1. Introduction
The Eurasian climate is strongly affected by atmospheric teleconnection patterns (Lu et al. 2002; Wu 2002; Ding and Wang 2005; Folland et al. 2009; Ding et al. 2011; Li and Lu 2017, 2018; S. Chen et al. 2019). The teleconnection patterns not only play an important role in affecting regional climate anomalies (Wang 1992; Iwao and Takahashi 2006, 2008; Chen and Huang 2012; Lin 2014) and extreme events (Cassou et al. 2005; Enomoto et al. 2009; Wulff et al. 2017; Thompson et al. 2019; K. Xu et al. 2019; Kim et al. 2020; Xu et al. 2020a), but also link climate anomalies in different regions (Schubert et al. 2011, 2014; Lin et al. 2017; Wu and Jiao 2017; Deng et al. 2018; P. Xu et al. 2019; Chen et al. 2020). An improved understanding of the roles of large-scale teleconnections is therefore of great importance.
There are two teleconnections that propagate zonally across the middle and high latitudes, respectively, of the Eurasian continent. The midlatitude one is called the Silk Road pattern (SRP; Lu et al. 2002; Enomoto et al. 2003), and the high-latitude one, which was recently found, is named differently in previous studies: the polar front jet wave train pattern in Xie and Kosaka (2016), the British–Baikal Corridor pattern (BBC) in P. Xu et al. (2019), and the high-latitude Eurasian teleconnection pattern in Li et al. (2020). Hereinafter, the high-latitude teleconnection is called the BBC, following P. Xu et al. (2019). There are some common features between the SRP and the BBC. First, the SRP and BBC are both meridionally confined and zonally oriented (Lu et al. 2002; Enomoto et al. 2003; Yasui and Watanabe 2010; Chen and Huang 2012; P. Xu et al. 2019; Li et al. 2020). The SRP propagates zonally along the upper tropospheric Asian westerly jet (around 40°N), with the latitude of the main body confined between 20° and 60°N, and the BBC propagates zonally along about 60°N, with the latitude ranging from 45° to 80°N. Second, both wave trains are characterized by alternate southerly and northerly anomalies from the western to eastern Eurasian continent, with an equivalent-barotropic structure. The SRP and BBC have been shown to be the dominant modes of upper tropospheric meridional wind anomalies over the middle and high latitudes, respectively, of the Eurasian continent (Sato and Takahashi 2006; Kosaka et al. 2009; Chen and Huang 2012; Hong and Lu 2016; P. Xu et al. 2019; Li et al. 2020). In addition, the maintenance mechanism of both the BBC and SRP can largely be explained by the interaction between stationary waves and the basic flow (Sato and Takahashi 2006; Kosaka et al. 2009; Yasui and Watanabe 2010; Song et al. 2013; P. Xu et al. 2019; Li et al. 2020). For the BBC, the nonlinear interaction with transients also plays an important role in driving the pattern (Xu et al. 2020b).
It has been well reported that the SRP exerts a strong influence on summer climate anomalies over a broad area of midlatitude Eurasia (Lu et al. 2002; Wakabayashi and Kawamura 2004; Chen and Huang 2012; Saeed et al. 2014; Wang and He 2015; Hong et al. 2017; Jin and Guan 2017). The BBC also significantly affects the summer surface air temperature (SAT) and rainfall anomalies along its wave route (P. Xu et al. 2019). An individual teleconnection can link climate anomalies in different regions in the zonal direction, but the concurrence of teleconnections in the middle and high latitudes may also link climate anomalies at different latitudes. For example, Iwao and Takahashi (2006, 2008) identified a seesaw pattern in the July precipitation north and south of Lake Baikal and found that this north–south seesaw pattern is associated with wavelike anomalies over northern Eurasia and along the midlatitude westerly jet. While previous studies have investigated the roles of each of the BBC and SRP in affecting the Eurasian climate, the effect on climate of the combined action of these two wave trains has not yet been addressed, which is the main concern of this study.
The motivation for this study is as follows. First, the BBC and SRP dominate the variance of circulation anomalies in high and middle latitudes, respectively. Therefore, the combination of the two wave trains may dominantly explain the mid- to high-latitude Eurasian climate. Second, the BBC and SRP both tend to be geographically fixed (Lu et al. 2002; Ding and Wang 2005; Kosaka et al. 2009; P. Xu et al. 2019; Xu et al. 2020b). Therefore, it is important to project climate anomalies on these two teleconnections. Third, there are some overlaps in the meridional scope of the BBC and SRP, and the concurrence of the two wave trains covers almost the whole mid- to high-latitude region of Eurasia. The combination of the BBC and SRP may coordinate the mid- and high-latitude climate anomalies and thus contribute to a more comprehensive understanding of the large-scale climate variability in Eurasia.
In this study, we mainly focus on the combined impact of the BBC and SRP on large-scale SAT anomalies, rather than rainfall anomalies. It has been reported that there is a good correspondence between large-scale SAT and geopotential height anomalies in the mid- to high latitudes of Eurasia in summer—that is, an increased SAT is associated with positive geopotential height anomalies, and vice versa (Pfahl and Wernli 2012; Chen et al. 2016; Park and Ahn 2016; Hong et al. 2017; R. Chen et al. 2019; K. Xu et al. 2019). By contrast, rainfall anomalies are more vulnerable to local factors and show a complex correspondence with large-scale circulation anomalies. For example, positive rainfall anomalies may be induced by either southerly anomalies or cyclonic anomalies, or both, and vice versa. In addition, the cause–effect relationship between the large-scale circulation and rainfall anomalies might be ambiguous. For instance, the Indian rainfall anomalies, which are greatly affected by the SRP, could, in turn, favor the downstream SRP (Ding and Wang 2005; Ding et al. 2011; Chen and Huang 2012; Greatbatch et al. 2013).
The rest of this paper is arranged as follows. Section 2 describes the data and methods. Section 3 investigates the individual and combined effects of the BBC and SRP on Eurasian SAT and presents that the combination of the BBC and SRP would result in two different large-scale temperature patterns. Section 4 further suggests that these two patterns resemble the two major modes of temperature anomalies in mid- to high-latitude Eurasia. Section 5 is devoted to a summary and discussion.
2. Dataset and methods
This study uses the monthly atmospheric circulation variables including winds and geopotential height on a horizontal resolution of 1.25° × 1.25° from the Japanese 55-Year Reanalysis (JRA-55; Kobayashi et al. 2015) datasets. Also employed is the monthly land surface air temperature data from the Climatic Research Unit high-resolution dataset (version 4.03; Harris et al. 2014) with a horizontal resolution of 0.5° × 0.5°, which is based on weather station records and starts from 1901 to 2018. Considering the current warming trend (e.g., Sutton et al. 2007; Dong et al. 2009; Boer 2011; Xie et al. 2015), we remove the 9-yr running mean of the SAT prior to analyses. The time span analyzed is selected from 1958 to 2018. In this study, we focus on the summer mean, which refers to the averages over the months June–August (JJA). In addition, to confirm the stability of the present results, we also enlarge the sample size using the NOAA–CIRES Twentieth Century Reanalysis V3 (20CRv3; Slivinski et al. 2019) dataset and repeat all the analyses during the period 1901–2015. The results for the extended-period analyses are very similar and partly shown in this paper.
We define two indexes to facilitate quantitative estimations of the BBC- and SRP-related anomalies. The BBC index (BBCI) is defined as the standardized difference in the JJA-mean 250-hPa meridional winds (V250) between 62.5°N, 46.25°E and 60°N, 90°E, following Li et al. (2020). The SRP index (SRPI) is defined as the standardized first principal component (PC1) of the empirical orthogonal function (EOF) mode of the JJA-mean 200-hPa meridional wind anomalies (V200) within the domain 20°–60°N, 0°–150°E, which is identical to a number of previous studies (e.g., Yasui and Watanabe 2010; Hong and Lu 2016; Hong et al. 2017). The BBC index has also been defined as the first EOF mode of V300 (Xie and Kosaka 2016) and V250 (P. Xu et al. 2019) over northern Eurasia, respectively. The correlation coefficients between the BBCI in this study and the wave train indexes defined by Xie and Kosaka (2016) and P. Xu et al. (2019) are 0.91 and 0.94, respectively, during the time period 1958–2018. We repeated the main analyses using these indexes and obtained similar results. Therefore, the present results are not sensitive to the choice of indexes.
The main statistical methods used include EOF, regression, correlation, and composite analyses. Student’s t test is used to determine the statistical significance in the analyzed results, and the method proposed by North et al. (1982) is used to test the statistical significance in the EOF analysis.
3. Individual and combined effects of the BBC and SRP on the SAT in Eurasia
a. Anomalies associated with the BBC and SRP
Figure 1 shows the time series of the BBCI and SRPI and the regressed 200-hPa horizontal wind anomalies. The BBCI shows no clear interdecadal variation (Fig. 1a), consistent with previous studies (P. Xu et al. 2019; Li et al. 2020). The SRPI shows both interannual and interdecadal variations (Fig. 1b), with regime shifts in the early 1970s and late 1990s, respectively, which have been reported in previous studies (Hong et al. 2017, 2018; Wang et al. 2017). In this study, we focus on the combined effect of the BBC and SRP on SATs. As illustrated in later analyses, the spatial patterns of temperature anomalies induced by the conjunction of the BBC and SRP are not sensitive to the existence of the decadal component of the SRPI. We therefore adopt the original SRPI without removing its decadal component. In addition, the BBCI and SRPI show a weak linear relationship during 1958–2018: the correlation coefficient between the BBCI and SRPI is 0.20, which is barely statistically significant at the 95% confidence level. However, their correlation coefficient is statistically significant at the 95% confidence level during 1901–2015 (0.21), suggesting that there is a possible linkage between the BBC and SRP.
Time series of the (a) BBCI and (b) SRPI during the boreal summer. Also shown are the 200-hPa horizontal wind anomalies (m s−1; vectors) and meridional wind anomalies (m s−1; shading) regressed onto the (c) BBCI and (d) SRPI. Only vectors >1.0 m s−1 and statistically significant at the 95% confidence level from Student’s t test are shown in (c) and (d).
Citation: Journal of Climate 34, 9; 10.1175/JCLI-D-20-0325.1
For the BBCI-related anomalies (Fig. 1c), a clear wave pattern appears zonally over the high-latitude Eurasian continent, consistent with previous studies (P. Xu et al. 2019; Li et al. 2020). For the SRPI-related anomalies (Fig. 1d), there is a wave train along the midlatitude Asian westerly jet (around 40°N), which has been reported in a number of previous studies (Lu et al. 2002; Yasui and Watanabe 2010; Chen and Huang 2012; Hong and Lu 2016; Hong et al. 2017). In this study, the spatial distributions shown in Figs. 1c and 1d are considered to be the positive phase of the BBC and SRP, respectively. The positive phase of the BBC corresponds to cyclonic anomalies over northwest Europe and central Siberia and anticyclonic anomalies over western Siberia, and vice versa (Fig. 1c). The positive phase of the SRP indicates cyclonic anomalies over Europe and central Asia and anticyclonic anomalies over western Asia and East Asia, and vice versa (Fig. 1d).
Figure 2 shows the SAT anomalies regressed onto the BBCI and SRPI, respectively. Significant 200-hPa geopotential height (H200) anomalies with absolute values >10.0 gpm are also shown. The BBCI-related SAT anomalies are characterized by a wavelike train in the high-latitude Eurasian continent (Fig. 2a), consistent with the study of P. Xu et al. (2019). The positive phase of the BBC corresponds to significantly anomalous cooling over northwest Europe and central Siberia and anomalous warming over western Siberia. These cold and warm anomalies are in good agreement with the negative and positive H200 anomalies and the cyclonic and anticyclonic anomalies, respectively (Figs. 2a and 1c). On the other hand, the SRP also induces large-scale SAT anomalies over the Eurasian continent, especially over the midlatitude regions (Fig. 2b). These SAT anomalies also show a wavelike train. Associated with the positive phase of the SRP, there are significant cold anomalies over Europe and central Asia and warm anomalies over western Asia. These cold and warm SAT anomalies also correspond well to the negative and positive H200 anomalies and the cyclonic and anticyclonic anomalies, respectively (Figs. 2b and 1d).
Summer SAT anomalies (°C; shading) regressed onto the (a) BBCI and (b) SRPI. Vertical hatching indicates that the anomalies are significant at the 95% confidence level from Student’s t test. The red and blue contours show the regions where the absolute values of the positive and negative 200-hPa geopotential height anomalies associated with the BBCI/SRPI are >10.0 gpm, respectively.
Citation: Journal of Climate 34, 9; 10.1175/JCLI-D-20-0325.1
Figure 3 shows the variance of the SAT and H200 anomalies explained by the BBCI and SRPI, respectively, with combinations of BBCI and SRPI shown in the bottom panels. For the SAT, the variance explained by the BBCI appears over the high-latitude Eurasian continent (Fig. 3a): northwestern Europe, western Siberia, and central Siberia. Especially over western Siberia, the maximum of the explained variance can reach up to 60%. The variance explained by the SRPI is large (10%–50%) in Europe and western and central Asia (Fig. 3c). These large centers correspond well to the SAT anomalies related to the BBC (Figs. 2a and 3a) and SRP (Figs. 2b and 3c), respectively. Compared with the individual variance, the combination of the BBCI and SRPI almost covers the whole mid- to high-latitude Eurasian continent (Fig. 3e). Large centers in the zonal direction of Eurasia appear over midlatitude Europe, western Siberia/western Asia, and central Asia. As shown in later analyses, the combined effect of the BBC and SRP on SAT anomalies reflects the synergetic variations of these large centers. The explained variance averaged over the domain 30°–75°N, 10°–120°E is 22.1%.
Variance of the summer (left) SAT and (right) 200-hPa geopotential height explained by the (a),(b) BBCI and (c),(d) SRPI, and (e),(f) their combinations (%). The combined variance is calculated from the sum of their individual variance.
Citation: Journal of Climate 34, 9; 10.1175/JCLI-D-20-0325.1
The distributions of the H200 variance explained by both the BBCI and SRPI correspond well to those of the SAT (left vs right panels), confirming the good correspondence between the SAT and H200 anomalies. For the BBC (Fig. 3b), large variances appear over high latitudes and the maximum of the explained variance can reach up to 50% over western Siberia. For the SRP (Fig. 3d), the large explained variance appears over Europe and western and central Asia. The variance explained by the BBC and SRP together is large over a broad area of the mid- to high latitudes of Eurasia (Fig. 3f) and the ratio of the explained variance averaged over 30°–75°N, 10°–120°E is 21.9%. Very similar results are obtained based on the extended-period analyses (not shown). For instance, the BBC and SRP together explain 21.5% of the variance of the SAT and 20.5% of the H200 over 30°–75°N, 10°–120°E. These results suggest that, when compared with the individual effects, the combination of the BBC and SRP can explain a wider range of the variance in the circulation and SAT anomalies over the Eurasian continent.
b. Anomalies associated with the combination of the BBC and SRP
We investigated the spatial patterns of circulation and SAT anomalies induced by different configurations of the BBCI and SRPI. As shown in the scatter diagram of the BBCI and SRPI (Fig. 4), all of the samples can be divided into four configurations. On the basis of the signs of the BBCI and SRPI, the number of years in each category are roughly similar: 15 years for BBCI+ SRPI+, 17 years for BBCI− SRPI−, 16 years for BBCI+ SRPI−, and 13 years for BBCI− SRPI+. Note that the in-phase (BBCI+ SRPI+ and BBCI− SRPI−) and out-of-phase (BBCI+ SRPI− and BBCI− SRPI+) configurations rely on the definition of the positive phases of the BBC and SRP—that is, the in-phase and out-of-phase configurations are reversed if the positive phase of the BBC or SRP is defined as opposite-signed anomalies to those shown in Fig. 2. Figure 4 suggests that more dots tend to be farther from the origin of the coordinates in the first and third quadrants than in the second and fourth quadrants. This is confirmed by the interannual standard deviations of the BBCI and SRPI. The standard deviations of the BBCI and SRPI are both larger for the in-phase configurations than for the out-of-phase configurations, with the value of the BBCI being 1.07 for the in-phase and 0.93 for the out-of-phase configurations and that of the SRPI being 1.16 for the in-phase and 0.81 for the out-of-phase configurations. The extended-period analyses show similar results: the standard deviations of the BBCI and SRPI for the in-phase cases are 1.01 and 1.16, respectively—both greater than those for the out-of-phase cases (0.97 for BBCI and 0.79 for SRPI).
Scatter diagram of the BBCI and SRPI (see the text for definitions).
Citation: Journal of Climate 34, 9; 10.1175/JCLI-D-20-0325.1
Figure 5 shows the composite 200-hPa horizontal wind (UV200) and meridional wind anomalies for the four configurations of the BBCI and SRPI and the difference between the two in-phase (BBCI+ SRPI+ minus BBCI− SRPI−) and two out-of-phase (BBCI+ SRPI− minus BBCI− SRPI+) configurations. The composite anomalies are well organized and statistically significant for all the configurations (Figs. 5a–d). However, the spatial patterns for the in-phase and out-of-phase configurations show distinctly different features (e.g., Figs. 5a,b). For the BBCI+ SRPI+ configuration (Fig. 5a), the V200 anomalies show a meridionally symmetrical feature over the mid- to high-latitude Eurasian continent: the high- and midlatitude anomalies merge with each other and exhibit as a whole wavelike train with a large meridional extension. The UV200 anomalies are associated with cyclonic anomalies over Europe and central Asia/central Siberia and anticyclonic anomalies over western Asia/western Siberia. By contrast, for the BBCI+ SRPI− configuration (Fig. 5b), the V200 anomalies show meridionally asymmetrical features: the high- and midlatitude anomalies are well separated and there are clearly two zonally oriented wave trains over high- and midlatitude Eurasia, respectively. The anticyclonic anomalies over western Siberia tend to be connected with those over midlatitude Europe and central Asia and these anomalies form an Ω-like pattern. The BBCI− SRPI− configuration is nearly opposite to the BBCI+ SRPI+ configuration (Figs. 5a,c) and the BBCI− SRPI+ is nearly opposite to the BBCI+ SRPI− configuration (Figs. 5b,d). In addition, it seems that the wavelength in high latitudes occupies a larger zonal scope than that in midlatitudes. This discrepancy results from the sphericity of Earth and would disappear under the Lambert projection (not shown).
Composite 200-hPa horizontal wind anomalies (m s−1; vectors) and meridional wind anomalies (m s−1; shading) for (left) in-phase and (right) out-of-phase configurations of the BBCI and SRPI. Only vectors that are greater than 1.0 m s−1 and statistically significant at the 95% confidence level from Student’s t test are shown. Shown are (a) positive BBCI and positive SRPI, (b) positive BBCI and negative SRPI, (c) negative BBCI and negative SRPI, and (d) negative BBCI and positive SRPI. Also shown are (e) the difference between (a) and (c) and (f) the difference between (b) and (d). “AC” and “C” in (e) and (f) represent the anticyclonic and cyclonic anomalies, respectively.
Citation: Journal of Climate 34, 9; 10.1175/JCLI-D-20-0325.1
It is notable that the circulation anomalies show some differences in the upstream region (i.e., over North Atlantic and Europe), between the in-phase and out-of-phase configurations (Figs. 5e,f). For the in-phase configuration, the cyclonic anomaly appears mainly over Europe and there tends to be an anticyclonic anomaly over the North Atlantic Ocean (Fig. 5e), whereas for the out-of-phase configuration the cyclonic anomaly tends to be larger, extending westward to the North Atlantic (Fig. 5f). This implies that the circulation anomalies have a smaller wavenumber for the out-of-phase configuration in the upstream region in comparison with the in-phase configuration. According to the theory, stationary Rossby waves with a smaller wavenumber propagate along a more meridionally curved route (Hoskins and Karoly 1981). Indeed, the circulation anomalies for the out-of-phase configuration, which are marked as cyclonic, anticyclonic, and cyclonic anomalies in Fig. 5f, tend to curve southward. By contrast, the circulation anomalies for the in-phase configuration are more like in a zonal route (Fig. 5e).
Figure 6 shows the composite 200-hPa geopotential height anomalies for the four configurations and the difference between the two in-phase and two out-of-phase configurations. Consistent with the UV200 anomalies, the H200 anomalies are also well organized and statistically significant for all the configurations (Figs. 6a–d). The in-phase and out-of-phase configurations have two distinctly different spatial patterns (e.g., Figs. 6a,b). For the BBCI+ SRPI+ configuration (Fig. 6a), the anomalies over the mid- to high latitudes merge with each other in the meridional direction, consistent with the meridionally symmetrical feature of the V200 anomalies. These anomalies are characterized by a tripole pattern in the zonal direction over the Eurasian continent. There are negative anomalies over Europe and central Asia/central Siberia and positive anomalies over western Asia/western Siberia. These anomalous centers are in good agreement with the large explained variance centers of H200 (Figs. 6a and 3f). By contrast, for the BBCI+ SRPI− configurations (Fig. 6b), the H200 anomalies are seen as a meridionally asymmetrical feature, and the signs of the anomalies over high latitudes are opposite to those over midlatitudes in the meridional direction. The positive anomalies over western Siberia are connected with those over midlatitude Europe and central Asia and these anomalies form an Ω-like pattern over the Eurasian continent. Besides, there are negative anomalies over northwestern Europe, western Asia, and central Siberia (Fig. 6b). The anomalous centers also correspond well to the explained variance centers (Fig. 3f). The BBCI− SRPI− and BBCI− SRPI+ configurations are also nearly opposite to the BBCI+ SRPI+ and BBCI+ SRPI− configurations, respectively (Figs. 6a,c vs. Figs. 6b,d). In comparison with the individual teleconnection, the combination of the BBC and SRP occupies a larger meridional scale and thus may affect the synergistic climate anomalies in mid- and high-latitude Eurasia.
As in Fig. 5, but for the 200-hPa geopotential height anomalies (gpm). The vertical hatching indicates that the anomalies with absolute values of >10.0 gpm are significant at the 95% confidence level from Student’s t test.
Citation: Journal of Climate 34, 9; 10.1175/JCLI-D-20-0325.1
Figure 7 shows the composite SAT anomalies for the four configurations and the difference between the two in-phase and two out-of-phase configurations. The SAT anomalies for all the configurations also show significantly well-organized features, but manifest as two distinctly different spatial patterns: a zonal tripole pattern for the in-phase configurations (left-hand panels) and an Ω-like pattern for the out-of-phase configurations (right-hand panels). For the BBCI+ SRPI+ configuration (Fig. 7a), the SAT anomalies exhibit as a large meridional scope. The zonally oriented tripole pattern is characterized by anomalous cooling over Europe and central Asia/central Siberia and anomalous warming over western Siberia/western Asia—that is, for this configuration, the large SAT variance center over western Siberia shows the opposite variations to those over midlatitude Europe and central Asia (Fig. 3e). For the BBCI+ SRPI− configuration (Fig. 7b), the SAT anomalies are seen as an asymmetrical feature in the meridional direction. The Ω-like pattern manifests as anomalous warming over western Siberia, midlatitude Europe, and central Asia—that is, the large SAT variance centers show consistent variations (Fig. 3e). In addition, there is anomalous cooling over northwestern Europe, western Asia, and central Siberia. The SAT anomalies for the BBCI− SRPI− (BBCI− SRPI+) configurations show a similar pattern to those for the BBCI+ SRPI+ (BBCI+ SRPI−) configurations, but with opposite signs (Fig. 7c vs Fig. 7a, and Fig. 7d vs Fig. 7b). For all of the configurations, increased and decreased SAT anomalies correspond well to positive and negative H200 anomalies, respectively. In addition, the SAT anomalies tend to be stronger in high latitudes than in midlatitudes, associated with the larger interannual variability of the geopotential height there (Li et al. 2020). Overall, these results indicate that the SAT anomalies in mid- and high latitudes can be coordinated together under the concurrent variation of the BBC and SRP.
As in Fig. 5, but for the surface air temperature anomalies (°C; shading). Vertical hatching indicates that the anomalies are significant at the 95% confidence level from Student’s t test. Red and blue contours denote the regions where the absolute values of significant positive and negative 200-hPa geopotential height anomalies are greater than 10.0 gpm, respectively.
Citation: Journal of Climate 34, 9; 10.1175/JCLI-D-20-0325.1
It seems that for all the configurations, the circulation and SAT anomalies are seen as the linear superposition of the individual effects of the BBC and SRP (e.g., Figs. 2 and 7a). To confirm this, we obtained the SAT anomalies regressed onto the BBCI and SRPI, respectively, and then the composite BBCI- and SRPI-related SAT anomalies for different configurations. Their corresponding linear superpositions (not shown) are very similar to the original composite ones (Fig. 7), confirming that the combined effect basically acts as a linear superposition of the individual effects. On the other hand, it seems that the anomalies show a slightly larger amplitude in the in-phase than out-of-phase configurations (e.g., Figs. 5e,f), which is consistent with the greater standard deviations of both the BBCI and SRPI for the in-phase configurations (Fig. 4).
Note also that the composite anomalies shown in this section are obtained by using all of the years in the individual quadrants. We performed a similar composite analysis using stronger BBCI and SRPI anomalies (e.g., >0.3 standard deviations) and obtained similar results, except for stronger amplitudes (not shown). Considering that the SRPI experiences decadal variations (Fig. 1b), we also repeated the major analyses using the interannual component of the SRPI, which is obtained by removing the 9-yr running mean of the original SRPI, and found similar patterns (not shown). This is probably because the SAT anomalies associated with the combination of the BBC and SRP show a larger amplitude at high latitudes than at midlatitudes (Figs. 7e,f). Besides, as the BBC does not show appreciable decadal changes, the BBC–SRP configurations would not exhibit appreciable variations on the decadal time scales. The extended-period analyses also obtain similar composite results (not shown).
The results in this section indicate that the combination of the BBC and SRP can result in significant and well-organized large-scale anomalies. The two in-phase configurations are nearly symmetrical, as are the two out-of-phase configurations. Therefore, different configurations of the BBC and SRP result in two kinds of large-scale circulation anomalies: a meridionally symmetrical pattern for the in-phase configurations, which corresponds to the zonal tripole pattern of the H200 anomalies, and a meridionally asymmetrical pattern for the out-of-phase configurations, which corresponds to the Ω-like pattern of the H200 anomalies. These circulation anomalies result in two different large-scale SAT anomalies over the mid- to high-latitude Eurasian continent: the zonal tripole pattern for the in-phase configurations and the Ω-like pattern for the out-of-phase configurations. These two patterns reflect the synergetic variations of the large SAT variance centers (i.e., midlatitude Europe, western Siberia, and central Asia) explained by the combination of the BBC and SRP. Overall, these results indicate that a combination of the BBC and SRP can coordinate the circulation anomalies over the middle and high latitudes and largely explains the large-scale Eurasian SAT anomalies.
4. Major modes of mid- to high-latitude Eurasian SAT anomalies and their associations with the concurrence of BBC and SRP
The results in the previous section indicate that the large-scale SAT anomalies over the mid- to high latitudes of Eurasia can be coordinated by the synergy of the BBC and SRP. In this section, we extract the leading modes of the large-scale SAT anomalies over the mid- to high-latitude Eurasian continent (20°–80°N, 15°W–150°E) by EOF analysis and investigate their associations with the concurrent variation of the BBC and SRP. Considering that the standard deviation of the SAT increases with latitude over the Eurasian continent (not shown), the SAT anomalies are normalized and weighted by the square root of the cosine of latitude before EOF analysis.
Figure 8 shows the two leading EOF modes associated with the large-scale Eurasian SAT anomalies during the boreal summer, which explain 13.6% and 11.3% of the total variance, respectively. Slightly broadening or narrowing the EOF domain would yield similar patterns. Both modes reflect the synergetic variations of the SAT anomalies in the middle and high latitudes. The spatial pattern of the first EOF mode resembles the zonally oriented tripole pattern of the SAT anomalies associated with the in-phase configurations of the BBC and SRP (Figs. 8a and 7e). There are negative anomalies over Europe and central Asia/central Siberia and positive anomalies in between (Fig. 8a). The second mode resembles the Ω-like pattern associated with the out-of-phase configurations of the BBC and SRP (Figs. 8b and 7f). There are positive anomalies over western Siberia and midlatitude Europe and negative anomalies over northwestern Europe, western Asia, and central Siberia (Fig. 8b). These results suggest that the zonal tripole pattern and the Ω-like pattern associated with the synergy of the BBC and SRP dominate the variance of large-scale Eurasian SAT anomalies.
(a),(b) The first two EOFs of the large-scale variations in the SAT over the region 20°–80°N, 15°W–150°E during 1962–2014 and (c),(d) the corresponding time series of the principal components. The 9-yr running mean of the SAT has been removed to obtain interannual components, and thus the first and last four years are excluded from the analysis. The percentage of the variance explained by each EOF is shown in the upper-right-hand corner of (a) and (b).
Citation: Journal of Climate 34, 9; 10.1175/JCLI-D-20-0325.1
Figure 9 shows the SAT and 200-hPa circulation anomalies regressed onto the normalized PC1 and PC2, respectively. The SAT anomalies are in good agreement with the H200 anomalies—that is, positive SAT anomalies correspond well to the positive H200 anomalies, and vice versa. For PC1, the SAT and H200 anomalies both exhibit as the zonal tripole pattern, as expected (Fig. 9a). These anomalies resemble those associated with the in-phase configurations of the BBC and SRP (e.g., Fig. 7e). The spatial correlation coefficient between Figs. 9a and 7e is 0.64 for the SAT anomalies and 0.66 for the H200 anomalies over 20°–80°N, 15°W–150°E. For PC2, the SAT and H200 anomalies are both characterized by the Ω-like pattern (Fig. 9c). These anomalies resemble those induced by the out-of-phase configurations of the BBC and SRP (e.g., Fig. 7f). The spatial correlation coefficient between Figs. 9c and 7f reaches 0.80 for the SAT anomalies and 0.76 for the H200 anomalies over 20°–80°N, 15°W–150°E. In addition, among the six cases with absolute PC1 > 1.5 standard deviation, five are seen as an in-phase configuration; and among the nine cases with absolute PC2 > 1.5 standard deviation, six are seen as an out-of-phase configuration. These results suggest that the EOF1 and EOF2 of large-scale SAT anomalies are dominated by the in-phase and out-of-phase configurations of the BBC and SRP, respectively.
(a),(c) Summer SAT anomalies (°C; shading), (b),(d) 200-hPa horizontal wind anomalies (m s−1; vectors) and meridional wind anomalies (m s−1; shading) regressed onto the normalized (top) PC1 and (bottom) PC2 during 1962–2014. In the left panels, vertical hatching indicates that the anomalies are significant at the 95% confidence level from Student’s t test. The red and blue contours show the regions where the absolute values of the positive and negative 200-hPa geopotential height anomalies associated with the PC1/PC2 are greater than 6.0 gpm, respectively. In the right-hand panels, only vectors that are statistically significant at the 95% confidence level from Student’s t test are shown.
Citation: Journal of Climate 34, 9; 10.1175/JCLI-D-20-0325.1
The UV200 and V200 anomalies associated with PC1 (PC2) also resemble those associated with the in-phase (out-of-phase) configurations of the BBC and SRP (e.g., Fig. 9b and 5e; Fig. 9d and 5f). For PC1 (Fig. 9b), there are zonally oriented wavelike anomalies over the mid- to high latitudes of Eurasia. The V200 anomalies are connected in the meridional direction, displaying a meridionally symmetrical feature. The spatial correlation coefficient of the V200 anomalies between Figs. 9b and 5e is 0.70 over 20°–80°N, 15°W–150°E, confirming that the in-phase configurations of the BBC and SRP play an important role in affecting EOF1 of large-scale SAT anomalies. By contrast, for PC2 (Fig. 9d), the zonally oriented wave train in high latitudes is separated from that in midlatitudes and is seen as a meridionally asymmetrical feature. The spatial correlation coefficient of the V200 anomalies between Figs. 9d and 5f is 0.72 over 20°–80°N, 15°W–150°E, confirming that the EOF2 of large-scale SAT anomalies over the Eurasian continent is dominated by the out-of-phase configurations of the BBC and SRP.
The first EOF mode is distinguished from the second EOF mode, but the latter is not separable from other higher EOF modes according to the criteria of North et al. (1982), suggesting that the leading modes may be sensitive to the analysis period and a larger sample size may provide a more reliable result. We therefore apply EOF analysis during a longer period (i.e., 1905–2011) by using the 20CRv3 dataset (Fig. 10). Note that the 9-yr running mean of the SAT has been removed prior to analyses. The first two modes resemble the zonal tripole pattern (Fig. 10a) and Ω-like pattern (Fig. 10b), respectively. In this circumstance, these two modes are both well separated from other modes according to the criteria of North et al. (1982) and they explain 12.1% and 10.1% of the total variance, respectively. Whether these two modes are mathematically distinguishable therefore may depend on the size of the sample.
As in Fig. 8, but during 1905–2011 on the basis of the 20CRv3 dataset.
Citation: Journal of Climate 34, 9; 10.1175/JCLI-D-20-0325.1
Figure 11 further shows the SAT and 200-hPa circulation anomalies regressed onto the normalized PC1 and PC2, respectively, during 1905–2011. The SAT and circulation anomalies are in good agreement with those in the shorter period (Fig. 9), for both the leading modes. The SAT and H200 anomalies associated with PC1 both exhibit as the zonal tripole pattern (Fig. 11a) and those associated with PC2 are both featured with the Ω-like pattern (Fig. 11c). The spatial correlation coefficient of SAT anomalies between the composite in-phase (out-of-phase) configuration and those associated with PC1 (PC2) is 0.85 (0.91) over 20°–80°N, 15°W–150°E, and that of H200 anomalies is 0.83 (0.87). On the other hand, the V200 anomalies associated with PC1 and PC2 resemble the in-phase and out-of-phase configurations of the BBC and SRP, respectively (Figs. 11b,d). The spatial correlation coefficient of V200 anomalies between the in-phase (out-of-phase) configuration and those associated with PC1 (PC2) is 0.87 (0.89) over 20°–80°N, 15°W–150°E. These results confirm that the interannual variability of these two major modes is dominated by the combination of the BBC and SRP.
As in Fig. 9, but during 1905–2011 on the basis of the 20CRv3 dataset.
Citation: Journal of Climate 34, 9; 10.1175/JCLI-D-20-0325.1
5. Conclusions and discussion
The BBC and the SRP manifest as zonally oriented teleconnections over the high- and midlatitude Eurasian continent, respectively, and exert a significant influence on climate anomalies along their wave routes. In this study, we investigated the combined effect of the BBC and SRP on the Eurasian SAT. It is found that compared with the individual effects, a combination of the BBC and SRP can coordinate the circulation and SAT variations in the mid- to high latitudes and evidently explain the large-scale variability of the SAT in Eurasia.
The combination of the BBC and SRP is categorized into four configurations: two in-phase and two out-of-phase configurations. The anomalies for the two in-phase configurations are nearly symmetrical, as are the two out-of-phase configurations. The combination of the BBC and SRP therefore results in two kinds of significant and well-organized large-scale circulation anomalies: a meridionally symmetrical pattern and asymmetrical pattern, or a zonal tripole pattern and Ω-like pattern in the H200 anomalies. For the former, the anomalies in the mid- to high latitudes have the same signs and merge with each other in the meridional direction and the H200 anomalies are characterized by opposite variations between western Siberia/western Asia and Europe/central Asia/central Siberia. For the latter, the anomalies are characterized by an Ω-like pattern and are seen as consistent variations over midlatitude Europe, western Siberia, and central Asia.
Correspondingly, these circulation anomalies result in two different large-scale SAT anomalies over the mid- to high latitudes of Eurasia—that is, the same zonal tripole pattern and Ω-like pattern. Further results indicate that these two patterns resemble the two major modes of SAT variability over the mid- to high-latitude Eurasian continent and the associated circulation anomalies display the in-phase and out-of-phase configurations of the BBC and SRP, respectively, confirming that the large-scale variability of the SAT in Eurasia is mainly explained by the concurrent variation of the BBC and SRP.
It is interesting that the major modes of the interannual variability of the Eurasian SAT can be explained by the combination of the BBC and SRP, as there are many other teleconnections or local factors that affect the SAT variability (e.g., Wakabayashi and Kawamura 2004; Wulff et al. 2017; Li and Ruan 2018; Chen et al. 2016; R. Chen et al. 2019; K. Xu et al. 2019). This is probably because these two wave trains exhibit as leading modes of circulation anomalies in the mid- and high-latitude Eurasia, respectively, and their combination covers almost the whole mid- to high latitudes of Eurasia.
We explored the mechanisms responsible for the concurrent variations of the BBC and SRP. Considering that the sea surface temperature (SST) or SAT may provide a favorable background for the in-phase or out-of-phase configurations, these factors are examined (not shown). The results indicate that the composite SST and SAT anomalies for the in-phase cases (BBCI+ SRPI+ and BBCI− SRPI−) and out-of-phase cases (BBCI+ SRPI− and BBCI− SRPI+), as well as their difference, are all weak. Besides, to check the role of the warming trend of SAT over the Eurasian continent, we divided the analysis period into two epochs of the same length (1958–87 and 1989–2018). It is found that the numbers of in-phase cases are similar in the two epochs, as are the out-of-phase cases. It therefore seems that the SST and SAT do not have a role in inducing an in-phase or out-of-phase configuration of the BBC and SRP. In addition to these background factors, as discussed in section 3, it seems that these two teleconnections tend to be reinforced in the in-phase configurations (e.g., Figs. 5e,f). The possible linkage between these two teleconnections requires further investigation.
Acknowledgments
We thank the three anonymous reviewers for their insightful comments, which helped to improve this paper. This work was supported by the National Natural Science Foundation of China (Grants 41905055 and 41721004), the Natural Science Foundation of Jiangsu Province (Grant BK20190500), the Fundamental Research Funds for the Central Universities (Grant B200202145), and the Korea Meteorological Administration Research and Development Program (Grant KMI2018-01213).
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