The Madden–Julian Oscillation, Barotropic Dynamics, and North Pacific Tropical Cyclone Formation. Part II: Stochastic Barotropic Modeling

Dennis L. Hartmann Department of Atmospheric Sciences, University of Washington, Seattle, Washington

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Eric D. Maloney Department of Atmospheric Sciences, University of Washington, Seattle, Washington

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Abstract

A stochastic barotropic model linearized about the 850-mb flow is used to investigate the relationship between wind variations associated with the Madden–Julian oscillation (MJO) and eddy kinetic energy variations in the Tropics. Such a model is successful in predicting the observed location of eddy kinetic energy maxima during the westerly phase of the MJO and the suppression of eddy activity during the easterly phase of the MJO. The concentration of eddy energy during the westerly phase results from the strong east–west and north–south gradients of the large-scale wind fields. The model shows that barotropic wave propagation and wave mean–flow interaction tend to concentrate small-scale Rossby wave energy in regions of convergence, which may be an important mechanism for organizing convection into tropical cyclones. The structure and barotropic energetics of the wave activity are similar to those observed, but the modeled eddies are smaller in scale and do not move westward as do the observed eddies. The eddies that dominate the observed correlations are heavily modified by convection, but barotropic processes can explain the localization of eddy energy by the MJO that is observed.

Corresponding author address: Prof. Dennis L. Hartmann, Department of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195-1640. Email: dennis@atmos.washington.edu

Abstract

A stochastic barotropic model linearized about the 850-mb flow is used to investigate the relationship between wind variations associated with the Madden–Julian oscillation (MJO) and eddy kinetic energy variations in the Tropics. Such a model is successful in predicting the observed location of eddy kinetic energy maxima during the westerly phase of the MJO and the suppression of eddy activity during the easterly phase of the MJO. The concentration of eddy energy during the westerly phase results from the strong east–west and north–south gradients of the large-scale wind fields. The model shows that barotropic wave propagation and wave mean–flow interaction tend to concentrate small-scale Rossby wave energy in regions of convergence, which may be an important mechanism for organizing convection into tropical cyclones. The structure and barotropic energetics of the wave activity are similar to those observed, but the modeled eddies are smaller in scale and do not move westward as do the observed eddies. The eddies that dominate the observed correlations are heavily modified by convection, but barotropic processes can explain the localization of eddy energy by the MJO that is observed.

Corresponding author address: Prof. Dennis L. Hartmann, Department of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195-1640. Email: dennis@atmos.washington.edu

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