Publication: The role of the divergent circulation for large-scale eddy momentum transport in the tropics. Part I: observations
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2019-04
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American Meteorological Society
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Abstract
This work investigates the role played by the divergent circulation for meridional eddy momentum transport in the tropical atmosphere. It is shown that the eddy momentum flux in the deep tropics arises primarily from correlations between the divergent eddy meridional velocity and the rotational eddy zonal velocity. Consistent with previous studies, this transport is dominated by the stationary wave component, associated with correlations between the zonal structure of the Hadley cell (zonal anomalies in the meridional overturning) and the climatological-mean Rossby gyres. This eddy momentum flux decomposition implies a different mechanism of eddy momentum convergence from the extratropics, associated with upper-level mass convergence (divergence) over sectors with anomalous westerlies (easterlies). By itself, this meridional transport would only increase (decrease) isentropic thickness over regions with anomalous westerly (easterly) zonal flow. The actual momentum mixing is due to vertical (cross isentropic) advection, pointing to the key role of diabatic processes for eddy–mean flow interaction in the tropics..
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© 2019 American Meteorological Society. We are grateful to Sukyoung Lee and two anonymous reviewers for suggestions that improved this manuscript. P.Z.-G. acknowledges financial support by Grant CGL2015-72259-EXP by the State Research Agency of Spain. This work benefited from helpful discussions with I. Held during a series of summer visits to Princeton funded by NSF Grant AGS-1733818.