Hadley cell and the subtropical high over the western. Pacific. Chang et al. [2000a, 2000b] suggested that the interdecadal climate variations in China were ...
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D19, 4614, doi:10.1029/2003JD003651, 2003
Long-term climate variations in China and global warming signals Zeng-Zhen Hu Center for Ocean-Land-Atmosphere Studies, Calverton, Maryland, USA
Song Yang Climate Prediction Center, NOAA, Camp Springs, Maryland, USA
Renguang Wu Center for Ocean-Land-Atmosphere Studies, Calverton, Maryland, USA Received 1 April 2003; revised 18 June 2003; accepted 1 July 2003; published 11 October 2003.
[1] In this work, the authors analyze the observed long-term variations of seasonal climate
in China and then investigate the possible influence of increases in greenhouse gas concentrations on these variations by comparing the observations with the simulations of the second phase of the Coupled Model Intercomparison Project (CMIP2). The long-term variations of precipitation and temperature in China are highly seasonally dependent. The main characteristic of summer precipitation in China is a drying trend in the north and a wetting trend in the central part. The precipitation in winter shows an increasing trend in southern and eastern-central China. Interesting features have also been found in the transitional seasons. In spring, precipitation variations are almost opposite to those in summer. In autumn the precipitation decreases in almost the whole country except for the middle and lower reaches of the Yangtze River Valley. In addition, the seasonality of precipitation has become slightly weaker in recent decades in southern and eastern China. Pronounced warming is observed in the entire country in winter, spring, and autumn, particularly in the northern part of China. In summer a cooling trend in central China is particularly interesting, and cooling (warming) trends generally coexist with wetting (drying) trends. The correlativity between precipitation and temperature variations is weak in spring, autumn, and winter. It has also been found that the long-term climate variations in winter and summer in China may be connected to the warming trend in the sea surface temperature of the Indian Ocean. A comparison between the observed seasonal climate variations and the CMIP2 simulations of 16 models indicates that the observed long-term variations of winter, spring, and autumn temperature in China may be associated with increases in greenhouse gas concentrations. However, such a connection is not found for the summer temperature. The tremendous uncertainties among the models in precipitation simulations make it difficult to link the precipitation variations to global INDEX TERMS: 1620 Global Change: Climate dynamics (3309); 3319 Meteorology and warming. Atmospheric Dynamics: General circulation; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 4215 Oceanography: General: Climate and interannual variability (3309); 1610 Global Change: Atmosphere (0315, 0325); KEYWORDS: global warming signals, Chinese climate, CMIP2 simulation Citation: Hu, Z.-Z., S. Yang, and R. Wu, Long-term climate variations in China and global warming signals, J. Geophys. Res., 108(D19), 4614, doi:10.1029/2003JD003651, 2003.
1. Introduction [2] The climate variations in China are well known for their complexity due to the influence of many factors over a wide range of spatial and temporal scales. They are linked to global teleconnections and the El Nin˜o-Southern Oscillation (ENSO) [Lau, 1992; Yang et al., 2002; Wu et al., 2003] and closely tied up with the variations of the Asian-Australian monsoon system. The climate in China differs significantly Copyright 2003 by the American Geophysical Union. 0148-0227/03/2003JD003651$09.00
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from that in South Asia and Australia, although climate variations in these regions interact strongly with each other [Tao and Chen, 1957; Matsumoto, 1992; Ding, 1994; Lau et al., 2000; Wang et al., 2001]. The variability of Chinese climate is due largely to the activities of the summer and winter monsoons [Ding, 1994]. Precipitation and temperature anomalies, especially the summer and spring floods and droughts in China, are intimately related to the country’s economy and people’s lives. These floods and droughts have often been considered among the most severe natural disasters for the country. Thus the long-term climate variations in China and their possible connections to the increases in
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HU ET AL.: GLOBAL WARMING IN CHINESE CLIMATE
greenhouse gas concentrations are subjects of considerable scientific and practical interest. [3] There have already been a few observational studies on the long-term climate variations in China. Zhu and Wang [2002] found that an 80-year oscillation is an important component of the summer precipitation variations in East Asia. Hu [1997], Chang et al. [2000a, 2000b] and Gong and Ho [2002] demonstrated the existence of an interdecadal shift of the climate in China in 1977 – 1978. Hu [1997] and Gong and Ho [2002] indicated that heating in the Indian Ocean may play a key role in the interdecadal variation of the summer climate in China through the changes in the Hadley cell and the subtropical high over the western Pacific. Chang et al. [2000a, 2000b] suggested that the interdecadal climate variations in China were related to interdecadal changes of the relationship between ENSO and the Chinese climate. Observational analyses of climate changes in East Asia during the last 100 years [Wang and Ye, 1993] showed a pronounced annual mean warming trend in northern China and a minor cooling trend in central China. Using stational data with a short interval, Chen et al. [1991] and Nitta and Hu [1996] found a similar trend pattern in Chinese summer climate. Wang and Gaffen [2001] show evidence of moisture increase over most of China in 1951 – 1994, and Zhai and Ren [1999] demonstrated the changes of maximum and minimum surface temperature in China in 1951 – 1990. In addition, the past 50-year trend of climate in western China is also described by Qin [2002]. [4] Recently, coupled general circulation model (CGCM) simulations have shown that increases in greenhouse gas concentrations intensify the South Asian summer monsoon and its variability [Meehl and Washington, 1993; Hu et al., 2000a] and diminish the Asian winter monsoon [Hu et al., 2000b]. After analyzing model simulations of climate change, Hulme et al. [1994] also pointed out the potential impact of global warming on the variations of temperature and precipitation in East Asia. The projection of climate in western China to the future 50 years is also made by Qin [2002]. These studies raise the questions of what the observed long-term variations of the seasonal climate in China are and whether these variations are connected to global warming. [5] Clearly, present CGCMs have serious defects in simulating regional climate. For example, none of the 10 models that participated in the Climate Variability and Predictability (CLIVAR)/Monsoon GCM Intercomparison Project can realistically reproduce the observed Mei-yu rainband [Kang et al., 2002]. The model deficiencies and the shortage of observational data make the detection and explanation of the long-term climate variations in China extremely difficult and uncertain. Nevertheless, the second phase of the Coupled Model Intercomparison Project (CMIP2) [Meehl et al., 2000] provides additional data to explore the problem. [6] To understand the long-term climate trends in China, it is necessary to analyze the trends for each season, because of the differences observed for each season. Furthermore, the potential impact of global warming on the seasonal climate variations in China may be understood by comparing the observed climate trends with those in the CMIP2 simulations. In this work, we analyze the observed long-term variations of the seasonal-mean precipitation and temperature in China using station data. We also explain the
projected seasonal climate variations and their uncertainty using the CMIP2 results. In section 2 we describe the main features of observed and simulated data used in this study. The observed long-term seasonal climate variations and the responsible mechanisms are investigated in section 3. Section 4 provides discussions of the projected climate change. The uncertainty of model simulations is also discussed in section 4 by analyzing the differences between individual simulations and by examining the composite features for some selected models. Section 5 provides a summary and further discussions of the results.
2. Observed and Simulated Data 2.1. Observed Data [7] We analyze the monthly precipitation and temperature data of 160 meteorological stations in China for the period from January 1951 to February 2000. The data were collected and edited by the China Meteorological Administration and were relatively homogeneously distributed, especially in eastern China (see Figure 1a). The temperature data have not been adjusted for urban warming biases. In addition, the observed monthly mean sea surface temperature (SST) data are applied to study the relationship of ocean warming with the climate variations in China. The SST data are the reconstructed data with a resolution of 2° 2° [Reynolds and Smith, 1994]. 2.2. Simulated Data [8] Simulations of the CMIP2 experiments by 16 models [Meehl et al., 2000] are used in the analysis (Table 1). Each model simulation consists of a control run with constant ‘‘present-day’’ atmospheric CO2 and a greenhouse run with a standard gradual (1% year1 compound) increase in CO2. Each experiment was run for 80 years. Details about the individual models and experiments can be found at http:// www-pcmdi.llnl.gov/cmip/ or in the work of Ra¨isa¨nen [2001, 2002] and Ra¨isa¨nen and Palmer [2001], and the references therein. Our analysis is focused on the comparison of the 20-year means for years 61– 80 centered at the CO2 doubling with the 80-year means of the control run. We choose seasonal mean precipitation and surface air temperature as the prime variables.
3. Observed Long-Term Climate Variation 3.1. Rainfall [9] Figure 1a shows the summer (JJA, June, July, and August) mean precipitation in China. It can be seen that the precipitation amount decreases from southern (>700 mm) to northern (0.5°C) are observed in part of southwest China. The above analyses show that the opposite trends of temperature and precipitation (the correlativity between temperature and precipitation) in China occur in summer but not in spring, autumn, and winter. Also, the warming trends are the largest in winter, and the smallest in summer. 3.3. Possible Connection With the Indian Ocean Sea Surface Temperature [16] Ocean temperature is an important factor in affecting the low-frequency variations of climate. Hu [1997] found that the interdecadal variability of the summer precipitation and temperature in East Asia is largely influenced by the changes of SST and convective activity over the tropical Indian Ocean and western Pacific. The convective activity is usually enhanced when the SST is warmer than normal, so the subtropical high over East Asia is intensified through the enhancement of the Hadley cell. As a result, subtropical East Asia, including south and southwest China and the Southwest Islands of Japan, is under the control of a positive height anomaly at 500 hPa that causes above normal temperature and below normal precipitation. The
Figure 4. Seasonality index (SI) for monthly mean precipitation averaged over (a) 1951 – 1970, (b) 1981 –2000, and (c) the difference between the two periods (Figures 4b and 4a). The contour intervals are 0.1 in Figures 4a and 4b and 0.02 in Figure 4c. Dark (light) shading marks the values greater (smaller) than 0.8 (0.6) in Figures 4a and 4b. Shading in Figure 4c denotes significant differences at the level of 95% using the T-test. See the text for the definition of SI.
(Figure 6b). The warming rates reach values larger than 3.5°C per 50 years in some regions of northern China and
