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Cross-Jet Lagrangian Transport and Mixing in a 2½-Layer Model

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  • 1 SAIC at Environmental Modeling Center, National Centers for Environmental Prediction, Camp Springs, Maryland
  • | 2 Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
  • | 3 University of North Carolina, Chapel Hill, North Carolina
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Abstract

Cross-stream mixing and Lagrangian transport caused by chaotic advection within a baroclinic (2½ layer) meandering jet are investigated. The quasi-steady meanders arise as a result of evolution from an initial small-amplitude instability. The investigation keys on the proposition, made in earlier work, that the cross-jet mixing and transport resulting from the meandering motions are maximized at a subsurface level. It is found that the results depend largely on the size of the shear between the two active layers (which are referred to as the upper and lower layer), as measured by a parameter α. For weak vertical shear (α greater than about 0.5) the primary instability is barotropic and there is no cross-jet transport in either of the active layers. Barriers to transport are identified as plateaus in the probability density function (PDF) of potential vorticity distributions. For stronger shear (α less than about 0.4), baroclinic instability comes into play, and the lower layer experiences barrier destruction followed by cross-jet exchange and mixing. The upper-layer barrier remains intact. The barrier destruction has a dynamical effect as evidenced by the decay of total variance of potential vorticity in the lower layer. Of interest is that the value of α estimated for the Gulf Stream lies in the range 0.4–0.5.

Corresponding author address: Dr. Guocheng Yuan, Bauer Laboratory, 7 Divinity Ave., Cambridge, MA 02138. Email: gyuan@cgr.harvard.edu

Abstract

Cross-stream mixing and Lagrangian transport caused by chaotic advection within a baroclinic (2½ layer) meandering jet are investigated. The quasi-steady meanders arise as a result of evolution from an initial small-amplitude instability. The investigation keys on the proposition, made in earlier work, that the cross-jet mixing and transport resulting from the meandering motions are maximized at a subsurface level. It is found that the results depend largely on the size of the shear between the two active layers (which are referred to as the upper and lower layer), as measured by a parameter α. For weak vertical shear (α greater than about 0.5) the primary instability is barotropic and there is no cross-jet transport in either of the active layers. Barriers to transport are identified as plateaus in the probability density function (PDF) of potential vorticity distributions. For stronger shear (α less than about 0.4), baroclinic instability comes into play, and the lower layer experiences barrier destruction followed by cross-jet exchange and mixing. The upper-layer barrier remains intact. The barrier destruction has a dynamical effect as evidenced by the decay of total variance of potential vorticity in the lower layer. Of interest is that the value of α estimated for the Gulf Stream lies in the range 0.4–0.5.

Corresponding author address: Dr. Guocheng Yuan, Bauer Laboratory, 7 Divinity Ave., Cambridge, MA 02138. Email: gyuan@cgr.harvard.edu

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