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 It has been newly found that the occurrence of cold surges in east Asia is significantly influenced by the Arctic Oscillation (AO). In the present study, the events of cold surge are objectively determined based on several synoptic criteria and the phase-dependency of its occurrences in association with the AO has been revealed. During the negative AO phase, the frequency of cold surge occurrence is relatively increased than that of neutral and positive AO phases. The variation of upper level trough and jet stream over east Asia and the Siberian High in association of AO are suggested to explain the change of cold surge occurrences.
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 The cold surge is the one of the most important extreme weather events which tremendously influences a number of socio-economical human activities in east Asia. As the most energetic sub-system of the east Asian winter monsoon, there have been great effort to understand its dynamical origin, and the various factors which affect the cold surge: i.e., the variation of the Siberian High [Zhang et al., 1997], interannual influences of El Nino/Southern Oscillation (ENSO) [e.g., Zhang et al., 1997; Chen et al., 2004], and intraseasonal modulation of the Madden and Julian Oscillation (MJO) [Madden and Julian, 1972; Jeong et al., 2005] so far. Several studies have explored the relationship between AO and the east Asian winter climate such as direct influence on the east Asian winter monsoon system [Wu and Wang, 2002] and indirect influence through changing the Siberian High activity [Gong et al., 2001]. But there is no study that has linked AO to the east Asian cold surges yet. In this context, we have focused on the relationship between the occurrence of cold surges in east Asia and AO in the present study.
2. Data and Method
 In this study, the daily-mean surface air temperature (SAT) of 172 Chinese and 5 Korean stations are derived from the China Meteorological Administration and the Korea Meteorological Administration, respectively. Also, the zonal and meridional wind, geopotential height (GPH), and mean sea level pressure (MSLP) obtained from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis [Kalnay et al., 1996] are utilized. The analysis period is confined to 44 winters from 1957/58 to 2000/01 in consideration of data availabilities especially for SAT. The wintertime is defined as November through March when the east Asian winter monsoon dominates [Zhang et al., 1997] and the amplitude of AO variability reaches its maximum. The monthly mean AO index, defined as the time-series of the leading principle mode for monthly SLP poleward of 20°N for wintertime (November to April) [Thompson and Wallace, 1998], is obtained via website at http://www.jisao.washington.edu/ao/#monthly.
 The occurrences of cold surges in east Asia are objectively determined based on several synoptic criteria. Generally, the typical characteristics of cold surges in east Asia are represented as a steep drop of SAT and an immigration of surface anticyclone from southern Siberia. Many earlier studies [e.g., Lau and Chang, 1987; Zhang et al., 1997; Chen et al., 2002] introduced a variety of criteria for clarifying the above features. Here, we focus our attention on the cold surges that affect the greatest part of the east Asian continent, thus adopted the criteria described by Zhang et al.  with some additional considerations as follows.
 Firstly, a surface anticyclone over the region of 95°–110°E, 40°–50°N, located south of Siberia, should be identified prior to the event of a cold surge over east Asia. The center of this surface anticyclone is found following the methodology of Zhang and Wang , i.e., a grid point showing maximum GPH relative to eight surrounding grid cells at the 1000 hPa level. In addition, the magnitude of the MSLP of this anticyclone center must be greater than 1035 hPa; the magnitude of the relative vorticity in this center exceeds a critical value of 1.0 × 10−5 s−1, remaining for at least 1 day. Secondly, a decreasing SAT value exceeds 1.5σ (σ, standard deviation of SAT anomaly for 44 winters) for two days in mid China (total 6 stations, 110°–115°E, 32.5°–37.5°N) or south China (total 7 stations, 115°–120°E, 22.5°–27.5°N).
 An event of strong cold surges is additionally defined among the above cold surge days using stricter criteria in that the SAT criterion is set to 2σ with an anomalous northerly wind speed exceeding 1.5σ in the middle of China. As a result, a total of 369 cold surge events (8.39 per year) and 54 strong cold surge events (1.23 per year) are identified in this study. These numbers are slightly smaller than those of previous studies, such as 13 events per year found by Zhang et al.  and 17.3 events per year noted by Chen et al. . This can be explained by the fact that we have focused on cold surges, which are sufficiently strong to dominate most parts of China. Thus, our criteria are stricter than those of aforementioned studies.
3. Number of Cold Surge Versus AO Phase
 As described in many studies [e.g., Thompson and Wallace, 1998; Rigor et al., 2000; Thompson et al., 2000], SAT in mid and high latitudes are largely influenced by AO-related atmospheric circulation anomalies. The SAT is an important criterion for defining a cold surge, thus we explore the variations of monthly-mean SAT anomaly in association with the AO firstly. Figure 1 depicts the distribution of monthly SAT anomaly composite for the positive and negative AO months. On the whole, a statistically noteworthy AO signal is found in high latitude north 35°N; strong cooling (warming) is observed over southern Korea and northeastern China during the negative (positive) AO phase. However, most of inland China, where the cold surge takes major influences on, shows weak indication related with AO.
 To explore the variation of the cold surge influenced by AO, we classify the number of occurrences in terms of the AO phase (Table 1). As seen in the table, among a total of 369 cold surge events during whole series of 44 winters, 251 cases have occurred in the neutral AO phase, 45 and 73 cases in the positive and negative phase, respectively. Because we categorize the AO phase using a value ±1σ of the AO index, most winter months belong to the neutral AO phase (150 months among 220 months) and relatively small fractions of the period belong to the positive (33 months) and negative AO phase (37 months). For an optimal comparison of three phases, the relative frequency of cold surge occurrence (the total number of cold surge occurrences in each AO category is divided by the total months of each AO) is presented in Table 1. Overall, the cold surges occur frequently in the negative (1.97 events per month) and neutral (1.67) AO phases compared to the positive (1.36) AO phase.
Table 1. Cold Surge Statistics Based on Monthly AO Index
AO Phase (Total Month)
 The most considerable phase-dependency is found in the occurrence of strong cold surges. The occurrences of strong cold surges depend significantly on AO phase so that most cases took place in the neutral (41 cases) and negative phase (11 cases), while only two cases occurred in the positive AO phase. On average, the number of events of strong cold surge is 0.30 and 0.27 events per month in the negative and neutral AO, 0.06 in the negative AO phase.
 These AO phase dependencies of cold surges can be more clearly seen in Figure 2, which depicts the histograms of the total number and frequency of cold surge occurrence based on monthly AO index. The total occurrence is slightly biased to negative AO index (Figure 2a) as expected from above results. In the distribution of occurrence frequency (Figure 2b), the negative relationship between AO and cold surge occurrence is distinct, especially for strong cold surge.
4. Dynamic Interpretation and Discussion
 It has widely been recognized that cold surges in Europe, North America, and sub-polar regions are tightly correlated with the phase of the AO or North Atlantic Oscillation, which has recently been perceived as a regional manipulation of AO [Wallace, 2000]. Generally, the polar jet, which encompasses a sub-polar region from the surface to the lower stratosphere, becomes weaker (stronger) and cold Arctic air reaches more (less) southerly latitude regions in the negative (positive) AO phase. However, the location of east Asia is too far from the sub-polar region to be directly affected by Arctic cold air. The occurrence of cold surges in east Asia may seem to be affected by the variation of AO-related mean circulation.
 The typical scenario of cold surge occurrence is summarized as follows. When the Siberian High reaches certain intensity, eastward moving upper level short-wave tough aloft over Lake Baikal deepens as it propagates toward quasi-stationary east Asian coastal trough. During the above process, surface anticyclone moves southward together with very cold air accumulated over eastern Siberia, which is called as a cold surge occurrence. Regarding above description, composites analysis are presented in Figure 3 to examine the variation of AO-related mean conditions which influence the phase-dependency of cold surge occurrence. During the negative phase of AO, it is clearly shown the deepening of upper level trough in east coast of Asian continent and strengthening of jet stream in composite monthly geopotential height at 500 hPa and zonal wind at 300 hPa respectively (Figures 3a and 3b). These conditions, which imply intense baroclinity and strong cold advection, are typical and favorable large-scale condition prior to cold surge occurrence as discussed by Lau and Li  and Boyle . The Siberian High, another important factor for cold surge occurrence, is also significantly changed in association with AO as discussed by Gong et al. . In the monthly MSLP composite (Figures 3c and 3d), mean MSLP over the climatological position of the Siberian High [40°–60°N, 80°–120°E] is strengthened (weakened) during negative (positive) AO period. Moreover, its day-to-day variability (root-mean-squared variance of MSLP) is also increased (decreased) over the nearly similar region during negative (positive) phase of AO. This pattern is similar to Figure 11 of Gong and Ho  which represents the leading principle mode of day-to-day variance of SLP in east Asia. The most significant signal is found over western Mongolia, and it is stretching southeastward to mid-China. This implies that the AO-related change is closely connected with the Siberian High because it bears dominant variability in those areas. Therefore, it is noted that the chance of intense Siberian High increases (decreases) during negative (positive) phase of AO.
 The relationship between the Siberian High and cold surge occurrence is investigated by several studies. Ding  has suggested that the possibility of cold surge occurrence is proportional to the intensity of the Siberian high pressure, and Zhang et al.  shows a certain intensity of the Siberian High is necessary condition of cold surge occurrence. Putting all the above results together, the characteristics of large-scale circulation and the Siberian High in association with AO provide favorable condition for cold surge occurrence during negative AO phase.
 The present study notes a possible linear connection between the AO and cold surges in east Asia. Several studies have reported the possibility of low-frequency components for an AO prediction [e.g., Baldwin et al., 2003; Charlton et al., 2003] recently. So if other low-frequency climate variabilities, which can affect extratropical circulation such as the MJO or ENSO, are combined with the present study, it will be more helpful for forecasting or alerting cold surges.
 This work was supported by the Meteorological Research Institute of Korean government. The first author was supported by BK21 Project of the Korean government. We thank the efforts of two anonymous reviewers in improving the clarity of the manuscript.