on sinking parcels of dense fluid. More quantitative results are derived from a series of numerical experiments using a zonally invariant, high-resolution, nonhydrostatic model. Convection occuring in a flow with tilted absolute momentum surfaces will mix properties along these slanted surfaces. This implies that the fluid can retain a weak vertical stratification while overturning and also, more importantly, that the evolution of the convective layer cannot be described in terms of one-dimensional, vertical mixing. The authors show, for conditions typical of the Labrador Sea, that the convective layer depth difference between that estimated by mixing vertically and one obtained allowing for slantwise mixing can be greater than 100 m; slantwise convection reaches deeper because of the reduced stratification along the slanted paths. An alternative slantwise mixing scheme, based on the assumption of zero potential vorticity of the convected fluid, is proposed.

Current affiliation: Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

Corresponding author address: Fiammetta Straneo, Dept. of Physical Oceanography, WHOI, MS 21, Woods Hole, MA 02543. Email: fstraneo@whoi.edu

Abstract

Intermediate, or deep, convection in a baroclinic flow occurs along slanted paths parallel to the alongflow absolute momentum surfaces. These surfaces are principally tilted due to the vertical shear in velocity but can be further modified by a nonvertical axis of rotation. An inviscid Lagrangian parcel model, using realistic parameters, is utilized to illustrate, qualitatively, the different scenarios resulting from the combined action of inertial and gravitational forces acting on sinking parcels of dense fluid. More quantitative results are derived from a series of numerical experiments using a zonally invariant, high-resolution, nonhydrostatic model. Convection occuring in a flow with tilted absolute momentum surfaces will mix properties along these slanted surfaces. This implies that the fluid can retain a weak vertical stratification while overturning and also, more importantly, that the evolution of the convective layer cannot be described in terms of one-dimensional, vertical mixing. The authors show, for conditions typical of the Labrador Sea, that the convective layer depth difference between that estimated by mixing vertically and one obtained allowing for slantwise mixing can be greater than 100 m; slantwise convection reaches deeper because of the reduced stratification along the slanted paths. An alternative slantwise mixing scheme, based on the assumption of zero potential vorticity of the convected fluid, is proposed.

Current affiliation: Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

Corresponding author address: Fiammetta Straneo, Dept. of Physical Oceanography, WHOI, MS 21, Woods Hole, MA 02543. Email: fstraneo@whoi.edu

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