Changes in Zonal Surface Temperature Gradients and Walker Circulations in a Wide Range of Climates
Variations in zonal surface temperature gradients and zonally asymmetric tropical overturning circulations (Walker circulations) are examined over a wide range of climates simulated with an idealized atmospheric general circulation model (GCM). The asymmetry in the tropical climate is generated by an imposed ocean energy flux, which does not vary with climate. The range of climates is simulated by modifying the optical thickness of an idealized longwave absorber (representing greenhouse gases). The zonal surface temperature gradient in low latitudes generally decreases as the climate warms in the idealized GCM simulations. A scaling relationship based on a two-term balance in the surface energy budget accounts for the changes in the zonally asymmetric component of the GCM-simulated surface temperature. The Walker circulation weakens as the climate warms in the idealized simulations, as it does in comprehensive simulations of climate change. The wide range of climates allows a systematic test of energetic arguments that have been proposed to account for these changes in the tropical circulation. The analysis shows that a scaling estimate based on changes in the hydrological cycle (precipitation rate and saturation specific humidity) accounts for the simulated changes in the Walker circulation. However, it must be evaluated locally, with local precipitation rates. If global-mean quantities are used, the scaling estimate does not generally account for changes in the Walker circulation, and the extent to which it does is the result of compensating errors in changes in precipitation and saturation specific humidity that enter the scaling estimate.
© 2011 American Meteorological Society. Manuscript received 30 August 2010, in final form 9 March 2011. We thank Simona Bordoni, Isaac Held, Yohai Kaspi, Xavier Levine, and Paul O'Gorman for helpful discussions and technical assistance. The comments of two anonymous reviewers helped clarify the presentation of our work. This work was supported by a National Defense Science and Engineering Graduate Fellowship, a National Science Foundation Graduate Research Fellowship, and a David and Lucile Packard Fellowship. The GCM simulations were performed on Caltech's Division of Geological and Planetary Sciences Dell cluster. The program code for the simulations, based on the Flexible Modeling System of the Geophysical Fluid Dynamics Laboratory, as well as the simulation results themselves are available from the authors upon request.
Published - Merlis2011p15998J_Climate.pdf