Theor Appl Climatol DOI 10.1007/s00704-013-1058-y
ORIGINAL PAPER
Teleconnection between rainfall over South China and the East European Plain in July and August Qin Su & Riyu Lu
Received: 27 August 2013 / Accepted: 27 November 2013 # Springer-Verlag Wien 2013
Abstract In the present reported work, we identified that there is a significant negative relationship between rainfall over South China (SC) and the East European Plain (EEP) in the months of July and August, and investigated the possible reason for this negative relationship. The correlation coefficients between SC and the EEP rainfall were calculated to be −0.42 for July and −0.35 for August, both significant at the 95 % confidence level. We report that a wave-like train of circulation anomalies and a pathway of wave-activity flux stretching from Europe to East China connect the anticyclonic anomaly over Europe and the cyclonic anomaly over central and southern China, which are responsible for less EEP rainfall and more SC rainfall. We suggest that the teleconnection between SC and EEP rainfall results from the extension of stationary Rossby waves in the mid-latitudes in the upper troposphere for both July and August. This stationary Rossby wave is contributed to by summer North Atlantic Oscillation (NAO) and its extension features are determined by the location and intensity of the climatological uppertropospheric westerly jet. Furthermore, we found that there was an interdecadal change around the mid-1970s in the negative SC–EEP rainfall relationship for both July and August. The negative correlation was significant and strong in the period 1976–2005, but much weaker in the period 1955–1975. The extension of stationary Rossby waves from
Q. Su : R. Lu (*) State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, P.O. Box 9804, Beijing 100029, China e-mail:
[email protected] Q. Su University of the Chinese Academy of Sciences, Beijing 100049, China
Europe to East China was responsible for the significant negative relationship during the period 1976–2005.
1 Introduction Teleconnection refers to the relationship between two or more climate anomalies separated by large distances. Many teleconnection patterns have been well documented, and the most well known is the Southern Oscillation, connecting sealevel pressure at Tahiti to that at Darwin, Australia. Teleconnection patterns with at least one foot in the tropics may help with climate predictability in remote locations because of the relatively higher predictability of tropical climate. For instance, predicting ENSO events enables the prediction of North American climate anomalies by the concept of the Pacific–North American Pattern. There are some teleconnection patterns during summer in the extratropics, such as North Atlantic Oscillation (NAO). It is characterized by a seesaw pattern between polar and midlatitude regions over the Atlantic Ocean (e.g., Barnston and Livezey 1987), and summer NAO has an influence over a large area. For instance, the climate over Northern Europe is directly influenced by the southern lobe of summer NAO (e.g., Folland et al. 2009). In addition, precipitation and surface temperature over Mediterranean regions and East Asia are also remotely affected (e.g., Sun et al. 2008; Blade et al. 2012; Sun and Wang 2012). Another extratropical teleconnection pattern has been identified along the upper-tropopsheric westerly jet in the midlatitudes and is called the "Silk Road Pattern" (Lu et al. 2002; Enomoto et al. 2003). The Silk Road Pattern is maintained by inertial dynamics, and tropical heating anomalies may play a role in triggering the teleconnection pattern (Sato and Takahashi 2006; Kosaka et al. 2009; Yasui and Watanabe 2010; Ding et al. 2011; Chen and Huang 2012). For example,
Q. Su, R. Lu
Yasui and Watanabe (2010) suggested that heating anomalies over the eastern Mediterranean region and equatorial Africa are effective at exciting this teleconnection along the Asian jet. Meanwhile, other studies have indicated that this teleconnection pattern has a great impact on the climate in East Asia. For instance, it contributes to the formation and variation of the Bonin High near Japan (Enomoto et al. 2003), the mid-July break of the Mongolian rainy season by forming a barotropic ridge over Mongolia (Iwasaki and Nii 2006), and an increase in rainfall over North China by developing a ridge in the upper troposphere over the Korean Peninsula (Liang et al. 2011). Other teleconnections have been identified as occurring during summer over the extratropics, but these tend to be less dominant in comparison with the above-mentioned NAO and Silk Road Pattern. For instance, Wakabayashi and Kawamura (2004) identified two teleconnection patterns in the mid-high latitudes of the Eurasian continent. In fact, these mid-high latitude teleconnections and the Silk Road Pattern may operate together to jointly affect regional climate (Iwao and Takahashi 2008; Moon et al. 2013). The present reported study detected a significant relationship in rainfall variability between South China (SC) and the East European Plain (EEP) in the months of July and August. SC is located at the southeastern-most edge of the Eurasian continent, while the EEP is situated in the northwest part of the continent. Between them, there is a long distance of about 7000 km. In this paper, we illustrate this SC–EEP rainfall relationship, and then examine the circulation anomalies that may be responsible for the relationship. SC is situated in the East Asian summer monsoon region, and as such experiences some of the greatest levels of precipitation in the country. The interannual standard deviations of SC rainfall in July and August are comparable to those in May and June, although the amounts of SC rainfall in July and August are smaller than those in May and June (Su et al. 2013). The amounts of SC rainfall are about 170– 180 mm month−1 and the standard deviations are around 55 mm month−1 in July and August. Su et al. (2013) indicated that the patterns of circulation anomalies associated with July and August SC rainfall variations are significantly distinct from those associated with May and June rainfall, implying that it is appropriate to distinguish July–August from May– June when circulation anomalies associated with SC rainfall are averaged over summer months. Located in the northwest part of the Eurasian continent, the EEP experiences much less rainfall in comparison with SC. Recently, Zveryaev and Allan (2010) indicated that the leading modes of European precipitation for both July and August are characterized by coherent precipitation variation over the EEP and a relatively weak and opposite variation over the Mediterranean region. They also suggested that these leading
modes are intimately associated with summer NAO. Moreover, Rossby waves propagating in the mid-high latitudes have an impact on the summer climate variability in Europe. For instance, the long-lasting blocking high over Eastern Europe, which is maintained by Rossby waves (Schneidereit et al. 2012), results in the well-known extreme heat wave event in the EEP in the summer of 2010 (e.g., Mokhov 2011; Lupo et al. 2012). In addition, sea surface temperature and sea ice concentration also play a role in affecting the EEP climate (Semenov et al. 2012). Sun et al. (2008) and Sun and Wang (2012) reported that a strong relationship between summer NAO and rainfall/ temperature in the northern part of East Asia has been established since the late 1970s, while no such relationship could be detected before that time. Therefore, the possible interdecadal change in the SC–EEP rainfall relationship is also discussed here. The rest of the paper is organized as follows. Section 2 describes the datasets used in the study. Section 3 examines the relationships between SC rainfall and EEP rainfall, especially their interannual variations. The atmospheric circulation anomalies associated with the relationships are also documented in this section. Section 4 presents the interdecadal change of the relationship between the interannual variations of SC rainfall and EEP rainfall. Section 5 is devoted to a summary.
2 Datasets Monthly precipitation data for the period 1951–2009 from the Climate Research Unit (CRU) high-resolution gridded dataset, CRU TS 3.1 (Harris et al. 2013), were used in the study. The resolution of this dataset is 0.5° (longitude) by 0.5° (latitude). Monthly rainfall data recorded at 160 stations provided by the Chinese Meteorological Data Center for the same period were also used. Monthly mean horizontal winds and geopotential heights in the upper troposphere for the period 1955–2005 were obtained from the National Centers for Environmental Prediction/National Center for Atmosphere Research (NCEP/NCAR) reanalysis datasets (Kalnay et al. 1996), which have a resolution of 2.5° (longitude) by 2.5° (latitude). The monthly NAO index (NAOI) used in the study was obtained from the Climate Prediction Center website (http://www.cpc.ncep.noaa.gov/ data/teledoc/nao.shtml), which is calculated according to Barnston and Livezey (1987). The time period used for NAOI was 1955–2005. In addition, we present the horizontal components of the wave-activity flux, which were calculated according to Takaya and Nakamura (1997) and were dependent on both zonally varying basic flow and stationary disturbances. The wave-activity flux is used to represent the local group velocity.
South China–East European Plain rainfall relationship
3 The relationship between SC and EEP rainfall Figure 1 shows the correlation coefficients between the SC rainfall index (SCI) and land rainfall. The SCI was obtained by averaging the precipitation at 29 stations over the SC region, which was defined as the southeast part of mainland China between the longitudes 105ºE and 120ºE and between the latitudes 21°N and 28°N. Large areas of significant positive correlation appear over SC in July and August (right panels of Fig. 1a and b), indicating that the SCI defined here exhibits a good representation of SC rainfall in both months. There is a weak and negative correlation north of the region of positive correlation. Interestingly, the interannual variations of SC rainfall exhibit a significant negative correlation with the variations of rainfall over the EEP, in both July and August (left panels of Fig. 1a and b). The significant negative correlation in July extends from the Black Sea to the EEP, and in August from the north of the Baltic Sea to Eastern Europe. There is also
Fig. 1 Correlation of SC rainfall with land rainfall: a July; b August. The contour interval is 0.2 and the contour line of zero has been omitted. The shading shows correlations significant at the 95 % confidence level
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relatively weak and positive correlation in the Mediterranean region (not shown). These patterns of correlation coefficients over Europe resemble the leading modes of European precipitation for both July and August, characterized by a large area of coherent precipitation variations from the British Isles to the EEP and a small area of opposite variation over the Mediterranean region (Zveryaev and Allan 2010). To facilitate further analyses, we specified the region between the longitudes 30°E and 50°E and between the latitudes 50°N and 60°N as the EEP, and defined the EEP rainfall index (EEPI) as the precipitation averaged over this region. Figure 2 shows the standardized SCI and EEPI in July and August, respectively. Standardized time series were used to facilitate comparison, since the standard deviations, as well as amounts, of SC rainfall are much greater than those of EEP rainfall. The standard deviations of EEPI are around 20 mm month−1 in July and August, being only about one-third of those for SCI. Similarly, the amounts of EEP rainfall in July and August
Q. Su, R. Lu
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Fig. 2 Standardized time series of rainfall anomalies over the EEP (solid line) and SC (dashed line): a July; b August
are around 65 mm month−1, being about one-third of SC rainfall. The negative relationship detected in Fig. 1 can be seen more clearly in the rainfall variations shown in Fig. 2. The variations of SCI and EEPI exhibit opposite signs in most years, and the correlation coefficients between these two indices are −0.42 for July and −0.35 for August, both of which are significant at the 95 % confidence level. SC summer rainfall exhibits clear interdecadal variation (Kwon et al. 2007; Wu et al. 2010; Liu et al. 2011; Chen et al. 2012; Ye and Lu 2012; Sui et al. 2013), as does EEP rainfall. Therefore, we separated interannual and interdecadal variations in SC and EEP rainfall, and primarily analyzed the interannual component. The interannual component was obtained by removing the interdecadal component from the original data, and the interdecadal component was simply defined as 9-year running averages. It should be noted that the interannual variations are dominant in the variations of both SC and EEP rainfall, with the interannual component explaining 93.8 % and 97.6 % of the total variance for July SCI and EEPI, respectively. For August, these numbers change to 97.5 % and 86.7 %, respectively. The correlation coefficients between the interannual variations of SCI and EEPI are −0.44 for July and −0.31 for August, significant at the 95 % confidence level.
Figure 3 shows the composite geopotential height anomalies and the corresponding wave-activity flux at 200 hPa, in association with the negative SCI–EEPI relationship. The composite anomalies refer to the differences of geopotential heights between the wet SC–dry EEP years and the dry SC– wet EEP years. The wet SC–dry EEP years during the period 1955–2005 were defined as when the interannual variations of SCI were greater than half the standard deviation, while the interannual variations of EEPI were less than minus half the standard deviation. The dry SC–wet EEP years were defined in a similar way. There are eight wet SC–dry EEP years and nine dry SC–wet EEP years for July, and seven years for both categories for August (Table 1). A wave-like train in the mid-high latitudes extending from Europe to East China characterizes the geopotential height anomalies associated with the negative SCI–EEPI relationship for both July and August (Fig. 3a and b). There is a positive height anomaly over Europe for both months, and it is stronger and occupies a larger area for August in comparison with July. East of this positive height anomaly, there is a negative anomaly. By comparing with the precipitation anomalies shown in Fig. 1, it is evident that these circulation anomalies are coherent with the precipitation anomalies. The northerly anomalies correspond well with the negative rainfall
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Fig. 3 Composite differences of geopotential heights (contours; units: gpm) at 200 hPa between the wet SC–dry EEP years and the dry SC–wet EEP years (see Table 1). The vectors show the corresponding waveactivity flux (units: m2 s−2) according to Takaya and Nakamura (1997): a July; b August. The contour interval is 15 gpm and the contour line of zero has been omitted. The wave-activity flux has also been omitted when its value was less than 10 m2 s−2
South China–East European Plain rainfall relationship Table 1 Wet SC–dry EEP years and dry SC–wet EEP years during the period 1955–2005 Wet SC–dry EEP years (SC+EEP−) July
1955, 1981, 1986, 1994, 1997, 1999, 2001, 2002 August 1955, 1959, 1972, 1979, 1985, 1996, 2002
Dry SC–wet EEP years (SC−EEP+) 1956, 1962, 1978, 1984, 1989, 1990, 1998, 2000, 2003 1956, 1962, 1981, 1986, 1987, 1998, 2003
Here, + and−indicate the interannual variations of SCI or EEPI greater than half the standard deviation and less than minus half the standard deviation, respectively
anomalies in both months, implying a balance between cold advection and adiabatic heating caused by descending flow, which can be identified by less rainfall occurring in the region. There is an anticyclonic anomaly over Mongolia and northern China, and a cyclonic anomaly over central and southern China. The wave-activity flux suggests that the above-mentioned circulation anomalies could be explained as an extension of stationary Rossby waves. The wave-activity flux shows that waves extend eastwards over Europe, turn southeastwards over West Asia, and propagate eastwards again along the westerly jet into eastern China, and southwards out of the jet into central and southern China. The pathways of stationary waves are similar between July and August, suggesting that stationary Rossby waves stretch from Europe to eastern China in both months. There is another obvious pathway of waves in the high latitudes over Eastern Europe and Siberia, but it may not be relevant to the relationship between SC and EEP rainfall. Figure 4 shows the 200-hPa geopotential height and wind anomalies associated with the interannual variations of SCI and EEPI, respectively. For convenience of comparison, the
Fig. 4 Geopotential height anomalies (contours; units: gpm) and horizontal wind anomalies (vectors; units: m s−1) at 200 hPa regressed upon SC rainfall in a July and c August, and upon EEP rainfall in c July and d August. EEP rainfall was multiplied by minus one before regression. The contour interval is 5 gpm and the contour line of zero has been omitted. The shading indicates significance at the 95 % confidence level
interannual variations of EEPI are multiplied by minus one, due to the negative SCI–EEPI relationship. A cyclonic anomaly appears over central China and the East China Sea in association with more SC rainfall for both months (Fig. 4a and c), and an anticyclonic anomaly appears over Europe in association with less EEP rainfall (Fig. 4b and d). The circulation anomalies associated with the interannual variations of SCI and EEPI are similar, and all exhibit a wavelike train stretching from Europe to East Asia (Fig. 4). These anomalies resemble those associated with the negative SCI– EEPI relationship (Fig. 3). Therefore, it is suggested that the stationary Rossby wave over the Eurasian continent influences rainfall variability over both SC and the EEP by modulating the upper-tropospheric circulation anomalies over Europe and East China. As a result, the extension of a quasistationary Rossby wave train from Europe to East China is responsible for the negative relationship between SC and EEP rainfall. The pattern of circulation anomalies associated with August SC rainfall (Fig. 4c) differs somewhat from other patterns shown in Fig. 4 and from those shown in Fig. 3, characterized by a too broad anticyclonic anomaly stretching from West Asia to East Asia. However, this pattern also includes an anticyclonic anomaly over Europe and a cyclonic anomaly over SC. The anticyclonic anomaly over Europe shown in Fig. 4b and d resembles well the one associated with the leading mode of rainfall over Europe (Zveryaev and Allan 2010). Since the leading mode of rainfall over Europe is associated with the summer NAO (Zveryaev and Allan 2010), we also compared EEP precipitation variability with the summer NAO. The correlation coefficients between the EEPI and NAOI are −0.34 for July and −0.41 for August, which are statistically significant at the 95 % and 99 % confidence levels, respectively. These coefficients are consistent with, but weaker than,
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Q. Su, R. Lu Fig. 5 Monthly wave-activity flux (vectors; units: m2 s−2) at 200 hPa determined by the wind anomalies for the high and low EEP–CI years. Monthly climatology of zonal wind at 200 hPa (units: m s−1) is also shown by contours: a May, b June, c July, d August. The contour interval is 3 m s−1, and contour lines less than 25 m s−1 have been omitted to highlight the jet. The shading indicates areas of zonal wind speed greater than 30 m s−1. The wave-activity flux has also been omitted when its value was less than 10 m2 s−2
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those between the leading European rainfall mode and the NAO reported in Zveryaev and Allan (2010). Westerly jets act as a waveguide and trap teleconnection patterns or Rossby waves (Hoskins and Ambrizzi 1993; Ambrizzi et al. 1995). Therefore, the existence and routes of teleconnection patterns may be significantly affected by changes in the shape of westerly jets (e.g., Yun et al. 2011). The Asian jet exhibits a marked subseasonal variation during summer. The core of the Asian jet retreats westwards from the western North Pacific at about 140°E to the Eurasian continent at about 90°E during mid-June to midJuly (Zhang et al. 2006). This subseasonal change in the Asian jet provides an extra opportunity to confirm the role of basic flow in affecting teleconnection patterns. For this purpose, we analyzed the May and June situations in addition to the July and August ones, and made comparisons between them. Considering the eastward propagation of Rossby waves, we used the most upstream cell of the teleconnection patterns identified in the present study as a reference, examined the monthly results, and defined an EEP circulation anomaly index (EEP–CI) by averaging the monthly 200-hPa geopotential height anomalies over the region between the longitudes 10°E and 40°E and between the latitudes 50°N and 60°N. The correlation coefficients between the EEP–CI and EEPI are −0.70, −0.68, −0.69 and −0.73 for May, June, July and August, respectively, indicating that the EEP–CI represents the large-scale circulation anomaly associated with EEP rainfall variability. Figure 5 shows the corresponding wave-activity fluxes at 200 hPa from May to August, which were determined by wind anomalies for the high and low EEP–CI years and the climatological winds in individual months. The high and low EEP– CI years were determined as the years when the EEP–CI was
greater or less than one standard deviation. According to this criterion, there are 11 high EEP–CI years and 11 low EEP–CI years for July, and nine high EEP–CI years and eight low EEP–CI years for August. There are similar numbers of high or low EEP–CI cases for both May and June, ranging from 9 to 12. The most striking difference in wave-activity flux, related to the SC–EEP teleconnection, is characterized by the
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Fig. 6 Running correlations between the interannual variations of SC rainfall and EEP rainfall with a 15-year window for the period 1955– 2005: a July; b August. The horizontal lines indicate significance at the 95 % confidence level
South China–East European Plain rainfall relationship Fig. 7 Correlation of the interannual variations of SC rainfall with land rainfall: a for July during 1955–1975; b for July during 1976–2005; c for August during 1955–1975; and d for August during 1976–2005. The contour interval is 0.2 and the contour line of zero has been omitted. The shading shows correlations significant at the 95 % confidence level
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different routes of wave-activity flux over China between May–June and July–August. The wave-activity flux passes eastwards into the western Pacific in May and June, but curves southwards in China and towards SC in July and August. This monthly difference in the route of wave-activity flux can be explained by the subseasonal change in the Asian jet. The Asian westerly jet is strong over East Asia and the western Pacific in May and June, with the core of the jet locating over the western Pacific (Fig. 5a and b). By contrast, the Asian jet is greatly weakened over East Asia in July and August in comparison with May and June, and shrinks significantly westwards onto the Tibetan Plateau (Fig. 5c and d). This subseasonal change in the Asian jet is consistent with Zhang et al. (2006). The significant weakness of zonal wind over East Asia deprives the Asian jet of its role in trapping stationary Rossby waves along it in July and August, and thus Rossby waves may escape from the Asian jet over East Asia and extend into SC. The southward propagation of Rossby waves over East Asia, rather than northward propagation, is in agreement with Kosaka and Nakamura (2006), who indicated a southward wave-activity flux in the upper troposphere over East Asia. This difference in the route of wave-activity flux between May–June and July–August can provide another reason for the SC–EEP rainfall relationship being significant in July and August, but not in May and June. Actually, the correlation coefficients between the SCI and EEPI are −0.04 and −0.02 for May and June, respectively, implying independence between SC and EEP rainfall variations for these 2 months. The common feature among the months is that the waveactivity flux enters southwards into the Asian jet roughly over the Mediterranean region and Middle East, and then moves along the jet. This confirms the role of the Asian jet as a waveguide.
4 The interdecadal change of relationship between the interannual variations It can be seen from Table 1 that most of the years with an evident negative SCI–EEPI relationship appear after the mid1970s. There are only eight cases from the period 1955–1975, but 23 cases from the period 1976–2005. The possibility for an inverse relationship is much lower for the former period, even considering that the latter period comprises more years (30 years) than the former period (21 years). This implies that the relationship between interannual variations of SC and EEP rainfall might experience interdecadal change. Figure 6 confirms the existence of interdecadal change in the SC–EEP rainfall relationship. An evident interdecadal change in the relationship between the interannual variations of SC and EEP rainfall occurs in the mid-1970s for both July and August. For July, a weak SCI–EEPI relationship is found before the mid-1970s: negative before the mid-1960s and positive from the mid-1960s to the mid-1970s. However, the relationship remains negative after the mid-1970s, becomes strengthened until about 1982, after which it remains stable and becomes statistically significant. For August, the correlation coefficients are close to zero before the mid-1970s, while
Table 2 Correlation coefficients between the interannual variations of SC and EEP rainfall during the periods 1955–1975 and 1976–2005
July August
1955–1975
1976–2005
−0.01 −0.12
−0.65 −0.40
The correlation coefficients that appear in bold are significant at the 95 % confidence level
Q. Su, R. Lu Fig. 8 Composite differences of geopotential heights (contour; units: gpm) between wet and dry EEP years defined based on the interannual variations of EEP rainfall for the periods 1955–1975 and 1976–2005 (Table 3). The vectors show the corresponding wave-activity flux (units: m2 s−2): a for July during 1955–1975; b for July during 1976–2005; c for August during 1955–1975; and d for August during 1976–2005. The contour interval is 15 gpm and the contour line of zero has been omitted. The wave-activity flux has also been omitted when its value was less than 10 m2 s−2
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a negative relationship appears and becomes stronger after the late 1980s. We divided the analysis period into two parts, 1955–1975 and 1976–2005, to facilitate the comparison between weak and strong relationships. The dividing point was selected as 1975/1976, since the 15-year running correlation coefficients become stable and strong after 1983 for July. The stable and negative relationship seems to appear a couple of years earlier for August. However, a slight modification in the dividing point does not change the present conclusions. Figure 7 shows the correlation coefficients between the SCI and rainfall over land for these two periods, respectively. It is evident that SC rainfall is negatively correlated to EEP rainfall in the period 1976–2005, but a significant relationship cannot be detected between them in the period 1955–1975. In the period 1976–2005, the patterns of the correlation coefficients in both July and August (Fig. 7b and d) are similar to those during 1955–2005 (Fig. 1). The correlation coefficients between the interannual variations of SCI and EEPI are −0.65 for July and −0.40 for August in this period (Table 2), significant at the 95 % confidence level. In contrast, in the period 1955– 1975, there are hardly any areas of significant correlation coefficient over the EEP (Fig. 7a and c). The correlation coefficients between the SCI and EEPI are −0.01 for July and −0.12 for August. The relationship is quite weak in this period. Thus, the inverse relationship in rainfall is contributed to mainly by the latter period. Figure 8 shows the composite differences of geopotential heights between the dry and wet EEP years and the corresponding wave-activity flux at 200 hPa during the periods 1955–1975 and 1976–2005. Note that the composite differences in this figure were obtained by the dry years minus the wet years to facilitate comparisons with other figures. The wet/dry EEP years were defined as those when the EEPI anomalies were greater/less than plus/minus half the standard
deviation during any individual period (1955–1975 or 1976– 2005), which are shown in Table 3. In the period 1976–2005, the composite geopotential height anomalies, which are characterized by a wave-like train extending from Europe to East China (Fig. 8b and d), are similar to those associated with the negative relationship between SC and EEP rainfall (Fig. 3). Also, the southeastward wave-activity flux stretching from Europe to SC, corresponding well with the wave-like train, resembles that associated with the negative relationship. Therefore, the connection between SC and EEP rainfall in this period is established by the aforementioned extension of Rossby waves. However, in the period 1955–1975, the wave-activity flux does not penetrate to SC in July, though a wave-like train also exists (Fig. 8a). There is neither a clear wave-like train nor pathway of wave-
Table 3 Wet and dry EEP years during the periods 1955–1975 and 1976–2005 Wet EEP years (EEP+) 1955–1975 July
1956, 1962, 1968, 1973, 1974 August 1956, 1960, 1961, 1962, 1967, 1969, 1975 1976–2005 July 1977, 1978, 1979, 1984, 1989, 1990, 1993, 1998, 2000, 2003, 2004 August 1976, 1980, 1981, 1986, 1987, 1988, 1993, 1998, 1999, 2003
Dry EEP years (EEP−) 1955, 1959, 1967, 1970, 1972, 1975 1955, 1959, 1963, 1972, 1974 1981, 1986, 1988, 1992, 1994, 1995, 1996, 1997, 1999, 2001, 2002 1979, 1983, 1984, 1985, 1992, 1996, 1997, 2002, 2005
Here, + and−indicate the interannual variations of EEPI greater than half the standard deviation and less than minus half the standard deviation, respectively
South China–East European Plain rainfall relationship
activity flux from Europe to East China in August (Fig. 8c). The wave-activity flux shows an evident break around 90°E for August. Thus, these circulation anomalies and waveactivity flux provide dynamical evidence for the stronger inverse relationship in rainfall during the period 1976–2005 and weaker relationship during the period 1955–1975. The interdecadal change in jet location around the mid1970s might be responsible for the above-mentioned changes in teleconnections and wave-activity flux. Zhang and Huang (2011) indicated that the East Asian jet axis has moved southward in July and August since the end of 1970s. This southward shift of the East Asian jet may be helpful for the extension of Rossby waves into SC and a resultant closer SC–EEP rainfall teleconnection. Furthermore, it should be noted that there are various factors affecting both SC rainfall and EEP rainfall. The effects of these factors on rainfall may exhibit a interdecadal change, and thus affect the SC–EEP rainfall relationship on the decadal timescale. For instance, Wu et al. (2012) indicated that the sea surface temperatures in the tropical Pacific and Indian Ocean have an opposite impact on SC rainfall before and after 1970s. More studies may be necessary to better gain a understanding of the interdecadal change in the SC–EEP rainfall relationship.
5 Conclusions The relationship between rainfall anomalies over SC and EEP in July and August and the possible reasons for the relationship have been revealed by using observational and reanalysis data for the period 1951–2009. The results showed that SC rainfall and EEP rainfall are significantly and negatively related. The correlation coefficients between the SCI and EEPI, which were used to represent rainfall anomalies over these remote regions, were −0.42 for July and −0.35 for August, all significant at the 95 % confidence level. These correlation coefficients were only slightly modified, being −0.44 for July and −0.31 for August, when interannual components — after removing interdecadal components — were used for the calculation. Analysis of the circulation anomalies and wave-activity flux indicated that the connection of interannual rainfall anomalies between SC and the EEP can be attributed to the extension of stationary Rossby waves in the mid-high latitudes in the upper troposphere for both July and August. There is a wave-like train of circulation anomalies and a pathway of wave-activity flux stretching from Europe to East China, linking the anticyclonic anomaly over Europe and the cyclonic anomaly over central and southern China, which are responsible for less EEP rainfall and more SC rainfall, respectively. In addition, the anticyclonic anomaly over Europe is associated with the summer NAO.
This wave-like pattern responsible for the SC–EEP rainfall relationship is, to some extent, determined by the shape of the Asian westerly jet, which can play a role as a waveguide. The Rossby wave is trapped in the Asian jet and penetrates westwards along it into the western North Pacific in May and June, and thus does not lead to an inverse relationship between SC and EEP rainfall. When the core of the Asian jet shifts westwards and the Asian jet is weakened over East Asia in July and August, the jet’s ability to trap the Rossby wave wanes, and thus the Rossby wave may escape from the jet over East Asia and extend southwards into central and southern China. It was further found that the relationship between the interannual variations of SC rainfall and EEP rainfall experiences an interdecadal change around the mid-1970s for both months. The correlation coefficients were calculated as −0.65 for July and −0.40 for August in the period 1976–2005, all significant at the 95 % confidence level, but were −0.01 for July and −0.12 for August in the period 1955–1975. The circulation anomalies and wave-activity flux for these two periods indicated that there was an extension of stationary Rossby waves from Europe to East China during the period 1976–2005, but not during the period 1955–1975, providing a dynamical explanation for the interdecadal change in interannual relationship between SC and EEP rainfall. Acknowledgements This study was supported by the National Basic Research Program of China (grant no. 2010CB950403).
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