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1982 JOURNAL OF CLIMATE VOLUME 22 Evaluating the Rich Get Richer Mechanism in Tropical Precipitation Change under Global Warming CHIA CHOU Research Center for Environmental Changes Academia Sinica and Department of Atmospheric Sciences National Taiwan University Taipei Taiwan J DAVID NEELIN Department of Atmospheric and Oceanic Sciences and Institute of Geophysics and Planetary Physics University of California Los Angeles Los Angeles California CHAO AN CHEN Research Center for Environmental Changes Academia Sinica Taipei Taiwan JIEN YI TU Department of Atmospheric Sciences Chinese Culture University Yang Ming Shan Taipei Taiwan Manuscript received 6 February 2008 in final form 24 October 2008 ABSTRACT Examining tropical 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 mechanism has 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 dynamic feedback is poorly understood and differs among models The argument outlined states that the thermodynamic component should be a good approximation for large scale averages this is confirmed for averages across convection zones and descent regions respectively Within the convergence zones however dynamic feedback 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 dynamic feedback is consistent with reduced gross moist stability due to increased moisture not being entirely compensated by effects of tropospheric warming and a 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 associated with a convergence zone shift 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 an 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 Corresponding author address Chia Chou Research Center for Environmental Changes Academia Sinica P O Box 1 48 Taipei 11529 Taiwan E mail chiachou rcec sinica edu tw DOI 10 1175 2008JCLI2471 1 2009 American Meteorological Society 15 APRIL 2009 CHOU ET AL 1 Introduction Predicting future temperature changes under global warming is a challenging task but predicting future precipitation changes may be even more difficult The agreement among climate model simulations on the spatial distribution of time mean precipitation changes tends 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 contribute moisture and energy budget analysis of balances and mechanisms contributing to such precipitation changes in the tropics Before addressing the specific questions associated with this we note that there are a number of aspects of precipitation change under global warming that are better documented A number of model studies indicate that increased precipitation intensity and decreased precipitation frequency will most likely occur associated with the warming recently Wilby and Wigley 2002 Trenberth et al 2003 Kharin and Zwiers 2005 Meehl et al 2005 Barnett et al 2006 Sun et al 2007 Evidence for such changes has also been sought in observations as reviewed in Trenberth et al 2007 Despite imperfect simulation of precipitation distributions in climate models e g Dai and Trenberth 2004 Dai 2006 Wilcox and Donner 2007 these effects are thought to be robust because of a simple underlying argument Moisture content available for extreme events tends to increase at a rate roughly governed by the Clausius Clapeyron equation while the energy available to drive convection for averages over sufficiently large scales that transports are negligible increases less quickly e g Allen and Ingram 2002 Meehl et al 2007 Increased precipitation at high latitudes and decreased precipitation in the subtropics is a common feature among climate models while in the annual average the deep tropics tend to have a precipitation increase Meehl et al 2007 Examining precipitation change averaged over latitudinal bands Zhang et al 2007 argued for detectable human impacts on precipitation with larger amplitudes in observations than in model simulations Even at regional scales a few areas exhibit precipitation change that is consistent among model simulations Christensen et al 2007 For instance relatively consistent projections are noted for twenty first century decreasing trends of precipitation in southern Europe Rowell and Jones 2006 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 region in summer Neelin et al 2006 where an observed trend is also noted 1983 In the tropics at large scales on the annual average Held and Soden 2006 hereafter HS06 sought robust features among climate models and found a weakening of the tropical circulation which tends to compensate the effect of the increased atmospheric moisture on tropical precipitation in convergence zones Chou et al 2007 studied hemispherical averages of tropical precipitation and found a widening of the seasonal


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