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Evaluating the ‘‘Rich-Get-Richer’’ Mechanism in Tropical PrecipitationChange under Global WarmingCHIA CHOUResearch Center for Environmental Changes, Academia Sinica, and Department of Atmospheric Sciences,National Taiwan University, Taipei, TaiwanJ. DAVID NEELINDepartment of Atmospheric and Oceanic Sciences and Institute of Geophysics and Planetary Physics,University of California, Los Angeles, Los Angeles, CaliforniaCHAO-AN CHENResearch Center for Environmental Changes, Academia Sinica, Taipei, TaiwanJIEN-YI TUDepartment of Atmospheric Sciences, Chinese Culture University, Yang-Ming Shan, Taipei, Taiwan(Manuscript received 6 February 2008, in final form 24 October 2008)ABSTRACTExamining tropical regional precipitation anomalies under global warming in 10 coupled global climatemodels, several mechanisms are consistently found. The tendency of rainfall to increase in convergence zoneswith large climatological precipitation and to decrease in subsidence regions—the rich-get-richer mechanism—has previously been examined in different approximations by Chou and Neelin, and Held and Soden. Theeffect of increased moisture transported by the mean circulation (the ‘‘direct moisture effect’’ or ‘‘thermo-dynamic component’’ in respective terminology) is relatively robust, while dynamic feedback is poorly un-derstood and differs among models. The argument outlined states that the thermodynamic component shouldbe a good approximation for large-scale averages; this is confirmed for averages across convection zones anddescent regions, respectively. Within the convergence zones, however, dynamic feedback can substantiallyincrease or decrease precipitation anomalies. Regions of negative precipitation anomalies within the con-vergence zones are associated with local weakening of ascent, and some of these exhibit horizontal dryadvection associated with the ‘‘upped-ante’’ mechanism. Regions of increased ascent have strong positiveprecipitation anomalies enhanced by moisture convergence. This dynamic feedback is consistent with re-duced gross moist stability due to increased moisture not being entirely compensated by effects of tropo-spheric warming and a vertical extent of convection. Regions of reduced ascent with positive precipitationanomalies are on average associated with changes in the vertical structure of vertical velocity, which extendsto higher levels. This yields an increase in the gross moist stability that opposes ascent. The reductions inascent associated with gross moist stability and upped-ante effects, respectively, combine to yield reducedascent averaged across the convergence zones. Over climatological subsidence regions, positive precipitationanomalies can be associated with a convergence zone shift induced locally by anomalous heat flux from theocean. Negative precipitation anomalies have a contribution from the thermodynamic component but can beenhanced or reduced by changes in the vertical velocity. Regions of enhanced subsidence are associated withan increased outgoing longwave radiation or horizontal cold convection. Reductions of subsidence are as-sociated with changes of the vertical profile of vertical velocity, increasing gross moist stability.Corresponding author address: Chia Chou, Research Center for Environmental Changes, Academia Sinica, P.O. Box 1–48, Taipei11529, Taiwan.E-mail: [email protected] JOURNAL OF CLIMATE VOLUME 22DOI: 10.1175/2008JCLI2471.1Ó 2009 American Meteorological Society1. IntroductionPredicting future temperature changes under globalwarming is a challenging task, but predicting futureprecipitation cha nge s m ay be eve n more di fficult. Theagreement among climate model simulations on thespatial distribution of time mean precipitation changestends to be very poor, especially at a regional scale(e.g., Cubasch et al. 2001; Allen and Ingram 2002;Stott and Kettleborough 2002; Neelin et al. 2006;Meehl et al. 2007). This paper aims to contributemoisture and energy budget analysis of balances andmechanisms contributing to such precipitation changesin the tropics.Before addressing the specific questions associatedwith this, we note that there are a number of aspects ofprecipitation change under global warming that arebetter documented. A number of model studies indicatethat increased precipitation intensity and decreasedprecipitation frequency will most likely occur associatedwith the warming (recently, Wilby and Wigley 2002;Trenberth et al. 2003; Kharin and Zwiers 2005; Meehlet al. 2005; Barnett et al. 2006; Sun et al. 2007). Evidencefor such changes has also been sought in observations,as reviewed in Trenberth et al. (2007). Despite imper-fect simulation of precipitation distributions in climatemodels (e.g., Dai and Trenberth 2004; Dai 2006; Wilcoxand Donner 2007), these effects are thought to be robustbecause of a simple underlying argument. Moisture con-tent available for extreme events tends to increase at arate roughly governed by the Clausius–Clapeyron equa-tion, while the energy available to drive convection (foraverages over sufficiently large scales that transports arenegligible) increases less quickly (e.g., Allen and Ingram2002; Meehl et al. 2007).Increased precipitation at high latitudes and decreasedprecipitation inthe subtropics is a common featureamongclimate models, while in the annual average the deeptropics tend to have a precipitation increase (Meehl et al.2007). Examining precipitation change averaged overlatitudinal bands, Zhang et al. (2007) argued for detect-able human impacts on precipitation, with larger ampli-tudes in observations than in model simulations. Even atregional scales, a few areas exhibit precipitation changethat is consistent among model simulations (Christensenet al. 2007). For instance, relatively consistent projectionsare noted for twenty-first-century decreasing trends ofprecipitation in southern Europe (Rowell and Jones2006), southwestern North America (Milly et al. 2005;Seager et al. 2007), parts of Southeast Asian dry seasons(Li et al. 2007), and the Caribbean–Central America re-gion in summer (Neelin et al. 2006) where an observedtrendisalsonoted.In the tropics, at large scales on the annual average,Held and Soden (2006, hereafter HS06) sought robustfeatures among climate models and found a weakeningof the tropical circulation, which tends to compensate theeffect of the increased atmospheric moisture on tropicalprecipitation in convergence zones.


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