Publication: The role of the divergent circulation for large-scale eddy momentum transport in the tropics. Part II: dynamical determinants of the momentum flux
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2019-04
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American Meteorological Society
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Abstract
This paper investigates the coupling between the rotational and divergent circulations aiming to explain the observations that show that the tropical eddy momentum flux is due to correlations between divergent eddy meridional velocities and rotational eddy zonal velocities. A simple linear model in which the observed eddy divergence field is used to force the vorticity equation can reproduce quite well the observed tropical eddy momentum flux. The eddy momentum flux in the model shows little sensitivity to the basic-state winds and is mainly determined by the characteristics of the divergent forcing. Vortex stretching and divergent advection of planetary vorticity produce eddy momentum flux contributions with the same sign but the former forcing dominates. It is shown that the main factor affecting the direction of the eddy momentum flux response to both forcings is the meridional tilt of the divergence phase lines, albeit with an opposite sign to the classical relation between rotational momentum flux and streamfunction phase tilt. How this divergent structure is determined remains an open question.
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© 2019 American Meteorological Society. We thank three anonymous reviewers for suggestions that improved the focus and the clarity of this manuscript. P.Z.-G. acknowledges financial support by Grant CGL2015-72259-EXP from the State Research Agency of Spain.