SCIENCE CHINA Earth Sciences • RESEARCH PAPER •
September 2013 Vol.56 No.9: 1566–1575 doi: 10.1007/s11430-012-4576-z
The zonal propagating characteristics of low-frequency oscillation over the Eurasian mid-high latitude in boreal summer YANG ShuangYan1,2, WU BingYi1*, ZHANG RenHe1 & ZHOU ShunWu2 2
1 Chinese Academy of Meteorological Sciences, Beijing 100081, China; Nanjing University of Information Science and Technology, Nanjing 210044, China
Received May 8, 2012; accepted August 31, 2012; published online January 15, 2013
Using 32-yr National Centers for Environment Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data, we investigated zonal propagation and circulation characteristics of the low-frequency circulation for the prevailing period over Eurasian mid-high latitude in boreal summer (May–August) in terms of empirical orthogonal function (EOF), linear regression, and phase analysis and so on. We found that the dominant periods of the low-frequency circulation are 10–30 days and it clearly shows meridional (southward) and zonal (westward) propagation features at the middle troposphere (500 hPa). The average zonal speed of the 10–30 days low-frequency oscillation (LFO) is about 9–10 longitudes per day. Further analysis shows that the southernmost part of the polar vortex in the northern hemisphere exhibits westward clockwise rotation in the eastern hemisphere in boreal summer. Also, the southernmost tips of 5400 and 5500 gpm contours, which indicate the site of the major trough in the eastern hemisphere, obviously move westwards. The southernmost tip of 5500 gpm contour line propagates westwards at the speed of about 9–10 longitudes per day, which is consistent with the mean zonal speed of the westward propagation of the low-frequency circulation. Moreover, the 10–30-day LFO-related cold air also shows west propagation feature with respect to LFO phases. The westward propagation of the LFO is the low-frequency-scale embodiment of the clockwise rotation of polar vortex. The cold air activities closely related to polar vortex or to ridge-trough system activities is the essential circulation of 10–30 days LFO circulation over the Eurasian mid-high latitude in boreal summer. 10–30 days LFO, the Eurasian mid-high latitude, propagation features, circulation characteristics Citation:
Yang S Y, Wu B Y, Zhang R H, et al. The zonal propagating characteristics of low-frequency oscillation over the Eurasian mid-high latitude in boreal summer. Science China: Earth Sciences, 2013, 56: 1566–1575, doi: 10.1007/s11430-012-4576-z
Atmospheric low-frequency oscillation (LFO) generally refers to the atmospheric oscillation above 10 days, but within 100 days and contains two well concerned bands with 30–60-day period (usually known as the intraseasonal oscillation, referred to as ISO) and 10–20-day period (usually known as the quasi-biweekly oscillation, referred to as QBWO) [1, 2]. In terms of the spectral analysis, Madden and Julian [3] first discovered the ISO over the tropical regions using observational data of Canton Island. Later, they [4] confirmed that the ISO exists over the global tropic. *Corresponding author (email:
[email protected])
© Science China Press and Springer-Verlag Berlin Heidelberg 2013
Subsequently, more and more researches indicated that the LFO exists not only over tropical [5–10] or subtropical zone [11–13] but also all over the world [14, 15]. Since the 1980s, the LFO has been investigated not only as a periodic phenomenon but also as an essential atmospheric motion, and its genesis, structures and activities have been discussed [16]. Most tropical LFO is triggered directly by convection [17–19]. Much progress has been made in studying the tropical or subtropical LFO. The QBWO exhibits an equivalent-barotropic structure over the tropical region [20, 21], whereas the ISO has a baroclinic structure first [22]. For the earth.scichina.com
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horizontal structure, the QBWO generally exhibits a zonal wavenumber-6 structure, whereas the ISO exhibits a zonal wavenumber-1 structure in boreal summer or winter [23, 24]. Over the tropical area, the QBWO mainly presents westward propagation feature, but sometimes shows eastward movement [8, 14, 20, 25] and distinctive northward propagation feature [8, 13]. The ISO primarily propagates eastwards and sometimes shows quasi-stationary or westward propagation feature over the equator [26], and mainly moves westwards over the subtropics [22, 26]. The ISO also clearly exhibits northward propagation feature [27, 28], but sometimes shows southward propagation feature over the equatorial or subtropical area [29]. Many studies have investigated the significant influence of the tropical LFO on tropical cyclones [30], monsoon [10, 31], and ENSO [32, 33]. The generation of the LFO over mid-high latitude may be related to the nonlinear interaction between atmospheric disturbance and basic flow. In contrast to the LFO over the tropical regions, there are few researches on the LFO over the mid-high latitude area (especially over high latitude region), although its prevalence was revealed in many studies [34, 35]. Does the LFO really exist over mid-high latitude? If the answer is yes, which are the prevailing bands? What are the propagation feature and the circulation characteristics of the LFO? In the current study, we focus on the zonal propagating characteristics of the LFO circulation of the prevailing period in middle troposphere (500 hPa) in boreal summer (May–August). Carrying out these studies could deepen the understanding of the low-frequency circulation structure over Eurasian mid-high latitude, and it is significant to explore the effect of the LFO on weather or climate in development.
1 Data and methodology The daily reanalysis data used here are from 1979 to 2010 with a latitude-longitude horizontal resolution of 2.5°×2.5° grid provided by the National Centers for Environment Prediction-National Center for Atmospheric Research (NCEPNCAR) [36]. The reanalysis data include geopotential height (Z500), zonal wind (U500), meridional wind (V500), and air temperature (T500) at 500 hPa level. The air temperature at sigma-995 level is also used to represent the surface air temperature (SAT). The power spectral analysis1) and morlet wavelet analysis [37] are first used to get the main period of the fields over the mid-high latitude area. The seasonal cycle and synoptic scale less than 7 days are first removed respectively and the anomaly fields are bandpass filtered by Lanczos band-pass filtering [38] (n=150) to obtain the leading low-frequency
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signal. The linear lagged-regression is utilized to analyze the evolution of the primary low-frequency circulation. A t-test is used to determine whether the regression coefficient is significant: r
t N 2
1 r2
(1)
,
where r is the correlation coefficient, N is the number of independent samples, and N2 is the degrees of freedom. However, observations of number N for filtered time series are often not independent. The estimation of the number of independent observations in the sample, that is, the effective sample size (ESS) for significance tests of the time series x (t ) , is [19, 39]: N*
N , | | 2 1 N ( N 1) N 1
(2)
where N* is the ESS, N is the number of the primary samples, and is the autocorrelation functions of x(t). The daily filtered time series from 1 May to 31 August each year (namely N=123×32=3936) are abstracted to denote the low-frequency time series in boreal summer (BS). Given the discontinuity of the daily time series (only 123 days from 1 May to 31 August are used each year), the ESS is first calculated individually for each year, and then the sum of them is regarded as the total ESS referring to Wen and Zhang [19]. For the current study, a time series of 32×123 days corresponds to a mean ESS of 800. A local t-test applied to the difference between two sample means is used to judge the statistical significance for the low-frequency phase composite [40]. The tj value is defined by the follow formula: n
t j ( j 1,28)
F i 1
i, j
/n
j 1/ n
,
(3)
where Fi,j is the field for composing, n is the sample number in each LFO phase, and j is the standard deviation of Fi,j. The tj value of composite field in each phase is calculated. Because the sample of each phase comes from discontinuous time series, they are independent of each other. Thus, the ESS namely is the length of the primary sample. In addition, there is not much sense to judge the significance of the original field, so the significance of the composite original fields is not tested in this paper.
1) The program is provided by Prof. Li Jianping in Institute of Atmospheric Physics, Chinese Academy of Sciences: http://ljp.lasg.ac.cn/dct/page/65539, and the results are conducted by Han-ning smoothy to reduce the errors.
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2 Result analyses 2.1
Zonal propagation feature of the LFO
Before analyzing the propagating characteristic of the LFO, one should determine the dominant period, so the low-frequency periods of Z500, U500 and V500 for each
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year are evaluated by power spectral analysis. It is found that each of them exhibits the LFO phenomenon obviously. Figure 1 shows their prevailing LFO period in a certain year (other years omitted). Figure 1 shows that the 10–30-day LFO does exist each year over the Eurasian mid-high latitude (EMHL). Moreover, by using the morlet wavelet
Figure 1 The power spectrum of Z500 (a), V500 (b), and U500 (c) over EMHL. Dashed line indicates the standard spectrum of red noise at 5% significant level, solid line represents the value of power spectrum, and only the spectrum of middle grids is plotted in each area, for example, the plot in the range of 30°–35°N, 40°–45°E indicating the spectrum of grid 32.5°N, 42.5°E.
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transformation [37], it is found that the main cycle of the Z500, U500 and V500 is also 10–30-day (figure omitted). Therefore, we will focus on the characteristics of 10–30-day LFO. In the rest of this paper, without being mentioned specifically, all the terms of ‘LFO’ are referred to 10–30-day oscillation. The empirical orthogonal function (EOF) is used to reveal the dominant patterns of the LFO over EMHL in BS. The structures of the first and second EOF modes (EOF1 and EOF2) of low-frequency Z500 and the time-lag correlation coefficients between the two principal components (PC1 and PC2) are shown in Figure 2. The variance contribution of the first two leading modes is 18.94% and 15.24% respectively. EOF1 and EOF2 are statistically independent from the higher modes by calculating the error range of eigenvalue proposed by North et al. [41]. As shown in Figure
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2(a) and 2(b), the positive or negative center presents zonal and meridional propagation between EOF1 and EOF2. In the rest of this study, we will focus on the features of the zonal propagation. As illustrated in Figure 2(c), when PC1 leads and lags PC2 for 5 days (about a quarter cycle), the correlation coefficient is maximum and minimum, respectively. The absolute value of the coefficient is close to 0.5 and the entire cycle is approximately 20 days. A significant lagged correlation between PC1 and PC2 suggests that physically EOF1 and EOF2 represent the different phases of the same LFO cycle [5, 42]. Thus, one can use PC1 to reveal the evolution characteristics of the entire LFO cycle [5]. Figure 3 portrays the regressed LFO circulation evolution at 500 hPa against PC1 associated with an entire cycle of the LFO from day 10 to day 10 (The time interval is 2 days). On day 10, to
Figure 2 EOF1 (a) and EOF2 (b) of the LFO-related Z500 and the correlation coefficients between PC1 and PC2 (c). Shaded area means the absolute value surpassed 0.01; a positive (negative) lagged time denotes that PC1 lags (leads) PC2. For example, ‘10’ means PC1 leading PC2 for 10 days.
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the north of 50°N, a positive height anomaly and corresponding anticyclonic circulation anomaly appear to the east of the Lake Baikal with the center located near the Novosibirsk islands. At the same time, a negative height anomaly appears to the west of the Lake Baikal and corresponds to a cyclonic circulation abnormality. The negative center is located in middle Barents Sea (Figure 3). In the subsequent days (from days 9 to 5), the positive and negative centers clearly propagate westwards simultaneously. On day 4, to the north of the 50°N, the most Eurasian region is controlled by positive height and anticyclonic anomaly. On day 2, a new negative height anomaly appears to the east of the Lake Baikal. At day 0, most high latitude region to the east of the Lake Baikal is controlled by the new anomaly. The positive anomaly to the west of the Lake Baikal is located in the middle Barents Sea. The circulation patterns of day 0 and day 10 are basically contradictory. The positive anomaly to the east of the Lake Baikal intensifies with the circulation propagating westwards. The circulation patterns from days 0 to 10 are contrary to those from days 10 to 0, respectively. That is, the negative abnormal center to the north of the Lake Baikal gradually propagates eastwards from the Novosibirsk islands nearby, and moves to the middle Barents Sea at day 10. The average propagating speed of the LFO is about 10 longitudes per day in an entire cycle. By assuming that the distance of each longitude approximately equals 111 km, the speed is equal to about 13 m s1, which is slightly faster than the zonal shifting speed of ISO received by Li [16] (about 10 m s1) by
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using the velocity potential, OLR and stream function data. One may notice from Figure 3 that the circulation patterns on day 0 and day 5 (The pattern of day 5 is similar to that of day 4 in Figure 3) are respectively similar to EOF1 and EOF2 patterns shown in Figure 2. This indicates that the two leading EOF modes reflect the different phases of the same LFO cycle. To prove that in development, one can choose PC2 as the referenced time series to reveal the evolution characteristics of the entire LFO cycle, also using the method of lagged-regression (Figure 4). As illustrated in Figure 4, there are two LFO height abnormal centers with reverse signs in the east-west direction, corresponding with LFO cyclone or anticyclone over EMHL in BS. From days 10 to 0, the positive abnormal center moves westwards from near 100°E to around 10°E at a speed of about 9 longitudes per day. From days 0 to 10, the negative normal center also propagates westwards from near 100°E to near 10°E at the same speed. Similarly, the assumption that the distance of one longitude approximately equals 111 km leads to that the speed is about 12 m s1. It is similar to Figure 3 that the circulation patterns from days 0 to 10 are basically contrary to those from days 10 to 0 in Figure 4. The 5-day led circulation pattern regressed against PC1 is close to the 10-day led pattern regressed against PC2 (It is noticed that PC1 leads PC2 for 5 days), which is in accord with the conclusion obtained from Figure 2, namely when PC1 leads PC2 for 5 days, the correlation coefficient is peak. In addition, the circulation patterns on day 5 (The pattern of day 5 is similar to that of day 4 in Figure
Figure 3 The lagged-regressed LFO-related Z500 (contour, the interval is 20 gpm, and all shaded area passes 5% significant level) and UV500 (vector: m s1, only the area passing 5% significant level is plotted) against PC1 (The pattern at day 0 is plotted twice in order to clearly show the zonal evolution versus time, and a positive (negative) lagged time denotes that circulation lags (leads) PC1, for example, ‘10’ means circulation leading PC1 for 10 days, the same as follows).
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4) and day 0 are respectively similar to EOF1 and EOF2 shown in Figure 2, which again indicates that the two leading EOF modes reflect the different phases of the same LFO mode. Thus, one may use any time series of PC1 or PC2 to reveal the evolution characteristics of the entire LFO cycle. Actually, no matter which time series one selects, one can obtain the similar results. Considering the slightly larger variance contribution of PC1 than that of PC2, we select PC1 as the reference series. In the rest of this study, without being mentioned specifically, all regression coefficients are relevant to PC1. To further explain the clear westward propagation of LFO negative/positive height anomaly and the corresponding cyclonic/anticyclonic circulation anomaly, which is shown obviously in Figures 3 and 4, the time-longitude section of regressed LFO-related and original U500 anomalies along 55°N is plotted in Figure 5. It is evident in Figure 5(a) that both easterlies and westerlies oscillations move west-
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wards conspicuously in BS over EMHL. Similarly, the regressed original zonal wind field (The original field means the field without any processed, the same as follows) also evidently exhibits westward propagation feature (Figure 5(b)). Similarly again, the regressed height and wind fields, whose seasonal cycle are removed only, also remarkably show westward propagate features (figure omitted). 2.2
Circulation feature of the LFO
The meridional and the zonal propagation features are the fundamental characteristics of the LFO (including the ISO and the QBWO), so any mechanism of the LFO should be able to explain the basic features [43]. The most tropical LFO is excited directly by convection [5, 17–19], but the mechanism of the mid-high latitude LFO is still not very clear. In order to investigate the LFO mechanism, we must clarify its circulation characteristics. By referring to the
Figure 4
As in Figure 3, but for the regressed fields against PC2.
Figure 5
The time-longitude section of lagged-regressed LFO-related U500 (a) and original U500 (b) against PC1 along 55°N (Contour interval: 1 m s1).
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method of Matthews [44], each LFO cycle can be categorized into four phases. The LFO at certain time is expressed as a two-dimensional spatial vector Z. Z (t ) [PC1(t ), PC2(t )],
(4)
A(t ) [PC12 (t ) PC22 (t )]1/ 2 ,
(5)
PC2(t ) , PC1(t )
(t ) tan 1
(6)
where A(t) is the amplitude, and (t) is the phase angle between PC1 and PC2. The four phases are named Phase 1, Phase 2, Phase 3 and Phase 4, respectively. Taking the 20-day cycle as an example, time length of each phase is about 5 days. When PC1, which leads PC2 for 5 days, is on day 10, PC2 is at day 5, and then their positive correlation coefficient reaches a peak (Figure 6(a)). The angle interval is /2 and the four phase angle range is [ / 2, 0), [0, / 2), [ / 2, ) and [ ,3 / 2], respectively (Figure
Figure 6 LFO.
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6(b)). Figure 7 shows the composite LFO-related Z500, U500, and V500 in each LFO phase. In the stage of phase 1, the area over EMHL is controlled mainly by positive height anomaly accompanying anticyclonic circulation anomaly. To the east of the anticyclonic circulation, it is controlled by the western edge of negative height anomaly in the northeast of the East Siberia Sea over EMHL. In phase 2, the negative anomaly moves westwards. It is controlled by negative anomaly in middle and eastern Russia accompanying cyclonic anomaly. In phase 3, the circulation pattern is opposite to that in phase 1. The negative anomaly continues to shift westwards leading to that the region over EMHL is controlled by negative anomaly accompanying cyclonic circulation anomaly. In phase 4, the center of the negative abnormality keeps shifting westwards. It is controlled by positive anomaly in middle and eastern Russia accompanying anticyclonic circulation anomaly. Therefore, From
Sample of time variation of PC1 and PC2 with respect to LFO phases (a) and the distribution of phase angels (b) based on a 20-day cycle of the
Figure 7 Composite LFO-related Z500 (contour, the interval is 5 gpm and the zero contours are omitted, and all shaded area passes 5% significant level) and UV500 (vector, only the area suppressed 5% significant level is shown).
Phase 1 to Phase 4, the LFO circulation gradually moves westwards, which is consistent with the conclusion obtained in section 2. Thus, the zonal propagation features of the LFO can be well revealed by the method of phase analysis. In order to discuss the circulation features of the LFO, the evolution features of original circulation corresponding to the zonal propagation feature of the LFO are analyzed. Figure 8 presents the composite original circulation in the four LFO phases. For clearly showing the evolution of original fields with respect to the LFO phases, the area whose original Z500 field is less than 5500 gpm, is shaded in Figure 8. This area represents circumpolar cyclonic vortex, namely the polar vortex in the northern hemisphere (NH). Figure 8 shows that the southernmost part (SMP) of the polar vortex in the eastern hemisphere (EH) is located at 90°–180°E in phase 1. In phase 2, the polar vortex shows clockwise rotation and its SMP is located at 80°–90°E. In phase 3 and phase 4, the polar vortexes continue to rotate clockwise and the SMPs are located at 30°–80°E and 0°–30°E in EH, respectively. Therefore, the polar vortex exhibits westward clockwise rotation with respect to the LFO phases in NH. Li [16] has pointed out that many sys-
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Figure 8 Composite original Z500 (contour: interval is 50 gpm) and UV500 (vector) (latitude range is 50°–90°N, shaded area indicating Z500 less than 5500 gpm).
tems over mid-high latitude present marked LFO phenomenon such as the polar vortex in NH. Consequently, the westward propagating characteristic of the LFO obtained in section 2 is closely linked with the westward clockwise rotation of the polar vortex. The westward propagation of the LFO is exactly the low-frequency-scale embodiment of the westward clockwise rotation of the polar vortex. The polar vortex is a symbol of mass cold air, so the westward propagation of LFO represents the activities of cold air. In order to further illustrate the westward movement of
Figure 9
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the polar vortex with respect to the LFO phases, the evolution of 5400 gpm and 5500 gpm contour lines is shown in Figure 9. The position of their SMPs implies the position of the major trough in EH, and the evolution of the SMPs position indicates the evolution of the major trough. During Phase 1 to Phase 4, the SMP of 5400 gpm isoline gradually moves westwards in EH, accompanying expansion to the lower latitude of the EH (Figure 9(a)). The expansion of the polar vortex affects the weather or climate significantly [45]. When evolving from Phase 1 to Phase 2, the SMP of 5500 gpm contour moves westwards from the Novosibirsk islands to near the Yenisei River of Russia at a speed of around 10 longitudes per day (Figure 9(b)). From Phase 2 to Phase 3, the SMP of 5500 gpm contour moves westwards to the south of Novaya Zemlya at a rate of about 9 longitudes per day. From phase 3 to phase 4, the SMP moves westward to the south of Norwegian Sean at an average speed of about 9 longitudes per day. From phase 1 to Phase 4, the average speed of the SMP is close to that of the LFO (9–10 longitudes per day), as concluded in the former discussion, which thoroughly explains that the westward propagation of the LFO is the embodiment of the westward clockwise rotation of polar vortex on low-frequency scale. The rotation of the polar vortex is closely related to cold air activities. Also, the variation of circulation trough-ridge system in association with westward clockwise rotation of the polar vortex center is very close to the LFO circulation over EMHL. The polar vortex is often regarded as the symbol of polar cold air mass, and the trough-ridge activities themself are closely related to the cold air activities, so the LFO circulation anomaly essentially represents quasi-periodic activities of cold air over EMHL. For revealing the westward propagation feature of cold air in development, Figure 10 depicts the evolution of the LFO-related SAT with respect to the LFO phases. It can be seen that the SAT anomaly shows conspicuous westward propagation feature. In phase 1, low anomaly center is located at near the Laptev Sea. In phase 2, the low center propagates eastwards to near Russian Khatanga and the whole area of middle and eastern Russia is controlled by low anomaly. In phase 3, the low center keeps moving westwards to the north of the Ural Mountains. In phase 4, the center moves to near the Baltic Sea. From Phase 1 to
Composite original 5400 gpm (a) and 5500 gpm (b) contours in the four LFO phases (latitude range is the same as Figure 8).
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Figure 10 Composite LFO-related SAT in the four LFO phases. Contour interval is 0.2°C, the zero contours are omitted, and all shaded area passes 5% significant level.
Phase 4, the westward propagating speed of the SAT anomaly is about 10 longitudes per day, which is in accordance with the propagating rate and direction of the polar vortex and trough-ridge system over EMHL in BS concluded in the previous discussion. The LFO-related T500 also shows the same propagating speed and orientation (figure not shown). According to all above results, it is true that the cold air activities are the essential circulation in BS over EMHL.
3 Discussion and conclusions By using NCEP/NCAR reanalysis data, the zonal propagation and circulation characteristics of the LFO at middle troposphere (500 hPa) in BS (May–August) over EMHL (particularly over the high latitude) are analyzed, based on the methods of EOF, linear regression and phase analysis and so on. Main findings of the current study are summarized as follows: (1) The Z500, U500, and V500 fields generally exhibit the noticeable LFO phenomenon and the dominant band is 10–30-day from 1979 to 2010, based on the power spectral analysis and morlet wavelet analysis (the seasonal cycle and synoptic scale less than 7 days are removed). (2) The LFO circulation apparently shows both meridional (southward) and zonal (westward) propagation features. The evolution of the regressed LFO circulation against PC1 for a whole cycle from day 10 to day 10 is analyzed. On day 10, to the north of 50°N, there is a positive and a negative height disturbed center respectively to the east and to the west of the Lake Baikal corresponding to anticyclonic and cyclonic circulation anomaly respectively. The positive and negative centers are located near the New Siberian
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Islands and the middle Barents Sea respectively. Subsequently, both the high and the low centers propagate westwards leading to the opposite weather situation on day 0 and day 10. On day 0, to the east of the Lake Baikal, most area over EMHL is controlled by negative height and cyclonic circulation anomalies. It is controlled by positive height and anticyclonic circulation anomaly over the middle Barents Sea. The circulation patterns from days 0 to 10 are contrary to those from days 10 to 0 respectively. The average speed of the westward propagation of the LFO circulation is about 9–10 longitudes per day (about 12–13 m s1). Choosing PC2 as the referenced time series draws the similar principal conclusion. (3) The zonal propagation feature of the LFO circulation in BS over EMHL can be well expressed by using phase analysis method. The SMP of the polar vortex in EH exhibits westward clockwise rotation with respect to the LFO phases in BS. The SMPs of 5400 and 5500 gpm contours in EH, which present the position of the major trough in EN, obviously move westwards with LFO phases. The SMP of 5500 gpm contour line moves at a speed of about 9–10 longitudes per day. This speed is approximately equal to the mean zonal speed of the propagation of the LFO circulation. Also, the activities of the LFO cold air show west propagation with respect to LFO phases. The westward propagation of the LFO is the low-frequency-scale embodiment of the clockwise rotation of the polar vortex in BS over EMHL. The LFO circulation anomaly essentially represents quasi-periodic activities of cold air over EMHL. Cold air activities closely related to the trough-ridge activities are the essential circulation of the LFO over EMHL in BS. The LFO circulation pattern is the cyclone-and-anticyclone wave train at least over wavenumber-2 on a global scale, which can be seen from Figures 3 and 8, especially from Figure 8. The wavenumber-1 dipole structure over EMHL shown in Figures 3 and 4 is only part of the global wave train. What and how the global wave train affects the wavenumber-1 dipole is open to discussion. The LFO possesses meridional propagation feature as well as zonal propagation feature as indicated in previous studies. The meridional moving feature of the LFO is even reverse visibly in different longitude areas over EMHL in BS. The characteristics of the meridinal propagation feature deserve further researches. We thank NOAA for providing the reanalysis data. Thanks particularly go to the anonymous reviewers for their valuable comments and to Prof. Wen Min for her help with the method of significant test. This work was supported jointly by the National Natural Science Foundation of China (Grant Nos. 40875052 & 41221064), the Calling Project of China (Grant Nos. GYHY200906017 & GYHY201006020), and the Basic Research Foundation of CAMS (Grant No. 2010Z003). 1
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