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Relationship between –Siberian Blocking and the East Asian Winter Monsoon in Relation to the Oscillation and the El Nin˜ o–Southern Oscillation

HO NAM CHEUNG AND WEN ZHOU Guy Carpenter –Pacific Climate Impact Center, School of Energy and Environment, City University of Hong Kong, Hong Kong, China

HING YIM MOK AND MAN CHI Hong Kong Observatory, Hong Kong, China

(Manuscript received 7 April 2011, in final form 14 December 2011)

ABSTRACT

This study attempts to assess the possible linkage between Ural–Siberian blocking and the East Asian winter monsoon (EAWM). During the boreal winter, the dominance of blocking thermally enhances cold advection downstream. The frequent occurrence of Ural–Siberian blocking potentially promotes a cold EAWM and vice versa. The seasonal blocking activity can be regarded as the combined effect of the Arctic Oscillation (AO) and the El Nin˜ o–Southern Oscillation (ENSO). Weakened (strengthened) meridional flow in the positive (negative) phase of the AO is unfavorable (favorable) for the formation of blocking highs. Because the AO shows a close relationship with the North Atlantic Oscillation (NAO), its teleconnection with Ural–Siberian blocking may exist in the form of an eastward-propagating wave train. Be that as it may, the wave train signal across may be disturbed by the external effect of a strong ENSO event, which probably enhances (weakens) the westerlies near in its warm (cold) phase. Consequently, the blocking–EAWM relationship is stronger (weaker) when the AO and ENSO are in phase (out of phase). If both AO and ENSO attain the positive (negative) phase, the Siberian high tends to be weaker (stronger) and the temperature tends to be higher (lower) in East Asia, with less (more) Ural–Siberian blocking. On the other hand, if they are out of phase, they are not strongly linked to the intensity of the Siberian high, and the blocking activity over Ural–Siberia is unclear.

1. Introduction (2005b) studied the impact of blocking highs on cold surges by classifying them based on their origin in the Atmospheric blocking (‘‘blocking’’) is made up of an- Atlantic or Pacific. The Atlantic group includes the highs ticyclonic and cyclonic vortices. Its dominance over the over the Eurasian , which is often regarded as extratropics characterizes a strong meridional-type flow a wave train signal propagating from the North Atlantic (Rex 1950) and promotes the advection of polar air Ocean (e.g., Joung and Hitchman 1982). The Pacific masses along its downstream cyclonic component. When group, on the other hand, refers to a quasi-stationary a blocking high is located upstream adjacent to Siberia blocking high over the North Pacific, which may exert an in the boreal winter, the enhanced northerly cold ad- influence on the East Asian trough and indirectly affect vection increases mass convergence in the upper tropo- the cold surge pathways in East Asia. No matter how sphere, which in turn intensifies the surface Siberian high strong the blocking high is, the Siberian high undergoes and triggers a cold air outbreak in East Asia (Ding 1990). substantial intensification only if there are preexisting This provides a fundamental linkage between the block- cold anomalies (Takaya and Nakamura 2005a). There- ing and the East Asian cold surge. Takaya and Nakamura fore, the Pacific group has a relatively less significant im- pact on the cold surges, and the focus of this study is on the blocking upstream of Siberia. Corresponding author address: Dr. Wen Zhou, Guy Carpenter Asia-Pacific Climate Impact Center, School of Energy and Envi- While strong blocking events were found to play a cru- ronment, City University of Hong Kong, Hong Kong, China. cial role in reinforcing severe cold surges in East Asia (e.g., E-mail: [email protected] Joung and Hitchman 1982; Takaya and Nakamura 2005a;

DOI: 10.1175/JCLI-D-11-00225.1

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Lu and Chang 2009), the frequent occurrence of Ural– North Pacific are higher (lower). As warmer (cooler) SSTs Siberian blocking was found to be the main contributor in La Nin˜a(ElNin˜o) years enhance (suppress) the con- to two prolonged cold periods in the 2000s, which hap- vection that drives the meridional circulation in East Asia, pened in February–March 2005 (Lu and Chang 2009) the ENSO may remotely control the sinking motion over and January 2008 (Zhou et al. 2009). In particular, the Siberia. 2008 episode combined with plentiful southwesterly Whereas the EAWM is dynamically connected with moisture transported from the Bay of and a strong blocking, it is teleconnected with the AO and ENSO. La Nin˜a event, resulting in freezing rain and snowstorms Thus, it raises the question of how the two climate fac- that lasted for three weeks in southern China (Wen et al. tors exert an impact on the relationship between Ural– 2009; Zhou et al. 2009). On the other hand, a lower fre- Siberian blocking and the EAWM. In addition, El Nin˜ o quency of Ural–Siberian blocking probably results in occurred more frequently in the positive phase of the a warmer East Asian winter climate. Wu and Leung Pacific decadal oscillation (PDO) from the middle 1970s (2009) showed that the longest cold spell in Hong Kong, to the 1990s (Mantua et al. 1997; X. Wang et al. 2009). a coastal city in southern China, tends to be shorter in What are the consequences of the increasing frequency a winter without a Eurasian blocking event, such that the of El Nin˜ o events and the positive phase of the AO? winter gets warmer. Moreover, the winter-mean inten- This paper is organized as follows. Data and methods sity of the Siberian high has been found to be directly are presented in section 2. Next, the two major geo- proportional to seasonal blocking activity over Ural– potential height modes over the Ural–Siberian Siberia (L. Wang et al. 2010). As the Siberian high is a and their associated impacts on Ural–Siberian blocking semipermanent feature of the East Asian winter mon- and the EAWM are introduced in section 3. Further soon (EAWM; e.g., Lau and Li 1984), a linkage appar- analyses consider the role of the AO and ENSO. Sec- ently exists between Ural–Siberian blocking and the tions 4 and 5 make the comparison between different EAWM. polarities of the AO and the ENSO, respectively, whereas The EAWM is the most energetic global monsoon section 6 concerns their combined effect. The results are system, involving interaction between the extratropics discussed and summarized in section 7. of the East Asian continent and the tropics of the Pacific Ocean. Therefore, an accurate forecast of the EAWM should consider climate factors from both sources. On 2. Data and methods one hand, climate variability over the extratropical re- a. Data gion can be captured by the Arctic Oscillation (AO; Thompson and Wallace 1998). On interannual time scales, The winter period covers the five consecutive months Gong et al. (2001) showed that it is negatively correlated from November through March (NDJFM) for the with the intensity of the Siberian high such that the AO 60 years from 1950 to 2009, where year 1950 represents mayexertanimpactontheEAWM(GongandHo2002). the five months from November 1950 to March 1951, After the climate shift in the middle 1970s, the stronger and so on. The 60 years of data were extracted from the polar westerlies under the predominantly positive phase National Centers for Environmental Prediction–National of the AO could have been a major factor responsible Center for Atmospheric Research (NCEP–NCAR) 2.58 for the weakening trend of the Siberian high (Gong latitude by 2.58 longitude gridded reanalysis datasets and Ho 2002). On the other hand, the El Nin˜ o–Southern (Kalnay et al. 1996). The raw data include monthly fields Oscillation (ENSO) plays a major role in interannual of temperature T, sea level pressure (SLP), geopotential climate variability over the tropical Pacific Ocean. The height Z,zonalwindU, and meridional wind V. In par- significant impact associated with a strong El Nin˜ o event ticular, daily fields of geopotential height at 500 hPa in 1982/83 (Quiroz 1983) motivated extensive work fo- (Z500) were also extracted for detecting blocking events. cusing on its impact on global climate. Previous re- Furthermore, the AO index and the ENSO index (SST searchers have provided evidence for the tendency of anomalies over the Nin˜o-3.4 region) were freely adopted the EAWM to be weak in El Nin˜ o years (e.g., Li 1990; from the Climate Prediction Center (CPC) website (http:// Zhang et al. 1997). Li (1990) suggested that the Ferrel www.cpc.ncep.noaa.gov/). cell is activated by the active Hadley cell in El Nin˜ o For standardizing the raw data, the following normal- years such that the strong westerlies near the surface izing procedure is performed. Normalizing daily data for inhibit the intensification of the Siberian high. Chen acalendardayx uses the 11-day samples from day x 2 5 et al. (2000) further showed that the EAWM tends to be to x 1 5 for the entire 60-yr study period (i.e., 660 data stronger (weaker) in La Nin˜ a(ElNin˜ o) years when points), while normalizing monthly data for a particular tropical sea surface temperatures (SSTs) in the western month uses all 60 samples from the same month (i.e., 60

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and 408N, respectively (Austin 1980; Tibaldi and Molteni 1990). Since the geographic location of the blocking an- ticyclone varies with the season, a fluctuation of 58 of lat- itude is accepted (B06), which is indicated by D.A longitude is said to be blocked (i.e., blocking longitude) when Eqs. (1) and (2) are simultaneously satisfied by at least one of the five referencing latitude pairs. As in B06’s study, a blocking region has to be made up of at least five consecutive blocking longitudes (i.e., 12.58 of longitude), except that a nonblocking longitude is not accepted in between. Once a new blocking region is identified, it marks the beginning of an event. Following that, the algorithm looks for the blocking region be- FIG. 1. Longitudinal frequency distributions (%) of winter-mean longing to the same event. If either (i) any longitude onset (solid) and decaying (dashed) longitudes of blocking highs. constituting the blocking region on calendar day x over- laps with that on the previous day x 2 1 or (ii) the centers between the blocking on the two days are closer than 258, then the event is considered persisting between data points). The raw data from 29 February of leap years the two consecutive days (day x 2 1todayx). If neither of are excluded in the normalization procedure. The nor- the two conditions (i) or (ii) is satisfied on day x, the event malized value of each of these days is taken as the mean is said to be terminated on day x 2 1. After the detection of the values for 28 February and 1 March. procedures, only those events with a duration of at least b. Detection of atmospheric blocking 4 days are retained in this study, which is designated by the characteristic time of the blocking region (Pelly and The blocking event detection algorithm basically Hoskins 2003). follows the one adopted by Barriopedro et al. (2006, hereafter B06), which can be viewed as a modified ver- c. Study region of blocking sion of Tibaldi and Molteni (1990). It begins with iden- The scope of this study is confined to the blocking tifying the longitude that potentially characterizes the events upstream of Siberia, where the downstream cy- blocking-type circulation by applying the following zonal clonic vortex of the blocking may exert a direct impact index equations to the daily field of Z500: on the climatological region of the Siberian high (408– 658N, 808–1208E). The region is selected based on the Z500(f ) 2 Z500(f ) N S , average blocking high movement deduced from Fig. 1, f 2 f N S which shows three local maxima over the Eurasian con- tinent. The first two are located in the eastern Atlantic where fN and fS represent the reference latitude at the Ocean and the European continent over the Euro– north and the south; Atlantic sector, while the third is over the Ural–Siberian region, which matches the secondary peak of blocking GHGN(l) frequency near the identified by others, Z500(l, f ) 2 Z500(l, f ) albeit in summer (e.g., Tibaldi et al. 1994; Diao et al. 5 N 0 ,210 m (8 lat)21 and f 2 f 2006). As a result, the area of interest is narrowed to 308– N 0 1008E. This region encompasses the regions considered (1) by previous studies focusing on the impact of blocking Z500(l, f ) 2 Z500(l, f ) on the East Asian winter climate—for example, 408– GHGS(l) 5 0 S . 0; (2) 808E in Wu and Leung (2009) and 308–908E in L. Wang f 2 f 0 S et al. (2010). This region also coincides with the ex- traordinary peak of blocking frequency in January 2008 where f 5 808N 1D, f 5 608N 1D, f 5 408N 1D, N 0 S identified by Zhou et al. (2009). and D5258, 22.58,08, 2.58, and 58. Three reference latitudes are selected along each d. EAWM indices longitude based on previous climatological studies. The central point f0 is chosen as 608N (Treidl et al. 1981), Surface air temperature is indicative of the coldness of while the northern and the southern points are set at 808 a region. Because the temperature variations are dissimilar

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FIG. 2. The (a) first and (b) second eigenvectors (EOF1 and EOF2) of winter-mean 2-m air temperature in the EAWM, with the explained variance shown at the top right. They are re- ferred to as TM1 and TM2. between the northern and southern portions of East Asia 3. Recurring blocking high centers over on interannual time scales (B. Wang et al. 2010), there is Ural–Siberia no unique EAWM index able to capture the temperature variability in the entire EAWM region (Wang and Chen An EOF analysis is first performed on the covariance 2010a). It is therefore necessary to use at least two in- matrix made up of the winter-mean Z500 with a domain dices (Wu et al. 2006; B. Wang et al. 2010). On one hand, enclosing 408–808N and 308–1008E. The first 10 EOFs are the strength of the EAWM can be measured by then extracted (EOF1–10). In particular, the first two the Siberian high index (SHI), which is defined as the modes (EOF1–2) are well separated from the remaining area-averaged SLP over 408–658Nand808–1208E eight modes. They account for 67.2% of the total vari- (Panagiotopoulos et al. 2005; L. Wang et al. 2010). On ance by displaying different orientations of an anoma- the other hand, the two temperature modes in the lous ridge–trough couplet (Figs. 3a,b). In each winter, one EAWM identified by B. Wang et al. (2010), which are dominant EOF mode stands out that contains the largest taken as the first two leadingmodesoftheempirical value in magnitude among the 10 principal components orthogonal function (EOF) applied to the covariance (PCs) of all extracted EOFs in that winter. Table 1 shows matrix of 2-m air temperature over 08–608N and 1008– that 44 out of 60 winters demonstrate dominance in the 1408E, are applicable for the purpose of this study. first two EOF modes. The table further divides each mode The two modes are illustrated in Fig. 2, where the first into two groups based on the polarity of the dominant mode [temperature mode 1 (TM1)] has a center of EOF. As shown in Fig. 4, it is noteworthy that blocking action over and the second mode frequency anomalies are significantly different over the [temperature mode 2 (TM2)] has a center of action central (western and eastern) portion of the Ural–Siberian over China. For convenience, the sign of the TM2 is region when the EOF1 (EOF2) mode predominates. It is reversed such that the positive (negative) phase of the suggested that the dominance of the two modes represents two modes represents higher (lower) temperatures in the recurrence of blocking highs over at most three geo- the northern and southern portions of the domain, re- graphic locations in the study region. As deduced by spectively. In addition, it should be emphasized that the Figs. 5 and 6, however, their dominance should represent signs to the north and the south of the TM2 are opposite a large discrepancy in blocking activity over the Ural (Fig. 2b). Mountains and eastern , respectively. The impacts

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FIG. 3. The first and second eigenvectors (EOF1 and EOF2) of the Z500 in the Ural–Siberian region. Their explained variance is indicated at the top right. arising from the persistence of blocking over these loca- suggesting higher blocking activity over . tionsontheEAWMarediscussedasfollows. Correspondingly, the northerly wind is stronger near the Ural Mountains and it brings much more intense cold a. The Ural Mountains air masses toward over 458–758N, 608– The two polarities of the EOF1 mode show a large 1058E (Fig. 6d). The cold anomaly region shifts west- discrepancy in blocking frequency near the Ural Moun- ward with respect to the climatological Siberian high tains (Fig. 4a). The more (less) frequent occurrence of region and extends toward Asia in its zonal but not its blocking during the negative (positive) EOF1 coincides meridional direction. On average, the temperature over with an abnormally high (low) Z500 center near the Ural only part of the northern region of East Asia is well Mountains, as shown in Fig. 5a (Fig. 5b). The associated below normal, but this still contributes to the strong stronger (weaker) meridional flow along its downstream linear relationship between the PC2 and the TM1, which edge enhances (inhibits) cold advection toward Siberia. is significant at the 95% confidence level. On the other Correspondingly, more (less) intense cold air masses ac- hand, the temperature to the south of 308N is close to cumulate over western Siberia, giving rise to a more (less) normal, suggesting a low impact on the temperature intense Siberian high and a cooler (warmer) East Asia anomaly in the southern part of East Asia. The significant (Figs. 6a,b). This is evidenced by high correlations be- correlation between the PC2 and the TM2 is noteworthy. tween the PC1 and both the TM1 and the SHI (Table 2). When the positive phase of EOF2 predominates, the However, the PC1 forms a very weak relationship with blocking frequency is lower than normal over the Euro– the TM2. Even though the surface cold (warm) anomaly Atlantic sector and slightly higher than normal over extends equatorward from western Siberia toward South Siberia and the western North Pacific (black line of China during the negative (positive) phase of EOF1, the Fig. 4b). In Fig. 6c, an anomalous anticyclonic flow and surface air temperature over the southern part of East pronounced warming can be observed over 608–908E, Asia is not significantly lower (higher) than normal. The but it is not associated with stronger northerly cold statistical results suggest that blocking near the Ural advection downstream, suggesting that the meridional- Mountains, in general, has a direct impact on the north- type circulation is not significantly stronger there. ern part of East Asia, but whether the impact can be Therefore, this group cannot be considered a recurring exerted on the southern part may depend on other factors geographic location of blocking over Siberia. Rather, it determining the cold air pathway. In short, the impact of is more representative of lower blocking activity over the EOF1 mode on the EAWM is analogous to the TM1. b. Eastern Europe TABLE 1. List of years for the period 1950–2009 showing dominance in the EOF1 and EOF2 modes. The two phases of the EOF2 mode differ in blocking frequency primarily over the Euro–Atlantic sector EOF1 1 (13 yr) 1957, 1962, 1963, 1975, 1978 1988, 1989, 1991, 1992, 1996, 1997, 2001, 2002 across the European continent and the western side of EOF1 2 (12 yr) 1950, 1954, 1960, 1966, 1969, 1976, 1979 the Ural-Siberian region (Fig. 4b). During its negative 1987, 2000, 2005, 2008, 2009 phase, there are two outstanding anomalous peaks over EOF2 1 (11 yr) 1951, 1961, 1964, 1967 the climatological peak and eastern Europe. The ab- 1980, 1981, 1982, 1983, 1985, 1994, 2007 normally high region at 500 hPa, however, is signified by EOF2 2 (8 yr) 1959, 1968, 1971, 1972, 1973 1984, 1990, 1993 a single center over 308E only as shown in Fig. 5d,

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FIG. 4. Longitudinal distribution of blocking frequency anomalies (days) over the for the winters demonstrating dominance in the positive (black) and negative (gray) polarities of the (a) EOF1 and (b) EOF2 modes, where the error bars represent the s deviation. eastern Europe due to a low anomaly near 308E (Fig. 5c) driving force exerted on the EAWM by extratropical that brings anomalous southwesterly warm advection and tropical climate systems, which will be analyzed in toward Siberia (Fig. 6c). Over the majority of East Asia, section 6. the surface air temperature is close to normal except for substantial cooling in the vicinity of Japan, which may be 4. The impacts of the AO attributed to the deepening of the East Asian trough as inferred from Fig. 5c. In other words, cooling is not In section 3, it was shown that the EOF1 mode is evidenced in the southern part of East Asia. Hence, both strongly related to the SHI and the TM1, whereas the the predominant positive and negative phases of EOF2 EOF2 mode may represent a dipole temperature pat- are not related to the size of the surface temperature tern of the EAWM. In addition to distinctive blocking anomaly in the region captured by the TM2. Therefore, activity near the Ural Mountains (eastern Europe) un- where does the linear relationship come from? der the predominant EOF1 (EOF2) mode, it is sug- By simply regressing the two time series by the least gested that the dominance of the two modes leads to two squares fit, a linear relationship exceeding the 99% different blocking–EAWM relationships. Indeed, the confidence level can be obtained and is shown in Fig. 7. Z500 patterns associated with the two EOF modes re- Specifically, there is one year with an exceptionally semble a Rossby wave train signal coming from the negative value of the PC2 and a largely positive value of North Atlantic Ocean (Fig. 5). The patterns are similar the TM2. If this point is removed from the regression, to the ‘‘North Atlantic–Ural–East Asia’’ teleconnection which is represented by the gray dotted line, the linear pattern depicted in Li et al. (2008), which may be ex- correlation is still significant at the 99% confidence plained by the North Atlantic Oscillation (NAO)—a level, albeit with a 6% decline in the explained variance. major teleconnection pattern over the Northern Hemi- Thus, the results suggest that the predominant EOF2 sphere (Hurrell 1995). In this study, another well-known mode (Fig. 3b) is capable of explaining part of the var- teleconnection pattern (AO) is considered instead since iance of the TM2 (Fig. 2b), but it may be due to the it is more representative of the extratropical climate yet north–south thermal contrast rather than the tempera- retains the characters of the NAO (Thompson and ture departure in the southern region. That is, the pos- Wallace 1998; Deser 2000). itive (negative) phase of EOF2 may be characterized by During the positive (negative) phase of the AO, its a cooler south (north) but a warmer north (south), as signature over the North Atlantic Ocean is character- evidenced by the cold anomaly in the vicinity of Japan ized by a stronger (weaker) seesaw pressure pattern, (western Siberia). This may be due to the incoherent with a stronger (weaker) subtropical ridge giving rise to

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FIG. 5. The Z500 anomaly in NDJFM for the years demonstrating dominance in (a) positive EOF1 mode, (b) negative EOF1 mode, (c) positive EOF2 mode, and (d) negative EOF2 mode. The shaded regions are signifi- cantly different from the 60-yr climatological mean at the 95% confidence level. an anomalous high (low) near the Iberian Peninsula circulating the polar region favor (inhibit) the advection of (Thompson et al. 2000). A Rossby wave train may be polar air southward, resulting in a cooler (warmer) generated there that propagates eastward across Eura- (Hurrell 1995; Thompson and Wallace 1998) sia. As a consequence, one of the Z500 patterns in Fig. 5 and a stronger (weaker) Siberian high (Gong et al. may appear to be deduced from the statistically signifi- 2001). These impacts are evidenced by the strong cor- cant correlations between the AO and both the PC1 relations between the AO and both the SHI and the and the PC2 listed in Table 2. Their impacts on Ural– TM1 (Table 3), which form the basic linkage between Siberian blocking can be determined by comparing the the AO and the EAWM. blocking frequency anomalies between the two phases of the AO, as shown in Fig. 8. Whereas the largest dif- 5. The impacts of the ENSO ference occurs in the North Atlantic Ocean, where the AO shows the strongest signature, the difference also ex- Much extensive work has been done on the relation- ceeds the 90% confidence level between 508 and 62.58E, ship between the ENSO and the strength of EAWM with a peak near the Ural Mountains. This suggests that (e.g., Li 1990; Zhang et al. 1997; Chan and Li 2004; Zhou the wave train signal is more likely to resemble the EOF1 et al. 2007). In Table 3, it is noticeable that the ENSO mode with a center of action near 608E (Fig. 3a). shows a substantial linkage only with the TM2. It ap- On the other hand, the AO is a measure of the strength pears that the correlations between the ENSO and any of the polar vortex. A larger amount of wave activity flux factors with the center of action over the extratropics are propagates upward from the lower troposphere such that very weak. Indeed, the impacts of the ENSO on Ural– the polar vortex gets warmer and weaker during its neg- Siberian blocking and the EAWM are evidenced by the ative phase, and vice versa (Chen et al. 2005; Chen and tendencies of the PC1 and the SHI, as implied by Figs. 9 Kang 2006). Accordingly, the weaker (stronger) westerlies and 10. During warm ENSO events (where the anomaly

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21 FIG. 6. Surface air temperature anomaly (shading; 8C) and 500-hPa wind anomaly (vector; m s ) in the winters demonstrating dominance in (a) positive EOF1 mode, (b) negative EOF1 mode, (c) positive EOF2 mode, and (d) negative EOF2 mode. The air temperature enclosed by the dotted regions and the vector wind are significantly different from the 60-yr climatology of air temperature and either zonal wind or meridional wind component, re- spectively, at the 95% confidence level. Note that the is shaded black. of the ENSO index is greater than 1.08C), the PC1 The impact of cold ENSO years on the extratropics is tends to be positive and the SHI tends to be negative. not as significant as that of the warm years. There are no These results agree with previous findings that the very clear tendencies of the PC1 and the SHI, as seen in EAWM tends to be weak in warm ENSO years (e.g., Figs. 9 and 10. Moreover, the likelihood of blocking can Li 1990; Chen et al. 2000). Furthermore, an anomalous be deduced by Fig. 11, which illustrates blocking fre- anticyclonic flow in the lower troposphere was found quency anomalies in warm and cold ENSO years, in over the Philippine Sea, and its northward propaga- which the anomaly of the ENSO index is greater than tion would weaken the climatological cyclonic flow 1.08C and less than 21.08C, respectively. The difference over East Asia such that the EAWM would tend to be between the two groups exceeds the 90% confidence weaker (Wang et al. 2000). The surface westerlies near

Siberia become stronger such that the cold air lacks TABLE 2. Linear correlations between the two EOF modes and the tendency to intrude southward (Li 1990). Among the EAWM indices, where the bold (italic) values exceed the 99% the nine warm ENSO years, there is only one excep- (95%) confidence level. tion (2009/10) with a largely negative PC1 and a strong SHI TM1 TM2 Siberian high (Figs. 9 and 10) contributed by the ex- tremely negative AO in December 2009 (Wang and PC1 20.653 0.683 20.094 PC2 0.214 0.296 20.449 Chen 2010b).

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FIG. 7. Scatterplot between standardized values of the PC2 and TM2 (dimensionless). level between 658 and 152.58E. However, the frequen- stronger than cold events such that their impacts on both cies in the entire region are slightly above normal in cold blocking and the EAWM are more pronounced. years, which is in contrast to a remarkable decrease in warm years. Therefore, it is suggested that the extra- 6. The combined effect of the AO and ENSO tropical circulation over East Asia is more sensitive to the AO. It may be modulated by the ENSO if the ENSO The results in the previous two sections reveal the pos- signal is strong enough to induce an anomalous circu- sible impacts of the AO and ENSO on the occurrence of lation over the western North Pacific to propagate to- Ural–Siberian blocking and the strength of the EAWM. ward East Asia. Comparatively, warm ENSO events are Since the EAWM interacts with both extratropical and

FIG. 8. (top) Longitudinal distribution of blocking frequency anomalies in the Northern Hemisphere under the positive phase of the AO (solid gray) and the negative phase of the AO (solid black), where the error bar represents the s deviation (days); (bottom) confidence level (%) for the difference in blocking frequency between the two polarities of the AO in the Student’s t test.

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TABLE 3. Linear correlations between the AO/ENSO and the correlations with the PC2 and both the TM1 and the two EOF modes and EAWM indices, where the bold (italic) values TM2 (Table 6). However, the correlation between each exceed the 99% (95%) confidence level. of these factors and the AO is opposite in sign to that ENSO PC1 PC2 SHI TM1 TM2 with the ENSO (Table 6). This may indicate that the AO 20.110 0.438 0.288 20.336 0.598 20.183 AO and ENSO exert an incoherent forcing on the ENSO 20.125 20.116 20.115 0.008 0.328 extratropical–tropical interaction over the East Asian continent. Hence, this may explain why the two climate factors form a weak linkage with the SHI (Table 6). In- tropical regions, it is affected by the combination of the deed, such a weak relationship is similar to Wu and Wang’s two climate factors on the blocking–EAWM relationship (2002) suggestion that the Siberian high is excluded from and whether they are in phase or out of phase. the relationship between the AO and the EAWM. In The major Ural–Siberian blocking pattern is deter- short, the blocking–EAWM relationship is stronger mined by comparing the magnitude of the first two (weaker) under the in-phase (out of phase) condition. leading EOFs listed in Table 4. It is suggested that the The above results show that the ENSO may modulate EOF1 (EOF2) mode tends to stand out for the in-phase the impact of the AO on the extratropical circulation (out of phase) condition. Therefore, the in- and out-of- over Eurasia and its interaction with the tropical region, phase conditions are accompanied by different blocking– which agrees with the findings in section 5. To further EAWM relationships, as revealed in section 3. More illustrate how the combined effect of the AO and ENSO evidence concerning the combined effect of the AO and establish the blocking–EAWM relationship, the analysis ENSO can be provided by the correlation analyses listed is restricted to those years in which an anomaly of the in Tables 5 and 6. When the AO and ENSO are of ENSO index exceeds 1.08C in magnitude. The years be- the same polarity, both are closely related to the PC1, longing to each of the four combinations are listed in the SHI, and the TM1, but are weakly correlated with Table 7. Their composites are shown in Fig. 12 and their the PC2 and the TM2 (Table 5). These relationships are longitudinal distributions of blocking frequency anoma- comparable to those with only the AO given by Table 3. lies are illustrated in Fig. 13. Since the correlation coefficient between each of these The prominent feature of the positive AO is a stron- factors and the AO is of the same sign as that with the ger Icelandic low and Azores high over the North At- ENSO, it is suggested that the ENSO collaborates with lantic Ocean, characterizing predominant zonal-type the AO in constructing the blocking–EAWM rela- circulation (Figs. 12a,b). As mentioned in section 4, tionship. When the AO and ENSO are of opposite po- such a flow pattern is not favorable for blocking forma- larity, on the other hand, both of them show significant tion over Ural–Siberia and is responsible for pronounced

FIG. 9. Scatterplot between the ENSO index and standardized values of the PC1, where the ENSO indices .1.08C(,21.08C) are enclosed by the black (gray) dashed line.

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FIG. 10. As in Fig. 9, but for the scatterplot between the ENSO index and SHI. warming over Eurasia. If a warm ENSO event also takes Conversely, the prevailing northeasterly wind near the place, there is an anomalous anticyclone extending pole- surface over the South China Sea is strengthened when ward from the equatorial Pacific in the upper troposphere associated with a cold ENSO event (Chen et al. 2000; Wu (Fig. 12a). This is accompanied by an anomalous southerly and Leung 2009; Wang et al. 2009a; Zhou et al. 2010). The wind near the surface, resulting in a weaker monsoonal stronger monsoonal flow in the southern region does not flow (Wu and Leung 2009). Together with the low advance the warming equatorward such that the warming anomaly over the Urals, the spatial pattern of the Z500 is localized to the extratropical region under the positive anomaly over the study region is a northwest–southeast- AO–cold ENSO condition (Fig. 12b). In comparison to the oriented dipole (Fig. 12a), which resembles the positive positive AO–warm ENSO condition, the anomalous an- phase of EOF1 (Fig. 3a) with an overall low blocking ticyclone over East Asia does not tilt southeastward such frequency over Ural–Siberia (solid gray line in Fig. 13). that the Z500 anomaly pattern is like the positive phase of

FIG. 11. (top) Longitudinal distribution of blocking frequency anomalies in the Northern Hemisphere during warm ENSO winters (solid gray) and cold ENSO winters (solid black), where the error bars indicate the s deviation; (bottom) confidence level (%) for the difference in blocking frequency between the two polarities of the ENSO in the Student’s t test.

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TABLE 4. Mean and standard error of the standardized values of TABLE 6. As in Table 5, but for the cases when the AO and ENSO the first two leading principal components under different combi- are out of phase. The bold italic values exceed the 90% confidence nations of the AO and ENSO. level. Note that the EOF2 is the dominant blocking pattern.

Combined effect PC1 PC2 PC1 PC2 SHI TM1 TM2 In phase AO 1 ENSO1 0.34 60.16 0.11 60.25 AO 0.380 0.449 20.287 0.657 20.411 AO 2 ENSO220.45 60.23 0.08 60.23 ENSO 20.065 20.338 0.192 20.334 0.377 Out of phase AO 1 ENSO2 0.24 60.28 0.28 60.27 PC1 20.642 0.743 20.049 AO 2 ENSO120.02 60.27 20.41 60.23 PC2 0.259 0.301 20.437

EOF2 (Fig. 3b). Blocking frequency is relatively larger in the low does not extend southeastward, the anomalous the study region (dashed gray line in Fig. 13), but it is Z500 pattern over Ural–Siberia as shown in Fig. 12c is still lower than in the cold ENSO condition, as shown in analogous to the negative phase of EOF2 (Fig. 3b). Fig. 11. Therefore, the blocking–EAWM relationship is more In the negative phase of the AO, cold air advances comparable to the EOF1 (EOF2) mode when the AO southward from the polar region (Figs. 12c,d). Over East and ENSO are in phase (out of phase), but what are Asia, the tendency of cold air to intrude equatorward the possible physical mechanisms responsible for such depends on the meridional-type circulation accompa- a relationship? nied by the external effect of a strong ENSO event. This is favored by a cold ENSO event, which enhances 7. Discussion and conclusions the upper-tropospheric mass convergence near Siberia and the near-surface northerly wind over East Asia a. Physical mechanisms of Ural–Siberian blocking (Chen et al. 2000). In Fig. 12d, the spatial pattern is The interaction between a cyclone and the planetary composed of an anticyclonic anomaly over the Urals and wave is one of the main physical mechanisms responsible a cyclonic anomaly over Siberia, which takes shape in for the establishment and maintenance of blocking the negative phase of EOF1 (Fig. 3a). This negative (Frederiksen 1982; Hansen and Chen 1982; Dole and EOF1-like pattern is signified by the more frequent Gordon 1983; Colucci 1985). In other words, blocking is occurrence of Ural–Siberian blocking (solid black line in a manifestation of quasi equilibrium between waves of Fig. 13). During a warm ENSO event, on the other hand, different spatial scales, as proposed by Charney and the northward extension of an anomalous anticyclone DeVore (1979). Specifically, intense cyclogenesis is an from the equatorial Pacific inhibits the southward in- essential element for the formation of blocking highs trusion of cold air, resulting in a cooler north and a (e.g., Rex 1950; Egger et al. 1986; Dole 1986), which warmer south temperature pattern (Fig. 12c). The often follow the intensification of a surface cyclone blocking signature is sharpest over the Atlantic sector (Colucci 1985; Tsou and Smith 1990; Alberta et al. 1991). (dashed black line in Fig. 13), with an anticyclonic The cyclone under intensification is a developing baro- anomaly over Greenland and a cyclonic anomaly to its clinic wave with a thermal trough lagging behind a south (Fig. 12c). In contrast, the blocking frequency is height trough (Holton 2004, his Fig 6.6). A thermal ridge quite low over Eurasia; it is as low as with the positive ahead of the trough advects the subtropical warm air AO–warm ENSO over Siberia. The infrequency of masses for the establishment of a warm ridge on its blocking may arise from the stronger midlatitude west- downstream side (Illari 1984; Mullen 1987). If the warm erly of a more active Ferrel cell driven by a more active ridge is located upstream of the Siberian high, it tightens Hadley cell in warm ENSO years (Li 1990). Because the zonal temperature gradient. The large thermal contrast may create a resonance with the topographic TABLE 5. Linear correlations between the AO/ENSO and the forcing of the Ural Mountains such that a blocking high two leading EOF modes and EAWM indices when the AO and ENSO are in phase, where the bold and italic values exceed the 99% and 95% confidence levels, respectively. Note that the EOF1 TABLE 7. List of strong ENSO years and their classification based is the dominant blocking pattern. on the polarity of the AO.

PC1 PC2 SHI TM1 TM2 Group Years AO 0.524 0.106 20.400 0.536 0.056 Positive AO, warm ENSO 1972, 1982, 1991 ENSO 0.394 0.190 20.421 0.527 0.261 Positive AO, cold ENSO 1973, 1975, 1988, 1999, 2007 PC1 20.665 0.644 20.180 Negative AO, warm ENSO 1957, 1965, 1986, 1997, 2002, 2009 PC2 0.199 0.234 20.444 Negative AO, cold ENSO 1955, 1970, 1984, 1998

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FIG. 12. Composite maps of Z500 anomalies (contour interval: 10 m) and standardized anomalies of surface air temperature (shading; 8C) under different combinations of the AO and ENSO. may form according to the postulation of Shabbar et al. Siberian winter. In the positive phase of the AO, however, (2001). Therefore, the blocking high over Ural–Siberia the cyclones tend to move northeastward over the polar may involve interaction between the planetary wave region, passing through the Kara Sea (Thompson and above the Ural Mountains and a developing cyclone Wallace 1998). The associated stronger westerly wind ad- upstream at midlatitudes. vects more warm marine air toward Siberia (Rogers and Thompson 1995; Thompson and Wallace 1998). Accord- b. Blocking–EAWM relationship under the ingly, there is less blocking in Ural–Siberia and the winter combined effect of the AO and ENSO gets milder in the extratropical region of East Asia. On the Rogers and Thompson (1995) found that more cyclones other hand, the westerly wind also becomes stronger in are formed over Europe, usually over the Mediterranean themidlatitudesduringwarmENSOyearsbecauseof Sea, and move eastward across the midlatitudes in a cold a stronger Ferrel cell (Li 1990). This anomalous circulation

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FIG. 13. Longitudinal distribution of blocking frequency anomalies (days) in different combinations of the AO and ENSO, with a box enclosing the study region. inhibits the interaction of the cyclone with the planetary blocking (Park et al. 2011). In addition to a decreasing wave. Thus, there is less blocking in warm ENSO years, number of cold surges in recent decades (Hong et al. 2008; independent of the polarity of the AO. Overall, the Zhai et al. 2008; Wang et al. 2009b), the EAWM has also blocking–EAWM relationship is stronger (weaker) when become weaker (Wang et al. 2009b; Hung and Kao 2010), the AO and ENSO are in phase (out of phase), where the which may be a consequence of the smaller impact exerted external forcing by the AO and ENSO are coherent by the cold surges because of less blocking under the (incoherent) over Ural–Siberia. The impacts of the AO predominant positive phase of the AO (Park et al. 2011). and ENSO on the blocking–EAWM relationship are c. Conclusions summarized in Fig. 14. A decadal climate shift took place around the mid- An overview of the relationship between Ural– 1970s that involved the AO and PDO changing from Siberian blocking and the EAWM has been introduced. their predominant negative phase to a positive phase. The spatial feature of Ural–Siberian blocking is acquired Since there are more warm (cold) ENSO events during by performing the EOF analysis on the winter-mean Z500. the positive (negative) phase of PDO (Mantua et al. 1997), The EOF1 (EOF2) mode represents the recurrence it is likely there will be more years of positive (negative) of blocking highs over the Ural Mountains (eastern AO together with a warm (cold) ENSO after (before) the Europe), which shows a stronger (weaker) linkage with climate shift. Therefore, the blocking–EAWM relation- the SHI and TM1 and a weaker (stronger) linkage with ship is not likely to undergo interdecadal variations. the TM2. Concerning the combined effect of the AO However, the blocking–EAWM relationship may help to and ENSO, the EOF1 (EOF2) mode dominates and the explain the decadal variations in the EAWM. If there are blocking–EAWM relationship is stronger (weaker) more warm ENSO events and the positive phase of the when the two factors are in phase (out of phase), which AO predominates, there would be a greater likelihood for is summarized in Fig. 14. Since the AO may undergo weak Ural–Siberian blocking and a weak EAWM. As sharp intraseasonal transitions because of tropospheric– suggested by L. Wang et al. (2010), more wave activity flux stratospheric interaction (Baldwin and Dunkerton 1999), is emitted from the Ural Mountains and propagates east- further work is needed to verify the blocking–EAWM ward toward East Asia rather than propagating upward relationship at subseasonal time scales. In fact, the toward the stratosphere. This probably reflects a stronger downward-propagating signal from the stratosphere is polar vortex with stronger westerlies near 558N(Chen probably an important precursor for cold surges in East et al. 2003, 2005) that contribute to a higher frequency Asia (Jeong et al. 2006; Wang and Chen 2010b). Apart of warm winters in East Asia after the climate shift. Fur- from the AO and ENSO, other factors such as the snow thermore, the impact of cold surges is larger during the cover over Eurasia and the sea surface temperature over negative phase of the AO because of the presence of the North Atlantic Ocean and are also

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