1. Introduction
The large-scale tilted ridge and trough (LSTR and LSTT, respectively) in the midtroposphere, with zonal extents larger than the wintertime climatological ones, sometimes form over mid- and high-latitude regions of the Eurasian continent. They are directly associated with large-scale cold air activities and often cause extensive and persistent low temperature events in Eurasian countries (Bueh et al. 2011a; Peng and Bueh 2012). For example, an extensive and persistent low temperature event occurred in southern China during mid- to late January of 2008, and its key circulation systems were realized as the LSTR, stretching from the Ural Mountains to northeastern Asia, and the accompanying LSTT that extended from middle Asia to East Asia (Tao and Wei 2008; Wen et al. 2009; Zhou et al. 2009; Bueh et al. 2011b, among others). Because of the enormous amount of damage to life and property it caused in China, this cold event aroused extensive public attention and scientific interest.
The long-lasting nature of the LSTR (or LSTT) and the persistent cold event concur with synoptic reasoning, since the time scale of a circulation system is positively proportional to its spatial scale. Further evidence for this observation came from Peng and Bueh (2011), who documented 52 extensive and persistent low temperature events in China during 1951–2008 using observed daily temperatures from 756 meteorological stations in China, and examined their key circulation systems. It was found that a southwest–northeast-tilted large-scale midtropospheric ridge was the key circulation feature common among the wintertime extensive and persistent low temperature events. With the large extent of southward cold advection in association with the LSTR, the Siberian high expanded and amplified with tightened sea level pressure gradients.
The dynamic features of the persistent large-scale ridge and trough system on submonthly time scales have been revealed in several studies. For the persistence of the tilted ridge and trough over the Northern Hemisphere, Zeng (1983) proposed that the ridge and trough tend to intensify barotropically at the expense of the westerly jet, if they are tilted in the northeast–southwest (northwest–southeast) orientation on the northern (southern) side of the westerly jet. On the other hand, Santos et al. (2009) revealed that the strong and persistent midlatitude large-scale ridge system over the northeast Atlantic plays a key role on the extremely dry episodes in western Iberia (Santos et al. 2007). Their results showed that the tropospheric Rossby waves originating from positive height anomalies over North America propagated eastward over the North Atlantic and contributed to the generation and maintenance of the strong and persistent ridge over the northeast Atlantic. Interestingly, they also found that anomalously strong and vertically extended stratospheric positive height anomalies over North America/North Pacific, which were in phase with their tropospheric anomalies, were favorable to the maintenance of the strong ridges over the northeast Atlantic by feeding anomalies at the Rossby wave origin. Wu et al. (2013) found a significant increasing trend of an anomalously strong ridge over northern Eurasia during winter since the late 1980s, and revealed that such a trend corresponded well with that of Arctic sea ice reduction. However, so far the involved dynamics for the wintertime LSTR and LSTT have not yet been systematically investigated in depth. This may be partly due to the lack of an appropriate objective identification method.
Historically, the depiction and thus the detection of the LSTRs and LSTTs have been quite challenging, and no objective identification method has been developed. Generally, over the mid- and high-latitude regions, the upright ridge (or trough) axis in a wavelike flow can be defined by a change in sign of the meridional wind change [i.e., υ = 0 m s−1; Reed et al. (1977)], or as the traverse ridge (or trough) axis by a change in sign of the zonal wind change [i.e., u = 0 m s−1; Li and Zhu (2010)]. However, these diagnostics do not work at the tilted ridge (or trough) axis. In this situation, the position of the tilted ridge (or trough) can be simply defined if the geostrophic vorticity advection disappears (i.e., 
Therefore, for real-time monitoring and advancement of the relevant research an objective method of identifying the LSTRs and LSTTs over mid- and high-latitude regions is necessary. In this paper, we propose a new objective identification method for the LSTR and LSTT that can be applied to any gridded dataset, and highlight the importance of this identification method through analyzing the correspondence between the zonal extent of LSTR (or LSTT) and that of the large-scale cold air activity. The paper is organized as follows: section 2 describes the objective identification method of the LSTR and LSTT. In section 3, we test the skill of this objective technique by comparing the observed and identified daily LSTRs and LSTTs during the winter of 2011/12 (15 November–15 March). In section 4, we apply the method to identify the LSTRs and LSTTs during a well-known East Asian cold event and examine their association with the large extent of cold spell. The final section consists of a summary and discussion.
2. Methodology
a. Background
It is desirable to have a diagnostic that can represent the actual position as well as the zonal extent of the LSTR and LSTT, which is easy to generate in real time with minimal computation from observed or modeled fields. For this purpose, in this study we chose the LSTR and LSTT axes at 500 hPa as the core diagnostics, as they have an intimate relationship with the extensive cold air activity in the lower troposphere.
In the present study, we adopted another strategy for the identification of large-scale ridges and troughs, which differs from that of the CVA method. Specifically, for identifying ridge or trough axes, we searched the lines (or axes) along which the contours of 500-hPa geopotential height field (Z500) had the local maximum curvatures.
As is empirically known, the LSTR often passes through the blocking high, such as the blocking high over the Ural Mountains, or the locally isolated cutoff high [it cannot be recognized as blocking when its longitudinal extent is not large enough to satisfy the blocking criteria; Tibaldi and Molteni (1990)]. Similarly, the LSTT also often goes across the regional cutoff low. Therefore, it is also necessary to properly identify the LSTR and LSTT axes within the regions of blocking high and cutoff low.
b. Identification
Based on the daily Z500 fields of the NCEP–NCAR reanalysis data (Kalnay et al. 1996), the identification process of the LSTR (or LSTT) axis consisted of four steps. First, the low-pass-filtered Z500 fields were obtained using a Lanczos low-pass filter (Duchon 1979), with the cutoff period of 8 days and 121 weights. (The frequency response of the filter is shown in Fig. A1 of the appendix.) Then, different types of contours on the filtered Z500 field were recognized and their constituting points were recorded accordingly. Subsequently, the ridge points and trough points on different types of contour were identified. Finally, each LSTR (or LSTT) axis was defined based on the identified ridge (or trough) points.
1) Different types of contours
In the Northern Hemisphere, we first found the contour with the minimum geopotential height, 
According to the relative position to the North Pole, the contour was simply recognized as either the circumpolar contour or the locally closed contour. All constituent points of a given circumpolar contour defined by longitudes and latitudes 
Example of the Z500 field on 7 Nov 2007. The circumpolar wavy contour (5720 m), the circumpolar contour containing the meridional overturning part (5280 m), and the locally closed contour (5200 m) are drawn in dotted purple, red, and blue, respectively. All discretized constituent points of a given contour are recorded at every 2.5° of longitude.
Citation: Monthly Weather Review 143, 12; 10.1175/MWR-D-14-00238.1
2) Ridge and trough points
(i) Circumpolar wavy contour
Then, each node point was simply defined by 
An arbitrary circumpolar wavy contour of the Z500 field and the slope S corresponding to each constituent point of the contour. The value of S is indicated on the right vertical ordinate axis.
Citation: Monthly Weather Review 143, 12; 10.1175/MWR-D-14-00238.1
A schematic diagram of the bending angle 
Citation: Monthly Weather Review 143, 12; 10.1175/MWR-D-14-00238.1
Since 
(ii) Meridionally overturned part of the circumpolar contour
For the meridionally overturned part of a circumpolar contour (see the dotted contour in red in Fig. 1), the setting of node points was not applicable, in contrast to the situation of the wavy part of the contour. As for the meridionally overturned part, it holds that 
The (a) anticyclonically and (b) cyclonically overturned contours. The meridionally overturned parts are indicated by the crossed open circles between points A and B.
Citation: Monthly Weather Review 143, 12; 10.1175/MWR-D-14-00238.1
(iii) Locally closed contour
Since the LSTR and LSTT axes often passed through the locally closed contours, we expected to recognize them within the locally closed contour. In this situation, the above identification method was not applicable. With the recognition of all locally closed contours, as introduced in section 2b(1), here we identified the ridge and trough points in terms of the local maximum curvature points on the contour, in which the local maximum curvature was determined by the local minimum bending angle 
(a) A schematic of the recorded points on a circle in every 2.5° longitude. (b) As in (a), but the points are recorded where the circle intersects the gridlines. (c) The convex point and concave point on a locally closed contour (dotted, 5120 m). This example is taken from the Z500 field on 3 Feb 1984, in which contours are drawn every 80 m.
Citation: Monthly Weather Review 143, 12; 10.1175/MWR-D-14-00238.1
As indicated in Fig. 5c, there were two types of local maximum curvature points on the locally closed contour, namely the convex point (see point A in Fig. 5c) and the concave point (see point B in Fig. 5c), respectively. In practice, a convex (concave) point was defined at Pi (ith point) if the central point (Pc1) of the segment connecting points Pi−1 and Pi+1, and the central point (Pc2) for points Pi−2 and Pi+2, were both situated inside (outside) the contour. Then, for a closed contour with geopotential height increasing outward (i.e., the closed low) a convex (concave) point Pi was recognized as the trough (ridge) point with an additional constraint of the bending angle 
3) Ridge and trough axes
The identification of the ridge (or trough) axis was based on the fact that it consists of a number of nearby ridge points (or trough points) on different contours and can be recognized by some objective criteria. As for the ridge axis, the algorithm started from each ridge point of the contour with 
In fact, condition A is a statement of the nearest principle for a ridge or trough axis, and implicitly assumes that there is only one ridge (or trough) axis within a horizontal distance of 
The identified ridge and trough axes on the Z500 field, with (a) the minimum distance condition A only, and (b) the minimum distance conditions A and B. Red and blue lines indicate the ridge and trough axes, respectively. This example is taken from the Z500 field on 13 Jan 2008, in which contours are drawn every 80 m.
Citation: Monthly Weather Review 143, 12; 10.1175/MWR-D-14-00238.1
Since the LSTR and LSTT were our primary concern, we eliminated the small ridge and trough axes with horizontal length less than 1000 km. The length of the ridge or trough axis was defined as the spherical distance between the start and end point of the same ridge or trough axis, and the spherical distance was determined only by the longitudes and latitudes of the start and end points. Furthermore, in order to keep the large-scale smooth feature of the LSTR and LSTT, every ridge or trough axis was smoothed by the 15-point smoothing method, through averaging longitudes and latitudes of the neighboring 15 ridge or trough points for a given ridge or trough point, with itself as the central point. Finally, we eliminated all ridge and trough points identified to the north of 80°N. The method is inadequate to depict the ridge and trough axes crossing the polar region, because its recording way of contour points relies on longitudes (e.g., every 2.5° longitude for the circumpolar contour).
3. Skill of the objective technique
In this section, we use the daily Z500 fields during the winter of 2011/12 (15 November–15 March) to test how well the objective identification method works, through comparing the identified LSTR and LSTT axes and their observed counterparts. Since the observed LSTR and LSTT axes are not available, their equivalents were subjectively plotted in the daily Z500 fields with a careful visual inspection, which may be considered to be those generated by an experienced analyst. Hereafter, such an equivalent is called the “observed” LSTR (or LSTT) axis. The test includes two parts. First, we evaluate the reliability of the identification method in terms of its hit rate (HR), false-alarm ratio (FAR), and threat score (TS); then, to the meridionally overturned part of a circumpolar contour, we examine if the identification is consistent with the Rossby wave–breaking feature in the mid- and upper troposphere.
a. Hit rate, false-alarm ratio, and threat score
To evaluate the reliability of the objective technique, following the notion of the skill in a probability forecast, we calculated the HR, FAR, and TS of the identification method, based on the numbers of the “observed” and identified LSTRs and LSTTs in the daily fields during the period from 15 November 2011 to 15 March 2012 (122 days). The data used here (and also in section 4) were the daily meteorological fields of the NCEP–NCAR reanalysis (Kalnay et al. 1996).
Figure 7 shows the identified LSTRs and LSTTs (left column) and those in the subjective plotting (right column). The daily plots are displayed once a week. Table 1 shows the HR, FAR, and TS of the identification method. If one visually checks the identified LSTRs and LSTTs (left column) and compares them to their counterparts in the subjective plotting (right column), the identification method performs reasonably well throughout the winter of 2011/12. It is also true if we check the identified LSTRs and LSTTs in all daily plots (not shown). In fact, as listed in Table 1, its HR for the LSTRs (LSTTs) is high, approximately 0.97 (0.96), consistent with its high TS of 0.92 (0.93). Even for the ridge (trough) axes with a zonal extent larger than 60° longitude, the method also shows a high HR of 0.94 (0.92).
(left) The identified ridge and trough axes in the Z500 fields and (right) their counterparts in the subjective plotting for the period from 15 Nov 2011 to 15 Mar 2012. Red and blue lines indicate the ridge and trough axes, respectively. Contours are drawn every 80 m. The ridge and trough axes with horizontal length less than 1000 km are eliminated. The region of an AWB (CWB) is crosshatched in pink (green).
Citation: Monthly Weather Review 143, 12; 10.1175/MWR-D-14-00238.1
The hit rate (HR), false-alarm ratio (FAR), and threat score (TS) of the identification method for LSTRs and LSTTs. The second column is for the ridges and troughs with a horizontal length larger than 1000 km, and the third to fifth columns are for those with zonal extents larger than 30°, 45°, and 60° longitude, respectively.
However, as seen in Table 1, as compared to those with a horizontal length larger than 1000 km, the ridge and trough axes with a zonal extent larger than 30° longitude were identified with a lower HR, and hence a lower TS. With careful checking, it is found that if an observed larger-scale ridge (or) trough axis goes across the very flat contours (the minimum 
The FAR for the method is relatively low, up to 5% (4%) for the LSTRs (LSTTs). In particular, for the ridge and trough axes with a zonal extent larger than 45° longitude, it performs without a false alarm. In addition, after examining the daily maps throughout the winter of 2011/12, we found that the identification method works remarkably well for the ridge and trough axes within locally closed contours, without a missing or false alarm. The HR, FAR, and TS for the test reveal that, on the whole, the method presented in section 2 has a reasonably good skill in the identification of LSTRs and LSTTs.
b. Meridionally overturned part of the contour and Rossby wave breaking
The meridionally overturned part of the Z500 contour, which occurred frequently during the period from 15 November 2011 to 15 March 2012 (Fig. 7), in fact, corresponds to a local reversal of potential vorticity (PV) gradient and thus reflects the presence of the Rossby wave breaking (Rivière 2009; Rivière et al. 2010). In this subsection, by examining the wave-breaking feature, we assess the robustness of the methodology in the identification of the LSTRs and LSTTs around the meridionally overturned parts of the contours.
In most of the current detection methods, the Rossby wave–breaking event was identified at the tropopause and the main emphasis was placed on the synoptic wave-breaking (e.g., Postel and Hitchman 1999; Martius et al. 2007; Strong and Magnusdottir 2008; Rivière et al. 2010; Ndarana and Waugh 2011). Therefore, it is not appropriate to apply them directly to evaluate the methodology in the present study, in which we used the low-pass-filtered Z500 fields. For our purpose here, we have developed a simple detection method of Rossby wave breaking, following the method of Rivière et al. (2010).
The detection method of Rossby wave breaking here consists of four steps. First, the PV fields were first calculated on the isobaric surfaces using the low-pass-filtered daily wind and temperature fields and then the resultant isobaric PV fields were linearly interpolated to the isentropic surfaces, as in Ndarana and Waugh (2011). The low-pass-filtered wind and temperature fields were obtained with the Lanczos low-pass filter in the appendix, which match well with the low-pass-filtered Z500 fields in our method in the identification of LSTRs and LSTTs. For the details of the PV calculation and interpolation, readers are referred to Ndarana and Waugh (2011). Second, on each of the isentropic surfaces of 320, 330, 340, and 350 K, the PV contours at 0.5-PVU (1 PVU = 10−6 K kg−1 m2 s−1) intervals over all values of PV present were generated. Then, the PV contours that are circumpolar were retained and those with isolated pockets of high- or low-PV air, which are not associated with wave breaking, were excluded, using the same technique as in Strong and Magnusdottir (2008) and Rivière (2009). At each day, on each isentropic surface, the points of the PV contour at which there is a meridional reversal of the PV gradient were identified to be related to be Rossby wave breaking, following the method of Rivière et al. (2010). As indicated by the schematic in Figs. 8a and 8b, the thick dashed parts (from Pi to Pf) stand for the wave-breaking portions of the PV contours. For more details in the identification of wave breaking, readers are referred to Rivière et al. (2010). Third, we divided wave breaking into the anticyclonic wave breaking (AWB) and cyclonic wave breaking (CWB) on each of the four isentropic surfaces. As shown in Fig. 8, if the first point Pi along the wave-breaking part of the contour is more to the south (north) than the point Pb, which is the previous point (searched backward in the reverse orientation of the arrows) along the PV contour at which the meridian (thin dashed line) passing through Pi intersects the PV contour, all of the points from Pi to Pf (the last point of the wave-breaking part) are considered to be related to an AWB (CWB). The region of AWB (CWB) is crosshatched in Fig. 8a (Fig. 8b) in pink (green). On each isentropic surface, the wave-breaking feature (anticyclonic or cyclonic) at each grid point was simply represented by a wave-breaking index (WBI). The WBI is set as 1 (−1) at the grid point that is related to an AWB (CWB) and as 0 at the grid point without wave breaking. Finally, at each grid point, we averaged the WBIs on the four isentropic surfaces with the same weight and determined the wave-breaking feature at that grid point. Specifically, the grid point is recognized as related to an AWB (CWB) if the averaged WBI at that grid point is not lower (higher) than 0.25 (−0.25). At a given grid point, the threshold value of the averaged WBI means that, even though the wave breaking occurs only on one of the four isentropic surfaces, it is also recognized as wave breaking. In addition, the importance of vertical averaging of the WBIs over several isentropic surfaces was discussed in detail in Michel and Rivière (2011).
A schematic describing the detection method of AWB and CWB. Each contour represents an isoline of PV on an isentropic surface and is oriented from west to east. The thin solid part of the contour corresponds locally to a poleward PV gradient, while the thick dashed part (from Pi to Pf) corresponds to a local reversal of the PV gradient. The latter part defines a wave-breaking region. The Pb is the previous point along the PV contour at which the meridian (thin dashed line) passing across Pi intersects the PV contour. When the first point Pi along the wave-breaking region is more to the south (north) than Pb, all of the points from Pi to Pf belonging to the thick dashed part are considered to be related to an AWB (CWB). The region of an AWB (CWB) is crosshatched in pink (green).
Citation: Monthly Weather Review 143, 12; 10.1175/MWR-D-14-00238.1
The regions of Rossby wave breaking in the daily fields during the winter of 2011/12 were indicated by crosshatches in the left column of Fig. 7. Generally speaking, the region of wave breaking is always embedded between the LSTR and LSTT, as indicated in Fig. 8. From mid- to late November (top three rows), Z500 contours were meridionally overturned over the regions from eastern Europe to middle Asia, corresponding to the occurrences of AWB (pink crosshatches). In this period, the LSTRs and LSTTs were identified between 20° and 80°E, matching well with the region of AWB. For example, on 22 November, a pair of southwest–northeast-oriented LSTR and LSTT were identified over the above-mentioned region within the meridionally overturned parts of contours and their locations and extents are consistent with the region of AWB. On 15 November, CWBs (green crosshatches) occurred over the northwest and northeast Atlantic, in good correspondence with the identified troughs and ridges. From 6 December 2011 to 10 January 2012, the Northern Hemisphere circulation mainly shows a wavelike structure and thus the observed meridionally overturned parts of Z500 contours have a smaller zonal extent. During this period, the identified ridges and troughs for the meridionally overturned parts of contours are consistent as well with the local wave-breaking features (AWB or CWB).
During 17–24 January [top two rows of Fig. 7 (continued)], an impressive CWB event occurred over northwestern Pacific, in conjunction with the southeastward intrusion of the cutoff low of polar origins. The identified trough and ridge axes over this region consistently clamped the region of CWB. The ridge and trough axes identified over Europe also correspond well with the regional AWB.
From late January to mid-February, most strikingly, the circulation over the mid- and high-latitude Eurasia primarily showed the wave-breaking feature in a large zonal extent. During this period, the Eurasian countries experienced a strong and persistent cold wave (WMO 2012; Vries et al. 2013). On 31 January, to the east of 60°E, the two transverse ridges identified over northern Asia and the troughs to the south are consistent well with the region of AWB. Over the northern part of Europe, there was a lack of ridge identification, whereas an AWB occurred over the region. As discussed in section 3a, this is primarily due to the sparse Z500 contours, within which the methodology fails to identify the ridge or trough axis. From 7 to 14 February, AWBs occurred locally over several parts of the Eurasian continent, matching as well with the identified LSTRs and LSTTs. Besides, the CWBs detected over the northeastern Pacific during this period also correspond well with the identified ridges and troughs.
From late February to mid-March, the Northern Hemisphere circulation was recovered from the striking wave-breaking state in the earlier period. Thus, the observed meridionally overturned parts of Z500 contours showed a smaller zonal extent. During this period, the identified ridges and troughs for the meridionally overturned parts of contours were also in good correspondence with the local wave-breaking features.
It should be pointed out that the detection of wave breaking is based on the PV fields and the latter have a finer scale than the Z500 fields (Hoskins et al. 1985), though the low-pass-filtered wind and temperature fields were used in the PV calculation. It means that, on a finer scale, the identified LSTRs and LSTTs in the present study are not necessarily in good agreement with the detailed features of wave breaking.
With a careful examination, the wave-breaking features in daily fields throughout the winter of 2011/12 reveal that the methodology performs remarkably well in identifying LSTRs and LSTTs for the meridionally overturned parts of the Z500 contours.
4. Application of the methodology to an East Asian cold event
In this section, we apply the objective identification method of large-scale tilted ridge and trough axes, as described in section 2, to a well-known East Asian cold event, to examine how the identified LSTR and LSTT are associated with persistent large-scale cold air activity. The case chosen here is the East Asian cold event that occurred during the period from 26 December 1954 to 17 January 1955. It was a long-lived cold spell that happened under the background of the protracted La Niña event of 1954–56 (Lau et al. 2006; Peng and Bueh 2011; Bueh et al. 2011a). In addition, this cold event was characterized by persistent and frequent LSTRs and LSTTs with a large zonal extent (Bueh et al. 2011a). The main purpose in this section is to highlight the importance of the identification method for the detection of the key circulation system typical of the extensive and persistent cold temperature event.
Figure 9 shows the evolution at 3-day intervals of the Z500 fields (contours) and the identified LSTRs (red lines) and LSTTs (blue lines) during 26 December 1954 to 17 January 1955. Throughout the cold event, LSTRs and LSTTs with a zonal extent larger than 30° longitude appeared persistently and frequently over the Eurasian continent and they are successfully identified by our objective identification method.
(a)–(j) The identified ridge and trough axes in the Z500 fields for the persistent East Asian cold event during 26 Dec 1954 to 17 Jan 1955. Day −3 in (a) stands for 23 Dec 1954 and day 0 in (b) the onset day of this cold event. Red and blue lines indicate the ridge and trough axes, respectively. Contours are drawn every 80 m. The ridge and trough axes with horizontal length less than 1000 km are eliminated.
Citation: Monthly Weather Review 143, 12; 10.1175/MWR-D-14-00238.1
A natural question that immediately follows is how these LSTRs and LSTTs are associated with the Siberian high (SH) and thus the extensive and persistent cold event over East Asia. Synoptically, influences of the LSTR and LSTT on the SH can be understood from the perspective of the well-known quasigeostrophic height tendency equation [see Eq. (8.4) of Martin (2006)]. According to the equation, amplification of the near-surface anticyclone results from cold temperature advection that increases with height from the surface to midtroposphere. The presence of the midtropospheric LSTR and LSTT couplet over the Eurasian continent would be consistent with a large extent of cold air advection aloft, as the near-surface temperature advection over the Siberian region, as a whole, is rather weak, thus causing an extensive amplification of the SH. On the intraseasonal time scale, Takaya and Nakamura (2005) investigated the amplification process of the SH using the PV inversion technique. They revealed that in the upper troposphere a blocking ridge over western and central Siberia and its accompanying trough to the southeastern side, as components of a quasi-stationary Rossby wave train propagating across the Eurasian continent, act to reinforce the SH [see Figs. 3 and 7 in Takaya and Nakamura (2005)]. Therefore, it suggests that the LSTR and LSTT couplet, as the large-scale example of the upper-tropospheric pattern associated with the strengthening of the SH, could possibly cause the extensive amplification of SH. This hypothesis was further supported by the observational evidences in Bueh et al. [(2011a), see their Figs. 1 and 3].
We now examine the LSTRs and LSTTs in the daily fields and their association with the cold temperature anomalies over East Asia during the cold case of 1954/55. The surface air temperature (SAT) anomaly fields for the case are displayed in Fig. 10, with the 1030-hPa isobar of sea level pressure superimposed (bold black contour). For brevity, day 0 refers to the starting day of the cold event and day N (−N) stands for the day that is N days after (before) day 0. At day −3, a southwest–northeast-oriented LSTR (zonal extent of ~80° longitude) straddled the regions from middle to northeast Asia and a zonally oriented trough was formed to its south (Fig. 9a). The latter was connected with a cold low over the northwestern Pacific, as indicated by the closed contours. At the same time, the SH was amplified and expanded, as indicated by the isobar of 1030 hPa (Fig. 10a), leading to extensive cold air accumulation around central Siberia. With the initiation of the cold event at day 0, the LSTR apparently extended northeastward to the northern side of the Bering Strait, spanning a zonal extent of ~140° (Fig. 9b). The cold SAT anomalies were strengthened over eastern Siberia and the Bering Strait, as the isobar of 1030 hPa still covered the region around the Bering Strait (Fig. 10b). Over northern China, both the isobar of 1030 hPa and the isoline of −4°C slightly moved southward. The above analysis suggests that the SH amplification is closely associated with the accompanying LSTR and LSTT. From days 3–6, the LSTR was slightly shortened, but still maintained its zonal extent of ~90° longitude (Figs. 9c,d). Correspondingly, the northeastern portion of the SH shrank westward (Figs. 10c,d). From days 9–12, the zonal extent of the tilted ridge was further shortened (Figs. 9e,f), matching well with the shrinking of the horizontal extents of both the SH and cold SAT anomalies over East Asia (Figs. 10e,f). During this stage, the tilted ridge slightly moved southward and the corresponding isobar of 1030 hPa and the contour of −4°C also approached the southeastern coast of China. So, it is shown that the horizontal extent of SH has a close association with the zonal extents of the accompanying LSTR and LSTT. Up to day 12, the LSTR and LSTT showed an AWB feature. During days 15–18 (Figs. 9g,h), such an AWB feature was considerably weakened and cold SAT anomalies mainly dominated over eastern China (Figs. 10g,h). The broader extent of the ridge corresponded well to the large extent of warm SAT anomalies over northern Asia (Figs. 10g,h). At day 21, the AWB appeared again between the LSTR and LSTT, and the related cold SAT anomalies were strengthened over the region of northeastern China and Yakutsk (Figs. 9i and 10i). With the prevalence of warm SAT anomalies over central Siberia at day 24, the cold surface temperature anomalies associated with the LSTR were basically restricted over southern China (Fig. 10j).
As in Fig. 9, but for the SAT anomalies, with a superimposition of 1030-hPa isobars of sea level pressure (thick solid lines). Isolines for SAT anomalies are drawn every 2°C and thick dashed lines represent isolines of −4°C. The gray shading denotes topography higher than 3000 m.
Citation: Monthly Weather Review 143, 12; 10.1175/MWR-D-14-00238.1
As shown in Figs. 9 and 10, it is clear that the identified LSTR and LSTT, on the whole, have a close association with the extensive amplification of the SH, as suggested by Takaya and Nakamura (2005). The LSTR and LSTT couplet corresponds considerably well with the large extent of the “cold south–warm north” SAT anomaly pattern over the Eurasian continent, which was recognized as the key characteristic of extensive and persistent low temperature events over East Asia (Tao and Wei 2008; Wen et al. 2009; Zhou et al. 2009; Bueh et al. 2011b).
Since blocking is closely associated with the cold air accumulation over central Siberia and its following outburst toward East Asia (Yihui 1990; Takaya and Nakamura 2005), here we examine its daily evolution during this event (Fig. 11). Two kinds of blocking indices were applied in the present study. The first was the well-known Tibaldi and Molteni (1990) index (referred to as the TM90 index), which was based on the fact that blocking occurs where the meridional Z500 gradients are locally reversed around 50°N. The second was developed by Pelly and Hoskins (2003) from the perspective of wave breaking (referred to as the PH03 index) and blocking was defined as where the meridional potential temperature gradient on the potential vorticity surface near tropopause (e.g., 2-PVU surface) is locally reversed. In fact, Rossby wave breaking (or blocking) measured by the PH03 index also reflected the local reversal of the meridional PV gradient, which was presented in section 3b of the present study (see also Fig. 8). For more details of these two blocking indices, readers are referred to Tibaldi and Molteni (1990) and Pelly and Hoskins (2003). From days −3 to 6, the blocking circulations over the Ural Mountains, central and eastern Siberia were well represented by the both TM90 and PH03 indices, consistent with the meridionally overturned contours of the Z500 fields (Figs. 9a–d). The blocking circulation over eastern Siberia from days 17 to 24 was also indicated by the PH03 index, whereas it was absent in the TM90 index configuration. For this cold event that persisted for 23 days, however, the whole story of the cold SAT anomalies could not be explained completely by regional blocking over northern Asia, although the latter corresponded well with the cold anomalies for some time slices of the whole event. For example, during days 9–17, blocking was absent according to the two indices (Figs. 11a,b). Instead, the cold SAT anomalies during this event were accompanied by the LSTR and LSTT (Fig. 9). This suggests that, for the extensive and persistent cold event over East Asia, the LSTR and LSTT axes might be considered as appropriate detection diagnostics.
Daily evolutions of the blocking indices for the persistent East Asian cold event during 26 Dec 1954 to 17 Jan 1955, as represented by (a) TM90 index (m degree latitude−1), and (b) PH03 index (K). Days 0 and 22 refer to 26 Dec 1954 and 17 Jan 1955, respectively, and they are indicated by the dashed lines.
Citation: Monthly Weather Review 143, 12; 10.1175/MWR-D-14-00238.1
5. Summary and discussion
a. Summary
We presented an objective identification method for the LSTR and LSTT axes in the midtropospheric circulation over mid- and high-latitude regions that has not previously been applied in either research or operational areas. The algorithm classified the contours into the circumpolar wavy contour, circumpolar contour containing the meridionally overturned part (wave breaking part), and the locally closed contour. Then, the ridge and trough points were defined on the three kinds of contour of the Z500 field and subsequently the ridge or trough axes were identified after connecting successively the nearest ridge or trough points of the neighboring contours under the minimum distance constraint.
The reliability of the objective identification method was evaluated in terms of its hit rate, false-alarm ratio, and threat score, based on the numbers of the “observed” and identified LSTRs and LSTTs in the daily Z500 fields during 15 November 2011 to 15 March 2012. The hit rate, false-alarm ratio, and threat score of the identification method are 0.97 (0.96), 0.05 (0.04), and 0.92 (0.93), respectively, for the ridge (trough) axes with the horizontal length larger than 1000 km. These scores reveal that the identification method performs well with a reasonably good skill. In addition, during the same period, we examined the wave-breaking features in daily fields and found that the method performs remarkably well in identifying LSTRs and LSTTs for the meridionally overturned parts (or wave-breaking parts) of the Z500 contours.
We also applied this objective technique to a well-known East Asian cold event and examined how the identified the LSTRs and LSTTs are associated with the large extent of cold air activity. The observed LSTRs and LSTTs during this event were successfully depicted by the identification method. It is shown that the LSTR and LSTT axes might be appropriate diagnostics for the detection of the key circulation system typical of the extensive and persistent low temperature event over East Asia. Yet, it should be pointed out that as the LSTR and LSTT have been recognized as the key circulation feature common among the extensive and persistent cold events over East Asia (Bueh et al. 2011a; Peng and Bueh 2011), it is still unknown whether they also work in the cases without cold air outbreak. This issue remains for future work.
b. Discussion
The CVA method was verified as effective in identifying the ridge and trough axes in the synoptic fields (Berry et al. 2007). To examine its performances in the instantaneous and the low-frequency fields, we show the identified troughs and ridges with the CVA method (see http://www.atmos.albany.edu/student/gareth/source.htm) in Figs. 12a and 12b. Figure 12a just corresponds to the instantaneous Z500 field, but Fig. 12b is created on the basis of the low-pass-filtered (with an 8-day cutoff) Z500 field, which could be directly compared with the corresponding result of the current study (Fig. 12c). As shown in Fig. 12a, the CVA method works almost perfectly in the instantaneous Z500 field (1200 UTC 15 January 1955), with a reasonable identification of the troughs and ridges everywhere. In the low-pass-filtered daily mean field (15 January 1955), as shown in Fig. 12b, the CVA method still works well where the troughs and ridges were not heavily tilted. However, the CVA method is somewhat inadequate to identify the heavily tilted ridges and troughs in the time mean (or low frequency) field, whereas they (LSTRs and LSTTs) are the primary concern in the present study. For example, the observed tilted ridge extending from middle Asia to far eastern Russia in Fig. 12b was not well identified with the CVA method. One may infer from Figs. 12a,b that the smaller-scale depiction of the curvature vorticity advection term in this method is an advantage in the instantaneous Z500 field, but it might be a disadvantage for portraying the LSTR and LSTT in the low-frequency field. It also reflects that the time mean Z500 field and its corresponding vorticity field show different horizontal-scale features. In comparison, the LSTRs and LSTTs were reasonably well illustrated by our identification method, as shown in Fig. 12c. It seems that, to depict ridge and trough axes, the curving of the Z500 contour itself is adequate for the large-scale circulation structure in the low-frequency field.
The identified ridge and trough axes with the CVA method, (a) in the instantaneous Z500 field at 1200 UTC 15 Jan 1955 and (b) in the low-pass-filtered daily mean field on 15 Jan 1955. (c) As in (b), but with the identification method in the present study. The ridge and trough axes are plotted as red and blue axes, respectively. The color shading indicates the values of curvature vorticity (10−5 s−1). The identified ridge and trough axes are eliminated in (a) where the curvature vorticity is more than −0.5 × 10−5 s−1 and less than 0.5 × 10−5 s−1 and in (b) where the curvature vorticity is more than −0.2 × 10−5 s−1 and less than 0.2 × 10−5 s−1. The ridge and trough axes with horizontal lengths less than 1000 km are eliminated in (c). The horizontal resolution of the data is 2.5° × 2.5° in (a) and (c), but 5° × 5° in (b).
Citation: Monthly Weather Review 143, 12; 10.1175/MWR-D-14-00238.1
In section 4, we have discussed the possible connection between the LSTR–LSTT couplet and the SH. Generally speaking, the zonal extents of the LSTR and LSTT axes have a good reference meaning to the horizontal extent of the SH. However, the amplitude and wavelength of the ridge–trough couplet are also closely related to the dynamical forcing for the SH, whereas our algorithm for detecting the LSTR and LSTT does not account for their amplitude and wavelength. In addition, a preconditioned surface cold anomaly over western Siberia is also very important for the intraseasonal amplification of the SH (Takaya and Nakamura 2005). Therefore, how the LSTR and LSTT, with other prerequisites, affect the SH deserves future investigation. We hope to address this issue in future work.
It has long been known that large-scale tilted (or transverse) ridge and trough systems are of essential importance to extreme climate and weather events (Section of Synoptic and Dynamic Meteorology, Institute of Geophysics and Meteorology, Academia Sinica 1958; Chang and Lau 1980; Uccellini et al. 1985; Bradbury et al. 2002; Peng and Bueh 2012; among others). However, the dynamics involved have not yet been understood in depth or systematically researched. One reason for this is possibly the lack of a statistical basis built on an objective identification method for large-scale tilted (or transverse) ridge and trough systems. Therefore, the identification method in the present study is expected to provide support for the further dynamical investigation of these large-scale systems.
Acknowledgments
We sincerely thank Prof. Tim D. Hewson and Dr. Ron McTaggart-Cowan, for giving us sound criticism and detailed comments that have led to the improvement of our paper. We also thank two anonymous reviewers for their valuable comments. We also convey our sincere thanks to Mr. Andrew Shearer for helping us to polish this paper. This work was jointly supported by the National Natural Science Foundation of China (Grant 41375064) and the National Key Technologies R&D Program of China (Grant 2015BAC03B03). The figures in this study are plotted using NCARG Command Language (UCAR/NCAR/CISL/VETS 2012).
APPENDIX
Low-Pass Filter
Frequency response of Lanczos low-pass filter. The frequency response at 0.125 day−1 (8 days) rises from zero to one.
Citation: Monthly Weather Review 143, 12; 10.1175/MWR-D-14-00238.1
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