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  • Monsoons in a changing world: A regional perspective in aglobal context

    Akio Kitoh,1 Hirokazu Endo,1 K. Krishna Kumar,2 Iracema F. A. Cavalcanti,3

    Prashant Goswami,4 and Tianjun Zhou5

    Received 15 September 2012; revised 8 February 2013; accepted 11 February 2013.

    [1] We provide a new view of global and regional monsoonal rainfall, and their changes inthe 21st century under RCP4.5 and RCP8.5 scenarios as projected by 29 climate modelsthat participated in the Coupled Model Intercomparison Project phase 5. The model resultsshow that the global monsoon area defined by the annual range in precipitation is projectedto expand mainly over the central to eastern tropical Pacific, the southern Indian Ocean,and eastern Asia. The global monsoon precipitation intensity and the global monsoon totalprecipitation are also projected to increase. Indices of heavy precipitation are projected toincrease much more than those for mean precipitation. Over the Asian monsoon domain,projected changes in extreme precipitation indices are larger than over other monsoondomains, indicating the strong sensitivity of Asian monsoon to global warming. Over theAmerican and African monsoon regions, projected future changes in mean precipitation arerather modest, but those in precipitation extremes are large. Models project that monsoonretreat dates will delay, while onset dates will either advance or show no change, resultingin lengthening of the monsoon season. However, models limited ability to reproduce thepresent monsoon climate and the large scatter among the model projections limit theconfidence in the results. The projected increase of the global monsoon precipitation can beattributed to an increase of moisture convergence due to increased surface evaporation andwater vapor in the air column although offset to a certain extent by the weakening of themonsoon circulation.

    Citation: Kitoh, A., H. Endo, K. Krishna Kumar, I. F. A. Cavalcanti, P. Goswami, and T. Zhou (2013), Monsoons in achanging world: A regional perspective in a global context, J. Geophys. Res. Atmos., 118, doi:10.1002/jgrd.50258.

    1. Introduction

    [2] The monsoon is conventionally defined as a seasonalreversal of prevailing surface winds, driven by the seasonalcycle of solar heating and difference in thermal inertias ofland and ocean that establish a land-sea temperaturedifference [Webster and coauthors, 1998]. This large-scalewind reversal creates favorable conditions for the occurrenceof convection in the summer hemisphere. As the monsoonseason matures, latent heat released by convection highabove the land surface helps to draw additional moisture overthe land from nearby oceans, maintaining the wet season.

    Monsoons are responsible for the majority of summer rainfallwithin the tropics, where billions of people depend on themonsoon-related rainfall. Patterns of projected global-scalechanges in the surface temperatures are robust among models,with greater warming over land than over ocean and at mosthigh latitudes [Meehl and coauthors, 2007]. Therefore, theland-sea temperature contrast is projected to become largerin summer, together with ample moisture available in awarming world, leading to a more intense monsoon. However,in spite of their common basic mechanism, monsoons overdifferent regions are subject to diverse forcing; thus theresponses of the various regional monsoons to a changingclimate are expected to vary [Meehl et al., 2007; Turner andAnnamalai, 2012].[3] The monsoon region is distributed over all the tropical

    continents, and over the tropical oceans in the western NorthPacific, eastern North Pacific, and the southern Indian Ocean[Wang and Ding, 2006]. Although the American monsoonsare not characterized by a seasonal reversal of wind, thereare large differences in precipitation, humidity, andatmospheric circulation between summer and winter [Veraand coauthors, 2006; Raia and Cavalcanti, 2008; Marengoand coauthors, 2010]. While different indices have beenused to identify the lifecycle of monsoons, the definition ofthe monsoon in terms of the precipitation characteristics[Wang and LinHo, 2002; Wang and Ding, 2006] rather than

    1Meteorological Research Institute, Tsukuba, Ibaraki, Japan.2Indian Institute of Tropical Meteorology, Pune, India.3Center for Weather Forecasting and Climate Studies (CPTEC), National

    Institute for Space Research (INPE), Cachoeira Paulista, SP, Brazil.4CSIR Centre for Mathematical Modelling and Computer Simulations

    (C-MMACS), Bangalore, India.5LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences,

    Beijing, China.

    Corresponding author: A. Kitoh, Climate Research Department, Meteoro-logical Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan.(kitoh@mri-jma.go.jp)

    2013. American Geophysical Union. All Rights Reserved.2169-897X/13/10.1002/jgrd.50258

    1

    JOURNAL OF GEOPHYSICAL RESEARCH: ATMOSPHERES, VOL. 118, 113, doi:10.1002/jgrd.50258, 2013

  • the wind reversal, permits the use of similar monsoon indi-ces for all the monsoon regions (keeping in mind that rainfallis the lifeline for billions of people living there). Thus, froma global point of view, precipitation characteristics overSouth Asia, East Asia, Australia, Africa, and the Americascan be viewed as an integrated global monsoon system, asso-ciated with a global-scale persistent atmospheric overturningcirculation [Trenberth et al., 2000]. Wang and Ding [2008]have demonstrated that the global monsoon is the dominantmode of the annual variation of the tropical circulation,characterizing the seasonality of the tropical climate.[4] Examination of historical precipitation records over

    the monsoon regions around the globe reveals a decreasingtrend in the global land monsoon precipitation over the lasthalf of the 20th century (19482003), with primary contribu-tions from a weakening of the summer monsoon systems inthe Northern Hemisphere (NH) [Wang and Ding, 2006;Ramesh and Goswami, 2007; Zhou et al., 2008b]. Changesin both the monsoon area and monsoon intensity contributeto this decreasing tendency [Zhou et al., 2008a]. However,the combined (oceanic and land) monsoon precipitationhas increased for the more recent 19792008 epoch mainlydue to a significant upward trend in the NH summermonsoon precipitation [Wang et al., 2012]. When thechange in monsoon-affected domain is considered, thefractional increase in monsoon area is greater than in totalprecipitation, so that the ratio of these two measures (whichserves as an index of the global monsoon intensity) exhibitsa decreasing trend for the most recent 30 year (19792008)period [Hsu et al., 2011].[5] Kim et al. [2008] investigated the reproducibility of

    global monsoon climate variability simulated by the coupledglobal climate models (CGCM) that participated in theCoupled Model Intercomparison Project phase 3 (CMIP3).They found that the CMIP3 CGCM multimodel ensemblesimulates a reasonably realistic climatology of the globalmonsoon precipitation and circulation, but models have com-mon biases such as a northeastward shift of the IntertropicalConvergence Zone (ITCZ) over the tropical North Pacific.Kim et al. [2008] also pointed out that models with higherresolution generally reproduced a better spatial pattern of theglobal monsoon precipitation compared to lower-resolutionmodels. Hsu et al. [2012] investigated global monsoonarea and precipitation in the late 21st century using threehigh-resolution atmospheric GCMs (40 km MPI ECHAM5,20 kmMRI/JMA, and 50 kmGFDLHiRAM) that were forcedby different future sea surface temperature (SST) warmingpatterns. They found a consistent increase in the globalmonsoon area, precipitation, and intensity among the modelprojections.[6] Recently, many climate modeling groups around

    the world have participated in the Coupled ModelIntercomparison Project phase 5 (CMIP5), under which aseries of experiments including the 20th century historicalsimulation and 21st century climate projections with fourdifferent representative concentration pathway (RCP)scenarios were performed [Taylor et al., 2012]. There isevidence that the CMIP5 models are better than the CMIP3models in simulating the Asian-Australian monsoon system.Several works [Li et al., 2012; Sperber et al., 2012] havereported that the CMIP5 multimodel mean is more skillfulthan the CMIP3 multimodel mean; however, the CMIP5

    models simulation of El Nio-Southern Oscillation-monsoonrelationships is still found to be poor (correlation is too weak).Lee and Wang [2012] and Hsu et al. [2013] found an increaseof global monsoon area and precipitation intensity under theRCP4.5 scenario in CMIP5 model simulations of futurechanges in the global monsoon. These results are essentiallyconsistent with CMIP3 model results [Hsu et al., 2012,2013] but with some regional differences [Lee and Wang,2012]. The objective of this paper is to provide an updatedview of global and regional monsoonal rainfall and changesin the 21st century as projected by 29 CMIP5 models underthe RCP4.5 and RCP8.5 scenarios: our analysis includes notonly mean precipitation but also some precipitation extremeindices and monsoon seasonality changes for seven landmonsoon regions across the world.

    2. Data

    2.1. Global Climate Model Data

    [7] Table 1 lists the models used in our analysis. We usedmonthly mean data from 29 CMIP5 models and dailyprecipitation data from 21 CMIP5 models. Results fromhistorical (19862005), RCP4.5 (20802099), and RCP8.5(20802099) experiments were investigated. Only oneensemble member from each model was used. The gridsquares where the land fraction exceeds 10% were regardedas land area in the analysis. Further information on theexperiments and the models is available from the CMIP5web site (http://cmip-pcmdi.llnl.gov/cmip5/).

    2.2. Observed Precipitation Data for Validation

    [8] We used Climate Predic

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