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A Climatology of Tropospheric Zonal-Mean Water Vapor Fields and Fluxes in Isentropic Coordinates

Schneider, Tapio and Smith, Karen L. and O'Gorman, Paul A. and Walker, Christopher C. (2006) A Climatology of Tropospheric Zonal-Mean Water Vapor Fields and Fluxes in Isentropic Coordinates. Journal of Climate, 19 (22). pp. 5918-5933. ISSN 0894-8755. http://resolver.caltech.edu/CaltechAUTHORS:SCHNjc06

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Abstract

Based on reanalysis data for the years 1980–2001 from the European Centre for Medium-Range Weather Forecasts (ERA-40 data), a climatology of tropospheric zonal-mean water vapor fields and fluxes in isentropic coordinates is presented. In the extratropical free troposphere, eddy fluxes dominate the meridional flux of specific humidity along isentropes. At all levels, isentropic eddy fluxes transport water vapor from the deep Tropics through the subtropics into the extratropics. Isentropic eddy fluxes of specific humidity diverge near the surface and in the tropical and subtropical free troposphere; they converge in the extratropical free troposphere. Isentropic mean advective fluxes of specific humidity play a secondary role in the meridional water vapor transport in the free troposphere; however, they dominate the meridional flux of specific humidity near the surface, where they transport water vapor equatorward and, in the solstice seasons, across the equator. Cross-isentropic mean advective fluxes of specific humidity are especially important in the Hadley circulation, in whose ascending branches they moisten and in whose descending branches they dry the free troposphere. Near the minima of zonal-mean relative humidity in the subtropical free troposphere, the divergence of the cross-isentropic mean advective flux of specific humidity in the descending branches of the Hadley circulation is the dominant divergence in the mean specific humidity balance; it is primarily balanced by convergence of cross-isentropic turbulent fluxes that transport water vapor from the surface upward. Although there are significant isentropic eddy fluxes of specific humidity through the region of the subtropical relative humidity minima, their divergence near the minima is generally small compared with the divergence of cross-isentropic mean advective fluxes, implying that moistening by eddy transport from the Tropics into the region of the minima approximately balances drying by eddy transport into the extratropics. That drying by cross-isentropic mean subsidence near the subtropical relative humidity minima is primarily balanced by moistening by upward turbulent fluxes of specific humidity, likely in convective clouds, suggests cloud dynamics may play a central role in controlling the relative humidity of the subtropical free troposphere.


Item Type:Article
ORCID:
AuthorORCID
Schneider, Tapio0000-0001-5687-2287
Additional Information:© 2006 American Meteorological Society Manuscript received 5 January 2006, in final form 27 February 2006 We are grateful for financial support by the Davidow Discovery Fund, by a Davidow Graduate Fellowship (K. Smith), by an Alfred P. Sloan Research Fellowship, and by the National Science Foundation (Grant ATM-0450059), and for computing resources and reanalysis data provided by the National Center for Atmospheric Research (which is sponsored by the National Science Foundation). T. Schneider gratefully acknowledges discussions with Isaac Held in 2001, in which the idea to analyze water vapor fluxes in isentropic coordinates originated, and with Adam Sobel, which helped to clarify the relation between our results and those of Galewsky et al. (2005). We thank Kerry Emanuel for helpful comments on the manuscript.
Record Number:CaltechAUTHORS:SCHNjc06
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:SCHNjc06
Alternative URL:http://dx.doi.org/10.1175/JCLI3931.1
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:6855
Collection:CaltechAUTHORS
Deposited By: Archive Administrator
Deposited On:29 Dec 2006
Last Modified:18 Mar 2015 22:05

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