Chia Chou, J. David Neelin, Chao-An Chen and Jien-Yi Tu:
J. Climate, revised.
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Abstract. Examining regional precipitation anomalies under global warming in 10 coupled global climate models, several mechanisms are consistently found. The tendency of rainfall to increase in convergence zones with large climatological precipitation, and to decrease in subsidence regions---the rich-get-richer mechanismhas previously been examined in different approximations by Chou and Neelin and Held and Soden. The effect of increased moisture transported by the mean circulation (the direct moisture effect or thermodynamic component in respective terminology) is relatively robust, while dynamical feedbacks are poorly understood and differ among models. An argument is outlined that the thermodynamic component should be a good approximation for large-scale averages, and this is confirmed for averages across convection zones and descent regions, respectively. Within the convergence zones, however, dynamical feedbacks can substantially increase or decrease precipitation anomalies. Regions of negative precipitation anomalies within the convergence zones are associated with local weakening of ascent, and some of these exhibit horizontal dry advection associated with the ``upped-ante" mechanism. Regions of increased ascent have strong positive precipitation anomalies enhanced by moisture convergence. This dynamical feedback is consistent with reduced gross moist stability due to increased moisture not being entirely compensated by effects of tropospheric warming and vertical extent of convection. Regions of reduced ascent with positive precipitation anomalies are on average associated with changes in the vertical structure of vertical velocity, which extends to higher levels. This yields an increase in the gross moist stability that opposes ascent. The reductions in ascent associated with gross moist stability and upped-ante effects, respectively, combine to yield reduced ascent averaged across the convergence zones. Over climatological subsidence regions, positive precipitation anomalies can be induced locally by anomalous heat flux from the ocean. Negative precipitation anomalies have a contribution from the thermodynamic component but can be enhanced or reduced by changes in the vertical velocity.Regions of enhanced subsidence are associated with increased outgoing longwave radiation or horizontal cold convection. Reductions of subsidence are associated with changes of the vertical profile of vertical velocity, increasing gross moist stability.
Citation. Chou, C., J. D. Neelin, C.-A. Chen and J.-Y. Tu: Evaluating the rich-get-richer mechanism in tropical precipitation change under global warming. J. Climate, revised.
Acknowledgments. This work was supported under National Science Council grant 95-2111-M-001-001 and National Science Foundation grant ATM-0645200 (JDN).We acknowledge the modeling groups for providing their data, PCMDI at Lawrence Livermore National Laboratories for collecting the model data, the JSC/CLIVAR Working Group on Coupled Modelling (WGCM) for organizing the model analysis activity, and the IPCC WG1 TSU for technical support. The IPCC Data Archive at Lawrence Livermore National Laboratory is supported by the Office of Science, U.S. Department of Energy.`