A projection of future changes in summer precipitation and monsoon ...

SCIENCE CHINA Earth Sciences • RESEARCH PAPER •

February 2010 Vol.53 No.2: 284–300 doi: 10.1007/s11430-009-0123-y

A projection of future changes in summer precipitation and monsoon in East Asia SUN Ying* & DING YiHui National Climate Center, Beijing 100081, China Received September 4, 2008; accepted March 2, 2009; published online December 19, 2009

The future potential changes in precipitation and monsoon circulation in the summer in East Asia are projected using the latest generation of coupled climate models under Intergovernmental Panel on Climate Change (IPCC) Special Report on Emission Scenarios (SRES) A1B scenario (a medium emission scenario). The multi-model ensemble means show that during the period of 2010–2099, the summer precipitation in East Asia will increase and experience a prominent change around the 2040s, with a small increase (~1%) before the end of the 2040s and a large increase (~9%) afterward. This kind of two-stage evolution characteristic of precipitation change can be seen most clearly in North China, and then in South China and in the mid and lower Yangtze River Valley. In 2010–2099, the projected precipitation pattern will be dominated by a pattern of “wet East China” that explains 33.6% of EOF total variance. The corresponded time coefficient will markedly increase after the 2040s, indicating a great contribution from this mode to the enhanced precipitation across all East China. Other precipitation patterns that prevail in the current climate only contribute a small proportion to the total variance, with no prominent liner trend in the future. By the late 21st century, the monsoon circulation will be stronger in East Asia. At low level, this is due to the intensification of southwesterly airflow north of the anticyclone over the western Pacific and the SCS, and at high level, it is caused by the increased northeasterly airflow east of the anticyclone over South Asia. The enhanced monsoon circulation will also experience a two-stage evolution in 2010–2099, with a prominent increase (by ~0.6 m s−1) after the 2040s. The atmospheric water vapor content over East Asia will greatly increase (by ~9%) at the end of 21st century. The water vapor transported northward into East China will be intensified and display a prominent increase around the 2040s similar to other examined variables. These indicate that the enhanced precipitation over East Asia is caused by the increases in both monsoon circulation and water vapor, which is greatly different from South Asia. Both the dynamical and thermal dynamic variables will evolve consistently in response to the global warming in East Asia, i.e., the intensified southwesterly monsoon airflow corresponding to the increased water vapor and southwesterly moisture transport. East Asian summer monsoon, precipitation, climate models, future projection

Citation:

Sun Y, Ding Y H. A projection of future changes in summer precipitation and monsoon in East Asia. Sci China Earth Sci, 2010, 53: 284–300, doi: 10.1007/s11430-009-0123-y

In East China, the East Asian summer monsoon plays an important role in the occurrence of summer precipitation. In the past 50 years, the East Asian summer monsoon has experienced prominent changes, with a transformation towards the reduced southwesterly airflow and weaker north-

ward moisture transport [1–4]. The precipitation distribution has turned to a pattern of “dry North and wet South”, i.e., below-normal precipitation in North China and abovenormal precipitation in South China, from that of “dry South and wet North” [5–7], exerting important impacts on the economic and social development in East Asia. In the future 100 years, many issues related to these changes, such

*Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2009

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as: what change will happen in precipitation and monsoon circulation, whether or not the major rainfall pattern will differ from the current one, whether or not the circulation in East Asia will be stronger, etc., are of great significance to the sustainable development of the society. The study on these issues not only can provide useful information for the scientific community but also can serve as the important scientific basis for policy-making in disaster prevention and relief. In early studies about future changes in East Asian summer monsoon and precipitation, the simple climate models were often used to make the idealized experiments due to greenhouse gases increase. Some authors have suggested different findings [8–12] about the future change of precipitation in East Asia because different models and methods were used. For example, Hulme et al. [8] showed that the CO2 doubling may result in the increase of precipitation in East Asia. However, Giorgi and Francisco [9] calculated the multi-model ensemble mean based on the IS92a transient experiments and suggested that the precipitation in East Asia may not change in the future when only greenhouse gases (no aerosols) were used as the driven factor in the models. Later on, Hu et al. [10] used the 16 climate models ensemble from CMIP2 and showed that the summer precipitation may increase when the CO2 is doubled, but also with a large inter-model difference. In recent years, with better understanding of climate system and continuous improvement of climate models, the implementations of many international model inter-comparison programs provide a great opportunity for the systematic analyses of summer monsoon and precipitation changes in East Asia. Based on different global climate models, many studies consistently show that in East Asia, both the temperature and precipitation will increase in the future [13–16]. The latest studies [17] and IPCC AR4 [18] further indicate that this result owns a good agreement among the newest generation of climate models. In addition, the results from the regional climate models have generally demonstrated that the CO2 doubling may cause the increases of the precipitation in South China and South Japan, and the number of rainy days in South China [19, 20]. Most studies on the future changes in monsoon circulation are mainly focused on the analyses of the change in South Asian summer monsoon. With the global warming, the temperature increase over land will be more rapid than that over the oceans, and the continental-scale land-sea thermal contrast will become larger in summer and smaller in winter. Thus, it follows that the summer monsoon will be stronger and the winter monsoon weaker in the future. However, the model results are not as straightforward as this, and the current models show that the intensity of South Asian monsoon will be weaker [18]. Therefore, some authors explored the reason behind this difference and pointed out that more precipitation for the South Asian monsoon results mainly from the atmospheric moisture buildup due to

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increased greenhouse gases and consequent temperature increase and a larger moisture flux, despite weakening of the dynamical monsoon circulation [21–23]. For example, Udea et al. [24] suggested that the increase of precipitation in South Asian monsoon regions is mainly caused by the enhanced moisture transport into the Asian monsoon regions. On the other hand, the change of East Asian summer monsoon circulation has not been well studied. Kimoto [25] showed that the precipitation increase along the Meiyu-Changma-Baiu rain belt in the future may be due to the intensification of anticyclonic circulations located in the south and north of the rain belt. Kurihara et al. [26] indicated that the frequent occurrence of El Niño-like sea surface temperature (SST) pattern may also result in the precipitation increase in East Asia since this pattern can cause the intensification of western Pacific subtropical high south of Japan. Based on these studies, although the change of summer precipitation in East Asia by the late 21st century has been investigated, few studies have focused on the discussion of the temporal evolution of precipitation in different areas of East Asia and the future change of major precipitation patterns in East Asia. Some unique issues in East Asia have not been addressed, such as: how the precipitation evolution in East Asia is related to the change of monsoon, how the key dynamic and thermal dynamic variables evolve in response to the global warming, and whether or not these characteristics are different from South Asia. The discussion of these issues not only can help further understand the evolution feature of climate system in East Asia and the relationships between the monsoon circulation and precipitation under the global warming but also is crucial to the policy making associated with climate change in this region. Thus, we investigate these issues based on the newest generation of climate models. In this paper, section 1 introduces data and calculating methods used in the study; section 2 briefly states the models simulation of climatic mean precipitation in East Asia; sections 3 to 5 examine the changes in precipitation, large-scale monsoon circulation, and water vapor in East Asia respectively; and sections 6 and 7 present discussions and conclusions.

1

Data and calculation methods

The data used in the study include the monthly data from 19 climate models (Table 1) for 2010–2099 under the IPCC SRESA1B scenario [27]. These models represent the newest generation of current climate models commonly used in the scientific community and have been substantially improved in the past five years compared with those used in the 3rd assessment report of IPCC. There have been ongoing improvements to resolution, computational methods, and parameterization, and additional processes (e.g., interactive

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Information of 19 climate models examined in the study Model

1 2 3 4 5 6 7 8 9 10 11

CGCM3.1 (T47) CGCM3.1 (T63) CNRM-CM3 CSIRO GFDL-CM2.0 GFDL-CM2.1 GISS-EH GISS-ER FGOALS-g1.0 INM-CM3.0 IPSL-CM4

12

MIROC3.2 (medres)

13

MIROC3.2 (hires)

14 15 16 17 18 19

ECHAM5/MPI-OM MRI-CGCM2.3.2 CCSM3 PCM UKMO-HadCM3 UKMO_hadgem1

Center, Country Canadian Centre for Climate Modeling & Analysis, Canada Canadian Centre for Climate Modeling & Analysis, Canada Météo-France/Centre National de Recherches Météorologiques, France CSIRO Atmospheric Research, Australia US Dept. of Commerce/NOAA/Geophysical Fluid Dynamics Laboratory, United States US Dept. of Commerce/NOAA/Geophysical Fluid Dynamics Laboratory, United States NASA/Goddard Institute for Space Studies, United States NASA/Goddard Institute for Space Studies, United States LASG/Institute of Atmospheric Physics, China Institute for Numerical Mathematics, Russia Institut Pierre Simon Laplace, France Center for Climate System Research (The University of Tokyo), National Institute for Environmental Studies, and Frontier Research Center for Global Change (JAMSTEC), Japan Center for Climate System Research (The University of Tokyo), National Institute for Environmental Studies, and Frontier Research Center for Global Change (JAMSTEC), Japan Max Planck Institute for Meteorology, Germany Meteorological Research Institute, Japan National Center for Atmospheric Research, United States National Center for Atmospheric Research, United States Hadley Centre for Climate Prediction and Research/Met Office, United Kingdom Hadley Centre for Climate Prediction and Research/Met Office, United Kingdom

aerosols) have been included in these climate models. Most models no longer use the flux adjustments, which were previously required to maintain a stable climate. Also, the experiments design for future climate change is more reasonable in order to provide the most comprehensive multimodel perspective on climate change [18, 28]. The more detailed description about the models can be found at http://www-pcmdi.llnl.gov/ipcc/about_ipcc.php and the information regarding the precipitation parameterization in the models is given in Sun et al. [29] and Dai [30]. For the convenience of analyses, all the model results are interpolated to the grid point of 2.5°×2.5°. At the same time, the monthly gridded data (2.5°×2.5°) from the 19 climate models and from GPCP (Global Precipitation Climatology Project, version 2) [31] for 1979–1999 are also used to analyze the model’s performance in simulating the current climate. All the data used for the analyses of temporally evolving series in 2010–2099 are smoothed by a 9-year running mean. There is no total column water vapor content (TCWV) available in the models UKMO-HadCM3 and UKMOHadGem1, so the multi-model ensemble of TCWV in Section 5 does not include the information from these two models. In this study, only the future change under SRESA1B scenario is discussed while the changes under other scenarios will not be analyzed. The East China region (22.5°–45°N，110°–120°E) is chosen as the targeted study area in East Asia, with three key areas specified: South China (22.5°–27.5°N, 110°–120°E), the medium and low reaches of the Yangtze

Atmospheric Resolution (long. × lat.) 3.75°×~3.75° 2.8°×~2.8° 2.8°×~2.8° 1.88°×~1.88° 2.5°×2.0° 2.5°×2.0° 5°×4° 5°×4° 2.8°×~2.8° 5°×4° 3.75°×2.5° 2.8°×~2.8° 1.125°×~1.12° 1.88°×~1.88° 2.8°×~2.8° 1.4°×~1.4° 2.8°×~2.8° 1.25°×1.25° 1.875°×1.25°

River Valley (the YRV, 27.5°–35°N, 110°–120°E), and North China (35°–45°N, 110°–120°E). The three periods are also selected: 2010–2099, 2030–2039 and 2080–2099, to analyze the geographical distribution of variables from the perspectives of near- and long-term change. The method of multi-model ensemble mean is used to analyze the future changes of all the variables. Many studies have shown that the performance of current climate models in simulating the intedecadal variations of East Asian summer monsoon is still unsatisfied and need to be greatly improved [32]. In this sense, the methods of ensembles of weighting models or well-performed models are not appropriate to utilize in this paper. Instead, we presume the output from each individual model as a possible projection and will be used as a member of multi-model arithmetic ensemble mean. The recent findings [33] also indicate that the multi-model ensemble mean holds a higher credibility than the individual model in reproducing the climatic annual and seasonal average precipitation and sea level pressure (associated with the monsoon change) in East Asia. In this paper, the analyses are mainly focused on the discussion of multi-model ensemble means, while the issues regarding the uncertainties are not examined in detail. The uncertainties from the emission scenarios and climate models still maintain as the important resources of future projection uncertainty. However, with the continuous improvements of climate models and other methods, the model results can be used as an important basis for the credible projections, the climate study science, and the relevant policy

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making issues.

2 East Asian summer precipitation as simulated by global climate models In recent 20 years, the substantial progress has been made in the climate model developments. In East Asia, the simulated 20-year mean June-July-August (JJA) precipitation from multi-model ensemble is similar to that from GPCP data (Figure 1). The models are able to simulate the gradual decrease of precipitation from eastern to western China, with a reasonable precipitation amount in terms of magnitude. In the eastern flank of the Tibetan Plateau, i.e., in central and western China, the false precipitation center still can be seen due to the impacts of high terrain. However, in eastern China, where the East Asian monsoon prevails in the sum-


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mer, the multi-year JJA mean precipitation can be simulated quite well. The average correlation coefficient between 19 models and GPCP observation are 0.61, in which seven models correlations with GPCP are above 0.70 [32]. This implies that the new generation of climate models owns a reasonably well performance in simulating the precipitation and circulation in East Asia and can be used to project their changes in the future.

3 Projected changes in summer precipitation in East Asia Figure 2 shows the JJA precipitation percentage change in 2010–2099 based on 19-model ensemble mean in East China and three key areas: South China, the YRV and North China. Overall, the precipitation will increase in the future

Figure 1 1979–1999 June-July-August (JJA) mean precipitation (mm d−1)based on GPCP (a) and 19-model ensemble mean (b). Shaded areas in (b) indicate the model ensemble mean and contours are the difference between the model ensemble and GPCP. From Sun and Ding [32].

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Figure 2 Percentage changes (%, relative to 1980–1999 average) of JJA precipitation for 2010–2099 averaged in East China (a), South China (b), the YRV (c) and North China (d).

100 years. An interesting feature is that a prominent change around the end of the 2040s can be seen in the precipitation changes in all the areas, with a small increase before the 2040s and a large increase afterward. This feature can be seen most clearly in North China, and then in South China and the YRV. It will be termed as “a two-stage evolution” hereafter. In the YRV, there will be another fluctuation around 2075 but not as clear as the one around the 2040s. By the end of 21st century, the precipitation will increase by ~9% for the average of all East China. All the regions in East China will move into a wet period, with more precipitation than the present. Compared with the previous findings, the present study shows a new feature of precipitation change in East Asia: the two-stage evolution in all the areas under SRESA1B scenario. However, this kind of feature is not seen in other areas, and thus may indicate a unique regional response feature in East Asia. On the other hand, the percentage increase (~9%) of precipitation in East China is larger than the global average (1%–2%), which is associated with a large amount of moisture convergence in this region in the future. In order to better understand the model difference on the above-described feature, Figure 3 displays the linear trends of precipitation changes in 2010–2099 in East China and three key areas for 19 individual models. Most of models consistently show a positive linear trend in these areas. In

South China, out of 19 models under examination, 15 have a positive trend and four have a negative trend. In the YRV, North China, and all East China, except two models, the rest 17 display a positive trend. There are 12 models that demonstrate a consistent positive trend in East China and all three key areas. This indicates that the climate models in general project an increase of precipitation in East Asia and the inter-model difference is very small. The two-stage evolution feature in 19 individual models is also further examined in Table 2. The method of running-t test is used to define the time of precipitation abrupt increase (with the significance level above 95%) in the models. In both South China and the YRV, out of 19 models, 10 show an abrupt increase of precipitation in 2010–2099 and 6 occur around 2040–2060. In North China, 9 models have an abrupt increase in 2010–2099 and 7 appear around 2040–2060. In all East China, 13 out of 19 models display an abrupt increase and 11 are seen around 2040–2060. At the same time, another method: MannKendall test, is utilized to verify the findings from the running-t test, given the limitations of many methods in examining the abrupt change of the time series [34]. Two methods show the similar results although a small difference exists in the definition of abrupt changing time. Both methods demonstrate an abrupt increase in more than half of the models in 2010–2099, especially the abrupt increase generally appears around the 2040s in East China. Fifteen

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−

Figure 3 The linear trend (% (10 a) 1) of JJA precipitation change (relative to 1980–1999 average) in 19 individual models for 2010–2099 averaged in East China (a), South China (b), the YRV (c) and North China (d).

Table 2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Time of abrupt increase of precipitation in East China and 3 key areas based on the running-t test with 95% significance levela) Model CGCM3.1 (T47) CGCM3.1 (T63) CNRM-CM3 CSIRO GFDL-CM2.0 GFDL-CM2.1 GISS-EH GISS-ER FGOALS-g1.0 INM-CM3.0 IPSL-CM4 MIROC3.2 (medres) MIROC3.2 (hires) ECHAM5/MPI-OM MRI-CGCM2.3.2 CCSM3 PCM UKMO-HadCM3 UKMO_hadgem1

South China − 2070 2078 − − − − 2032 2056 2058 2051 − − 2046 2029, 2049 2042 2065 − −

YRV 2072 2054 − − 2054 2031, 2070 − 2063 − 2058 − − 2081 − 2050 2043 2046 − −

North China

East China

− 2040 − − − − 2046, 2066 2048 − − − 2071 2057 − − 2046 2083 2051 2047

− − − − 2054 2069 2044, 2074 2066 − 2058 − 2049 2051 2046 2052 2046 2046 2050 2024, 2047

a) The hyphen “−” indicates the inexistence of abrupt increase or insignificant increase.

out of 19 models show an abrupt increase in East China and 12 show the abrupt change around 2040–2060 based on Mann-Kendall test. This certifies the findings from the running-t test that the abrupt increase of precipitation around

the 2040s can be examined in most models in East Asia. A noteworthy thing is the large linear trends in all areas in CCSM3 and PCM. If one relates this to the large precipitation increase (Figure 3) and to significant abrupt change

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(above 99% level) in these two models, it indicates a great contribution of CCSM3 and PCM to the feature of two-stage evolution (increase) in the multi-model ensemble. Another important issue is the projection of major precipitation pattern in East China. Figure 4 displays an empirical orthogonal function (EOF) analysis of precipitation change for 2010–2099 in East China. The first component of EOF (EOF1) shows a “wet East China” pattern, which is characterized by the above-present precipitation in all East China areas and accounts for 36.6% of total variance. The corresponding time coefficient of EOF1 will maintain at a


positive value in 2010–2099 and increase abruptly after the end of 2040, indicating the enhanced precipitation in all East China due to this component. The contribution from the second component (EOF2) is 11.9% to the total variation, which reveals a “wet South and dry North” pattern (dipole pattern), with a fluctuating corresponding time coefficient and no clear trend. The EOF3 demonstrates a pattern with dry South, wet YRV and dry North (dry-wet-dry pattern) and only contributes 9.1% to the total variance, also with a negligible trend found in 2010–2099. A comparison of the precipitation patterns in the future

Figure 4 The first three components of EOF analysis based on percentage change of JJA precipitation in East China for 2010–2099. (a) The first component (EOF1, 33.6% of total variance); (b) time coefficient of EOF1; (c) the second component (EOF2, 11.9% of total variance); (d) time coefficient of EOF2, (e) the third component (EOF3, 9.1% of total variance); (f) time coefficient of EOF3.

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with those in the past 50 years [35] shows that the first 3 EOF modes are quite similar but with different contribution to the total variance. The EOF1 will change to “wet East China” pattern from a dipole pattern in the past [35], indicating a transformation of dominant precipitation pattern in the future. The similar temporal evolutions between precipitation in East China and EOF1 time coefficient clearly shows a large contribution from this component to the “two-stage evolution” feature and overall increase of precipitation in East China. Figure 5 further shows the geographical distribution of the precipitation percentage change in 2010–2019, 2030–2039 and 2080–2099, which can be treated as the results from the perspectives of near- and long-term climate change. In 2010–2019, with the exception of negative change in southern, central, and northwestern China, the positive change will appear in most areas of China. In 2030–2039, the increase of precipitation will gradually shift into higher latitudes and the more precipitation will be seen in more areas. By the late 21st century, the enhanced precipitation will prevail across China, with more increase in eastern than western parts. The magnitude of precipitation increase is larger than any periods before, showing a consistent change with those from EOF analysis.

Figure 5


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4 Projected changes in summer monsoon circulation in East Asia Several representative monsoon indices are chosen to calculate and analyze the future change in East Asian summer monsoon circulation. Some authors have shown that the monsoon index defined by Webster and Yang [36] in South Asia (WYMI) and by Wang and Fang [37] in Southeast Asia (WFMI) displays a good ability in describing the zonal component of monsoon circulation in the corresponding monsoon regions, respectively. In East Asian monsoon region, however, there are still many arguments about the definition of monsoon index because of the complexity of circulation and topography in this region. Generally, the indices defined by Guo [38] (GuoMI) and Shi et al. [39] (ShiMI) are able to display the variation of meriodional component of monsoon circulation and correlate with the precipitation variation in the YRV quite well [40, 41]. The index defined by Lu and Chan (LuMI) [42] is based on the meriodional wind at low troposphere in South China and thus has a straightforward implication to the variation of monsoon meriodional circulation in East Asia. Therefore, in the present study, the WYMI and WFMI are calculated to examine the monsoon changes over South Asia and South-

Percentage changes (%) of JJA precipitation (relative to 1980–1999 average) for 2010–2019 (a), 2030–2039 (b) and 2080–2099 (c).

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east Asia, and GuoMI, ShiMi and LuMI are used to discuss the monsoon change in East Asia. In the calculation of LuMI, the meriodional wind at 925 hPa is used since the data at 1000 hPa are not available in the models. Figure 6(a) shows that both the WYMI and WFMI will be declining in 2010–2099, indicating that the monsoon in South Asia and Southeast Asia will be weaker than the present. Since the definitions of these two indices are based on the zonal wind in these regions, this also denotes a weakening of zonal monsoon circulation in the future, which is consistent with the previous studies [18]. As for the East Asian summer monsoon, both GuoMI and ShiMI demonstrate the similar temporal evolution with a negligible trend in the future, due to their similar definitions based on the sea level pressure. Note that the GuoMI here is multiplied by −1 to keep the same implication of the sign with ShiMi. So, the strong monsoon should be GuoMI > 1.1 or ShiMi > 0.71, and weak monsoon is GuoMI < 0.9 or ShiMI < −0.6 based on the standards by Guo [38] and Shi et al. [39]. Then, according to this, the monsoon in the future will fluctuate in the range of normal value, without abnormally strong or weak monsoon in general, although a large and small index seen around 2040 and 2050, respectively. On the other hand, the change in LuMI shows that the meriodional wind of East Asian summer monsoon will go up with an increase around the 2040s. Although a large value can be seen around 2020, the increase ~0.1 m s−1 after the 2040s is


obvious overall. Based on the changes of these monsoon indices, both South Asian and Southeast Asian summer monsoon displays a clear declining trend and will be weaker in the future. However, for East Asian summer monsoon, since the uncertainty in the definition of monsoon index, the future changes are quite different based on different monsoon index. From the perspective of circulation change, LuMI should have a good implication to the monsoon change since its definition is based on the low-level winds. From the perspective of precipitation change, although GuoMI and ShiMI can well reflect the precipitation variation in the YRV in the current climate, they do not have any good correlation with the precipitation change in all the areas of East China if one compares Figure 6(b) with Figure 2(c). In this sense, these two indices are unable to represent the future monsoon change both in circulation and in precipitation. In order to understand the future change in East Asian monsoon circulation, a straightforward way is to examine the temporal evolution of winds since the monsoon circulation is mainly consists of low-level southwesterly and high-level northeasterly winds. As displayed in Figure 7, the low-level southwesterly winds will intensify and experience a substantial increase around the 2040s before reaching a relatively large value. The maximum increases of zonal wind and meriodional wind will be ~0.6 m s−1 and ~0.4 m s−1, respectively, by the end of the 21st century. Compared

Figure 6 The evolution of summer monsoon indices in 2010–2099 in South Asia defined by Webster and Yang [36] (WYMI, solid line) and in Southeast Asia by Wang and Fang [37] (WFMI, dashed line) (a), in East Asia by Guo [38] (GuoMI, solid line, left ordinate ) and by Shi et al. [39] (ShiMI, dashed line, right ordinate) (b), and in East Asia by Lu and Chan [42] (LuMI) (c).

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Figure 7 Time-latitude cross-section of changes (m s 1) in JJA winds at 850 hPa (a) and 100 hPa (b) averaged in 110°–120°E for 2010–2099 (relative to 1980–1999 average).

with the observed change in monsoon in the past 50 years, the magnitude of future increase in monsoon is quite weak. At 100 hPa, the northeasterly wind will also be strengthened and show a prominent increase around the 2040s, especially the increase in the easterly winds north of 30°N is obvious. The strengthened northeasterly winds will be corresponded to the strong southwesterly wind at low level, indicating an intensification of summer monsoon circulation in East Asia. It can easily be found that the temporal evolution of the low- and high-level wind in East Asia is consistent with the precipitation and both show an almost simultaneous prominent increase around the 2040s. Figure 8 shows the geographical distribution of wind changes at 850 hPa in 2010–2019, 2030–2039 and 2080–2099. The common feature for these three periods is that the southwesterly wind in East Asia will become stronger and the westerly winds over the tropical Indian

Ocean will be weaker than the present. This clearly demonstrates that the future monsoon over these two areas will evolve reversely, with an increased westerly wind in East Asia and decreased westerly wind in South Asia. Figure 8 further displays that the appearance of an anomalous anticyclone over the South China Sea (SCS) is responsible for the intensification of southwesterly wind over East Asia. This anticyclone will prevail over the SCS in 2010–2099 and will become stronger and cover a larger area in 2080–2099, along with a movement southward. The existence of this anticyclone implies an important role of the northwestern Pacific anticyclone in the intensification of the East Asian summer monsoon, and a negligible impact of the westerly winds originating from the Southern Hemisphere. In addition, the high-level wind change at 100 hPa is examined (not shown) and displays that an anomalous anticyclone will be dominating over South Asia, with an increased

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Figure 8

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Changes (m s 1) of JJA winds at 850 hPa for 2010–2019 (a), 2030–2039 (b) and 2080–2099 (c) (relative to 1980–1999 average).

northeasterly winds in the eastern flank. Corresponding to the change in low-level circulation, this anticyclone will also become stronger with time evolution and thus provide a favorable background for the increased low-level airflow.

circulation will be weakening in the future (c.f., Figure 8). In East Asian monsoon region, Figure 9 displays a substantial increase of precipitable water in 2010–2099 over

5 Projected changes in water vapor in East Asia The change in water vapor undoubtedly plays a very important role in the future climate change in East Asia. Based on the Clausius-Claperon equation and many studies from the observations and models, the temperature rise will cause the increase of the global average water vapor content. Due to the impacts of dynamical circulation, this will cause some places to become drier and some wetter, and thus lead to a change in large-scale hydrological cycle in the future. In South Asian monsoon region, many authors have pointed out that the buildup of water vapor and a larger water vapor flux are the dominant factors causing the increase of precipitation in the region, although the dynamical monsoon

−

Figure 9 Time-latitude cross-section of changes (kg m 2) of JJA precipitable water averaged in 110°–120°E for 2010–2099 (relative to 1980－ 1999 average).

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East China. By the late 21st century, the increase will be more than 10% in most areas, indicating a good relationship between the water vapor content and temperature rises over this region. Figure 10 further demonstrates the geographical distribution of water vapor change over the Asian region. In general, the water vapor increase will be more and more from the low to mid- and high-latitudes. With the temporal evolution, the change of water vapor in mid- and high-latitudes will become larger. By the period of 2080–2099, the prominent water vapor increase can be found in all the monsoon regions, providing a favorable local and remote condition for the buildup of water vapor in East Asia. Figure 11 shows the time-latitude cross-section of water vapor transport at 850 hPa in East Asia. In 2010–2020, the southwesterly water vapor transport displays a slight intensification in the regions between 20°N and 30°N. Then the northward transport in East Asia will be overall strengthened after the 2040s, reaching a maximum transport more than 1.2 m s−1 by the late 21st century. This change is consistent with the temporal evolution of precipitation in East China, showing that the precipitation increase in this region results from not only the more water vapor at local area but also the increased water vapor transported from the low and subtropical latitudes.

Figure 10

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The geographical distribution of water vapor transport at 850 hPa (Figure 12) clearly demonstrates these changes. In 2010–2019, the water vapor transported into China mainly comes from the SCS and the northwestern Pacific. In 2030–2039, this transport will be slightly weakened while the water vapors from the Arabian Sea and the Indian Ocean becoming stronger. By 2080–2099, a westward moisture transport conduct will form from the Arabian Sea through the Indian Ocean, and all the way to the SCS. A large amount of water vapor coming from the SCS, the Indian Ocean and the western Pacific will be transported into East China and provide a rich water vapor resources for the precipitation in China. Interestingly, if one compares the geographical distributions of changes in moisture transport and winds at 850 hPa (Figure 8), it can be found that the moisture transport will change reversely with the wind fields over South Asia, the Arabian Sea, and the Indian Ocean. The southwesterly water vapor transport will be stronger while the low-level southwesterly wind will be weaker than the present over these regions, which may result from a relatively large increase of water vapor in contrast with a small change of low-level winds. However, in East Asia, a very different situation can be seen. The changes in water vapor (Figure 8)

Changes (kg m 2) of JJA precipitable water for 2010-2019 (a), 2030–2039 (b) and 2080–2099 (c) (relative to 1980–1999 average).

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Figure 11 Time-latitude cross-section of changes (kg m 1 s 1 hPa 1) of 850 hPa water vapor transport averaged in 110°–120°E for 2010–2099 (relative to 1980–1999 average).

and in the winds (Figure 6) are consistent, with intensified southwesterly winds corresponding to the increased moisture transport and precipitation. In addition, their changes are all characterized by a two-stage temporal evolution with a substantial increase around the 2040s. These indicate that both the hydrological and dynamical fields in East Asia will be intensified in the future. The precipitation change in East Asia is due to the increases in both monsoon circulation and water vapor. The thermal dynamic and dynamic variables will evolve consistently in response to the global warming.

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Discussions

The present study shows that the dominant precipitation pattern in East Asia will transform to a pattern with precipitation increase in all East China (“wet East China”) in the future. This pattern is similar to the third EOF component of precipitation in the past 50 years [35] but accounts for a different contribution to the total variance. In contrast with the changes in circulation and water vapor, this pattern may represent a typical mode of precipitation change in East Asia in response to the global warming and mostly reflect the impacts from the increased radiative forcing due to the greenhouse gases emissions. Compared with the precipitation variation in East China in the past 50 years, this also may reflect an in-phase augment of the precipitation

changes caused by anthropogenic and natural factors. Some studies [5–7] have shown that the precipitation variation in East China in the past 50 years is characterized by an increase in South China and a decrease in North China, which greatly differ from the precipitation change in the same latitude of the Northern Hemisphere. The latter is mainly characterized by a precipitation increase in the mid- and high latitudes and a decrease in the lower latitudes between 10°–30°N. Thus, this may imply a subdominant role of greenhouse gases increase in the precipitation change in East China. Although some authors [7] have suggested the effects of aerosols on precipitation formation in East Asia, recent studies [2] show that the impacts from natural factors, such as the variations of snow cover in the Tibetan Plateau and SST in the Pacific, may be more important and have a more straightforward influence on the precipitation change there. Thus, if one considers an 80-year period existing in the precipitation variation in East China [35] and the precipitation having reached the peak around 1960, it is reasonable to assume that the precipitation change caused by the anthoropogenic factors may be augmented by an in-phase natural precipitation variation, and thus may result in an overall intensification of precipitation in East China after the 2040s. In relation to the current precipitation distribution in East China, this increase of precipitation may mitigate the severe droughts in North China but also may increase the flooding risks in the present wet regions.

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Figure 12 Changes (kg m 1 s 1 hPa 1) of JJA water vapor transport at 850 hPa for 2010–2019 (a), 2030–2039 (b) and 2080–2099 (c) (relative to 1980–1999 average).

The physical reasons behind the changes in circulation and precipitation are also noteworthy to explore and take into account. Although the physical representation and regional response in East Asia are different from model to model, the multi-model ensemble generally reflects the change in external forcing (signal). To explore the relevant issues, two factors that may affect the summer circulation and precipitation in East Asia are analyzed. Figure 13(a) shows the change in thickness between 200 and 500 hPa over the Tibetan Plateau and Asian continent. It can be seen that the thickness will substantially increase over these regions in the future, indicating a warming in the mid- and high troposphere. Under this situation, the thermal impacts of the Tibetan Plateau and Asian continent will be intensified with the increased thickness. Figure 13(b) displays the correlation between the JJA precipitation change averaged in East Asia and JJA SST change in the global oceans. Obviously, a good correlation (~0.6) can be found in most oceans. For the time series with a length of 90 years, such a good correlation (~0.6–0.7) is much larger than the value needed for a statistical test with 95% significance level. However, if one thinks about it carefully, this correlation is actually mainly due to the almost linear increase in both JJA

precipitations in East Asia and JJA SST in the globe. It actually mostly reflects the relationship between two linear increasing series due to global temperature rise and may reflect the dominant role of global warming in the model future projections. Thus, this should be kept in the mind when one explains the role of increased SST in the enhanced precipitation in East Asia from the viewpoint of dynamical circulation change, which will be discussed in the following paragraph. On the other hand, the feature of two-stage evolution of precipitation change around the 2040s is also examined. It shows that this feature cannot be found in the changes in thickness over the Plateau and Asia and in SST over most oceans. This suggests that the abrupt change around the 2040s in East Asia is not directly caused only by a factor but by many factors and their interactions. Based on the above analyses from Figure 13, a general picture of future precipitation change can be depicted. In 2010–2099, the impacts of global warming due to the greenhouse gases increase play a dominant role in the multi-model ensemble projection. As temperature rises, the middle and high troposphere in the Tibetan Plateau and East Asia will become warmer, and the global SST will increase with a weak shift towards average background conditions,

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Figure 13 (a) Changes (gpm) of JJA 200–500 hPa thickness for 2010–2099 averaged over the Tibetan Plateau (75°–105°E, 25°–35°N, solid line) and Asian region (70°–130°E, 20°–45°N, dashed line) (relative to 1980–1999 average); (b) correlation coefficient between JJA precipitation averaged in East Asia (110°–120°E, 22.5°–45°N) and JJA SST in the globe for 2010–2099.

which may be described as ‘El Niño-like’ pattern [18]. All these two factors may exert important influence on the precipitation change in East Asia. The warming of mid- and high troposphere over the Plateau can enhance the Asian continental low in the summer and thus cause the intensification of East Asian summer monsoon. The frequent occurrence of El Niño-like SST pattern [18] can strengthen the downward airflow over the tropical western Pacific and the western subtropical high, thus leading to a stronger southwesterly airflow into East Asia. All these show a positive role of these two factors in the monsoon and precipitation changes in East Asia. With the global warming, they may interact with each other and result in an increase and abrupt change of precipitation in East Asia. As some studies [2–4] pointed out, the climate variations in East Asia in the past 50 years are associated with the large-scale cooling over East Asia, the weakened thermal impacts over the Tibetan Plateau and SST change in the tropical and subtropical oceans. This supports the notion that the climate change in East Asia is a result of multi-factor interaction, not only at present but also in the future.

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Conclusions

The future changes in summer precipitation and monsoon circulation are of great significance to the future social and

economic development in East Asia. The frequent occurrence of “wet South and dry North” in recent decades cause the severe droughts in North China and flooding in South China, as well as the increasing contradictions between supply and demand of water resources. Thus, it is necessary to study the issues of the future change in precipitation in East Asia. The present study has analyzed the 19-models ensemble mean based on the newest generation of climate models under SRESA1B, and shown that the precipitation will increase and the southwesterly monsoon airflow will be stronger in East Asia in the future. The major conclusions are as follows: (1) In East China, the precipitation will increase in 2010–2099 and experience a prominent change around the end of the 2040s. This two-stage evolution is characterized by a small increase before the 2040s and a large increase afterward, which can be most clearly seen in North China, and then in South China and the YRV. After the 2040s, an overall precipitation increase will be seen across China. In 2010–2099, the projected precipitation pattern will be dominated by a pattern of “wet East China”, which is featured by the above-present precipitation across all East China, along with a time coefficient markedly increasing after the 2040s. The other precipitation patterns that prevail in the current climate only contribute a small proportion to the total variance, with no prominent liner trend in the future.

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(2) By the late 21st century, the monsoon circulation will be stronger in East Asia and weaker in South Asia. Although a few monsoon indices do not show a good representation in projecting the monsoon change in East Asia, the analyses of low- and high-level winds display that both the low-level southwesterly airflow and high-level northeasterly airflow will become increasingly stronger in the future. At low level, this is due to the intensification of southwesterly airflow north of the anticyclone over the western Pacific and the SCS, and at high level, it is caused by the increased northeasterly airflow east of the anticyclone over South Asia. In addition, the monsoon circulation will experience a prominent increase around the 2040s, which is consistently corresponded to the two-stage change in precipitation. (3) The atmospheric water vapor content in East Asia will greatly increase in 2010–2099, similar to the change of global average. During 2010–2099, the water vapor transported northward into East China will gradually be intensified and experience an abrupt increase around the 2040s similar to the precipitation change. The southwesterly water vapor transport will reach over 1.2 m s−1 in East China by the late 21st century. This indicates that the precipitation increase in East China results from not only the water vapor buildup at local area but also the more water vapor transported from the low and subtropical latitudes. (4) By comparing the changes in monsoon circulation and water vapor, it can be concluded that the enhanced precipitation over East Asia is caused by the increases in both monsoon circulation and water vapor, which is greatly different from South Asia. The good consistency in temporal evolution and the spatial distribution of winds and water vapor demonstrate that the dynamical and thermal dynamical variables will evolve consistently in response to the global warming in East Asia, i.e., the intensified southwesterly monsoon corresponding to the increased southwesterly water vapor transport. Finally, the present study only represents the current understanding of future projection in precipitation and circulation over East Asia based on the newest-generation climate models under SRESA1B scenario. Many studies have shown that there still exist many uncertainties in the emission scenarios and climate models, such as the physical and chemical representations of processes in the models and the methods of ensembles, etc. Currently the climate models are evolving towards greater comprehensiveness, including the finer spatial resolution and more complicated processes, and will be able to better simulate observed climate. With these improvements and a better understanding of climate system, more credible and more detailed information will emerge in the near future. We thank two anonymous reviewers for helpful comments and suggestions. We acknowledge the international modeling groups for providing their data for analysis, the Program for Climate Model Diagnosis and Intercomparison (PCMDI) for collecting and archiving the model data, the


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JSC/CLIVAR Working Group on Coupled Modelling (WGCM) and their Coupled Model Intercomparison Project (CMIP) and Climate Simulation Panel for organizing the model data analysis activity, and the IPCC WG1 TSU for technical support. The IPCC Data Archive at Lawrence Livermore National Laboratory is supported by the Office of Science, U. S. Department of Energy. And thanks also go to ECMWF for providing the ERA-40 data used in this study. This work was supported by National Natural Science Foundation of China (Grant No. 40605020), National Basic Researth Program of China (Grant No. 2006CB403604), National Key Science and Technology Program (Grant No. 2007BAC03A01). 1 2

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