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 The phase asymmetry in a downstream pattern of the North Atlantic Oscillation (NAO) is examined during boreal winter time. We compared the downstream anomaly patterns associated with the positive and negative persistent NAO events over several days. Both phases of the persistent NAO events accompany quasi-stationary wave trains in downstream upper levels. However, a distinct low level anomaly appears only for the negative persistent NAO events. The low level anomaly migrates eastward as a surface anticyclone that is confined to the low troposphere. A few days later, the surface anticyclone reaches central Siberia and induces cold advection over East Asia. Therefore, the phase-dependent downstream development of the NAO expands the impact of the NAO to the East Asian region. We also discuss the possible interaction between the upper and lower level anomalies, a necessary condition for the amplification of the Siberian high.
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 As the major source of climate variability in the Northern Hemisphere, the North Atlantic Oscillation (NAO)/Northern Hemisphere annular mode (NAM) has attracted much attention from researchers [e.g., Thompson and Wallace, 1998; Ambaum et al., 2001, Wallace and Thompson, 2002]. In contrast to the NAM which is considered to be a global phenomenon, the NAO is usually understood to be a local phenomenon that primarily affects the regional climate adjacent to the North Atlantic region [Lamb and Peppler, 1987; van Loon and Rogers, 1978; Qian and Saunders, 2003]. However, some studies have indicated that there are remote climate impacts and teleconnection patterns related to the NAO [Hong et al., 2008; Branstator, 2002; Watanabe, 2004]. Recently, Feldstein and Franzke  argued that the NAO and NAM persistent events are indistinguishable. In addition, their study showed that the NAO/NAM events are neither confined to the North Atlantic nor are annular. In this study, we also consider whether or not the NAO is confined to the North Atlantic region by analyzing the downstream circulation pattern and its impact on the East Asian winter climate. While previous studies have dealt with the relation on interannual or decadal time scales, this study focuses on the downstream impact of the NAO in a submonthly time scale. In section 2 of this paper, we present our data and the definition of a persistent NAO event. The characteristics of the downstream pattern of the NAO are described in section 3. This study also suggests close relation between persistent NAO events and the East Asian winter Monsoon (EAWM). The summary and discussions are given in section 5.
2. Data and Definitions
 In order to analyze the circulation pattern of the NAO, daily mean reanalysis data sets from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) [Kalnay et al., 1996] were used. The analysis period is from the 1948/49 winter to the 2004/05 winter, which is defined as the period from November through March (NDJFM). Because the reanalysis data could include problems caused by the model assimilation process, we analyzed several variables including sea level pressure (SLP) and air temperature, which are derived mostly from the observations [Kalnay et al., 1996]. In order to examine the vertical structure of the downstream anomaly of the NAO, geopotential height fields were analyzed additionally. Surface air temperature (SAT) observations over East Asia were also examined. The station-observed SAT data sets were obtained from the Korea Meteorological Administration and China Meteorological Administration for the periods 1949–2004 and 1951–2001, respectively. The SAT observations at 12 and 75 stations were averaged into two time series to represent the temperature variations over Korea and China, respectively.
 This study defines the NAO pattern as a leading empirical orthogonal function (EOF1) for the monthly mean sea level pressure (SLP) fields over the North Atlantic region (20N–80N, 90W–40E) [Jia et al., 2007]. Seasonal cycle and linear trend were removed at each grid point prior to the EOF analysis. The EOF1 explained 33.6% of the total variance. In order to obtain the daily NAO index, the EOF1 was projected onto the map time series of the daily SLP anomaly from which the seasonal cycle had been removed. The seasonal cycle was defined as the 31 day running mean of the calendar day climatology.
 We compared the positive and negative persistent NAO events and their downstream circulation patterns. Using Feldstein's  definition of a persistent event, the onset of the persistent NAO event was defined as the point at which the NAO index exceeded 1σ(σ: standard deviation for the entire analysis period) and persists for 5 days or more exceeding the threshold. If a persistent event took place within 10 days from the end of the previous event with the same phase, the latter event was discarded in order to guarantee a single development of the persistent event. In this study, 43 and 57 positive/negative persistent events were chosen, respectively.
3.1. Phase Asymmetric Downstream Development of the NAO
 Many studies have examined the dynamical characteristics of persistent NAO events [Feldstein, 2003; Feldstein and Franzke, 2006; Jia et al., 2007]. In previous studies, the positive and negative phases of the persistent NAO events demonstrate both symmetric and asymmetric features in several aspects. Feldstein  showed the differences in the growth rate and intensity between the positive and negative persistent NAO events. Jia et al.  also revealed an asymmetric spatial structure during the developing stages of the positive and negative persistent NAO events. In Jia et al.'s study, the subpolar center of the negative NAO phase migrates from northern Russia, while the positive NAO develops from an anomaly which originates in the western North Atlantic. The differences in the origin of the northern center of the NAO seem to affect not only the Atlantic region, but also the remote downstream regions, as noted below.
 In this study, we examined the spatial evolution of the SLP for positive and negative persistent NAO events and compared the differences between them. In order to examine the spatial evolution since the onset of persistent events, lag composite maps were constructed for the SLP field during the positive and negative persistent NAO events (Figure 1). Figures 1a–1d show the daily SLP lag composites for the positive persistent NAO events from the onset to lag +7 day, and Figures 1e–1h depict the SLP pattern associated with the negative persistent NAO events. Figures 1a–1d present the typical positive NAO patterns which have a subpolar cyclonic anomaly centered over Iceland. The cyclonic anomaly exhibits circular structure, and the overall patterns seem to be quasi-stationary after the onset of positive persistent NAO events. On the contrary, there is a noticeable difference between the positive and negative NAO events. In the negative events composite, the downstream SLP anomalies develop across the Eurasian continent (Figures 1e–1h), while the positive NAO does not exhibit any significant downstream development. During the negative persistent NAO events, the subpolar anticyclonic anomaly extends over northern Europe and Iceland (Figure 1e). The extended structure of the subpolar anomaly is different from that of the positive composite. This feature is in agreement with the results of Jia et al. , which indicate the northern center of the NAO originates in northern Europe during the negative phase. The center of the anticyclonic pressure anomaly gradually intensifies over Iceland, and a separate branch of the anticyclonic anomaly forms over northern Russia during lag +3 day ∼ lag +5 day (Figures 1f and 1g). That branch of the anticyclonic surface anomaly moves eastward and reaches central Siberia and Mongolia at lag +7 day (Figure 1h). The main difference in the downstream development according to the different NAO phases seems to originate due to the existence of the anomaly over northern Russia. The importance of the SLP anomaly over northern Russia is discussed further in section 3.2 of this paper.
Figure 2 shows the vertical structures of the downstream anomaly accompanied with the positive and negative persistent NAO events. The geopotential height anomalies were averaged across the Eurasian continent between 40°N ∼ 60°N, because the downstream anomaly moves along the midlatitude. At lag +3 day, significant anomalies appear between 0°E and 30°E in the composites of the both phases, at the same time as the southern anomalies of the NAO form over western Europe, as shown in Figures 1b and 1f. The barotropic structures of those anomalies in Figures 2a and 2b are in agreement with the characteristics of the NAO. In the east of the southern NAO anomalies, there are opposite-signed significant anomalies. Both phase composites seem to be similar but have distinct differences; The anticyclonic pressure anomaly induced by the negative persistent NAO events has a center near surface, while the cyclonic anomaly of the positive events is centered at the upper levels. The upper level anomalies for both phases of the persistent NAO events seem like wave trains that are quasi-stationary. On the contrary, the surface anticyclonic anomaly accompanied with the negative persistent NAO events migrates eastward. Note that the anticyclonic height anomaly of the negative NAO events is the strongest near the surface which implies that a lower-level process is important for this migrating feature. The center locations of the surface anticyclonic anomaly are in agreement with those of the anticyclonic anomaly in Figures 1f–1h. The surface anticyclonic anomaly expands toward central Siberia at lag +5 day. As the center of the anticyclonic anomaly moves eastward, the upper level ridge seems to intensify (Figure 2d). At lag +7 day of the negative NAO events, the upper level trough deepens after the upstream ridge as the surface anticyclonic center of the negative NAO extends further over 100°E near East Asia (Figure 2f). The overall structures of the downstream anomalies accompanied with the negative NAO events can be categorized into two types: an upper level quasi-stationary wave train and a lower-level anticyclone migrating eastward. This structure of the negative persistent NAO is distinctly different from that of the positive NAO. In addition, another noteworthy feature is found in the negative NAO composite. The surface anticyclone is generally confined to the lower level [Bluestein, 1993]; however, as shown in Figure 2b, the upper level ridge and surface anticyclone are present within the in-phase structure. The coexistence of the upper level ridge and the surface anticyclone is an important precondition for the amplification of the Siberian high [Takaya and Nakamura, 2005].
 In order to examine the detailed structure of the eastward migrating surface anticyclone accompanied with the negative NAO events, tendencies of the SLP and the temperature at 850 hPa are shown in Figure 3. The color-shadings and contours indicate the temperature and SLP tendencies, respectively, after the onset of the negative persistent NAO events. The anticyclonic anomaly intensifies over Kazakhstan where the cold tendency (blue shading) appears on the day after lag +3 day, as shown in Figure 3a. As is evident in Figure 1f, this cold tendency is induced by the northerly advection due to the subpolar anticyclone over northern Europe. Two days later (between lag +5 day and lag +6 day), the cold tendency appears over Lake Baikal, and the anticyclonic pressure anomaly also intensifies over the lake (Figure 3b). The intensities of the cold and high-pressure anomalies become even stronger than the strengths shown in Figure 3a. The cold tendency reaches the East Asian region at lag +8 day (Figure 3c). It is clear that the tendencies of the pressure and temperature spatially coincide. The eastward migrating features of the anomalies seem to be related to the anticyclonic circulation which induces northerly cold advection in the east and southerly warm advection in the west. Cold advection results in a high-pressure anomaly over the eastern side of the surface anticyclone due to the thermal balance. This process could cause the eastward self-migrating features of the pressure and temperature anomalies. The eastward migrating feature of the surface anticyclone is described in detail by Bluestein . He showed that the effect of temperature advection usually causes surface anticyclones to move toward the southeast in the Northern Hemisphere, although other factors could also affect this movement. Takaya and Nakamura  also discussed a thermal Rossby wave that is trapped near the surface. According to Takaya and Nakamura , anomalous cold and warm advections to the east and west of the cold anomaly center, respectively, cause the existing cold anomalies to move eastward. This eastward self-migrating surface thermal Rossby wave or the surface anticyclone can extend the impact of the NAO to the East Asian region as is shown in Figure 3c. The impact of the negative persistent NAO events is described in more detail in section 3.2.
3.2. Impacts on the EAWM and Possible Link With the Siberian High
 In the previous section, we described the phase asymmetric downstream features of persistent NAO events. The asymmetric spatial structure of the NAO also causes distinct differences in the downstream impacts. The SAT variation over East Asia supports an asymmetric relationship between the downstream development of the persistent NAO events and the EAWM. Figures 4a and 4b show the daily lag composites of the SAT representing temperature fluctuations over East Asia following the positive and negative persistent NAO events, respectively. The black and green dashed lines represent the temperature anomalies over China and Korea, respectively. The red dashed line depicts the temperature anomaly over northeastern China, where the strongest temperature anomaly appears in the lag composite map for the 850 hPa temperature (data not shown). Within the confines of northeastern China, 15 weather stations are included (area marked with a red box in Figure 3c). While the positive NAO events did not show significant temperature fluctuations, Figure 4b depicts the distinct cold anomalies over both Korea and China after the +8 lag day of negative persistent events. This asymmetric influence of the NAO is caused by the asymmetric surface pattern of the downstream anomalies.
 The thin solid lines representing the SAT composites were calculated in the same manner as the dashed lines, except the seasonal means were removed. The seasonal mean was defined as the NDJFM average. The results after removing the seasonal means also show significant temperature variation, although the intensities are somewhat weakened. These results indicate that the lagged relationship between the NAO and temperature fluctuation over East Asia is due to the intraseasonal variation of the NAO, although the interannual relationship might make a minor contribution. The lagged relationship is still useful in weather prediction even if the interannual relationship is included, because the seasonal mean does not need to be taken into consideration for practical medium-range forecasts. For example, Sung et al.  suggested that the North Atlantic variability could be used as a predictor of the medium-range forecast for the East Asian winter weather variation due to the significant impact of the Eurasian pattern (EU).
 There are two possible processes to explain the significant SAT variation related to the NAO. One is a direct process caused by the cold advection accompanied with the surface anticyclone after the onset of the negative persistent NAO events. The other is an indirect process due to an interaction between the surface anticyclone and the Siberian high. In section 3.1, we mentioned the in-phase structure of the upper level ridge and surface anticyclone. The possible interaction noted in the indirect process is based on this vertical structure. Takaya and Nakamura  explained a specific mechanism of intraseasonal amplification for the Siberian high. According to their study, the upper level wave train that originates in the Atlantic interacts with the surface thermal Rossby wave as follows. The upper level ridge reinforces the surface anticyclone by counteracting the eastward migrating surface thermal tendency. Therefore, the surface cold anomaly is intensified, and in turn, reinforces the upper level blocking ridge. As a result, a strong amplification of the Siberian high appears. The upper and lower level anomaly patterns shown in the present study are very similar to the patterns presented by Takaya and Nakamura .
 However, Takaya and Nakamura  considered the upper and lower level anomalies to be independent components. More specifically, they interpreted that the upper level Rossby wave is synchronized with the lower level anomaly by chance. In contrast to the results of their study, we propose that both the upper and lower level anomalies could originate from the NAO variation. If the Siberian high is amplified through the interaction between the upper and lower level anomalies, then it is possible that the downstream development pattern of the NAO exerts a dominant influence on the EAWM. In fact, the lagged relationship between the negative persistent NAO events and an intensification of the Siberian high was significant at 95% confidence level. However, we did not find any significant relationship between the positive NAO events and the Siberian high.
 To confirm the possible relation between the NAO downstream development and the Siberian high, composites were constructed based on one standard deviation of the Siberian high index (as defined by Gong et al. ). Figure 5 shows the SLP composite pattern prior to an intensification of the Siberian high. The composite maps for lag −5 day ∼ lag −3 day were averaged to obtain the initial pattern related to the intensification of the Siberian high (thin solid lines). The thick solid lines represent the SLP climatology during winter, and indicate the climatological distribution of the Siberian high. For several days before the intensification of the Siberian high, a significant anticyclonic pressure anomaly is found over northern Europe. As shown in Figure 1, the negative persistent NAO events also exhibit distinct anomalies over northern Europe, which differed from the positive NAO. It appears that the pressure anomalies over northern Europe can play a key role in generating asymmetric downstream developments between the NAO phases. In addition, the anticyclonic anomaly over the upstream region of the Siberian high extends eastward and induces abnormal development of the Siberian high. This pattern confirms the possible link between the NAO and the Siberian high, as well as the EAWM.
4. Summary and Discussions
 In this study, we examined the downstream circulation patterns of both positive and negative persistent NAO events and their impacts on the EAWM on a submonthly time scale. While previous studies that investigate the relationships between the NAO/NAM and the EAWM had focused primarily on interannual or longer time scale relationships [e.g., Wu and Wang, 2002; Jeong and Ho, 2005, Hong et al., 2008], we studied the relationship on a submonthly time scale. After positive and negative persistent NAO events occur, quasi-stationary wave trains commonly appear in the upper troposphere. However, the negative NAO events show significant surface anomalies over the downstream regions, while the positive NAO events do not show any counterpart. This asymmetry in the downstream pattern between the positive and negative NAO events brings a different downstream impact on the EAWM. The surface anticyclone, associated with the negative NAO events, migrates eastward and reaches over central Siberia. Several days later, significant cold advection is induced over East Asia. While the negative persistent NAO events accompany distinct downstream impact due to the surface anticyclone, positive NAO events do not exert any significant impact on East Asia. The results of this study are expected to contribute to the medium-range weather forecasts.
 Besides the direct impact of the eastward migrating surface anticyclone on the EAWM, the upper level quasi-stationary wave might exert indirect impact on the EAWM through a possible interaction with the surface anticyclone. According Takaya and Nakamura , the coexistence of the upper level stationary wave and the surface anticyclone is a critical precondition for the strong amplification of the Siberian high. Although the previous study regards the upper and lower level anomalies as independent components, the present study suggests that both of the anomalies can be accompanied by the NAO variation. The possible interaction between the upper and lower level anomalies implies important impact of the NAO on the EAWM.
Feldstein  presented a vorticity budget related to persistent NAO events and found that low-frequency forcing is vital to the development of the NAO. In this study, we demonstrated that the anticyclonic pressure anomaly over northern Europe precedes the development of the negative persistent NAO events. This quasi-stationary pressure anomaly over northern Europe may function as a low-frequency forcing component which contributes to the development of the negative NAO. The anticyclonic anomaly also appears to serve as a link between the NAO and the Siberian high. This study evokes further questions to understand the specific processes which result in the upstream and downstream expansions of the anticyclonic anomaly over northern Europe.
 The authors appreciate two anonymous reviewers for their specific and helpful comments. Their insightful advices were very useful for improving the quality of our work. This work was supported by the Korea Meteorological Administration Research and Development Program under grant CATER 2006–2201. This research was partially supported by the Brain Korea 21(BK21) program for the fellowship G.-H. Lim.