We are grateful for support by the National Science Foundation (Grants ATM-0450059 and AGS-1019211) and by a David and Lucile Packard Fellowship. The two-dimensional turbulence simulations were performed using the spectral quasigeostrophic model developed by Shafer Smith. Thanks to Tim Merlis for helpful comments.
Couhert, A., , T. Schneider, , J. Li, , D. E. Waliser, , and A. M. Tompkins, 2010: The maintenance of the relative humidity of the subtropical free troposphere. J. Climate, 23, 390–403.
Derbyshire, S. H., , I. Beau, , P. Bechtold, , J. Y. Grandpeix, , J. M. Piriou, , J. L. Redelsperger, , and P. M. M. Soares, 2004: Sensitivity of moist convection to environmental humidity. Quart. J. Roy. Meteor. Soc., 130, 3055–3079.
Dessler, A. E., , and S. C. Sherwood, 2000: Simulations of tropical upper tropospheric humidity. J. Geophys. Res., 105, 20 155–20 163.
Frierson, D. M. W., 2007: The dynamics of idealized convection schemes and their effect on the zonally averaged tropical circulation. J. Atmos. Sci., 64, 1959–1976.
Frierson, D. M. W., , I. M. Held, , and P. Zurita-Gotor, 2006: A gray-radiation aquaplanet moist GCM. Part I: Static stability and eddy scale. J. Atmos. Sci., 63, 2548–2566.
Galewsky, J., , A. Sobel, , and I. Held, 2005: Diagnosis of subtropical humidity dynamics using tracers of last saturation. J. Atmos. Sci., 62, 3353–3367.
Held, I. M., , and A. Y. Hou, 1980: Nonlinear axially symmetric circulations in a nearly inviscid atmosphere. J. Atmos. Sci., 37, 515–533.
Hurley, J. V., , and J. Galewsky, 2010a: A last saturation analysis of ENSO humidity variability in the subtropical Pacific. J. Climate, 23, 918–931.
Hurley, J. V., , and J. Galewsky, 2010b: A last-saturation diagnosis of subtropical water vapor response to global warming. Geophys. Res. Lett., 37, L06702, doi:10.1029/2009GL042316.
O’Gorman, P. A., , and T. Schneider, 2006: Stochastic models for the kinematics of moisture transport and condensation in homogeneous turbulent flows. J. Atmos. Sci., 63, 2992–3005.
O’Gorman, P. A., , and T. Schneider, 2007: Recovery of atmospheric flow statistics in a general circulation model without nonlinear eddy–eddy interactions. Geophys. Res. Lett., 34, L22801, doi:10.1029/2007GL031779.
O’Gorman, P. A., , and T. Schneider, 2008: The hydrological cycle over a wide range of climates simulated with an idealized GCM. J. Climate, 21, 3815–3832.
Pierrehumbert, R. T., , and R. Roca, 1998: Evidence for control of Atlantic subtropical humidity by large scale advection. Geophys. Res. Lett., 25, 4537–4540.
Pierrehumbert, R. T., , H. Brogniez, , and R. Roca, 2007: On the relative humidity of the atmosphere. The Global Circulation of the Atmosphere, T. Schneider and A. H. Sobel, Eds., Princeton University Press, 143–185.
Salathé E. P., Jr., , and D. L., Hartmann, 1997: A trajectory analysis of tropical upper-tropospheric moisture and convection. J. Climate, 10, 2533–2547.
Schneider, T., , and C. C. Walker, 2006: Self-organization of atmospheric macroturbulence into critical states of weak nonlinear eddy–eddy interactions. J. Atmos. Sci., 63, 1569–1586.
Schneider, T., , K. L. Smith, , P. A. O’Gorman, , and C. C. Walker, 2006: A climatology of tropospheric zonal-mean water vapor fields and fluxes in isentropic coordinates. J. Climate, 19, 5918–5933.
Schneider, T., , P. A. O’Gorman, , and X. Levine, 2010: Water vapor and the dynamics of climate changes. Rev. Geophys., 48, RG3001, doi:10.1029/2009RG000302.
Seidel, D. J., , Q. Fu, , W. J. Randel, , and T. J. Reichler, 2008: Widening of the tropical belt in a changing climate. Nat. Geosci., 1, 21–24.
Sherwood, S. C., 1996: Maintenance of the free-tropospheric tropical water vapor distribution. Part II: Simulation by large-scale advection. J. Climate, 9, 2919–2934.
Sherwood, S. C., , W. Ingram, , Y. Tsushima, , M. Satoh, , M. Roberts, , P. L. Vidale, , and P. A. O’Gorman, 2010a: Relative humidity changes in a warmer climate. J. Geophys. Res., 115, D09104, doi:10.1029/2009JD012585.
Sherwood, S. C., , R. Roca, , T. M. Weckwerth, , and N. G. Andronova, 2010b: Tropospheric water vapor, convection, and climate. Rev. Geophys., 48, RG2001, doi:10.1029/2009RG000301.
Smith, K. S., , G. Boccaletti, , C. C. Henning, , I. Marinov, , C. Y. Tam, , I. M. Held, , and G. K. Vallis, 2002: Turbulent diffusion in the geostrophic inverse cascade. J. Fluid Mech., 469, 13–48.
Sukhatme, J., , and W. R. Young, 2011: The advection-condensation model and water vapour PDFs. Quart. J. Roy. Meteor. Soc., 137, 1561–1572.
Thomson, D. J., 1987: Criteria for the selection of stochastic models of particle trajectories in turbulent flows. J. Fluid Mech., 180, 529–556.
Wright, J. S., , A. Sobel, , and J. Galewsky, 2010: Diagnosis of zonal mean relative humidity changes in a warmer climate. J. Climate, 23, 4556–4569.
The distance to saturation also plays a key role in the expressions for the mean flux (A1) and condensation rate (A2). In the limit of small distance to saturation, these only depend on local derivatives of the humidity fields (rather than also directly depending on the specific humidity at lower latitudes), consistent with less poleward influence, although these expressions do not make clear that poleward influence is completely absent in the limit of vanishing distance to saturation.
On some dry isentropes (particularly in the Northern Hemisphere in summer), there is more than one inflection point in a given hemisphere. We resolve the ambiguity by showing the inflection point corresponding to the maximum rate of poleward decrease in saturation specific humidity. We focus on the DJF season in which identification of the appropriate inflection point in the NH is relatively straightforward. It has previously been noted that the relative humidity minima in Earth’s atmosphere are close to the positions of maximum curvature of the zonal-mean dry isentropes (Sherwood et al. 2010b). These positions are not sufficiently different in Earth’s atmosphere from the positions given by our inflection point criterion to allow for a strong argument that either criterion is more accurate.