JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, D19114, doi: /2011jd015696, 2011

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi: /2011jd015696, 2011 Correlation of surface sensible heat flux in the arid region of northwestern China with the northern boundary of the East Asian summer monsoon and Chinese summer precipitation Hui Wang 1 and Dongliang Li 1 Received 25 January 2011; revised 20 July 2011; accepted 24 July 2011; published 13 October [1] Northwestern (NW) China is the typical arid region of central Asia, and its surface sensible heat (SSH) anomaly significantly affects the Chinese climate and the atmospheric circulation of East Asia. In this study, we investigated the relationship between the SSH flux in the NW arid region of China and the northern boundary of the East Asian summer monsoon (EASM) and Chinese summer rainfall using a climatic diagnosis analysis method. Then the causes of formation were analyzed from the changes of the transfer of water vapor, geopotential height field, and the upper and lower level atmospheric circulation fields, and so on. It is found that during years of unusually weak (strong) SSH flux, the northern boundary of the EASM shifts northward (southward) than in normal years. There is an interplay between the SSH in the NW arid region of China and the precipitation in the northern boundary zone of the EASM: In the early stage of the monsoon, the SSH inhibits the latter precipitation, and during the peak of the monsoon, the precipitation suppresses the SSH. The teleconnection wave train structure of the geopotential height field at 500 hpa and the upper lower level atmospheric circulation fields above the Eurasian continent exhibit profound changes when summer SSH fluxes are unusually weak and strong. These changes are accompanied by significant alterations to the vertical velocity field and the water vapor field above northern China. The combination of these changes thereby contributes to the unusually southward shift of the northern boundary of the EASM. Citation: Wang, H., and D. Li (2011), Correlation of surface sensible heat flux in the arid region of northwestern China with the northern boundary of the East Asian summer monsoon and Chinese summer precipitation, J. Geophys. Res., 116,, doi: /2011jd Introduction [2] China is located in the East Asian (EA) monsoon region. The beginning of the rainy season in eastern China corresponds to the onset of the summer monsoon; the summer monsoon precipitation develops northward and reaches its northernmost boundary (the Hetao, Northern China, and northeastern (NE) China) after mid July. Subsequently, the rain belt retreats southward along with the monsoon. The evolution and progression of the summer monsoon have a profound influence on some crucial weather and climate phenomena, including regional precipitation, drought, flood, and northern sandstorms in China. Since the EA summer monsoon (EASM) shows annual fluctuations and exhibits an unstable northern boundary, a monsoon boundary zone with high annual variability in precipitation ranges from the upper 1 Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China. Copyright 2011 by the American Geophysical Union /11/2011JD Yellow River to northern China [Xu and Qian, 2003]. Previous studies of the influence of the EASM on Chinese climate have primarily focused on the major precipitation regions, including southern China and the Yangtze Huaihe River basin [Guo, 1983; Shi et al., 1996; Huang and Yan, 1999; Huang et al., 1999a], but ignored rainfall in the northern monsoon boundary region. There are two major climatic systems that influence the northern boundary zone of the EASM: the northern arid climatic system and the southern monsoon climatic system. This boundary zone has an annual precipitation of mm [Shi, 1996], so the impact of the change in precipitation on vegetation is more significant, and the presence (absence) of summer monsoon rainfall will bring wet (dry) climate conditions in this region. This monsoon boundary zone is not only semiarid but also a climatically sensitive and ecologically vulnerable zone [Ou and Qian, 2006]. In addition, this area is prone to natural disasters [Shi et al., 1994; Huang and Zhou, 2002]. Hence, it is scientifically and practically important to study the boundary zone of the EASM. [3] The monsoon is an atmospheric consequence of seasonal variation in land sea thermal differences, and, as such, 1of15

2 Table 1. Correlation Coefficients Between the Northern Boundary of the EASM and the SSH for Different Time Intervals Average in the NW Arid Region NW Arid Region May June May July May Aug. June Aug. May Sept. May June July Aug. Sep. Eastern a a a a a a a Western Total b b a 0.01 confidence by t test, a 0.01 = b 0.05 confidence by t test, a 0.05 = it is strongly influenced by the thermal situation on land. Many studies have indicated that thermal abnormalities in the Qinghai Tibet Plateau have a significant impact on the formation and evolution of the Asian summer monsoon [Nitta, 1983; Luo and Yanai, 1984; Huang, 1984, 1985; Yanai et al., 1992; Molnar et al., 1993; Wu and Zhang, 1998] and lead to aberrant atmospheric circulation in the Northern Hemisphere as well as irregular climatic patterns in China [Zhao and Chen, 2001a, 2001b; Li et al., 2001; Liu et al., 2002; Duan et al., 2003; Zhao et al., 2003; Ning and Qian, 2006; Zhao and Qian, 2007, 2009; Tang and Yu, 2008]. Northwestern (NW) China is the typical arid zone of central Asia and is characterized in the summer by extremely strong surface sensible heat (SSH); the strength of SSH even surpasses that of some plateau areas. The studies by Huang et al. [2006] and Zhou and Huang [2008] have indicated that an annual variation in the SSH on Chinese NW arid and semiarid areas during the spring and summer is one of the major factors contributing to annual fluctuations in climatic hazards, including drought and flood. Gao et al. [2008] used numerical experiments to reveal that unusual SSH in the NW arid regions significantly affects summer rainfall in China. Unfortunately, not much work on the SSH flux in vast arid areas in China and its influences have been carried out. We analyze the relations between and the causes of the aberrant SSH on arid regions of NW China and the northern boundary of the EASM and summer precipitation in China. We wish to provide a theoretical basis for drought and flood predictions during the flood season and the environmental protection in the farming grazing transitional belt in China. 2. Examination of the Influence of SSH Flux on NW Arid Regions [4] The monthly surface thermal transfer coefficients of 84 meteorological stations in the arid region of NW China are calculated with the aid of the normalized difference vegetation index (NDVI) data from 1982 to 2006 observed by National Oceanic and Atmospheric Administration (NOAA) remote sensing satellites and C h I NDV parametric relational expressions rather than taking C h as fixed value [Wang and Li, 2010]; then the monthly SSH fluxes are calculated using the bulk transfer method with the observations from the meteorological stations [Wang and Li, 2011]. Monthly precipitation data were collected from 743 weather stations in China. Rain belt classification data were obtained from the National Climate Center of China. The data concerning the northern boundary of the EASM were provided by Huang et al. [2009] Relation to the Northern Boundary of the EASM [5] Important achievements have been made in studies of the definition of northern boundary of the EASM and the variations in its position [Tang et al., 2006, 2009; Jiang et al., 2006; Hu and Qian, 2007; Huang et al., 2009]. However, differences in data collection methods and representations have led to large discrepancies in the proposed northern Figure 1. Evolution of the northern boundary of the EA summer monsoon (solid line) and the summer (JJA) average SSH in the eastern part of the NW arid region (dotted line) from 1982 to of15

3 Figure 2. Changes of the northern boundary zones and their locations of the EASM. (a) Total precipitation from May to September of , average, (b) integrated data for years of unusually strong SSH, (c) integrated data for years of unusually weak SSH, (d) locations of the northern boundary of the EASM (dotted line is 100 mm summer isohyet of , average, and the continuous thin line (heavy solid line) is for years of unusually strong (weak) SSH). 3of15

4 Table 2. Rain Belt Types and SSH Anomalies for Years of Unusually Strong (Weak) May September SSH Years of Strong H H Anomaly Rain Belt Type Years of Weak H H Anomaly Rain Belt Type boundary of the summer monsoon. Using the standard of soaking rainfall (i.e., total precipitation of 20 mm during a continuous rainfall) by Huang et al. [2009], this paper defines the northern boundary zone of the summer monsoon and the northern limit of the boundary zone as the locations where there are six times the amount of soaking rainfall and three times the amount of soaking rainfall, respectively, from April to October in the span of one year. The CresDan function was used to interpolate data from over 700 sites nationwide onto a horizontal grid. The northern boundaries were the latitude value that is closest to the northern boundary zone along 110 E in a given year. Therefore the northern boundary of East Asian summer monsoon has one latitude value in every year. Using the soaking rainfall as the criterion to demarcate the monsoon zone, the impact of the summer monsoon on plants, particularly crops, has underscored the monsoon boundary zone of China. [6] The studies by Wang and Li [2011] have indicated that considering 97.5 E as the boundary in the NW arid region of China, the SSHs in its eastern and western parts have different interannual variation trends in the four seasons. So this paper discusses the two distinct parts respectively. Correlation coefficients between the northern boundary of the EASM and the SSH flux in the eastern and western parts of the NW arid region in China, as well as the entire NW arid region, during various intervals between May and September are listed in Table 1. Table 1 demonstrates that the northern monsoon boundary has a strong and consistent negative correlation with the SSH in the eastern part of the NW arid region over these intervals. Specifically, the average coefficients from May to July, from May to August, and from May to September are all less than Those average values in summer (June, July, and August (JJA)) and from May to June are around 0.65, and the coefficients in May and June are both close to All of these values are valid at the 1% confidence level. As for the total NW arid region, only the coefficients in May and the average value from May to June exceed the 5% confidence level, whereas the rest of the periods display poor correlation. Although SSH transfer in the western part of the region is stronger than SSH transfer in the eastern part, it exhibits a weak correlation with the northern boundary of EASM. Together, the data indicate that the south north migration of the northern boundary of the EASM is markedly influenced by SSH transfer in the eastern part of the NW arid region but not by that in the western part. [7] Figure 1 clearly illustrates that when the summer average SSH flux in the eastern part of the NW arid region is strong, the northern boundary of the EASM shifts southward, and vice versa. Data from other time intervals also display a similar pattern (figures not shown). Rainfall is one of the significant characteristics of the monsoon; as such, precipitation can be used to characterize the monsoon. Tang et al. [2006] indicated that a 100 mm isohyet in summer, Figure 3. Correlation between summer SSH in the eastern part of the NW arid region and summer precipitation in China (shaded areas are statistically significant at the 0.05 confidence level; solid lines indicate positive correlations while dotted lines indicate negative correlations). 4of15

5 Table 3. Correlation Between SSH in the Eastern Part of the NW Arid Region from May Through September (H 5 H 9 ) and Rainfall in Hetao and Northern China Regions from May to September (R 5 R 9 ) Correlation Coefficients H 5 H 6 H 7 H 8 H 9 R R R R R similar to the monsoon in its annual migration (northward or southward), can be used to approximate the northernmost boundary of the summer monsoon. On the other hand, Shi [1996] defined the monsoon boundary zone of China as the areas in which there is annual precipitation of mm (or 500 mm). Our analysis of monthly precipitation data from 743 sites nationwide showed that the May September precipitation on average accounts for 75.1% of the annual precipitation nationwide and over 80% of the annual precipitation in northern China. Hence, we use the mm isohyet from May to September to approximate the northern boundary of the summer monsoon in this paper; specifically, according to the standard that the average SSH abnormal flux from May to September in the eastern part of the NW arid region was over 2 W/m 2 (one standard deviation) higher (lower) than during regular years; five years (1982, 1991, 1997, 2005 and 2006) with unusually strong SSH as well as five years (1985, 1988, 1992, 1994 and 2002) with unusually weak SSH were integrated and analyzed. The threshold 2 W/m 2 was chosen because one standard deviation is an objective value for climatic anomaly analysis [Nagura and Konda, 2007]. [8] Figure 2 shows the changes of the northern boundary zones and their locations of the EASM in different SSH transport conditions of the eastern part of the NW arid region. Figure 2a illustrates the average May September precipitation distribution from 1982 to 2006 in China, the precipitation distribution when the SSH of the eastern part of the NW arid region was unusually strong (Figure 2b), and the precipitation distribution when the SSH was unusually weak (Figure 2c). By comparing Figures 2b and 2a, it can be seen that when there is strong SSH in the eastern part of the NW arid region, the 100 mm and 200 mm isohyets apparently migrate southward, the 400 mm isohyet shifts slightly southward, and the northern boundary zone of the summer monsoon slightly widens. Further, Figures 2c and 2a reveal that when there is weak SSH in this area, the 100 mm, 200 mm, and 400 mm isohyets all clearly shift northward, and the northern boundary zone of the summer monsoon narrows. Figure 2d clearly exhibits that when the summer average SSH flux in the eastern part of the NW arid region is unusually strong (weak), the northern boundary location of the EASM moves southerly (northerly). During years of aberrant SSH, the Hetao region and northern China display the most significant migrations of the northern boundary zone of the summer monsoon (Figure 2). These observations indicate that strong (weak) SSH results correspond to southward (northward) shift of isohyets that indicates the northern boundary zone of the summer monsoon in China. Table 2 lists the rain belt types and the anomalies when the average SSH from May to September in the eastern part of the NW arid region is unusually strong and unusually weak. These data appear to reveal a pattern that unusually strong SSH leads to a type II or III rain belts (i.e., rainfall concentrated in the Yangtze River basin or southern China) and that very weak SSH leads to a type I rain belt (i.e., rainfall concentrated in northern China). Nevertheless, a type III rain belt also appears in some years of weak SSH, which may stem from the northwestward shift of the subtropical high. This shift causes a double rain belt in both northern and southern China, which is consistent with the analysis of the rain belt type in China when the northern boundary of the summer monsoon shifts northward or southward [Huang et al., 2009] Correlation Analysis of Summer Precipitation in China [9] Summer precipitation in China exhibits an apparent annual variation. There was a significant climate shift in 1976, and since 1977 there has been a drastic reduction of summer rainfall and continuous drought in northern China coupled with increased rainfall and frequent flooding in the Yangtz Huaihe River basin [Huang et al., 1999b; Zhou and Huang, 2003]. Furthermore, these studies indicated that such a climatic evolution was heavily influenced by aberrant SSH in the arid regions of China. As discussed above, during the years of aberrant strong (weak) SSH, the northern boundary zone of the EASM usually shifts southward (northward), resulting in more frequent precipitation in the south (north). Hence, the correlation between the summer sensible heat flux and summer precipitation fields at over 700 sites is examined (Figure 3). As shown in Figure 3, the total summer precipitation and SSH variation exhibit negative correlations in the Hetao region (the Hetao region is located in the area that looks like Л in shape and its neighboring basin of the Yellow River), northern China, NE China, and the area south of the Yangtze, yet the two are positively correlated in the Yangtze Huaihe River basin and in the plateau regions, which can be generalized into a plus minus plus ( + ) pattern from south to north. The correlation coefficients between the total summer precipitation and SSH in the entire Hetao region are valid at the 5% confidence level and at the 1% level in most areas there. [10] Table 3 presents the correlations between the May September SSH (H 5 H 9 ) in the eastern part of the NW arid region and the May September rainfall in the Hetao region and northern China (R 5 R 9 ). The rainfall data from May to September of 20 sites (Jingtai, Jingyuan, Yuzhong, Lintao, Narenbaolige, Darhanlianheqi, Hohhot, Otogqi, Dongsheng, Alashanzuoqi, Yinchuan, Zhongning, Yanchi, Wuqi, Hengshan, Haiyuan, Tongxin, Guyuan, Huanxian, and Lanzhou) that exhibit a high correlation (1% confidence level) with the SSH during the same period were selected in the Hetao region and in northern China between 1982 and Monthly average data were generated from the precipitation data before further analysis. As shown in Table 3, the SSH in the eastern part of the NW arid region exhibits the strongest negative correlation (all values are valid at the 1% confidence level) with precipitation in the Hetao region and in northern China during the corresponding period. The correlation coefficient is highest in June ( 0.871). In addi- 5of15

6 Figure 4. Distribution of the composite moisture flux anomaly vertically integrated from the ground surface to 300 hpa during years with (a)unusually weak and (b) unusually strong SSH fluxes in summer (unit: kg m 1 s 1). (Shaded areas are statistically significant at the 0.10 confidence level.) 6 of 15

7 Figure 5. Same as Figure 4 but for an anomaly distribution of the geopotential altitude field at 500 hpa (unit: geopotential meters). tion, a sound negative correlation is found between the SSH in May and precipitation in June and between the SSH in August and precipitation in July (1% confidence level). These results suggest that, during the early summer (May), which corresponds to the onset of the monsoon, the SSH in the eastern part of the NW arid region inhibits the latter precipitation. On the other hand, during the peak of the monsoon (July), rainfall in the northern boundary zone of the monsoon suppresses increases in the SSH in the arid region. Thus, the two factors exhibit a mutual influence. [11] The next question is, how does an unusually weak (strong) SSH affect atmospheric water vapor transport over the Chinese mainland? To address this question, according to the standard that the average summer SSH abnormal flux in the eastern part of NW arid region was over 2 W/m 2 higher (lower) than during regular years, five years (1984, 1988, 1992, 1994 and 1996) with unusually weak SSH and six years (1982, 1991, 1997, 2001, 2005 and 2006) with unusually strong SSH are selected and analyzed. The alculation formula for moisture flux is Q ¼ 1 g Z Ps 300 VqdP: ð1þ [12] V is the meridional wind or zonal wind speed (m/s), q is specific humidity (g/kg) and P is atmospheric pressure (hpa). All of the variable field data were derived from the NECP Reanalysis Data Set II. The anomaly distributions of the summer moisture flux vertically integrated from the ground surface to 300 hpa reveal that water vapor that originated from the western Pacific Ocean and the South China Sea can reach the Hetao region, northern China, and even NE China via the Yangtze Huaihe River basin during years with unusually weak SSH fluxes in the eastern part of the NW arid region (Figure 4a). The accumulation of water vapor is conducive to summer rainfall in the Hetao region and northern China. There is insufficient water vapor in the Yangtze Huaihe River basin, resulting in poor summer precipitation in this region. Therefore, during years of weak SSH flux, type I or III rain belts are generally observed. During years of strong sensible heat flux (Figure 4b), water vapor that originates from the Sea of Japan can reach the Yangtze Huaihe River basin. On the other hand, water vapor that originated from the Bay of Bengal can reach the southern Qinghai Tibet Plateau and the South China Sea. Therefore, during these years, water vapor that originated from the Bay of Bengal and the South China Sea cannot reach the areas north of the Yellow River, 7of15

8 Figure 6. Same as Figure 4 but for an anomaly distribution of the wind field at 850 hpa (unit: m s 1 ). leading to poor summer precipitation in northern China and producing type II or III rain belts. [13] The above analysis demonstrates that a relatively weak (strong) SSH flux in the summer in the eastern part of the NW arid region is favorable (unfavorable) for the northward transportation of water vapor and results in unusually high (low) precipitation in the Hetao region and in northern China but abnormally low (high) rainfall in the Yangtze Huaihe River basin. Correspondingly, the northern boundary of the summer monsoon shifts unusually far southward (northward). Thus, the fluctuation of SSH in the NW arid region of China closely correlates with summer rainfall in China. The study of SSH in the arid region may provide an explanation for mechanisms of flood or drought development in northern China. 3. Analysis of Atmospheric Circulation Types During Years of Unusually Strong (Weak) SSH [14] Our above analyses reveal a sound correlation between the aberrant change in the SSH in the NW arid region of China and the northern edge of the EASM as well as the summer rainfall in China. Thus, it is necessary to analyze the cause of atmospheric circulation based on its properties during years when the summer sensible heat flux is unusually strong (weak). In this section, all of the variable field data were derived from the NECP Reanalysis Data Set II Changes of Geopotential Height Field at 500 hpa [15] Figure 5 illustrates the anomaly distribution of the 500 hpa geopotential height field in summer during years of weak (strong) sensible heat flux. During years with weak sensible heat flux, the midlatitude area at the anomalous 500 hpa displayed a + + wave train structure from west to east, in which negative anomaly centers of the geopotential height field appear in West Africa, the Arabian Peninsula, Lake Baikal, the western Pacific Ocean, and the east Siberian Sea and positive anomaly centers emerge in the western European continent, the Tian Shan mountains, the Central Siberian Plateau and the Japanese islands (Figure 5a). During these years, the eastern part (100 E 110 E) of the midlatitude to high latitude region contains a long wave anomaly trough. The subtropical high shifts northwestward and is relatively strong; its western boundary harbors water 8of15

9 Figure 7. Same as Figure 4 but for an anomaly distribution of the zonal wind field at 200 hpa (unit: m s 1 ). vapor migrating northward. There is an updraft in front of the long wave trough, which results in frontal uplift after clashing with the strong cold air force behind the trough. This anomaly increases precipitation in the areas of the Hetao region and in northern China [Qian, 2004], and the northern boundary zone of the EASM therefore shifts northward. On the other hand, during the years of strong sensible heat, we can see from Figure 5b that the midlatitude area develops a wave train structure of + + from west to east. Specifically, West Africa, the Arabian Peninsula, Lake Baikal, and northern China are the positive anomaly centers, and the Mediterranean Sea, the West Siberian Plain and the Japanese islands represent the negative anomaly centers. During these years, the subtropical high over the western Pacific is unusually weak and the geopotential height is unusually high in northern China, which are unfavorable to the northward migration of the summer monsoon and to subsequent precipitation; the northern boundary zone of the EASM shifts southward. Our analyses indicate that, in comparison with the years with weak sensible heat, the anomalous 500 hpa geopotential height field exhibits conspicuous changes in the Eurasian continent during years with strong sensible heat, and the northern China area transitions from a negative anomaly to a positive anomaly Changes at Low Level and Upper Level Atmospheric Circulation [16] Because the wind field at 850 hpa perfectly illustrates the characteristics of low level monsoon circulation, we examine the anomaly pattern of the wind field at 850 hpa (Figure 6) during years in which the summer sensible heat is unusually weak and unusually strong. As shown in Figure 6a, during years with weak sensible heat, two monsoon circulation wave trains appear above the Eurasian continent and the western Pacific Ocean, respectively, and display aberrant teleconnection distributions. The midlatitude wave train exhibits an anticyclonic circulation anomaly above the Alps Mountains, a cyclonic circulation anomaly above the Caspian Sea, an anticyclonic circulation anomaly above Lake Balkash, a cyclonic circulation anomaly above the Mongolian Plateau, and an anticyclonic circulation anomaly above the area east of Japan. The low latitude wave train displays an anticyclonic circulation anomaly above West Africa and the Arabian Peninsula, a cyclonic circulation anomaly above the Arabian Sea, an anticyclonic circulation anomaly above the South China Sea, and a cyclonic circulation anomaly above the western Pacific Ocean. At the same time, a southerly flow anomaly throughout NE China, northern China, and eastern China is favorable for the 9of15

10 Figure 8. Same as Figure 4 but for an anomaly distribution of the meridional wind field at 200 hpa (unit: m s 1 ). northward migration of the summer monsoon as well as the northward transportation of water vapor originating from the East China Sea and the Bay of Bengal. In the Hetao region and northern China, the water vapor meets with a northern dry and cold flow stemming from the cyclonic circulation anomaly above the Mongolian Plateau and results in rich summer rainfall. During years with strong sensible heat (Figure 6b), however, the two monsoon circulation wave trains that emerge above the Eurasian continent and the western Pacific Ocean display the opposite teleconnection distribution. Specifically, the midlatitude circulation wave train exhibits cyclonic circulation anomalies above the Alps Mountains, the West Siberian Plain and the area east of Japan, but it exhibits anticyclonic circulation anomalies above the Caspian Sea and the Mongolian Plateau. The lowlatitude wave train exhibits cyclonic circulation anomalies above the Arabian Peninsula and the South China Sea and anticyclonic circulation anomalies above West Africa, the Arabian Sea, and the western Pacific Ocean. As such, a northerly flow disturbance appears above southern China, which severely affects the migration to the Hetao region and northern China of a warm and wet flow originating from the South China Sea and the Bay of Bengal, and thus reduces local rainfall and leads to continuous drought. [17] Large differences were observed in the low level atmospheric circulation above the Eurasian continent and the western Pacific Ocean between years with strong and weak sensible heat. At 850 hpa wind field, the southerly flow anomaly above the eastern Chinese mainland in years with weak SSH changes to a northerly flow anomaly in years with strong SSH. The change is detrimental to the northward migration of the EASM and results in an aberrant southward shift of the EASM s northern boundary zone. [18] However, changes in the SSH are not only influenced by low level atmospheric circulation but are also strongly correlated with upper level circulation. Thus, it is also important to investigate the evolution of upper level atmospheric circulation during years in which the summer SSH fluxes in the NW arid region of China were unusually weak or strong. Figures 7 and 8 illustrate the integrated anomaly distribution of zonal and meridional wind fields at 200 hpa during years with aberrant SSH. Figure 7a shows that during years with weak SSH, easterly anomalies emerge above the Iranian Plateau in the midlatitude Asian continent and around the Sea of Japan, indicating that westerly circulation is weaker than the circulation in regular years. The airflow between 40 N and 55 N exhibits a westerly anomaly, that is, the westerly jet is shifting northward. On the right hand 10 of 15

11 Figure 9. Same as Figure 4 but for anomaly altitude longitude profiles of vertical velocity (w 100) in the zonal 35 N 45 N area. side of the entrance region of the westerly jet, the pressure of the low level atmosphere decreases because of the convergent movement, and there are ascending flows in the Hetao region and northern China that make the summer precipitation increase in these regions [Zhou, 2009; Fang et al., 2009]. During years with strong SSH (Figure 7b), however, the 200 hpa zonal wind field exhibits significant changes, which include the emergence of an easterly 11 of 15

12 Figure 10. Same as Figure 4 but for anomaly altitude latitude profiles of vertical velocity (w 100) in the zonal 105 E 120 E area. anomaly north of 30 N and the appearance of a westerly anomaly between 15 N and 30 N (the westerly jet is shifting southward). This change is detrimental to summer rainfall in the Hetao region and in northern China [Zhou, 2009; Fang et al., 2009]. The 200 hpa zonal wind field above the Chinese mainland exhibits profound changes in years with aberrant SSH. A weak (strong) SSH results in southward (northward) shifting of the westerly jet, which is 12 of 15

13 favorable (unfavorable) for summer rainfall in the Hetao region and in northern China. As such, the northern boundary zone of the EASM aberrantly shifts southward (northward). [19] Figure 8a illustrates the anomalous distribution of the 200 hpa meridional wind field during years with weak SSH and reveals a wave train structure of + from west to east in the midlatitude area. During years with strong SSH, the wind field changes drastically such that the wave train structure in the midlatitude area becomes + from west to east. Lu et al. [2002] reported that a teleconnection pattern exists in the meridional wind anomalies in the westerly jet of the upper troposphere above East Asia. This study discovered that this teleconnection pattern in meridional wind anomalies is present not only above East Asia, but also in the upper troposphere above the midlatitude Eurasian continent. In addition, this teleconnection pattern changes significantly between years of weak and strong summer SSHs in the NW arid region of China, and these changes heavily influence summer climate anomalies in China. [20] Taken together, years with unusually strong and unusually weak summer SSH fluxes correspond to profound changes in low level and upper level atmospheric circulations above Eurasia (especially midlatitudinal East Asia). This result indicates that unusual SSH in the NW arid region of China not only correlates with the low level circulation anomaly above East Asia, but also closely corresponds to aberrant quasi steady perturbations in the westerly jet at 200 hpa Changes of Vertical Velocity [21] Our above analyses show that the low level and upper level summer atmospheric circulations above Eurasia both exhibit profound changes between years with unusually strong and unusually weak summer SSH fluxes. These changes in circulation patterns will inevitably result in changes in the vertical velocity. This section examines changes in the integrated vertical velocity field above Asia during years with unusually weak and unusually strong summer SSH fluxes. [22] Figure 9 illustrates the altitude longitude profile of the average vertical velocity in the zonal 35 N 45 N area. w >0 corresponds to sinking motion, while w < 0 indicates ascending motion. Figure 9a shows that during years of weak sensible heat, a negative anomaly in the vertical velocity appears around 105 E 125 E, which indicates ascending motion in the Hetao region and in northern China. Increased flow convergence is beneficial for local precipitation. On the other hand, a positive anomaly in the vertical velocity emerges in the eastern part of the NW arid region of China (95 E 105 E), which results in sinking airflow. During years with strong sensible heat, the vertical velocity field exhibits the opposite pattern: A positive anomaly arises between 100 E and 140 E, while a negative anomaly appears in the NW arid region (80 E 100 E). Thus, there is significant sinking flow in the Hetao region and in northern China and ascending flow in the NW arid region. [23] Figure 10 illustrates the altitude latitude profile of the average vertical velocity in the zonal 105 E 120 E area. Figure 10a shows that during years of weak sensible heat, the negative anomalies in the vertical velocity appear around 15 N 25 N and 32.5 N 42.5 N, which indicate ascending motion in the southern and northern China. The northern boundary of ascending motion reaches to 42.5 N, indicating that the location of rain belt is northward. On the other hand, a positive anomaly in the vertical velocity emerges in the Yangtze Huaihe River basin (25 N 32.5 N), which results in sinking airflow and decreasing rainfall. During years with strong sensible heat, a negative (positive) anomaly appears in south (north) of 27.5 N and the northern boundary of ascending motion reaches only to 27.5 N, indicating that the location of rain belt is southward. [24] The vertical velocity fields in NW China, the Hetao region, and northern China are clearly different during years of strong SSH and weak SSH. Weak (strong) SSH leads to apparent ascending (sinking) airflow in Hetao and northern China, which is beneficial (detrimental) to local rainfall in the summer and causes a northward (southward) shift in the northern boundary zone of the EASM. This is consistent with the altitude latitude profile of the average vertical velocity in the zonal E area. 4. Conclusions and Discussion [25] In this paper, we utilize the calculated value of SSH fluxes in the NW arid region of China, the location of the northern boundary zone of the EASM, the monthly average precipitation from 743 weather stations in China, and the NCEP Reanalysis Data Set II to investigate how the SSH flux in the NW arid region is correlated with the northern boundary of the East Asian summer monsoon and the summer rainfall in China and its causes. Two major conclusions are drawn. [26] First, we separately examined the influences of aberrant SSH flux in the eastern and western parts of the NW arid region of China, generating better results than those that are obtained simply by studying the entire region. There is a consistent and significant negative correlation between the May September SSH in the eastern part of the NW arid region of China and the location of the northern boundary zone of the EASM. During years of unusually weak (strong) SSH flux, the northern boundary zone of the EASM shifts northward (southward) and becomes narrower (wider) than in normal years. The changes in the SSH during summer negatively correlates with the total summer precipitation in the Hetao region, northern China, NE China, and the south area of the Yangtze River but positively correlates with the total summer precipitation in the Yangtze Huaihe River basin. The correlation coefficients in the Hetao region and northern China are valid at the 1% confidence level. Furthermore, there is interplay between the SSH in the eastern part of the NW arid region of China and precipitation in the northern boundary zone of the EASM. In the early stage of the monsoon, the SSH inhibits the latter precipitation, and during the peak of the monsoon, the precipitation suppresses the SSH. [27] Second, the wave train structure of the teleconnection at the 500 hpa geopotential height field and the upper and low level circulation fields above the Eurasian continent exhibit profound changes when the summer SSH fluxes are aberrantly weak and strong. These changes are accompanied by significant alterations to the vertical velocity field and the water vapor field above the Hetao region and northern China. In the 500 hpa geopotential height field, a negative 13 of 15

14 anomaly in years with weak SSH changes to a positive anomaly in years with strong SSH above northern China. In the 850 hpa low level circulation field, a southerly flow anomaly similarly changes to a northerly flow anomaly. The upper level 200 hpa zonal circulation field exhibits a shift in the westerly jet from north to south. The wave train structure in the midlatitude area of the Eurasian continent circulation field switches from a + pattern to a + pattern, and the vertical velocity field above the Hetao region and northern China changes from ascending flow to sinking flow. The areas that are rich in water vapor move from the Hetao region, northern China, and NE China to the Yangtze Huaihe River basin. All of these changes are unfavorable for precipitation in the Hetao region and in northern China and are also detrimental to the northward migration of the EASM, and the combination of these changes thereby contributes to the aberrant southward shift of the northern boundary zone of the EASM. [28] Acknowledgments. The authors wish to thank NOAA/DOE for providing access to the meteorological data and two anonymous reviewers for their assistance in evaluating this paper. This work was supported by the National Natural Science Fund ( ), Jiangsu Province General Graduate Student Science Research Innovation Program Common Fund 2009 (CX09B 218Z), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), China. References Duan, A. M., Y. M. Liu, and G. X. Wu (2003), The sensible heat conditions of 4 6 months over Qinghai Tibet plateau and the anomalies of midsummer precipitation and atmospheric circulation in East Asia [in Chinese], Sci. China, Ser. D, 33, Fang, X. J., X. M. Zeng, and X. Q. Chen (2009), Spatial and temporal distribution characteristics of westerly jet in 200 hpa in East Asia and the analysis of relationship between it and summer precipitation in China [in Chinese], Meteorol. Environ. Sci., 32, Gao, R., W. J. Dong, and Z. G. Wei (2008), Numerical simulation of the impact of abnormity of sensible heat flux in northwest arid zone on precipitation in China [in Chinese], Plateau Meteorol., 27, Guo, Q. Y. (1983), The summer monsoon intensity index in East Asia and its variation [in Chinese], Acta Geogr. Sin., 38, Hu, H. R., and W. H. Qian (2007), Confirmation of the north boundary belt for East Asian summer monsoon [in Chinese], Prog. Nat. Sci., 17, Huang, F., et al. (2009), Determination on the north boundary of summer monsoon in East Asian with soaking rainfall [in Chinese], J. Appl. Meteorol. Sci., 20, Huang, G., and Z. W. Yan (1999), East Asian summer monsoon circulation anomaly index and its interannual variability, Chin.Sci.Bull., 44(14), , doi: /bf Huang, R. H. (1984), The characteristics of the forced planetary wave propagations in the summer northern hemisphere, Adv. Atmos. Sci., 1, Huang, R. H. (1985), Numerical simulation of the three dimensional teleconnections in the summer circulation over the Northern Hemisphere, Adv. Atmos. Sci., 2, 81 92, doi: /bf Huang, R. H., and L. T. Zhou (2002), Research on the characteristics, formation mechanism and prediction of severe climatic disasters in China [in Chinese], J. Nat. Disasters, 11, 1 9. Huang, R. H., G. Huang, and B. H. Ren (1999a), Advances and problems needed for further investigation in the studies of the East Asian summer monsoon [in Chinese], Adv. Atmos. Sci., 23, Huang, R. H., Y. H. Xu, and L. T. Zhou (1999b), The interdecadal variation of summer precipitations in China and the drought trend in north China [in Chinese], Plateau Meteorol., 18, Huang, R. H., R. S. Cai, J. L. Chen, and L. T. Zhou (2006), Interdecadal variations of drought and flooding disasters in China and their association with the East Asian climate system [in Chinese], Chin.J.Atmos. Sci., 30, Jiang, Z. H., et al. (2006), Northerly advancement characteristics of the East Asian summer monsoon with its interdecadal variations [in Chinese], Acta Geogr. Sin., 61, Li, D. L., G. L. Ji, and L. Z. Lv (2001), The effect of surface sensible heat flux of the Qinghai Tibet plateau on general circulation over the northern hemisphere and climatic anomaly of China [in Chinese], Sci. Chin., Ser. D, 31, supplement, Liu, X., W. P. Li, and G. X. Wu (2002), Interannual variation of the diabatic heating over the Qinghai Tibet plateau and the northern Hemispheric circulation in summer [in Chinese], Acta Meteorol. Sin., 60, Lu, R. Y., J. H. Oh, and B. J. Kim (2002), A teleconnection pattern in upperlevel meridional wind over t he North African and Eurasian continent in summer, Tellus, Ser. A, 54, Luo, H. B., and M. Yanai (1984), The large scale circulation and heat source over the Tibetan Plateau and surrounding areas during the early summer of 1979, Mon. Weather Rev., 108, Molnar, P., P. England, and J. Martinod (1993), Mantle dynamics, uplift of Tibetan Plateau and the Indian monsoon, Rev. Geophys., 31, , doi: /93rg Nagura, M., and M. Konda (2007), The seasonal development of an SST anomaly in the Indian Ocean and its relationship to ENSO, J. Clim., 20, 38 52, doi: /jcli Ning, L., and Y. F. Qian (2006), Oscillation characteristics of sensible heat in North Africa and Qinghai Xizang plateau and their impacts on the rainfall in East China [in Chinese], Plateau Meteorol., 25, Nitta, T. (1983), Observational study of heat sources over the eastern Tibetan Plateau during the summer monsoon, J. Meteorol. Soc. Jpn., 61, Ou, T. H., and W. H. Qian (2006), Vegetation variations along the monsoon boundary zone in East Asia [in Chinese], Chin. J. Geophys., 49, Qian, W. H. (2004), Chinese precipitation, in Synoptic Meteorology, pp , Peking Univ. Press, Beijing. Shi, N., Q. G. Zhu, and B. G. Wu (1996), The East Asian summer monsoon in relation to summer large scale weather climate anomaly in China for recent 40 years [in Chinese], Sci. Atmos. Sin., 20, Shi, Z. T. (1996), Regional characters of natural disaster in marginal monsoon belt of China [in Chinese], J. Arid LandResour. Environ., 10, 4 7. Shi, Z. T., L. Y. Zhang, and G. W. Su (1994), Natural disasters and their formation causes on Chinese monsoon marginal belt [in Chinese], J. Catastrophol., 9, Tang, X., W. H. Qian, and P. Liang (2006), Climatic features of boundary belt for East Asian summer monsoon [in Chinese], Plateau Meteorol., 25, Tang, X., et al. (2009), Definition and featuers of the north edge of Asian summer monsoon [in Chinese], Acta Meteorol. Sin., 67, Tang, Y., and J. H. Yu (2008), A study on the correlation between Tibetan plateau surface sensible heating and the precipitation over north China during rainy season [in Chinese], Sci. Meteorol. Sin., 28, Wang, H., and D. L. Li (2010), Estimation of the surface thermal transfer coefficients over the arid region of Northwest China with the aid of satellite remote sensing and field observations [in Chinese], Chin. J. Atmos. Sci., 34, Wang, H., and D. L. Li (2011), Characteristics of the surface sensible heat flux calculated with the aid of satellite remote sensing and field observations over the arid region of northwest China in summer [in Chinese], Arid Land Geogr., 34, Wu, G. X., and Y. S. Zhang (1998), Tibetan Plateau forcing and timing of south Asia monsoon and South China Sea monsoon, Mon. Weather Rev., 126, , doi: / (1998)126<0913:tpfatt>2.0. CO;2. Xu, Y., and W. H. Qian (2003), Research on East Asian summer monsoon: A review [in Chinese], Acta Geogr. Sin., 58, supplement, Yanai, M., C. Li, and Z. S. Song (1992), Seasonal heating of the Tibetan Plateau and its effects on the evolution of the Asian summer monsoon, J. Meteorol. Soc. Jpn., 70, Zhao, P., and L. X. Chen (2001a), Interannual variability of atmospheric heat source/sink over the Qinghai Xizang (Tibetan) Plateau and its relation to circulation, Adv. Atmos. Sci., 18, , doi: /s Zhao, P., and L. X. Chen (2001b), The relationship between the climate characteristics of atmospheric heat source over Qinghai Tibet Plateau and Chinese precipitation in recent 35 years in Chinese], Sci. Chin., Ser. D, 31, Zhao, S. R., Z. S. Song, and L. R. Ji (2003), Heating effect of the Tibetan Plateau on rainfall anomalies over North China during rainy season [in Chinese], Chin. J. Atmos. Sci., 27, Zhao, Y., and Y. F. Qian (2007), Relationships between the surface thermal anomalies in the Tibetan Plateau and the rainfall in the Jianghuai area in summer [in Chinese], Chin.J. Atmos. Sci., 31, of 15

15 Zhao, Y., and Y. F. Qian (2009), Relationship between the Tibetan Plateau surface thermal anomalies and the summer circulation over East Asia and rainfall in the Yangtze and Huaihe River areas [in Chinese], Acta Meteoro., Sin., 67, Zhou, L. T. (2009), Circulation anomalies pattern causing the persistent drought in North China [in Chinese], Clim. Environ. Res., 14, Zhou, L. T., and R. H. Huang (2003), Research on the characteristics of interdecadal variability of summer climate in China and its possible cause [in Chinese], Clim. Environ. Res., 8, Zhou, L. T., and R. H. Huang (2008), Interdecadal variability of sensible heat in arid and semi arid regions of northwest China and its relation to summer precipitation in China [in Chinese], Chin.J.Atmos.Sci., 32, D. Li and H. Wang, Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, Jiangsu, China. 15 of 15