Articles Atmospheric Science

May 2010 Vol.55 No.14: 1437−1444 doi: 10.1007/s11434-009-0613-5

SPECIAL TOPICS:

Thermal difference between the Tibetan Plateau and the plain east of Plateau and its influence on rainfall over China in the summer ZHU YanFeng1*, ZHANG Bo2 & CHEN LongXun2 1 2

The Laboratory of Climate Study of China Meteorological Administration, National Climate Center, Beijing 100081, China; Chinese Academy of Meteorological Sciences, Beijing 100081, China

Received March 4, 2009; accepted August 14, 2009

There exist thermal differences between the Tibetan Plateau (TP) and the plain east of the TP, and between land and sea in East Asia. The influence of the land-sea thermal contrast on the precipitation in East China has been widely investigated; however, a few studies have paid attention to the role of the TP-plain thermal difference. Thus, using the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data and the observation data of China from 1951 to 2007, the area-mean temperature difference between the TP (27.5°−40°N, 80°−100°E) and the plain (27.5° −40°N,110°−120°E) at 500 hPa is defined as an index (dexT-C) of the TP-plain thermal difference in East Asia. The relationship between the dexT-C and East Asian general circulation and the rainfall in China in the summer has been explored. Diagnostic analysis and numerical simulation show that the TP-plain thermal difference is closely related to the rainfall over West China (90°−110°E) in the summer. High values of the dexT-C correspond to the large thermal difference between the TP and the plain, strengthening the heat low over the TP, southward of the northwestern Pacific subtropical high, southern flood and northern drought in West China (90°−110°E), and vice versa. From 1951 to 2007, the variation in dexT-C exhibits a remarkable oscillation and ascending trend, and the abnormal rainfall pattern over West China (90°−110°E) also changes from “northern flood and southern drought” to “southern flood and northern drought”. The above research is favorable to knowing how the two-stage thermal differences influence summer rainfall in China. Tibetan Plateau, plain, thermal difference, inter-decadal change of rainfall, summer season Citation:

Zhu Y F，Zhang B, Chen L X. Thermal difference between the Tibetan Plateau and the plain east of Plateau and its influence on rainfall over China in the summer. Chinese Sci Bull, 2010, 55: 1437−1444, doi: 10.1007/s11434-009-0613-5

East Asia, facing the Pacific on its east, is a famous monsoon climate zone due to the seasonal cycle of land-sea thermal contrast. China lies in the East Asian monsoon area, which leads to the close relationship between the anomalous summer rainfall over China and the East Asian summer monsoon [1]. Taking into account the fact that the land-sea thermal contrast is the key driving force of the East Asian monsoon, the researchers defined various indices of land-sea thermal contrast, and analyzed their connection with the summer rainfall in China. For example, someone defined the East Asian summer monsoon index by the sea level pressure (SLP) gradient between the East Asian con*Corresponding author (email: [email protected])

© Science China Press and Springer-Verlag Berlin Heidelberg 2010

tinent and the ocean [2,3], while Sun et al. [4] defined it by the surface temperature difference between land and sea. Results showed that the above-mentioned indices are not significantly related with the rainfall over West China (west of 110°E or 105°E), although the variation in land-sea thermal contrast had remarkable influence on the summer rainfall in East China. Being heated (cooled) by the TP in western China, there exists a thermal difference between the TP and its surrounding areas. In addition, the seasonal change in the thermal difference can augment the complexity of the East-Asia monsoon by changing the air flow. The influence of the anomalies of the thermal state over the TP on the variability of the summer rainfall in China has been elucicsb.scichina.com

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dated by many authors. For example, the authors suggested that an anomalous heating source over the TP can cause drought/flood in the Yangtze River-Huaihe River valley [5−8] and North China [9]. Further research showed that the thermal anomaly of the TP gives impacts to the summer rainfall in China by forcing the atmospheric circulation over East Asia [10−14]. In the summer, as a “heating island” up to the free air in the middle level of troposphere, the thermal anomaly over the TP has obvious influence on the climate, which has been widely considered in the current research. We infer that the TP-plain thermal contrast may exert more influence on the weather and climate in the areas surrounding the plateau than that of the TP alone, which is similar to the fact that the land-sea thermal contrast exerts more influence on East Asian monsoon than that of land or sea alone. Li and Yanai [15] analyzed the difference of the mean upper-troposphere (200−500 hPa) temperature between 30°N and 5°N and found that the onset of the Asian summer monsoon concurred with the reversal of the meridional temperature gradient south of the TP in late May. Based on the analysis of the climatic temperature latitudinal deviation in the middle troposphere, Qi et al. [16] advised that the seasonal cycle formed by the zonal thermal contrast between the Asian continent and West Pacific may be an independent driving force of the East Asian subtropical monsoon. Although their research discussed the effects of the land-sea thermal contrast on the seasonal transition in terms of land-sea temperature difference at the upper-troposphere (200−500 hPa), there are only a few studies about the influence of TP-plain thermal difference. The major objective of the present work is to examine the influence of the TP-plain thermal difference. Firstly, we analyzed the features of the variability of the TP-plain thermal difference in the period of 1951–2007 using the NCEP data; secondly, we defined an index of the TP-plain temperature difference to discuss its relationship with general circulation and precipitation; finally, we simulated the probable process by use of a numerical model.

1 1.1

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vestigating weather and climate, is adopted in this work. It is a global spectral atmospheric model using the σ-p hybrid coordinate, with a horizontal T42 spectral resolution, 26 vertical levels, and a rigid lid at 2.194 hPa. Compared with the previous version, the major changes are the modifications to the descriptions of clouds, precipitation, aerosols, and radiation parameterization schemes, which have improved several aspects of the simulated climate including more realistic tropical tropopause temperatures, boreal winter land surface temperatures, surface insolation, and the clear sky surface radiation in the polar regions [17,18].

2 Analysis of thermal contrast between the TP and the plain east of the TP The TP has a mean elevation in excess of 4500 m. In the summer, as a heating surface, the air temperature above the TP is much warmer than that over surrounding at the middle level of troposphere. Figure 1 shows the distribution of the summer (June-July-August) mean 500 hPa air temperature latitudinal deviation (TLD) (defined as the temperature difference between a certain longitude and the average of 70°–140°E) in East Asia. We can see that the TP is a warm area, while the cool area lies in the regions from eastern China to the western Pacific in the regions south to 40°N, with the critical boundary close to 100°–110°E. The index of the temperature difference between the Plateau (27.5°−40°N, 80°−100°E) and the plain (27.5°−40°N, 110°−120°E) at 500 hPa is defined as dexT-C after normalization. Figure 2 depicts the variation of dexT-C from 1951 to 2007. The high index years (dexT-C≥1) expressing the temperature difference between the TP and the plain is higher than normal, such as 1954, 1966, 1968, 1969, 1970, 1974, 1986, 1991, 1998, 2000, 2001, and 2002. The low index years (dexT-C≤−1) indicate that the Plateau-plain temperature difference is lower than normal, such as 1953, 1958,

Data and model

Data

Data used in this study are from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis datasets, including temperature, meridional wind, zonal wind, and geopotential height fields (horizontal resolution 2.5°×2.5°, with 17 vertical levels) from 1951 to 2007; and the 1951−2007 monthly precipitation data of 160 stations over China provided by China Meteorological Administration (CMA). 1.2

Model

CAM3.1/NCAR, the fifth general circulation model for in-

Figure 1 Distribution of the summer TLD on 500 hPa (Unit: K). The 3000 m terrain contour is indicated by a thick broken line.

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Figure 2 Variations in dexT-C from 1951 to 2007 (solid line for polynomial fitting curve).

1959, 1961,1963, 1964, 1975, 1976, 1979, 1983, and 1984. We noticed that the dexT-C experiences a remarkable inter-decadal variation. The high index years mostly existed in the period from the mid-late 1960s to the early 1970s, and from 1985 to 2002, which might have been caused by more warming over the TP than that in the vicinity of TP [19]. The low index years mainly emerged from the midlate 1950s to the early 1960s, and from the mid-late 1970s to the early 1980s. Consistent with the information mentioned above, the polynomial fitting curve of dexT-C exhibits an obvious trend of rise and fluctuation in the last 57 years. Additionally, it should be noted that the dexT-C has exhibited a descending trend after 2001.

3 Relation of dexT-C to the summer rainfall in China and East Asian circulation 3.1

The relationship of dexT-C with precipitation

Simultaneous correlation analysis is conducted between the dexT-C and the summer rainfall at 160 stations in China from 1951 to 2007 (Figure 3(a)). The result shows that the corre-

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lation coefficients between the precipitation in the 43 stations and dexT-C are significant at the 0.05 level of signifycance (alpha level), and the distribution of these stations, which account for 27% of 160 stations, are more concentrated. It can be seen that the negative correlation appears over the regions north of the Yangtze River except the southern part of Northeast China and some areas of western Xinjiang, with the notable negative correlation in the northwestern part of Sichuan basin, the region from the east-central of Northwest China to central Inner Mongolia. The positive correlation emerges over south of the Yangtze River, with the notable positive correlation located southeast of the TP, southern Sichuan Province to northern Yunnan Province, some areas of Guangxi and Guizhou Province, and the middle and lower reaches of the Yangtze River. Based on the variation of dexT-C in the last 57 years, we selected 10 years separately with the high and low dexT-C as follows. The 10 high dexT-C years appeared in 2001, 1969, 2002, 1974, 1954, 1986, 1968, 1970, 1991, and 1966, and the 10 low dexT-C appeared in 1961, 1953, 1976, 1964, 1975, 1963, 1969, 1984, 1983, and 1979. The above years of high/low dexT-C are chosen for the composite analyses of the summer precipitation percentage anomalies over China. Figure 3(b) shows that in comparing the low dexT-C years, summer rainfall decreases over most areas north of the Yangtze River and increases over south of the Yangtze River in high dexT-C years. The areas where rainfall has a remarkable difference between the two groups (passing the confidence test with a confidence level of 95%) are consistent with the regions with a remarkable correlation in Figure 3(a). This verifies that the high correlation between dexT-C and the rainfall is credible. In addition, the correlation coefficients between the summer rainfall of the 160 stations and the area-mean 500 hPa temperature over the TP and the plain are calculated, respectively. The results show that the relationship between the summer rainfall and temperature is unremarkable when only the temperature over the TP or the plain is considered.

Figure 3 (a) Simultaneous correlation coefficients of the dexT-C to the summer rainfall at 160 stations (Correlations at 0.05 significance level are shaded); (b) differences in the anomalous percentage of summer precipitation between the high and low dexT-C years (%). The shaded areas represent passing the confidence test with a confidence level of 95%.

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Figure 4 (a) Spatial pattern of the first EOF mode for summer rainfall in western China; (b) temporal coefficients the first EOF mode (solid line) and variation in the dexT-C (dashed); (c) variation in the anomalous percentage (%) of summer precipitation over the northeastern TP (solid line) and the southeastern TP (dashed line) (%).

In Figure 3 we find that most of the remarkable correlation are located over the west of 110°E. At present, most of studies about abnormal precipitation are focused on eastern

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China. Some scholars have pointed out that the variations in precipitation and the winds in the middle and lower level over the west of 110°E are obvious difference from those over the east of 110°E [20−22]. Therefore, the EOF analysis is applied to the anomalous percentage of summer precipitation using the data of 67 stations located west of 110°E. The spatial distribution of the first mode, which accounts for 12% of the total variation, shows an out-of-phase variation in rainfall existing between the regions north and south of 35°N (Figure 4(a)). The corresponding temporal coefficients express a notable weakening trend (Figure (b)). This seems to indicate that the summer rainfall over West China (90°−110°E) shows the trend of flooding in the north and drought in the south to the opposite of the spatial distribution. The correlation coefficient between the temporal coefficients of the first mode and dexT-C is −0.51 (passing the significance test of α=0.001). According to the distribution of the high load values of the first mode, we selected 9 stations in the northern and southern part of West China (90° −110°E), respectively. The 9 stations located northeast of the TP are Baotou, Mianyang, Tianshui, Yan’an, Lanzhou, Zhongning, Yinchuan, Xining and Ningxia, while the 9 stations located southeast of the TP are Lhasa, Changdu, Lijiang, Huili, Xichang, Kangding, Rongjiang, Guiyang, and Baise. The variations of the anomalous percentages of summer precipitation over the two regions are calculated. The change in precipitation over the northern and southern part of West China (90° –110°E) has the opposite trend (Figure 4(c)). The variation in precipitation over the northeast of the TP shows a decreasing trend from 1951 to 2007, which is accompanied by the increase of precipitation over the southeast of the TP. The correlation coefficients of dexT-C with rainfall over the two regions are −0.48 and 0.58, respectively. Using the 1959 to 1994 grid datasets, Wang and Wu [23] analyzed the spatial pattern of the abnormal summer rainfall over China and indicated that there is an opposite correlation between the southern and the northern part of the central-east of the TP, which is consistent with the above results. However, they did not make further investigation on the mechanism of the abnormal rainfall in West China. We believe that “northern flood and southern drought” turned into the opposite pattern over West China (90°–110°E) during 1951 to 2007, which is closely related to the variation of the TP-plain thermal difference. Furthermore, after 2001, the dexT-C shows a distinct decreasing trend. At the same time, precipitations are observed to increase over the northeastern TP and decrease in the southeastern TP. Therefore, do the above phenomena mean that the abnormal rainfall pattern of “southern flood and northern drought” in West China (90°–110°E) is changing? 3.2 The characteristics of the general circulation in the years of high and low dexT-C As the TP exerts profound influence on climate through its

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thermal and mechanical forcing, the general circulation over the plateau and its adjacent regions shows its distinctive features. Scholars have suggested that a shallow thermal depression exists below the 500 hPa level above the TP. Airflow at the low level of the troposphere obviously converges to the TP, which is expressed as the cyclonic circulation around the TP at 700 hPa. These circulations can impact the weather and climate over the TP and its adjacent areas [20]. Figure 5 shows the composite wind anomalies at 500 hPa and 700 hPa in the years of high and low dexT-C, respectively. In the years of high dexT-C, at the 500 hPa level, an abnormal anticyclone appears from the northern TP to Lake Baikal; an abnormal cyclone emerges over the region from the eastern TP to the Korean Peninsula; a weak abnormal cyclone appears over the southern TP, which indicates that the intensity of the thermal depression over the TP is strong; the Northwest Pacific subtropical high (NPSH) shifts more southward than normal during summer; and the abnormal northeasterlies control most of North China, which are unfavorable to spread northward of the East Asian summer monsoon. As far as the abnormal winds at 700 hPa are concerned, there are abnormal divergence flows over the northeastern TP and abnormal convergence flows over the southeastern TP. The abnormal circulation in the middle and low troposphere mentioned above is propitious to flood

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in the southeastern TP and drought in the northeastern TP in summer. In the low dexT-C years, the pattern of abnormal circulation is opposite to that of high dexT-C years. At 500 hPa, the abnormal cyclone appears from the northern TP to Lake Baikal; the abnormal anticyclone appears over the eastern plateau; and the NPSH shifts more northward than the normal, which is propitious to the abundant rainfall in the northeastern TP contributed by the convergence of abnormal moist and warm air from west of NPSH and abnormal northwesterlies. The abnormal anticyclone over the southern TP signifies a weak thermal depression. At 700 hPa, the intensity of the cyclonic around the TP decreases, resulting in the appearance of abnormal divergence flows and drought in the southeastern TP.

4 Numerical experiments The diagnostic analysis mentioned above shows that the variation in the TP-plain thermal difference is closely related to the circulation over the TP and its adjacent areas, and the precipitation in West China (90°−110°E) in the summer. To verify the results of the diagnostic analysis, we performed two numerical tests, which revealed the variation in the general circulation and precipitation over the plateau and its adjacent areas when the temperature difference between

Figure 5 The 500 hPa ((a),(c)) and 700 hPa ((b),(d)) composite anomalous winds for high ((a),(b)) and low ((c),(d)) dexT-C. The shaded areas represent passing the confidence test with confidence level of 95% (Unit: m/s).

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the TP and the plain was increased. The climatological monthly sea surface temperatures (SST) were used in CAM3.1. The control experiment and sensitivity experiment were designed as follows: Control experiment: A 365-days integration was carried out with the CAM3.1 initial dataset, which was simulated from 1 September. The monthly mean results were then output. Sensitivity experiment: To present the influence of the increasing TP-plain thermal difference, we designed a sensitivity experiment with a 3 K temperature difference between the TP and the plain, which was double the original value. In the numerical experiments, the scopes of the plateau and the plain were consistent with the previous diagnostic analysis. This experiment took the same initial datasets and integration time as those in the control experiment. From 1 June to 31 August in the sensitivity experiment, we prescribed the TP-plain temperature difference at 500 hPa to be 3 K. Because there was a temperature difference (TCHA) between the area-mean temperature over the TP (AVEQZ) and over the plain (AVELAND) at 500 hPa, that is, TCHA= AVEQZ–AVELAND. By adding TB (TB=3 K–TCHA) to the temperature of each grid at 500 hPa over the TP in each step, the temperature difference between the TP and the plain was maintained at 3 K. The differences between the sensitivity experiment and the control one show that precipitation decreases in northeast of the TP and increases in southeast of the TP when the TP-plain temperature difference is increased (Figure 6). At 500 hPa, the cyclonic difference circulation covers the eastern TP, and an anticyclonic difference circulation emerges in the north of the TP. At 700 hPa, the cyclonic difference circulation around the TP is remarkable. The simulation results show that the variation of the TP-plain thermal difference is closely related to the circulation over the TP and its adjacent areas, and precipitation over the eastern TP, which is consistent with the diagnostic analysis. However, there are some differences between the numerical experiment and the diagnostic analysis, such as the abnormal southerlies appearing over East China (Figure 6), which is opposite to the distribution in Figure 5. We consider that the general circulation in East Asia is jointly affected by the TP-plain and land-sea thermal contrast. The TP-plain thermal difference mainly influences the circulation over the TP and its adjacent areas, and the land-sea contrast affects the circulation in East China. In this work, we designed the numerical experiment only to consider the variation in TP-plain thermal difference, resulting in better simulation results near the TP and worse results in East China. Therefore, we should conduct further investigation on the joint influence of the two-stage thermal differences of TP-plain and land-sea on the East Asian general circulation.

5

Summary and discussion

Using the NCEP reanalysis and data from 160 stations in

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Figure 6 Difference value of rainfall in west China (a), winds at 500 hPa (b) and at 700 hPa (c) between the control and sensitivity experiment (Unit: mm/d; m/s).

China from 1951 to 2007, the TP-plain thermal difference and its relationship with the precipitation in China in the summer have been researched by the diagnostic analysis and numerical experiment. The major findings of this work are summarized as follows, with suggestions for further research. (1) In the summer, along the latitudes (27.5°−40°N) wherein the main part of the TP is situated, the TP is a warm center, and the regions from east of the TP to the

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West Pacific are cool areas. The boundary between the two areas is along 100°−110°E. The area-mean temperature difference between (27.5°−40°N, 80°−100°E) and (27.5°−40°N, 110°−120°E) at 500 hPa is defined as an index (dexT-C), which approximately represents the TP-plain thermal difference. Analyses suggest that there is a remarkable connection between the TP-plain temperature difference and the summer rainfall in West China. (2) Variation in summer rainfall in southern West China (90°−110°E) is opposite to that in northern West China. Specifically, the opposite correlation is remarkable between the northeastern and the southeastern TP. In the last 57 years (1951−2007), the precipitation pattern in West China (90°−110°E) had changed from “northern flood and southern drought” to “southern flood and northern drought”, which is closely connected with the interdecadal variation of the dexT-C. (3) In the years of high dexT-C, the thermal low over the TP and the high pressure ridge near Lake Baikal strengthened; the NPSH shifts more southward than the normal; and the abnormal divergence low flows over the northeastern TP and abnormal convergence flows over the southeastern TP, which are beneficial to the appearance of southern flood and northern drought in West China (90° −110°E). In the low dexT-C years, the anomalous distributions of rainfall and circulation are opposite to those in high dexT-C years. This study demonstrates that the variability of rainfall in West China (90°−110°E) is closely associated with the variability in the thermal difference between the TP and the plain. Previous studies have concluded that the land-sea thermal contrast can impact the precipitation in East China. Thus, what relationship and distinction exist between the sea-land and TP-plain thermal contrast? Furthermore, how can they impact the summer rainfall of China together? Recently, Yu et al. [24,25] pointed out that the inter-decadal variation of temperature in the upper troposphere (200−500 hPa) over eastern Asia is strongly linked to the variability of rainfall in China. Thus, is the variability of temperature in the upper troposphere connected to the variability of the TP-plain temperature difference? Moreover, what is the mechanism of the inter-annual and inter-decadal variation of the dexT-C? After 2001, the dexT-C showed a distinctly decreasing trend. At the same time, precipitations were observed to increase in the northeastern TP and decrease in the southeastern TP. Therefore, do the above phenomena mean that the abnormal rainfall pattern of “southern flood and northern drought” in West China (90°−110°E) is changing? All the above-mentioned issues need further research in the future.

The authors are grateful to Dr. Zhao Bin for his help. They would also like to thank the two anonymous reviewers for their valuable suggestions and

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comments. This work was supported by the National Natural Science Foundation of China (Grant Nos. 40505014 and 90711003) and Special Scientific Research Project for Public Interest (Grant No. GYHY20070 6010).

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May

2010

Precise dating of abrupt shifts in the Asian Monsoon during the last deglaciation based on stalagmite data from Yamen Cave, Guizhou Province, China········································································································································································· 633 Origin of CO2 in natural gas from the Triassic Feixianguan Formation of Northeast Sichuan Basin··························································· 642 Ionic conductivity anomaly in soil cover—Exploration of blind mineralization beneath regolith cover ······················································ 649 SHRIMP U-Pb dating and Hf isotopic analyses of Middle Ordovician meta-cumulate gabbro in central Qiangtang, northern Tibetan Plateau ··························································································································································································· 657 Effects of hydrocarbon physical properties on caprock’s capillary sealing ability ······················································································· 665 Heat flow and its coalbed gas effects in the central-south area of the Huaibei coalfield, eastern China······················································· 672 Spatial-temporal variation in soil respiration and its controlling factors in three steppes of Stipa L. in Inner Mongolia, China Starch grains from dental calculus reveal ancient plant foodstuffs at Chenqimogou site, Gansu Province ·················································· 694 The fluxes and controlling factors of N2O and CH4 emissions from freshwater marsh in Northeast China ················································· 700 Assessing and regulating the impacts of climate change on water resources in the Heihe watershed on the Loess Plateau of China ·········· 710 Retrieving crop leaf area index by assimilation of MODIS data into a crop growth model ········································································· 721 Wide area real time kinematic decimetre positioning with multiple carrier GNSS signals ·········································································· 731 Principles and methods for the validation of quantitative remote sensing products ····················································································· 741 An emission source inversion model based on satellite data and its application in air quality forecasts ······················································ 752 A one-dimensional heat transfer model of the Antarctic Ice Sheet and modeling of snow temperatures at Dome A, the summit of Antarctic Plateau ····················································································································································································· 763 Turbulent intensity and its similarity function over an Inner Mongolian grassland during spring ······························································· 773 The gravitational gradient tensor’s invariants and the related boundary conditions ····················································································· 781