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Global Summer Monsoon Rainy Seasons INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. (2008) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/joc.1659 Global summer monsoon rainy seasons Suping Zhanga*andBinWanga,b a Physical Oceanography Laboratory and Ocean – Atmosphere Interaction and Climate Laboratory, Ocean University of China, Qingdao, China b Department of Meteorology and International Pacific Research Institute, School of Ocean and Earth Science and Technology, University of Hawaii, Hawaii ABSTRACT: A concise and objective definition of monsoon rainy season characteristics is proposed for worldwide monsoon regions. The result highlights six major summer monsoon rainy season domains and the mean dates of the local onset, peak and withdrawal phases of the summer monsoon rainy season. The onset phases occur progressively later poleward in the continental domains but primarily eastward in the oceanic monsoon regions. The rainy season retreats equatorward over the continental and oceanic monsoon regions. The length of the rainy season decreases poleward and shorter rainy season can also be found over the outskirts of warm water. Some exceptions exist in terms of the characteristics of rainy season, e.g. the westward advance of rainy season over North Africa and an apparently prolonged rainy season in the Korean peninsula. The results here are basically compatible with those obtained in previous studies on regional monsoons. A definition of the seasonal wind overturning is proposed. Combining rainfall and winds, we stratify the global monsoon into strong and weak categories. The strong monsoons are typically in the regions with both concentration of summer rainfall and annual reversal of low-level winds, while the weak monsoon features only a contrasting wet–dry season. Seemingly, some mid-latitude regions with wind reversals are not monsoonal because of the reversals being opposite to the monsoon overturning and the rainfall patterns being more or less Mediterranean. The comparison between the monsoon domains derived from Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) and the 40-year European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40), the Japanese 25-year Reanalysis (JRA-25) and National Centers for Environmental Prediction (NCEP) datasets show good capabilities of the reanalyses in demarcation of the major monsoon rainy season domains in the tropics and subtropics. But the reanalyses are less realistic in the mid-latitudes of Eurasia and North America. The result here provides the simple yet objective definitions of monsoon domain, onset, peak and withdrawal which are useful for validation of GCMs. Copyright 2008 Royal Meteorological Society KEY WORDS global monsoon rainy season; domain; onset; withdrawal Received 22 July 2006; Revised 8 October 2007; Accepted 31 October 2007 1. Introduction In fact, monsoon distribution indicates the location of the atmospheric heat source that drives tropical circu- Monsoon covers both oceans and continents. The term lation; and thus, rainfall variation reflects the variabil- ‘monsoon’ originally stems from the Arabic word ity of the entire monsoon circulation system (Wang and ‘mausim’, meaning seasonal prevailing winds (www.met Ding, 2006). office.gov.uk). That is why early studies dealt with mon- Monsoon arises from the annual reversals in heating soon in terms of wind frequency (e.g. Ramage, 1971). and temperature gradients between continental regions Nowadays, monsoon is more generally applied to tropical and the adjacent oceans with the progression of the and subtropical seasonal reversals in both the atmospheric seasons (Trenberth et al., 2006). Trenberth and his col- circulation and associated precipitation (Trenberth et al., laborators have depicted the seasonal variations of the 2000; Trenberth et al., 2006). People are more concerned about the precipitation not only because natural disasters precipitation–evaporation and the associated transport of like flooding and drought that affect the planting, har- the latent energy. They pointed out the importance of vesting, economy and casualty of life are tightly linked treating monsoon in a global perspective from the con- to monsoon rainfall, but also because the huge amount servation of water and energy (Trenberth et al., 2000; of latent heat released in the process of precipitation Trenberth et al., 2006). Wang and Ding (2006) further plays an important part in the global diabatic heating. defined the global monsoon summer rainy season domain in terms of objective thresholds depicting the strength and seasonal distribution of precipitation. They also pro- * Correspondence to: Suping Zhang, Physical Oceanography Labora- posed a metrics to measure the variability of the global tory and Ocean – Atmosphere Interaction and Climate Laboratory, Ocean University of China, Qingdao, PRC. 266003, China. monsoon strength and found that in the last 25 years, E-mail: [email protected] the global continental monsoon precipitation has been Copyright 2008 Royal Meteorological Society S. ZHANG AND B. WANG levelled off whereas the oceanic monsoon precipitation we elaborate on the global summer monsoon regimes. has strengthened. And the results derived from different datasets are com- So far, majorities of the literatures on monsoon rain- pared. The last section summarizes our conclusions. fall climatological characteristics have been confined to regional monsoons, such as the Indian monsoon (e.g. Rao, 1976; Lau and Yang, 1997), the East Asian monsoon 2. Data (e.g. Chen et al., 1991), the Indonesian and Australian The primary datasets analysed in this study include monsoon (e.g. Wang et al., 2003; Wheeler and McBride, the Climate Prediction Center (CPC) Merged Analy- 2005), the South African monsoon (e.g. Cook, 2000), the sis of Precipitation (CMAP) (Xie and Arkin, 1997), South American monsoon (e.g. Rao et al., 1996; Charles reanalysis data from the National Centers for Envi- et al., 2002), the West African monsoon (e.g. Grodsky ronmental Prediction–National Center for Atmospheric and Carton, 2001; Sultan and Janicot, 2003a) and the Research (NCEP–NCAR) (Kalnay et al., 1996), the North American monsoon (e.g. Adams and Comrie, 1997; 40-year European Centre for Medium-Range Weather Anderson et al., 2000). Wang and LinHo (2002) pro- Forecasts reanalysis (ERA-40) (Simmons and Gibson, posed a set of simple objective criteria to describe the 2000) and the Japanese 25-year reanalysis (JRA-25) Asian summer monsoon (ASM) rainy seasonal character- (Sakamoto et al., 2007) monthly mean data. The CMAP istic and link the continental monsoon with the adjacent is derived by merging rain gauge observations, five satel- oceanic monsoon regions. While Wang (1994) delin- lite estimates and numerical model outputs. The CMAP eated the global tropical monsoon regimes by outgoing climatological pentad (5-day) mean (CPM) and monthly longwave radiation (OLR) and highly reflective clouds mean precipitation rate values are constructed from 1979 (HRC), our knowledge on global monsoon rainy season to 2004 with global coverage 88.75 °N to 88.75 °Sand characteristics has long been falling short due to lack of 1.25 °E to 358.75 °E(eastward)on2.5 × 2.5 grids. The long-term reliable data, especially the observations over NCEP and ERA-40 monthly mean data covers the same the monsoon ocean regions. grid points 144 × 73 on 2.5 × 2.5 geographical degrees, Because it has not been possible, in the past, to obtain and the JRA-25 are gridded by 320 × 160. a reliable precipitation over the ocean, a variety of proxy Over the ocean, the accuracy and reliability of the pen- variables and ways have been used to define monsoon tad mean CMAP on small spatial scales has not been fully domain as well as its onset, peak and withdrawal phases, determined because of the lack of ground truth obser- thus, leading to different results. The discrepancies are vations. Since the CMAP dataset merges multi-source already significant when we merely talk about the Asian estimates, the uncertainties contained in each individual monsoon as discussed in detail in Wang and LinHo estimate are significantly reduced. A comparative study (2002). So, it is necessary to develop a universal defi- of CMAP and land-based rain gauge data shows that the nition of monsoon and criteria for quantitative descrip- two datasets yield similar large-scale pictures over land tion of rainfall climatology in the entire global monsoon areas (Wang and LinHo, 2002). Thus, it is meaningful to regime. describe large-scale rainfall characteristics using CMAP. One of the objectives of this study is to extend the work The reconstructed climatological pentad mean (RCPM) of Wang and LinHo (2002) to investigate the feasibility rainfall time series resulting from Fourier truncation with of using a concise yet pertinent rainfall parameter and the sum of the first 12 harmonics (period longer than a set of universal criteria to quantify rainy season char- 1 month), and consisting of both the seasonal and sub- acteristics for all monsoon regions worldwide. Another seasonal signals and removing the sub-monthly vibration, purpose is to use the precipitation data derived from satel- is best suited for our purpose. In the following analysis lite observations over the oceans along with rain gauge of rainfall, we will use the reconstructed pentad rainfall observations over the continent to build up a unified pic- data series to define the onset, peak and withdrawal of ture of the spatial and temporal structures of the mean
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