Recent change of the global monsoon precipitation (1979–2008)
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The global monsoon (GM) is a defining feature of the annual variation of Earth’s climate system. Quantifying and understanding the present-day monsoon precipitation change are crucial for prediction of its future and reflection of its past. Here we show that regional monsoons are coordinated not only by external solar forcing but also by internal feedback processes such as El Niño-Southern Oscillation (ENSO). From one monsoon year (May to the next April) to the next, most continental monsoon regions, separated by vast areas of arid trade winds and deserts, vary in a cohesive manner driven by ENSO. The ENSO has tighter regulation on the northern hemisphere summer monsoon (NHSM) than on the southern hemisphere summer monsoon (SHSM). More notably, the GM precipitation (GMP) has intensified over the past three decades mainly due to the significant upward trend in NHSM. The intensification of the GMP originates primarily from an enhanced east–west thermal contrast in the Pacific Ocean, which is coupled with a rising pressure in the subtropical eastern Pacific and decreasing pressure over the Indo-Pacific warm pool. While this mechanism tends to amplify both the NHSM and SHSM, the stronger (weaker) warming trend in the NH (SH) creates a hemispheric thermal contrast, which favors intensification of the NHSM but weakens the SHSM. The enhanced Pacific zonal thermal contrast is largely a result of natural variability, whilst the enhanced hemispherical thermal contrast is likely due to anthropogenic forcing. We found that the enhanced global summer monsoon not only amplifies the annual cycle of tropical climate but also promotes directly a “wet-gets-wetter” trend pattern and indirectly a “dry-gets-drier” trend pattern through coupling with deserts and trade winds. The mechanisms recognized in this study suggest a way forward for understanding past and future changes of the GM in terms of its driven mechanisms.
There is no climate variability that will impose greater impacts on society than changes in monsoon precipitation which exists as the life blood of about two-thirds of the world’s population. Precipitation also plays an essential role in determining atmospheric general circulation and hydrological cycle, and in linking external radiative forcing and the atmospheric circulation. In order to predict the future changes of the global monsoon precipitation (GMP), it is crucial to determine the response of the GMP to the recent global warming.
In physical essence, monsoon is a forced response of the climate system to annual variation of insolation. The annual variation of solar radiation is a fundamental driver and a necessary and sufficient condition for the existence of monsoon. The land–sea thermal contrast is critical for the location and strength of the monsoon but it is neither a necessary nor a sufficient condition. Based on this viewpoint, monsoon must be a global phenomenon. Trenberth et al. (2000) depicted the global monsoon (GM) as the global-scale seasonally varying overturning circulation throughout the tropics. Wang and Ding (2008) have demonstrated that the GM represents the dominant mode of the annual variation of the tropical precipitation and low-level winds, which defines the seasonality of Earth’s climate. Thus, the annual variation of all regional monsoons can be viewed as an integrated GM system. In fact, it has been well established that over the last glacial-interglacial transition, the observed changes in regional monsoons are evidently synchronized by known changes in orbital forcing (Kutzbach and Otto-Bliesner 1982; Liu et al. 2004). Such coordinated variations of regional monsoons have also recently shown to occur on centennial-millennial time scales (Liu et al. 2009).
However, studies of monsoon interannual-interdecadal variations have primarily focused on regional scales, e. g, South Asia (e.g., Webster et al. 1998), East Asia (e.g., Tao and Chen 1987), Australia (e.g., McBride 1987), Africa (e.g., Nicholson and Kim 1997), North America (e.g., Higgins et al. 2003), and South America (e.g., Zhou and Lau 1998). It has been a traditional notion that each regional monsoon has indigenous characteristics due to its specific land–ocean configuration and orography and due to differing feedback processes internal to the coupled climate system. Although linkages among various regional monsoons have been noticed (e.g., Meehl 1987; Wang et al. 2001; Lau and Weng 2002; Biasutti et al. 2003), the recent variations of monsoon precipitation have not been studied on a global scale over both land and ocean areas. Besides, prior to the satellite era beginning in 1979 (Wentz et al. 2007), our knowledge of global precipitation variation has been limited to land areas (Dore 2005; Zhang et al. 2007; Wang and Ding 2006). Hitherto little is known about how the GMP has responded to the last three-decade period of rapid global warming.
The present study deals with the GM, the integral of all regional monsoon systems. This broader perspective allows us to view changes in the total system due to anthropogenic effects, very large scale natural long-term oscillations and the influences of major interannual variations [e.g., El Niño-Southern Oscillation (ENSO)]. Only in the context of the GM, can a more holistic overview be made and new findings revealed. Changes in the regional monsoons (say over India) cannot be fully understood unless we considered within a global perspective.
The primary objective of the present study is to document interannual and decadal variations (or 30-year trends) of the GMP and to understand fundamental drivers for the GMP variability using modern global observation (1979–2008). Efforts are made to address the following specific questions: To what extent can the internal feedback processes drive global-scale monsoon variability? Are there any trends in global-scale monsoons during recent decades? What give rise to the trends, if any? Are there any differences between the northern hemisphere summer monsoon (NHSM) and southern hemisphere summer monsoon (SHSM) variability? What cause the differences, if any? What roles does GM change play in global precipitation change? These questions are discussed in Sects. 4 through 7.
2 Data and methodology
There are two global (land and ocean) precipitation datasets available: Global Precipitation Climatology Project (GPCP, Huffman et al. 2009) and Climate Prediction Center merged analysis of precipitation (CMAP, Xie and Arkin 1997). For description of climatology and interannual variations, an arithmetic mean of the two datasets was used. But to determine the long-term trend we primarily used GPCP data because CMAP is calibrated to atoll gauge data and this calibration may have introduced inhomogeneity in long-term changes (Yin et al. 2004). However, in the post-SSM/I (Special Sensor Microwave/Imager) era (i.e., after 1987) the two datasets tend to have consistent trends and interannual variation on global scale. The circulation data used are also an arithmetic mean dataset, based on National Centers for Environmental Prediction-Department of Energy reanalysis 2 (NCEP2, Kanamitsu et al. 2002) and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA) which is a combination of the 40-year reanalysis (ERA-40, Uppala et al. 2005) and Interim ERA (1989–2008). Monthly mean sea surface temperature (SST) data were obtained from the National Oceanic and Atmospheric Administration (NOAA) extended reconstructed SST (ERSST) version 3 (Smith et al. 2008). The data periods are from 1979 to 2009 with horizontal resolution of 2.5° except ERSST of which the grid spacing is 2.0°.
Two methods were used to test the significance of linear trends: The trend-to-noise ratio and the nonparametric Mann–Kendall rank statistics (Kendall 1955). The maximum covariance analysis (MCA) (Wallace et al. 1992; Ding et al. 2011) was used to identify important coupled modes of variability between GMP and underlying SST fields. The empirical orthogonal function (EOF) analysis is used to show the importance of the MCA mode.
3 Definition of GM domain and GMP intensity
In this paper, the GMP means the annual mean monsoon precipitation measured for each monsoon year. To reflect the monsoon strength, we use local summer monsoon rainfall, which provides a good measure of the annual range of precipitation because the annual range is dominated by local summer rainfall in monsoon regions. The NHSM precipitation intensity (NHMPI) can be measured by the total monsoon precipitation (or area averaged mean monsoon precipitation rate) in the NH monsoon domain during boreal summer (MJJAS). Similarly, the SHSM precipitation intensity (SHMPI) can be measured by the total monsoon precipitation (or area averaged mean monsoon precipitation rate) in the SH monsoon domain during austral summer (NDJFM). The GMP intensity (GMPI) is, then, defined by the sum of NHMPI and SHMPI.
4 Interannual variations of GMP
The spatial pattern of interannual variation of monsoon precipitation (Fig. 3a) is overwhelmed by suppressed rainfall in majority of regional monsoons except the southwestern Indian Ocean and the southern part of the South American monsoon. In particular, the suppressed precipitation dominates over nearly all continental monsoon regions. The overall drying pattern is associated with a warm phase of ENSO (Fig. 3b). During the development (boreal summer) and mature phases (austral summer) of El Niño episodes, the warming in the eastern-central Pacific displaces the North Pacific intertropical convergence zone and the South Pacific convergence zone equatorward, thereby directly reducing Australian and North American monsoon rainfall (Webster et al. 1998). The suppressed maritime continental precipitation induces anticyclonic anomalies in the off-equatorial South Asian summer monsoon region, suppressing South Asian monsoon rainfall (Wang et al. 2003). El Niño also reduces rainfall over continental Africa and northern South America through atmospheric teleconnections (Enfield 1996; Nicholson and Kim 1997).
Since ENSO is a consequence of the internal ocean–atmosphere feedback processes, the result here suggests that the regional monsoons can be coordinated not only by external forcing on centennial and orbital time scale but also, to a large extent, by internal feedback processes. Results in Fig. 3 also suggest that during the recent three decades, the NHSM is better coordinated by ENSO than the SHSM, because in the southern African and South American monsoon regions, the ENSO-induced precipitation anomalies tend to have a dipolar structure, rather than a uniform pattern (Fig. 3a).
5 The decadal variations and 30-year trends (1979–2008) of GMP
Monsoon precipitation intensity trends (in units of mm day−1 decade−1) and the statistical significance levels for the trend‐to‐noise ratio (T2N) and Mann–Kendall rank statistics (MK) tests
6 Cause of the recent GMP change
To understand the cause of the increasing trend in GMPI, a starting point is to consider the recent global warming effect, which increases moisture contents and hence enhances precipitation, including monsoon rainfall. However, the processes controlling monsoon rainfall change is far more complex than this simple thermodynamic argument. It is important to account for the spatial and temporal structure of the change.
Trends (1979–2008) of land-minus-ocean T2, NH-minus-SH T2, zonal SST gradient index (ZSSTGI), and Indo-Pacific zonal oscillation index (ZOI) and their confidence levels determined by the trend-to-noise ratio (T2N) test
Land-minus-ocean T2 (°C)
Hemispheric T2 (°C)
7 Impact of GMP amplification on global precipitation change
We have demonstrated that internal feedback processes such as ENSO can cause a coherent variation among various regional monsoons, especially over the continental monsoon regions (Fig. 3). Therefore, regional monsoons are coordinated not only by external forcing on the diurnal, annual (Wang and Ding 2008), orbital (Liu et al. 2004), and centennial-millennial (Liu et al. 2009) time scales, but also by internal feedback processes and atmospheric teleconnections. This result also implies that the change of ENSO properties in the past [such as a permanent El Niño reported by Wara et al. (2005)] may induce monsoon changes on the globe scale.
We also found that the intensification of the annual cycle of the earth’s climate system over the past 30 years is due primarily to the enhanced GMP, especially the NHSM (Figs. 4, 5). This poses a statistical perspective for climate models that are used to predict decadal variation and near-term projection planned for the IPCC Fifth assessment.
The intensification of the GMP originates primarily from an enhanced east–west thermal contrast in the Pacific Ocean, which is coupled with a rising pressure in the EP and decreasing pressure over the Indo-Pacific warm pool. This enhanced Pacific zonal thermal contrast tends to amplify both NHSM and SHSM. The enhanced Pacific zonal thermal contrast is largely a result of natural variability, i.e., the IPO.
Over the past three decades, the NHSM has a significant upward trend, while the SHSM trend is relatively weak. This difference cannot be explained by the east–west thermal contrast trend which is symmetric about the equator. Rather, it is caused by the relatively large warming trend in the NH, which creates a hemispheric thermal contrast that favors intensification of the NHSM but weakens the SHSM. The enhanced hemispherical thermal contrast is likely due to anthropogenic forcing.
We have shown that the wet-get-wetter and dry-gets-drier precipitation trend over the past 30 years is likely a result of the intensifying global summer monsoon precipitation (Figs. 10, 11) through a monsoon-desert coupling mechanism (Fig. 12). While this trend bears similarity to the precipitation trend pattern projected by the majority of the global climate models under increasing anthropogenic forcing, we should note that the observed “wet-gets-wetter” pattern is associated with enhanced Walker circulation, while the similar pattern found in the future projections is associated with weakening Walker circulation and mainly caused by increased water vapor (Rodwell and Hoskins 2001; Held and Soden 2006).
The findings described here are helpful in determining what may have occurred to the GM during the last period of secular warming in the mid-twentieth century (Polyakov et al. 2003). Furthermore, a comparison of the GM in the most recent period of warming with the mid-twentieth century warming may help determine the proportion of climate change that is attributable to anthropogenic effects and that is a part of long-term internal variability of the complex climate systems.
This work is supported by US NSF award #AGS-1005599 (BW) and NSF-ATM 0965610 (PJW), the National Research Programs of China (Award Nos. XDA05080800, 2010CB950102, 2011CB403301 and 2010CB833404) (JL and BW), and Environment Research and Technology Development Fund (A0902) of the Japanese Ministry of the Environment (HJK). BW and HJK acknowledge support from the International Pacific Research Center which is funded jointly by JAMSTEC, NOAA, and NASA. This is publication no. 8535 of the School of Ocean and Earth Science and Technology and publication no. 837 of the International Pacific Research Center.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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