ADVANCES IN ATMOSPHERIC SCIENCES, VOL. 26, NO. 5, 2009, 1027–1041
Structural Variation of an Atmospheric Heat Source over the Qinghai-Xizang Plateau and Its Influence on Precipitation in Northwest China
ï
WEI Na1,2 (
1 2
¯ ), and HE Jinhai
), GONG Yuanfa∗2,3 (
3
(
)
Shaanxi Provincial Climate Center, Xi’an 710015
Center for Plateau Atmospheric and Environmental Research, Department of Atmospheric Science, Chengdu University of Information Technology, Chengdu 610225
3
College of Atmosphere Sciences, Nanjing University of Information Science and Technology, Nanjing 210044 (Received 14 June 2008; revised 16 December 2008) ABSTRACT NCEP/NCAR reanalysis data and a 47-year precipitation dataset are utilized to analyze the relationship between an atmospheric heat source (hereafter called < Q1 >) over the Qinghai-Xizang Plateau (QXP) and its surrounding area and precipitation in northwest China. Our main conclusions are as follows: (1) The horizontal distribution of < Q1 > and its changing trend are dramatic over QXP in the summer. There are three strong centers of < Q1 > over the south side of QXP with obvious differences in the amount of yearly precipitation and the number of heat sinks predominate in the arid and semi-arid regions of northwest China (NWC), beside the northern QXP with an obvious higher intensity in years with less precipitation. (2) In the summer, the variation of the heat source’s vertical structure is obviously different between greater and lesser precipitation years in eastern northwest China (ENWC). The narrow heat sink belt forms between the northeast QXP and the southwestern part of Lake Baikal. In July and August of greater precipitation years, the heating center of the eastern QXP stays nearly over 35◦ N, and at 400 hPa of the eastern QXP, the strong upward motion of the heating center constructs a closed secondary vertical circulation cell over the northeast QXP (40◦ –46◦ N), which is propitious to add precipitation over the ENWC. Otherwise, the heating center shifts to the south of 30◦ N and disappears in July and August of lesser precipitation years, an opposite secondary circulation cell forms over the northeast QXP, which is a disadvantage for precipitation. Meanwhile, the secondary circulation cell in years with more or less precipitation over the ENWC is also related to the heat source over the Lake Baikal. (3) The vertical structure of the heat source over the western QXP has obvious differences between greater and lesser precipitation years in western northwest China in June and July. The strong/weak heat source over the western QXP produces relatively strong/weak ascending motion and correspondingly constructs a secondary circulation cell in lesser/greater precipitation years. Key words: Qinghai-Xizang Plateau, atmospheric heat source/sink, greater/lesser precipitation years, northwest China Citation: Wei, N., Y. F. Gong, and J. H. He, 2009: Structural variation of an atmospheric heat source over the Qinghai-Xizang Plateau and its influence on precipitation in northwest China. Adv. Atmos. Sci., 26(5), 1027–1041, doi: 10.1007/s00376-009-7207-7.
1.
Introduction
A strong heat source in the middle troposphere is common in the summer over the Qinghai-Xizang Plateau (hereafter QXP to save space). It has great in∗ Corresponding
author: GONG Yuanfa,
[email protected]
fluences on the atmospheric circulation over the northern hemisphere and the global circulation. As early as the 1950s, Yeh et al. (1957) and Flohn (1957) pointed out that QXP acts as a strong heat source in the summer, with upward flow prevailing over its main block
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and downward flow prevailing over its south and north sides. Many scholars have also studied the features of this atmospheric heat source over QXP and over the adjacent Asian monsoon regions and their relationship with the atmospheric circulation in East Asia. For example, Liu et al. (2002) along with Chen and Li (1982) found that there are two atmospheric heat source centers over Asia in the summer, one in the northeastern Bay of Bengal (BOB) and the other in the eastern South China Sea (SCS). In addition, Zhao and Chen (2001) analyzed the features of the atmospheric heat source over QXP and its relationship with precipitation in China. Duan et al. (2005) and Ma et al. (2003) studied the relationship between the sensible heat over QXP from April to June with precipitation and the atmospheric circulation anomaly over East Asia in July. He et al. (1987) also analyzed the features of the vertical circulation in the regions surrounding QXP, pointing out that QXP together with the BOB and the plain regions in South China, are ascending areas, whereas the Indian Desert, the Turkestan desert, and the Qaidam Basin located in the north side of QXP, northwest, and north China are descending areas. Concerned with the mechanism of drought in northwest China (NWC), Wu and Qian (1996) and Qian et al. (2001) demonstrated that there is a very close relationship between the heat source intensity over QXP and the dry-wet state in NWC, and that in the summer exists arid and semi-arid meridional circulations over the arid regions of NWC and the semiarid regions of North China, respectively, and an enormous eastward zonal circulation to the south of 32.5◦ N over QXP. Based on numerical simulations (Wang et al., 2002), conclusions show that the dispersion of the QXP thermal forcing intensifies the high pressure system on the north side of QXP, and leads to the maintenance of the drought circulation pattern. Moreover, other scholars studied the causes of the precipitation change over the arid regions of NWC from the view of snow cover variation on QXP (Wu et al., 1998; Wang et al., 2000), the intensity of the East Asian monsoon (Zhang et al., 2002; Wang et al., 2004), the Plateau’s Outing Longwave Radiation (OLR) index (Zhu et al., 2000), and the surface heating field anomaly (Sun, 1997). These studies suggested that the heat source variation over QXP plays an important role in the drought trend over the arid regions of NWC. More recently, Shi et al. (2002), Ma et al. (2005) and Ma and Fu (2006) showed that there is an increasing trend of the precipitation and the climate is changing into a warm-wet pattern in some western NWC regions (WNWC); but quite conversely, a dramatic drought trend is appearing in the eastern NWC areas (ENWC), which is due to higher temperatures
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and less precipitation during the past 20 years. In summary, there are many factors that are responsible for the dry-wet change in NWC, but the heat source variation over QXP should be one of the most important factors. Precipitation serves as a basic factor reflecting a dry-wet status, however rather few have been the focus of previous studies with regards to the structural variation of the atmospheric heat source over QXP and the resultant variation of the vertical circulation in the surrounding areas between greater and lesser precipitation years in NWC. Given this, in this paper we started with analyzing the overall precipitation changes of the past 50 years over NWC, and then some representative years are selected, which are more or less precipitation years since the dramatic drought trend of the 1980s. The horizontal and vertical structure variation of the heat source over QXP and the vertical circulation variation in its surrounding areas are studied by the method of composite analysis between the lesser and greater precipitation years. We expect our work will provide some useful references for further investigations into the genesis of a dry-wet change in NWC. 2.
Materials and methods
The data employed in this study include a 47year daily precipitation dataset of the 145 meteorological stations (including Xinjiang, Qinghai, Gansu, Shaanxi, Ningxia, and western Inner Mongolia of China) from 1959 to 2005, and the NCEP/NCAR reanalysis data from 1959 to 2005, with a horizontal resolution of 2.5◦ ×2.5◦ . The daily precipitation data was provided by the Chinese National Meteorological Information Center. The reverse calculation method put forward by Yanai et al. (1973) was employed in our study to calculate the atmospheric heat source/sink. 3.
Variation features of summer precipitation in NWC
The NWC region is so broad and topographically complex that it varies significantly in climate from one part to another. To clarify the features of the regional precipitation variation in the summer, a precipitation dataset from May to September during 1959 and 2005 is analyzed using the rotated empirical orthogonal function (REOF) method. The results show a considerable regional departure in summer precipitation over NWC, as the cumulative variance contribution of the first seven eigenvectors only accounted for exceeding 50% of the total. It is featured by a local spatial distribution of ENWC, northern Xinjiang, and the so-
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Fig. 1. (a) REOF1 and (b) REOF2 of the total precipitation of May to September over NWC during 1959 to 2005. Isotimic is the correlation coefficient of the precipitation with the time series of this spatial pattern.
called “Three-river Source” in Qinghai, etc. Figure 1 shows the first (REOF1) and the second (REOF2) eigenvectors, whose variance contributions are 14.8% and 7.9%, respectively. As shown in Fig. 1a, eastern Qinghai, southeast Gansu, Ningxia, western Inner Mongolia of China, and northern Shaanxi constitute a region with an isotimic value exceeding 0.5, which covers most of the Loess Plateau, and the center (value exceeding 0.8) is located in Ningxia. This spatial pattern is the one with a maximal precipitation anomaly variation in NWC, which is generally called an ENWC pattern. The second spatial pattern of the summer precipitation anomaly appears on the north side of the Tianshan Mountains in Xinjiang, as shown in Fig. 1b, with an isotimic value exceeding 0.5 and a central value exceeding 0.9, which is called WNWC. Our results are in good agreement with findings by Song and Zhang (2003) and Huang et al. (2004). Climate changes in NWC become rather complex due to the co-effect of the mid-latitude westerly, subtropical areas, and the QXP circulation systems. Using temperature anomalies, precipitation anomalies, the surface humid index, and the Palmer index, Ma and Fu (2006) comparatively analyzed the dry-wet state in various regions of northern China. All indexes, which change almost in the same tendency, suggest an increasingly dry trend in ENWC and a wet trend in WNWC since the 1980s. Because precipitation served as the most important indicator for a dry-wet status, in our study 32 meteorological stations with an isotimic value exceeding 0.6 in Fig. 1a (local variance contribution exceeding 36%) are selected as the representative stations, and then are analyzed to investigate the causes of the drought trend in ENWC. Meanwhile, 28 meteorological stations in Fig. 1b are selected as the representative stations in WNWC. Figure 2a depicts the average total precipitation anomaly of the 32 meteorological stations from May to September during
1959 to 2005 in ENWC. The horizontal dashed line is one standard deviation of the anomaly. As revealed in Fig. 2a, there is a noticeable annualand inter-decadal variation in summer precipitation over ENWC. Before 1985, there are only 3 summers in which the negative precipitation departure is more than one standard deviation, while there are 4 summers in which the positive precipitation departure is more than two standard deviations, indicating plenty of precipitation and a wet climate in ENWC during this period. However after 1985, there are 14 years with a negative precipitation departure, noteworthy in 5 years, the departure is more than one standard deviation, and 6 years with a positive precipitation departure. Hence, less precipitation and an obvious drought trend appears in ENWC since 1985. Figure 2b depicts the average total precipitation anomaly of the 28 meteorological stations in WNWC. After 1985, there are only 2 summers in which the negative precipitation departure is more than one standard deviation, while 7 summers in which the positive precipitation departure is more than one standard deviation, indicating plenty of precipitation in WNWC during this period. We are attempting to investigate the causes for a precipitation decline over ENWC and an increase over WNWC since the 1980’s. According to the reliability of the reanalysis data, 5 years (1981, 1983, 1984, 1988, and 2003, respectively), in which the positive precipitation departure exceeds one standard deviation in Fig. 2a, are selected to represent the greater precipitation years; another 5 years (1986, 1991, 1994, 1995, and 1997, respectively), in which the negative precipitation departure exceeds one standard deviation, are chosen to represent the lesser precipitation years over ENWC. For WNWC, 3 years (1980, 1985, and 1997) are selected to represent the lesser precipitation years and another 7 years (1987, 1988, 1992, 1993, 1998,
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Fig. 2. Average total precipitation anomaly of the (a) 32 representative meteorological stations over ENWC and (b) 28 stations over WNWC from May to September during 1959 to 2005. Units: mm.
1999, and 2003) are chosen to represent the greater precipitation years. In the following sections, the variations of the heat source structure and vertical circulation over QXP will be discussed separately for lesser or greater precipitation years in ENWC and WNWC, respectively. 4.
Structural variation of the atmospheric heat sources over QXP and its influence on the ENWC
Previous relevant studies (Duan and Wu, 2003; Liu et al., 2007) have demonstrated that an atmospheric heating anomaly over different QXP regions corresponds with different atmospheric circulations over East Asia. On the basis of numerical experiments, Zhao et al. (2003) arrived at the conclusions that a thermal anomaly over QXP in the summer is closely related to the precipitation of North China during the flood season. To study the relationship between precipitation over ENWC (35◦ –45◦ N, 95◦ –110◦E) in lesser or greater precipitation years and an atmospheric heat source variation over QXP and its neighboring areas, in following sections, we analyzed the variational features of the horizontal and vertical distribution of the atmospheric heat source in lesser/greater precipitation years. 4.1
Difference of the horizontal distribution of the atmospheric heat sources
Figure 3 presents the composite
distribution over QXP and its adjacent regions from July to August in greater or lesser precipitation years. In July and August, the heat source over QXP reaches its maximum intensity, coinciding with relatively concentrated precipitation in NWC, and as shown in Fig. 3, there exists a significant difference of between greater and lesser precipitation years. In July (Figs. 3a and 3b), there are three strong
atmospheric heat sources on the south side of the main block of QXP. The strongest one is located near Bangladesh (25◦ N, 90◦ E), with its center exceeding 600 W m−2 in either lesser or greater precipitation years. The other two centers are located in between 10◦ –20◦ N, one on the western peninsular of India and the other in between the Bay of Bengal and the IndoChina Peninsula, each presenting a “western intensity higher than eastern” trend in greater precipitation years, with central exceeding 500 Wm−2 for the former center and only 300 W m−2 for the latter. However in lesser precipitation years, they hardly show a noticeable difference, with central being 400 W m−2 for either center. The atmospheric heat source is relatively weak over the main block of QXP, but a little stronger in its southeastern part, without a significant difference between greater and lesser precipitation years. However, a noteworthy phenomenon is that the Central Asia (35◦ –55◦ N, 50◦ –80◦ E) and northeast side of QXP (corresponding to an arid region of Central Asia and an arid and semi-arid region of NWC, respectively) is predominated by a heat sink, with an obvious higher intensity in lesser precipitation years. Especially, there is a larger range of the heat sink of −50 W m−2 to the west of “the Hetao area of the Huanghe River” in lesser precipitation years. In August (Figs. 3c and 3d), the strong heat source center located near Bangladesh in the south side of QXP remains almost unchanged in range, but its central intensity becomes slightly weaker as compared with that in July. In addition, as for the other two heat source centers (10◦ –20◦ N, one in the western peninsular of India and the other in the western IndoChina Peninsula), the central intensity is pretty identical between lesser and greater precipitation years, with the central value being 300 W m−2 for the former center and 400 W m−2 for the latter. The heat source over the main block of QXP is weaken-
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Fig. 3. Composite distribution of the greater or lesser precipitation years over QXP and its adjacent regions. Shaded areas are the heat sink areas with negative , the isotimic interval is 50 W m−2 ; the other area isotimic interval is 100 W m−2 . (a) July of greater precipitation years, (b) July of lesser precipitation years, (c) August of greater precipitation years, (d) August of lesser precipitation years.
ing in August, however, this tendency is particularly apparent in the lesser precipitation years. Whereas in Central Asia and the north and northeast side of QXP, a slight increase of the heat sink is seen in both range and intensity, and in addition, there is a larger range of the heat sink of −50 W m−2 over ENWC in lesser precipitation years. 4.2
Difference of the atmospheric heat sources vertical structure over eastern QXP
In the study on the thermal structure over QXP in the summer and its interactions with large-scale circulations, Yeh (1988) found that plateau heating is of significant importance not only in maintaining the large-scale vertical circulation in Asia and the Pacific regions, but also in the desert formation in North Africa and the Middle East and the drought climate in NWC. Jian and Luo (2002) also analyzed the daily vertical structure variation of the atmospheric heat sources over the eastern QXP in the summer and its relation with the plateau circulation. As our anal-
ysis on the vertical structure variation of the atmospheric heating rate over the eastern QXP is revealed, the overall changing trend of the atmospheric heating over QXP from spring to autumn differs quite a bit between the greater and lesser precipitation years, so does the intensity variation. In spring (from March to May), the difference is general negligible between the greater and lesser precipitation years. Over the eastern QXP, the lower troposphere from the surface to 300 hPa is covered by a heat source, with the center located somewhere between 400 hPa and 500 hPa and the intensity being 1.5–2.5 K d−1 ; while the middleand-upper troposphere, above 300 hPa, is covered by a heat sink, with the center located at 250 hPa and the intensity being −3.5 K d−1 . In June, the eastern QXP welcomes a significantly enhanced atmospheric heat source, which spreads above 150 hPa to the troposphere. From July to August appears the most obvious difference in the changing trend and the intensity of the heat sources over the eastern QXP between the lesser and greater precipitation years. However, the difference is narrowed in September, since the atmo-
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(a) March of greater precipitation years
(b) March of lesser precipitation years
(c) April of greater precipitation years
(d) April of lesser precipitation years
(e) May of greater precipitation years
(f) May of lesser precipitation years
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Fig. 4. Latitude-pressure cross-sections of the composite atmospheric heating rate (Q1 /cp ) of greater or lesser precipitation years for the spring over eastern QXP (averaged between 92.5◦ E and 102.5◦ E). The isotimic interval is 0.5 K d−1 , the color shaded areas are the heat sources, and the black shaded area is the QXP topography.
spheric heat source is weakening quickly at that time. 4.2.1 Differences of the spring (from March to May) Figure 4 is the latitude-pressure cross section of the composite atmospheric heating rate (Q1 /cp ) over eastern QXP(92.5◦–102.5◦E)in the spring (from March to May) of greater or lesser precipitation years. In March, over QXP between 30◦ –40◦ N, the lower troposphere from the surface (600 hPa) to 300 hPa is covered by an atmospheric heat source, which is centered at 400 hPa with an intensity at 1.5 K d−1 in greater precipi-
tation years (Fig. 4a), and as weak as only 0.5 K d−1 in lesser precipitation years (Fig. 4b), while the middleand-upper troposphere above 300 hPa is covered by a heat sink, whose intensity is somewhat stronger in lesser precipitation years. In the south side of QXP (10◦ –20◦ N), the atmospheric heat source developed from 700 hPa to the middle troposphere of 400 hPa, and for either lesser or greater precipitation years, is not strong except near the surface (1.0–1.5 K d−1 ). The north side of QXP (north of 40◦ N) is exclusively a heat sink, with the center intensity being −2.5 K d−1
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and −1.5 K d−1 for lesser and greater precipitation years, respectively. To sum up, in March the vertical structure of the atmospheric heat source varies slightly between greater and lesser precipitation years. The heat source in April (Figs. 4c and 4d) differs from that in March in terms of intensity and range, but the difference between the greater and lesser precipitation years is not too significant. Two heat source centers have basically taken shape over the eastern QXP and its south side between the eastern Bay of Bengal (BOB) and the Indo-China Peninsula, and another center, still weak then, is developing in the lower troposphere of the western part of Lake Baikal (50◦ – 55◦ N). Concretely speaking, the heat source over QXP becomes stronger (1.5 K d−1 at 400 hPa) and expands above 300 hPa in its vertical distribution. On its south side (10◦ –20◦ N), the heat source is expanded above 150 hPa, almost covering the whole middle troposphere. Its intensity, however, is still as weak as 1.0 K d−1 at 400–500 hPa. A weak heat source is developing on its north side at the lower troposphere, with the central intensity being 1.5 K d−1 at 700 hPa (50◦ –55◦ N), however the mid-and-upper troposphere is dominated by a heat sink, which is centered at 200 hPa. In May (Figs. 4e and 4f), the two heat source centers over eastern QXP and its south side have intensified, and are above 2.5 K d−1 (33◦ N, 400 hPa) and over 3.5 K d−1 (10◦ –20◦ N, 400–500 hPa), respectively. However, there is no significant difference between the lesser and greater precipitation years. Furthermore, on the south side of QXP, the heat source has expanded above 100 hPa, covering the whole middle troposphere. However, another heat source center, located on the north side of 40◦ N between the north side of QXP and the western part of Lake Baikal, is featured by an even distribution of a heat source/sink in greater precipitation years, with a weak heat source of 1.0 K d−1 at the lower-middle troposphere below 300–400 hPa and a weak heat sink of −1.0 K d−1 at the middleupper troposphere above 300 hPa. However, the heat source/sink is disorderly distributed in lesser precipitation years: at the lower-middle troposphere (300–700 hPa), there exists a heat sink between 40◦ –48◦ N, a heat source between 48◦ –55◦ N, and a heat sink again to the north of 55◦ N, while at the middle-upper troposphere above 300 hPa there exists only one heat sink, which is also distributed disorderly, and the central value is −2.0 K d−1 over ENWC (40◦ N, 250 hPa). 4.2.2
Differences of the summer (from June to August)
Figure 5 shows the latitude-pressure cross section of the composite summer (from June to August) atmo-
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spheric heating rate (Q1 /cp ) over eastern QXP (92.5◦ – 102.5◦E) in greater or lesser precipitation years. In June (Figs. 5a and 5b), the heat source center over eastern QXP (30◦ –40◦ N) reaches its peak, which, however, differs significantly between the greater and lesser precipitation years, and expands to the upper troposphere. In greater precipitation years, it is relatively weak at the upper troposphere above 200 hPa, and the center is located at 400h Pa between 32◦ –35◦ N with the value being 3.5 K d−1 . In lesser precipitation years, it expands rapidly to the upper troposphere above 150 hPa, and the central value is 4.5 K d−1 . The eastern Bay of Bengal and the Indo-China Peninsula are locations of the strongest heat source center over the Asian monsoon regions, with the central value being 5.5 K d−1 at 400 hPa in greater precipitation years, and 5.0 K d−1 , a slightly lower value, in lesser precipitation years. But in lesser precipitation years, there also exists another heat source center of 3.0 K d−1 in the upper troposphere above 150 hPa. On the north side of QXP, the heat source over the southwestern part of Lake Baikal (centered at 50◦ N) developed rapidly to the upper troposphere (above 200 hPa). Consequently, a narrow heat sink belt formed between the eastern QXP and the southwestern part of Lake Baikal (50◦ N). It is always weaker in greater precipitation years than in lesser precipitation years (−2.0 K d−1 vs. approximately −3.0 K d−1 at 200 hPa), no matter in the lower-middle or middle-upper troposphere. In July to August, the vertical structure of the atmospheric heat source exhibits considerable differences not only in intensity, but also in terms of a changing trend between greater and lesser precipitation years. In the eastern QXP between 30◦ –40◦ N, the heat source (Fig. 5c) reaches the upper troposphere above 150 hPa in July of greater precipitation years, which begins to decrease in intensity in August (Fig. 5e), but its center is still maintained over QXP at 400 hPa in the vicinity of 35◦ N. Whereas in July of lesser precipitation years, the center has shifted southward to 30◦ N (Fig. 5d), and disappeared in August over the main block of QXP (Fig. 5f). Another heat source center locates between the eastern Bay of Bengal and the Indo-China Peninsula (10◦ –20◦ N) and is 4.0 K d−1 at 400 hPa in July, and 5.0 K d−1 in August of greater precipitation years, respectively, slightly lower than the maximum value appears in June. While in lesser precipitation years, it increases to 6.0 K d−1 at 400 hPa in July, and in August there is a much wider range of the strong heat source over 5.5 K d−1 . On the north side of QXP (near 50◦ N), the heat source is intensified in July of both lesser and greater precipitation years, with the central value being above 2.0 K d−1 from the lower troposphere to the middle-
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(a) June of greater precipitation years
(b) June of lesser precipitation years
(c) July of greater precipitation years
(d) July of lesser precipitation years
(e) August of greater precipitation years
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(f) August of lesser precipitation years
Fig. 5. Same as Fig. 4, but for the summer.
upper troposphere above 300 hPa. In August, the heat source decreases quickly in greater precipitation years, with the central value being only 1.0 K d−1 , while in lesser precipitation years, only the middle-upper troposphere showed a slight decrease, and the central value is still maintained at 2.0 K d−1 in the lowermiddle troposphere. On the northeast side of QXP (40◦ –45◦ N) and ENWC, the narrow heat sink belt becomes much more obvious, and its intensity in the middle-upper troposphere is always stronger in lesser precipitation years. Especially in August of lesser precipitation years, the heat sink belt below −0.5 K d−1 has occupied the whole lower-middle troposphere. Once autumn arrives, the atmospheric heat source decreases violently in intensity and range. In September (figures omitted), the heat source over QXP de-
clines below 250 hPa, and is approximately 4.0 K d−1 between the eastern Bay of Bengal and the Indo-China Peninsula, and regions around 50◦ N are predominantly the heat sink. The previous analysis, which leads to further questions, are if the annual variation of the vertical structure of the atmospheric heat source over the northeast side of QXP relate directly to the drought circulation cell in ENWC? In addition, whether such a variation is one of the factors responsible for the summer precipitation in ENWC? 4.3
Variation of the vertical circulation
In section 4 we analyzed the differences of the horizontal and vertical structure of the atmospheric heat source over QXP and its north-and-south sides be-
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(a)
(c)
(b)
(d)
Fig. 6. Latitude-pressure cross sections of the composite meridional circulation of greater or lesser precipitation years over eastern QXP (100 E). Black shaded area is the Plateau topography. (a) July of greater precipitation years; (b) July of lesser precipitation years; (c) August of greater precipitation years; (d) August of lesser precipitation years.
tween lesser and greater precipitation years. The question still remains as to whether the difference of the atmospheric heat sources over QXP would bring about changes in the vertical circulation in the surrounding areas? To answer this, we therefore proceed to analyze the departure of the meridional and zonal circulations between lesser and greater precipitation years, using July and August as the representative months. The summer precipitation is rather concentrated over NWC in July and August, and there appears the most obvious structural variation of the heat source, as discussed in section 4. 4.3.1
Meridional vertical circulation
Figure 6 depicts the meridional circulation distribution over the eastern QXP (100◦ E) in July and August. In the southern part of QXP and its south side (to the south of 35◦ N), there is strong upward flow in both months due to the strong heating effect previously described, and it reaches the equatorial regions in the horizontal direction. This upward flow is fea-
tured by a south wind at the bottom of the troposphere over the South Asian Subcontinent, and a north wind in the upper troposphere, forming a very obvious summer monsoon circulation cell. However, the difference between greater and lesser precipitation years is not noticeable. In the northern part of the main block of QXP and its north side (to the north of 35◦ N), the vertical circulation shows great differences between the greater and lesser precipitation years. As shown in Fig. 6a, a strong ascending motion is seen in the northern part of the main block of QXP (35◦ –38◦N) in July of greater precipitation years, which could reach above 200 hPa in the vertical direction. This should be associated with the maintenance of the heat source center over 35◦ N QXP in July of greater precipitation years (Fig. 5c). On the north side of QXP (40◦ –46◦N) there exists a small but obvious secondary closed circulation cell in the lower-middle troposphere below 500 hPa, which owns its existence to the strong ascending motion of the heat source over the main part of QXP. The ascending motion is very strong in the northern
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(a)
(b)
(c)
(d)
Fig. 7. Longitude-pressure cross sections of the composite zonal circulation of greater or lesser precipitation years over eastern QXP (37.5◦ N). Black shaded area is the Plateau topography. (a) July of greater precipitation years; (b) July of lesser precipitation years; (c) August of greater precipitation years; (d) August of lesser precipitation years.
part of this cell (40◦ –43◦ N), which relates to ENWC and leads to more precipitation over this region. In July of lesser precipitation years (Fig. 6b), the ascending motion over the northern part of the block of QXP (35◦ –40◦ N) is weaker when compared with that in the greater precipitation years, and there appears an obvious southward airflow above 400 hPa. It is probably related with the southward shift of the heat source center to southern QXP, south of 30◦ N (Fig. 5d), and the weakening of the heat source over northern QXP. Furthermore, because the heat source over the southwestern part of Lake Baikal (around 50◦ N) is intensified, a closed circulation cell, which is opposite to that in greater precipitation years, is formed on the north side of QXP (between 40◦ –70◦ N and 850–300 hPa). A very obvious descending motion is seen in the southern part of the circulation cell (41◦ –44◦ N), which would result in less precipitation in ENWC. This meridional circulation cell results from the co-action of the weakening of the heat source over northern QXP (decreased ascending motion or relative descending motion), cou-
pled with the increasing of the heat source over regions near 50◦ N (enhanced ascending motion). In August, there still exists strong ascending motion in the lower-middle troposphere below 600 hPa and downward flow at 600–300 hPa on the north side of QXP (40◦ –45◦ N) in greater precipitation years. While in lesser precipitation years (Fig. 6d), the ENWC on the north side of QXP is characterized by downward flow from 700–250 hPa, with the exception of the weak ascending motion near the surface. This agrees well with the variation of the vertical structures of the heat sources between lesser and greater precipitation years in section 4.2, or in other words, it is caused by the variation of the vertical heat source structure over the northeast side of QXP. 4.3.2 Zonal vertical circulation Figure 7 shows the zonal circulation distribution on the north side of QXP (37.5◦ N) in July and August of lesser or greater precipitation years. As shown in the figure, there is little difference in the zonal circulation
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over the western QXP (to the west of 90◦ E) between greater and lesser precipitation years. Over the eastern QXP, there is a straight zonal westerly above 300 hPa and shows little difference between the greater and lesser precipitation years, while the opposite is true below 300 hPa. In July of greater precipitation years, a weak sinking motion appears below 400 hPa in the east side of QXP (to the east of 100◦E), forming a weak circulation cell below 400 hPa between 100◦– 110◦ E, furthermore the ENWC is located just on the north side of this circulation cell (Fig. 7a). In July of lesser precipitation years, a strong sinking motion appears below 300 hPa to the east of 100◦E, also forming a zonal circulation cell between 100◦–110◦E, and the ENWC is in the downward flow of the circulation cell. In August, the differences between the greater and lesser precipitation years are even widened. The zonal circulation cell in the east side of QXP (100◦ – 110◦ E) disappears in greater precipitation years, leaving extensive weak downward flow between 700–500 hPa and weak ascending motion near the surface, and the downward flow is becoming obviously stronger near 120◦ E (Fig. 7c). While in lesser precipitation years, there still exists the zonal circulation cell in the east side of QXP (100◦ –110◦E), which seems to be stronger than that in July, and the downward flow is extended to 120◦ E (Fig. 7d). 5.
Variation of the atmospheric heat source over western QXP
In section 4, we found that the structural variation of the atmospheric heat source over eastern QXP has an obvious influence on precipitation in ENWC. In this section, we proceed to analyze the relationship between the variation of the atmospheric heat source and the circulation over western QXP and precipitation in WNWC in the summer. 5.1
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Difference of the atmospheric heat source structure
Figure 8 shows the latitude-pressure cross section of the composite summer (from June to August) atmospheric heating rate (Q1 /cp ) over western QXP 80◦– 90◦ E in greater or lesser precipitation years. In Fig. 8, the distribution of the atmospheric heat source in WNWC is similar with that in ENWC in the mass, however there also exists an obvious difference between greater and lesser precipitation years. In this section we briefly depict this difference in greater and lesser precipitation years. In June (Figs. 8a and 8b), the heat source over western QXP between 30◦ –40◦ N reaches its peak, and the center is located at 500 hPa with a value of 2.5
K d−1 in greater precipitation years and 3.5 K d−1 in lesser precipitation years. On the south side of QXP, the heat source intensifies in June, and the center is located at 300 hPa (near 10◦ N) with a value of 2.5 K d−1 in greater precipitation years and 3.0 K d−1 in lesser precipitation years. The atmospheric heat source over western QXP (near 45◦ N) is stronger in greater precipitation years than in lesser precipitation years in the middle troposphere. At the upper troposphere, the heat sink distributes conversely, which is stronger in greater precipitation years and weaker in lesser precipitation years. In July, the atmospheric heat source over western QXP relatively decreased, and the center shifted southward, but the intensity is still stronger in lesser than greater precipitation years. In the south side of QXP, the heat source intensifies in July, however the difference is not noticeable between greater and lesser precipitation years. In the north of QXP, the heat sink region over WNWC is relatively stronger in July of lesser precipitation years. Once in autumn, the heat source decreases violently in intensity and range. In September (omitted figures), the heat source over western QXP declines below 300 hPa, and is 2.5 K d−1 in between the eastern Bay of Bengal and the Indo-China Peninsula, and the regions around 40◦ N are predominantly the heat sink. Could this difference in the vertical structure of the atmospheric heat source between greater and lesser precipitation years bring out the changes in the vertical circulation in WNWC? 5.2
Difference of the vertical circulation
We analyzed the departure of the meridional circulation between lesser and greater precipitation years, using June and July data. The precipitation over WNWC is relatively concentrated in June and July, and there appears the most obvious structural variation of the heat source, as discussed in section 5.1. Figure 9 depicts the meridional circulation distribution over western QXP (85◦ E) in June and July. To the south of 25◦ N, there is no noticeable difference between lesser and greater precipitation years. However, to the north of 30◦ N over western QXP, the vertical circulation shows a great difference between lesser and greater precipitation years. As shown in Figs. 9a and 9b, on the north side of QXP (35◦ –45◦ N) there exist secondary closed circulation cells, respectively, which are weaker in greater precipitation years and stronger in lesser precipitation years. The downward flow of the northern part of these two circulation cells (near 45◦ N) are corresponding with WNWC, but in lesser precipitation years, the downward flow is obviously violent.
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(a) June of greater precipitation years
(c) July of greater precipitation years
(e) August of greater precipitation years
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(b) June of lesser precipitation years
(d) July of lesser precipitation years
(f) August of lesser precipitation years
Fig. 8. Same as Fig. 4, but over western QXP (average between 80◦ E and 90◦ E) in the summer.
In July of greater precipitation years (Fig. 8c), on the north side of 35◦ N there still exists the small secondary circulation cell, which owns to the ascending motion of the heat source over western QXP. The ascending motion is in the southern of cell (38◦ –45◦N), which relates to WNWC and leads to more precipitation over this region. In July of lesser precipitation years, the WNWC (near 45◦ N) is characterized by downward flow from 900–300 hPa, with an exception near the regions (50◦ –60◦N) with ascending motion. This agrees well with the variation of the vertical structure of the heat source between lesser and greater precipitation years in section 5.1.
6.
Conclusions and discussion
Our detailed analysis in the previous paragraphs is sufficient for the below conclusions: (1) In the summer, both the horizontal distribution of and its changing trend varied dramatically over QXP in ENWC. There exist three strong heat source centers on the south side of QXP: the maximum appears near Bangladesh; the other two centers, one in the western Indo-China Peninsular and one in between the Bay of Bengal and the Indo-China Peninsula, present a “western intensity higher than eastern” trend in greater precipitation years. How-
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(a)
(b)
(c)
(d)
Fig. 9. Same as Fig. 6, but over western QXP (85◦ E) in June and July. (a) June of greater precipitation years; (b) June of lesser precipitation years; (c) July of greater precipitation years; (d) July of lesser precipitation years.
ever, they hardly show a noticeable difference in lesser precipitation years. The main block of QXP is a weak heat source, without differences between greater and lesser precipitation years. The northeast side of QXP (the arid and semi-arid areas of northwest China) is predominated by a heat sink, with an obvious higher intensity in lesser precipitation years. Especially, there is a larger range of the heat sink to the west of the “Hetao areas of Huanghe River” in lesser precipitation years. (2) In June, the variation of the vertical structure is increasingly evident between lesser and greater precipitation years in ENWC. The heat source center over the eastern QXP (35◦ N, 400 hPa) reaches its maximum level of 3.5 K d−1 and approximately 4.5 K d−1 in greater and lesser precipitation years, respectively. The eastern BOB (10◦ –20◦ N) is featured by a stronger heat source center (5.5 K d−1 at 400 hPa) in greater precipitation years and a lower one (5.0 K d−1 ) in lesser precipitation years. On the northwest side of QXP (40◦ –45◦ N), a narrow heat sink belt forms between QXP and the southwestern part of Lake Baikal, with the central value at 200 hPa being −2.0 K d−1 and a stronger one, −3.0 K d−1 , in greater and lesser precip-
itation years, respectively. (3) In July to August, the vertical structure of the heating source exhibits variation not only in intensity, but also in terms of a changing trend between greater and lesser precipitation years. The atmospheric heat source over the eastern QXP begins to decrease in greater precipitation years, but the center remains over QXP at 35◦ N, 400 hPa. While in July of lesser precipitation years, it shifts southward to 30◦ N, and disappears in August. However, the opposite changing trend of the heating source is demonstrated over the eastern Bay of Bengal: weaken in greater precipitation years but strengthen in lesser precipitation years. The narrow heat sink belt on the northwest side of QXP (40◦ –45◦ N) becomes much more obvious, with a stronger intensity in lesser precipitation years. In the southwestern part of Lake Baikal, the heat source peaks in July, which then decreases quickly in August of greater precipitation years, but in lesser precipitation years, only decreases slightly in the middle-upper troposphere, and the central value in the lower-middle troposphere remains at 2.0 K d−1 . (4) The vertical structure variation of the heat source over the eastern QXP actually brings about
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great structural variation of the meridional vertical circulation between lesser and greater precipitation years, which is obvious over the northeast side of QXP (the north of 35◦ N). In July of greater precipitation years, the heating centers remain near 35◦ N and the northern QXP (35◦ –40◦N) have a strong ascending motion, and therefore a secondary closed circulation cell (40◦ – 46◦ N), which owns its existence to the strong ascending motion over the main part of QXP, forms on the north side of QXP, and propitious to add precipitation over ENWC. While in July of lesser precipitation years, a closed circulation cell, which is opposite to that in greater precipitation years, forms over the north side of QXP (40◦ –47◦N, 850–300 hPa), resulting in less precipitation in ENWC. This meridional circulation cell results from the co-action of the weakening of the heat source over northern QXP (decreased ascending motion or relative descending motion), coupled with the increasing of the heat source over regions near 50◦ N (enhanced ascending motion). (5) In the northeast side of QXP (below 300 hPa, 100◦ –110◦E), the zonal circulation has a similar relationship as that between the meridional circulation and the precipitation in ENWC in July and August. (6) The vertical structure variation of the heat source over western QXP has the same influence on WNWC. In June, the strong heat source over western QXP produces ascending motion and forms the secondary circulation cell. The downward flow of the northern part of the secondary circulation cell controls the WNWC region, and is stronger in lesser precipitation years than in greater precipitation years. In July of greater precipitation years, the relative weak circulation cell is still evident. The ascending motion is in the southern part of the cell (38◦ –45◦ N) and leads to more precipitation in this region. In July of lesser precipitation years, the WNWC (near 45◦ N) is characterized by downward flow from 900–300 hPa, with the exception near the regions (50◦ –60◦ N) with ascending motion. In summary, our work has added some new elements to the traditional belief, which holds that the structural variation of the heat source/sink over QXP and its surrounding areas play an important role in precipitation in NWC. We also found that the variation of the heat source over the southwestern part of Lake Baikal has certain influences on the wet-dry status in NWC. However, we only analyzed the relationship between the structural variation of the heat source over QXP and the vertical circulation. The mechanism responsible for the dry-wet changes in NWC requires further studies. Acknowledgements.
This research was supported
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