2. Shenyang Regional Climate Center, Shenyang 110166;
3. Jilin Meteorological Observatory, Changchun 130062;
4. Shenyang Agricultural University, Shenyang 110866
Blockings are a characteristic type of meridional circulation, whose establishment, collapse, and retreat are often accompanied by abrupt changes in large or even hemispheric-scale circulation systems. At the same time, they can cause weather and climate anomalies in surrounding and nearby areas, and sometimes even severely disastrous weather (Zhu et al., 2007). Blockings possess noticeable features in time, space, and geographical distribution, and are closely correlated with the occurrence of the Meiyu season in China, summer drought in North China, and floods, cold-air outbreaks, and other disasters (Zhao and Chen, 1990; Li et al., 2012). Scientists at home and abroad have long been aware of the importance of blockings in Asia, and have thus carried out a series of studies on these systems to further our knowledge on them. Early research in this regard was mainly in the form of statistical analyses, while more recent work has focused on the underlying mechanisms involved and their numerical simulation (Elliot et al., 1949; Rex, 1950; Sumner, 1954; Tang, 1957; Ye et al., 1962; Shi et al., 1984; Shi, 1989, 2002; Xu and Zhao, 1994; Liu et al., 1995; Gao et al., 1998; Li, 2004; Duan, 2012; Liu et al., 2012).
Statistical analysis has revealed that, geographically, blocking high activities that significantly affect summer weather in China primarily concentrate in the areas of the Sea of Okhotsk, the Ural Mountains, and Lake Baikal (Tang, 1957; Bi and Ding, 1992). In terms of occurrence frequency, the annual mean blocking frequency over the Ural Mountains area is the highest, while in summer, the Okhotsk blocking happens most frequently (Li et al., 2012). Regarding the temporal distribution of blockings in Eurasia, the blocking activity in summer lasts longest on average; and meanwhile, spring and summer blockings are most frequently seen, and the number of autumn blockings is the smallest (Huang and Jiang, 2002). In June, blocking activities occur mostly in the Ural Mountains and the Sea of Okhotsk region, mainly in the form of double blockings (Li and Ding, 2004). Alongside the above, many scientists have also become interested in researching the formation mechanisms of blocking highs. As early as in 1949, Ye Duzheng, from the viewpoint of energy dispersion, investigated the forming and fading mechanism of blocking highs for the first time. Following his pioneering work, other scholars launched a series of studies, establishing many kinds of theories, such as multi-equilibrium theory, soliton and dipole theory, synoptic-scale disturbance excitation, and so on. Accordingly, progress regarding the formation and maintenance of blocking situations quickly reached a very high level, thus laying solid foundations for later studies (Rossby, 1950; Charney and DeVore, 1979; Zhu and Zhu, 1982; Miao, 1984; Shutts, 1986; Holopainen and Fortelius, 1987; Bi and Ding, 1992; Wu et al., 1994).
As research in this field moved forward, scientists found that, in the case of a single blocking high, the Ural Mountains–Sea of Okhotsk blocking high performs a very important function in the evolution of atmospheric circulations in the eastern part of China, as well as the entire northeast of Asia. As one of the major meteorological disasters in China, floods are mainly affected by the blocking-high activities over the Ural Mountains and Sea of Okhotsk (Li et al., 2012). Besides, the occurrence of low temperature in summer and cold vortex activities in Northeast China have been found to be closely related to the Okhotsk blocking high (Wang, 2002; Wang et al., 2005; Liang et al., 2009; Liu et al., 2012). Similarly, when double blockings appear over the Asian region, their impact on the weather and climate in China is particularly significant. Shi (2007) pointed out that, in years when the number of days of double blockings in June and July is obviously greater in the Eurasian continent, the quantity of precipitation over the Yangtze River valley is also much more than normal. Yao et al. (2005), through analyzing rainstorms in the Huaihe River basin during June–July 2003, found that the establishment of high-latitude double-blocking type highs, which are strong and stable, is beneficial to maintaining a relatively stable state of the subtropical high, creating persistent and abnormal rainfall over the Huaihe River basin, and thus strengthening the westward extension of the subtropical high. Wang (2010) analyzed the anomalous features of the western Pacific subtropical high during periods of torrential rain in the Huaihe River basin along with abnormally high temperature across the Jiangnan region (the area south of the Yangtze River) and South China during June–July 2007, and found that the establishment of mid–high latitude double-blocking type highs helps the strengthening and development of the western Pacific subtropical high. Yu and Lin (2006) revealed that, when there are blocking highs over Europe and the Sea of Okhotsk, that is, the double-blocking type high in the Eurasian mid–high latitudes, the Huaihe River basin will receive flood-inducing rainfall and the precipitation amount over the whole of China will be greater than usual. However, if only the Okhotsk blocking high, that is, an "East Blocking" type high, exists, the Yangtze River valley and Huaihe River basin will suffer from flood-inducing rainfall while North and South China will experience less rainfall than usual. Gong et al. (2004) studied the circulation characteristics in mid–high latitudes in the summer of 2003 and the pentad variation of a blocking-high index over the three regions of the Ural Mountains, the Sea of Okhotsk, and Lake Baikal, and found that, from late June to early July, mid–high latitudes in East Asia are under the control of a "double-blocking" situation, which causes sustained (for more than a month) and concentrated heavy rainfall over the Huaihe River basin. Therefore, we can see that the configuration of flow regimes, in various local domains in Asia, not only have an important influence on Northeast Asia (Shen et al., 2011), but even on the anomalies of summer weather and climate over the whole of China (Ding, 2005).
To date, most investigations on the double-blocking high system have been separate in their approach, with few taking a more holistic perspective on the basis of case studies. Based on previous research results, we began by analyzing the blocking-high activities over the Ural Mountains (named "Urals" for short) and the Sea of Okhotsk (named "Okhotsk" for short) and, under the coexistence of the two blocking highs, investigated the evolution and climatological changes of the Urals–Okhotsk double-blocking type activities. In this work, we regarded the double-blocking disturbance as a quasi-stationary process. Through case analyses, we explored the atmospheric energy dispersion and propagation characteristics of quasi-stationary planetary waves in the double-blocking situation, so as to provide a reference for further revealing the effect of Urals–Okhotsk double-blocking activities in the atmospheric circulation of the Eurasian mid–high latitudes.2 Data and methods
The data used in this study were the NCEP–NCAR 2.5° × 2.5° daily geopotential height, wind, and temperature data for the Northern Hemisphere during 1948–2009 (Kalnay et al., 1996). Based on the definition of blocking-high synoptic meteorology and its existence condition given by Rex (1950) and Zhu et al. (2007), the criteria for mid–high-latitude blocking high processes in the Northern Hemisphere are that: a high pressure center exists in the 500-hPa geopotential height field; the daily moving range of the pressure center is no more than 12.5 degrees of longitude; and the high pressure center maintains for at least 3 days. According to these criteria, the Urals–Okhotsk double-blocking high process over 45°–75°N, 40°–160°E during June–August of 1948–2009 was retrieved and analyzed.
Relying on the classification of the Eurasian blocking high established by Liu et al. (2012), we objectively identified the Urals–Okhotsk double-blocking high process, obtaining its formation, development, and dissipation processes, and determining its onset, end, and maintenance times, the cumulative number of days, the frequency, and the distribution of the double-blocking activities through statistical analysis. Characteristics of the meteorological variables of the double-blocking configuration were analyzed synthetically. In addition, two typical double-blocking cases were selected (one that maintained for 8 days from 29 June to 6 July 1964, and the other that lasted for 11 days from 14 to 24 July 2008), and the wave activity flux diagnosis method (Plumb, 1985) was used to analyze the dynamic characteristics of the establishment and maintenance of the double-blocking high.3 Distributional features of the Urals–Okhotsk double blockings
The number of days and frequency of the Urals–Okhotsk regional blocking high, from June to August 1948–2009, were statistically analyzed and the Mann–Kendall (MK) method was used to test the interannual variation of the number of days and occurrence frequency of the double blockings, separately (Fig. 1). As can be seen from Fig. 1a, the average number of days of double blockings is characterized by a remarkable abrupt change around 1977. Before 1977, the average number of days of double-blocking activities was 16.7 days, whereas after 1977, the number was only 5.8. To test the stability of the abrupt change, we used the STARS (Sequential T-test Analysis of Regime Shift method (Rodionov, 2004, 2006). According to the different cut lengths, the regime shift index (RSI) values under different parameters were calculated, and the finding was that the abrupt change signal of the double-blocking process in 1977 is credible. Figure 1b shows the results of the MK variation test for the interannual change in the frequency of the double-blocking configuration, revealing that the occurrence frequency of the double-blocking activity also experienced an abrupt change around 1977.
Based on the timing of the change in double-blocking activity, the difference between the blocking high intensity (maximum value in the 500-hPa height field) and the mean latitude and longitude positions before and after the abrupt change was analyzed (Fig. 2). Based on the variation of the blocking high intensity (Figs. 2a, d), the intensity of the Urals (Okhotsk) blocking high strengthened significantly from an average of 5796.6 gpm (5749.2 gpm) before the abrupt change, to 5817.6 gpm (5774.9 gpm) after the change. The changes in the longitude and latitude locations, where the blocking-high activities occurred (Figs. 2b, e, c, f), illustrate unobvious variation. The Urals blocking shifted eastward slightly after the abrupt change, while the Okhotsk blocking moved westward. The longitude position of the Urals (Okhotsk) blocking activity was on average 56.3°E (117.9°E) before the change, but moved eastward to 57.2°E (westward to 111.7°E) after the change. Thus, through analyzing the evolutionary characteristics of the Urals–Okhotsk double-blocking activity, we have discovered the timing of an abrupt change (i.e., around 1977) and the differences of the double-blocking high before and after that change. Next, taking the year when the abrupt change occurred as the boundary, we used composite analysis to investigate the climatological characteristics and differences of the double-blocking highs before and after the abrupt change.3.1 Distribution features of height and wind fields
Figure 3 shows the composite distribution of the 500-hPa height and wind fields of the Urals–Okhotsk double-blocking activities before and after the abrupt change. Before the abrupt change (Fig. 3a), the area from the Caspian Sea to the Ural Mountains and Sea of Okhotsk had two strong high-pressure ridges, with the 5700-gpm contour extending to the north near 65°N and an anticyclonic center near the Sea of Okhotsk. Thus, at 500 hPa, the blocking-high situation over the Ural Mountains was more remarkable than the Okhotsk high. After the abrupt change (Fig. 3b), the peripheral 5700-gpm contour of the Urals–Okhotsk blocking moved near to 70°N, showing that the Urals–Okhotsk high's ridge extended northward to higher latitudes. The Urals blocking high became more intensive, with the 5760-gpm contour of the center intensity extending to 50°N and the area further north, and the center longitude moving near 60°E. Meanwhile, the 5700-gpm contour of the Okhotsk blocking extended obviously to the northeast, and the anticyclone center became stronger. The composite height difference distribution (Fig. 3c) shows the area north of the Caspian Sea and Lake Balkhash, the northern section of the Sea of Okhotsk and the eastern Siberia region as the maximum negative-value areas of difference, at –40 and –60 gpm, respectively, both passing the 0.05 significance test. Therefore, it can be seen that after the abrupt change, the double-blocking activities reduced significantly in terms of the number of days, but the blocking high in the double-blocking activity area was considerably stronger.3.2 Distribution features of the 325-K isentropic potential vorticity
To further investigate the atmospheric circulation features of the double-blocking activities before and after the abrupt change, we analyzed the variational features of the isentropic potential vorticity (IPV) during the Urals–Okhotsk double-blocking events. In the Northern Hemisphere, the value of the isentropic surface is usually 315 K in winter and 325 K in summer. On this basis, we present the 325-K isentropic surface vorticity and wind field features on the double-blocking days from June to August before and after the abrupt change (Fig. 3). Before the change (Fig. 3d), the 1-PVU contour over the Ural Mountains lay near 55°N and the 1.5-PVU contour near 60°N; whereas, in the Sea of Okhotsk area, the 1.5-PVU contour was located near 65°N. Furthermore, all contours were saddle-backing toward high latitudes, showing characteristics of minimum potential vorticity, and the wind field was in the clockwise direction, strengthening the development of the anticyclonic circulation in this region.
After the abrupt change (Fig. 3e), the 1-PVU isoline near the Ural Mountains retreated southward to the vicinity of 53°N, and the 1.5-PVU isoline near 60°N. In comparison, the 1.5-PVU contour over the Sea of Okhotsk area shifted little. The distribution of the difference values (Fig. 3f) shows large areas of the Caspian Sea–Balkhash Lake and its northern region as having a positive center of difference, having passed the 0.05 significance test. This indicates that the potential vorticity in this region diminished significantly after the abrupt change while the anticyclonic circulation strengthened, contributing to the maintenance and intensification of the Ural Mountains blocking high. In the region of the Sea of Okhotsk and to the north, the 325-K IPV difference also shows a positive distribution.3.3 Dynamic diagnosis
Figure 4 displays the 500-hPa wave activity flux and its divergence distribution before and after the abrupt change of the double blockings over the Ural Mountains and the Sea of Okhotsk region. In general, wave energy propagated from upstream to downstream, and the characteristics of energy transfer from lower layers to middle layers were consistent, which was favorable for the maintenance of the blocking situation. Specifically, the wave energy coming from the upstream and lower level (Figs. 4a, b) converged in the western section (40°–55°N, 120°–140°E) and diverged in the eastern section (55°–70°N, 140°–155°E) of the two blocking highs before the abrupt change. Thus, the wave energy had consistent convergence features from low to high levels, which was conducive to the maintenance of deep systems. However, after the abrupt change, wave energy convergence area in the upstream of the Urals blocking high was reduced, causing the convergence center to shift westward, and making the divergence area in the downstream enlarged and the divergence intensity weakened, and then the divergence expanded northward (Fig. 4c). As seen from Fig. 4d, the upstream convergence of the Urals blocking high strengthened obviously, with its area expanding northward and the downstream divergence being more significant; the convergence of wave energy in the western part of the Okhotsk blocking high was significantly strengthened, and meanwhile the divergence in its eastern part also strengthened (Figs. 4c, d). Thus, the configuration of wave energy propagation after the abrupt change helped enhance the intensity of the double-blocking high and its extension northward. The major difference in energy propagation before and after the abrupt change lies in the fact that the path of energy dispersion from the Ural Mountains to the Sea of Okhotsk turned from the mid–low latitudes to high latitudes, which is of great significance in our quest to understand the dynamical mechanisms involved in the double-blocking high activities before and after the abrupt change.
To highlight the situational characteristics of the Urals–Okhotsk double blockings, we further analyzed the composite distributional characteristics of the 500-hPa height and wind fields, and the 325-K IPV and wind field in the case of no double-blocking activities, based on atmospheric circulations with/without double blockings, before and after the abrupt change (Fig. 5). It is apparent that obviously large-or low-value centers do not exist in the composite height field (IPV) on the days without double blockings, regardless of the occurrence of abrupt changes, and all the isolines are distributed evenly in the latitudinal direction, except that the Sea of Okhotsk height field extends slightly to the north after the abrupt change. Meanwhile, the wind field essentially matches with the height field, whose zonal degrees are bigger but meridional degrees are smaller. Figure 6 shows the composited distributions of the 500-hPa wave activity flux and its divergence in the process without double blockings before and after the abrupt change. From Fig. 6, we can see that, before and after the abrupt change, wave energy spread from high to low latitudes, and also from low to high levels. In addition, the convergence and divergence intensity of the wave energy were weaker in comparison.4 Case analysis of the Urals–Okhotsk double blockings 4.1 Height and wind fields before the abrupt change
Figure 7 reveals the characteristics of meteorological variables at 500 hPa during the Urals–Okhotsk double blockings from 29 June to 6 July 1964. As can be seen from Fig. 7, the double blockings were established on 29 June (Fig. 7a). Affected by the high-latitude low-value system in the upstream of the Ural Mountains, a weak anticyclonic center appeared in the north of the Ural Mountains (60°–70°N, 50°–70°E). The 5680-gpm contour extended northward to 70°N and a strong high ridge formed in the area southwest of the contour and over western Black Sea. The 5760-gpm contour extended close to 55°N. The area north of the Sea of Okhotsk featured a closed anticyclonic circulation, with the Chersky Mountains at the center, and in its southwest part, the Northeast Asian coast had a strong high ridge, with its 5760-gpm ridge line going northward to 55°N. To the west of the Da Hinggan Mountains, China, there was a strong cyclonic center (Northeast China cold vortex). On 2 July (Fig. 7b), due to the eastward movement of the low-value system in the upstream of the Ural Mountains, the Ural anticyclone moved further eastward. Affected by the terrain, the Urals blocking high attained a quasi-stationary state, causing the area and central intensity of the anticyclone to increase continuously. As a result, the central intensity exceeded 5760 gpm and a closed high-pressure center was born, making the Urals blocking high maintain and strengthen further. The sustained Urals blocking generated strong northerly winds on its leeward slope, providing energy for the development and maintenance of the downstream low-value system, which was stable and less dynamic. On the one hand, the southwest airflow in front of the trough powered the maintenance and development of the Okhotsk blocking high and, on the other hand, the existence of the Okhotsk blocking high made the low-value system maintain for a long time. On 6 July (Fig. 7c), the upstream low-value system gradually weakened and the Urals blocking high dispersed, causing the northerly winds on the leeward slope to disappear and the downstream low-value system to weaken and collapse. Besides, the southwest air flow in front of the trough faded, gradually disappearing, and along with it the Okhotsk blocking high weakened. Finally, the Urals–Okhotsk double blocking situation weakened and collapsed.4.2 Diagnostic analysis of wave activity flux in a case before the abrupt change
Through the preliminary analysis of the characteristics of meteorological variables, we obtained the evolutionary features of the atmospheric circulation during the process of double-blocking activity. Next, we further diagnosed and analyzed the energy propagation features of the double-blocking case via the wave activity flux method. Figure 8 shows the 500-hPa streamfunction field, wave activity flux, and divergence during the process of establishment, development, and weakening of the double-blocking case. On 29 June (Figs. 8a, b), the Urals–Sea of Okhotsk region was covered by a large-value zone of streamfunction, representing anticyclonic activity. The wave energy from the upstream low-value system of the Ural Mountains spread to the anticyclonic center of the mountains, causing convergence, whilst downstream of the Ural Mountains was a weak divergence zone. Such propagation characteristics of the wave energy were conducive to the maintenance and development of the Urals blocking. The upstream wave energy of the Ural Mountains was divided into two branches, one north and another south, by the Urals blocking, dispersing to the downstream. The energy was transmitted to low latitudes more obviously, spreading eastward along 50°–60°N, and finally dispersing downstream to the Sea of Okhotsk area via the Northeast China cold vortex. The wave propagation process provided kinetic energy not only for the development and maintenance of the Northeast China cold vortex but also, indirectly, for the downstream Okhotsk blocking activities. Over the Sea of Okhotsk region, however, it was wave energy convergence that kept the system developing further and maintaining. On 2 July (Figs. 8c, d), the double blocking stepped into the developing period, and the large-value streamfunction center in the Ural Mountains and Sea of Okhotsk increased further. As the upstream low-value system of the Ural Mountains deepened further and moved eastward, the wave propagation originating from the upstream became much stronger and broader, forming a ribbon of energy transmission along a route from the Caspian Sea to Volga River and Beloye More. This converged in the Urals blocking area and continued to propagate downstream along the blocking edge, so that the Urals blocking high developed further. The Northeast China cold vortex remained stable, and a southwest–northeast energy transmission belt was established in the fourth quadrant of the Northeast China cold vortex activities, enhancing the Okhotsk blocking situation. By 6 July (Figs. 8e, f), the low-value system in the Ural Mountains upstream was crippled, causing the energy source to die down. Although the Urals blocking high remained, the central energy became dispersed, and thus the wave energy propagating downstream was significantly weakened. As a result of the cut-off of the energy source, the Northeast China cold vortex was gradually undermined, and ultimately disappeared. The intensity of the Okhotsk blocking high then developed in a similar way and finally, the situation of the double-blocking high was seriously impacted until its collapse. Therefore, during the whole life cycle of double-blocking activity, the low-value system energy source in the upstream of the Urals blocking high, and its downstream dispersion, played crucial roles in the establishment and maintenance of the double-blocking high, while the existence of the Okhotsk high was affected by the low-level wave energy (Figs. 8b, d, f).4.3 Analysis of the height and wind fields in a case after the abrupt change
A double-blocking case after the abrupt change (14–24 July 2008) was selected for comparative analysis with the case before the abrupt change. Figure 9 shows the evolution of meteorological variables during the generation, development, and weakening of the double-blocking case after the change. When the double-blocking high was established on 14 July (Fig. 9a), there was a high pressure ridge over the northwest of the Caspian Sea, and the 5840-gpm contour extended northward, close to 60°N. In the vicinity of East Siberia–Sea of Okhotsk area, a closed high-pressure center was clearly present, whose 5840-gpm contour extended northward to 75°N, and thus a cyclonic center formed between the two highs. During the development stage of the double blocking on 19 July (Fig. 9b), the upstream low-value system shifted eastward and southward, pushing the Caspian Sea high pressure to move eastward and northward. Then, obstructed by terrain, the Urals blocking high was generated. The high center remained stable and less dynamic, but its central intensity increased gradually, causing the Urals blocking high to develop further, and a prominent closed high-pressure center to appear. At the same time, the downstream cold vortex system stepped eastward and southward, and together with the Okhotsk high, formed an east–west positive–negative distributional pattern. Such a situation was conducive to the stable maintenance of the cold vortex system, and the southerly airflow in the front of the cold vortex was helpful for the development and maintenance of the Okhotsk blocking high. The modulation effect of the two made the Okhotsk blocking move southward and the 5840-gpm contour to shift to 65°N, forming a high-pressure center. On 24 July (Fig. 9c), as the low-value system in the upstream of the Ural Mountains weakened, the Urals blocking high gradually weakened, retreating southward, and the 5840-gpm contour moved southward to around 60°N. Affected by this situation, the downstream vortex shifted northward, causing the dynamic conditions for the Okhotsk high to gradually weaken. The 5840-gpm contour moved southward to 55°N, the closed center of high pressure disappeared, and the double-blocking situation faded, before ultimately collapsing.4.4 Diagnostic analysis of wave activity flux in the case after the abrupt change
Figure 10 shows the 500-hPa streamfunction field, wave activity flux, and its divergence characteristics for the course of establishment, development, and weakening of the double-blocking case during 14–24 July 2008. The double-blocking high was established on 14 July (Figs. 10a, b), when high-value zones of streamfunction existed over the region north to the Black Sea in the Ural Mountains upstream, and in the eastern Siberia region north to the Okhotsk Sea. The high-value zones were characterized by anticyclonic circulation, which derived from the wave energy propagation of the low-value system in the Ural Mountains upstream, to the western section of the Ural Mountains anticyclone, producing convergence, and thus contributing to the formation and development of the downstream anticyclone. Wave energy divergence occurred in the southeast side of the streamfunction high-value zone, driving the downstream energy in the blocking system to transmit to the southeast. In fact, similar characteristics were found in the central Siberia depression and eastern Siberia high, but the divergence (convergence) varied in intensity. Over the Ural Mountains and the Sea of Okhotsk area, the wave energy divergence in the downstream was equivalent to the convergence intensity in the upstream, which was favorable for the maintenance of the system. Viewed from the wave energy propagation direction: 1) a Rossby wave train propagated from the Urals upstream to the area with Urals blocking-high activities, and further dispersed to the northeast of the Caspian Sea; and 2) the Rossby wave generated in the southeast side of the Ob River downstream was transmitted to the west side of the Siberia vortex, and then to the Okhotsk blocking area through the dispersion of this system. The corresponding longitude–height profile (Fig. 10b) shows that all of the three systems (two ridges and one trough) were very profound, the wave energy convergence (divergence) from low to high levels possessed consistent propagation characteristics, and the center was located near 300–500 hPa. Meanwhile, the wave energy from low levels spread to the blocking-high area of the Ural Mountains and the Sea of Okhotsk. On 19 July (Figs. 10c, d), impacted by the southeast movement of the Ural Mountains upstream low-value system, the wave energy spread to the Urals blocking area from the upstream of the Urals blocking high and generated a strong convergence, strengthening the Urals blocking high and lifting it northward. In its southeast side, divergence occurred, propagating eastward into the vortex on the northwest side of Lake Baikal, which kept the vortex developing stably and moving to the southeast. In the Urals blocking-high area, the intensity and range of wave energy convergence exceeded the divergence strength on the eastern side of the Urals blocking, which was good for the Urals blocking high to maintain and develop. On the west side of the Okhotsk blocking, there was still wave energy convergence, but the divergence on the east side was relatively weak. This helped to further the development of the Okhotsk blocking high. The corresponding longitude–height profile (Fig. 10d) was the same as the configuration of the system on 14 July (Fig. 10b), which means that the low-level wave energy spread to the double-blocking area of the Ural Mountains and the Sea of Okhotsk, and the wave energy divergence (convergence) center lay in the vicinity of 250–400 hPa. On 24 July (Figs. 10e, f), the low-value system in the upstream of the Ural Mountains weakened gradually, making the energy source for the Urals blocking high die away. The wave energy convergence in the west side of the Urals blocking weakened, while the divergence in the east side strengthened, which was unfavorable for the persistence of the Urals blocking high. The wave energy in the downstream low-value system mainly featured intense convergence, but the energy dispersion to the downstream of the Sea of Okhotsk region weakened gradually, which was not beneficial to the maintenance and development of the Okhotsk blocking. Thus, the Okhotsk blocking weakened and the double-blocking situation faded until its collapse.
The corresponding longitude–height profile (Fig. 10b) shows that all of the three systems (two ridges and one trough) were very profound, the wave energy convergence (divergence) from low to high levels possessed consistent propagation characteristics, and the center was located near 300–500 hPa. Meanwhile, the wave energy from low levels spread to the blocking-high area of the Ural Mountains and the Sea of Okhotsk. On 19 July (Figs. 10c, d), impacted by the southeast movement of the Ural Mountains upstream low-value system, the wave energy spread to the Urals blocking area from the upstream of the Urals blocking high and generated a strong convergence, strengthening the Urals blocking high and lifting it northward. In its southeast side, divergence occurred, propagating eastward into the vortex on the northwest side of Lake Baikal, which kept the vortex developing stably and moving to the southeast. In the Urals blocking-high area, the intensity and range of wave energy convergence exceeded the divergence strength on the eastern side of the Urals blocking, which was good for the Urals blocking high to maintain and develop. On the west side of the Okhotsk blocking, there was still wave energy convergence, but the divergence on the east side was relatively weak. This helped to further the development of the Okhotsk blocking high. The corresponding longitude–height profile (Fig. 10d) was the same as the configuration of the system on 14 July (Fig. 10b), which means that the low-level wave energy spread to the double-blocking area of the Ural Mountains and the Sea of Okhotsk, and the wave energy divergence (convergence) center lay in the vicinity of 250–400 hPa. On 24 July (Figs. 10e, f), the low-value system in the upstream of the Ural Mountains weakened gradually, making the energy source for the Urals blocking high die away. The wave energy convergence in the west side of the Urals blocking weakened, while the divergence in the east side strengthened, which was unfavorable for the persistence of the Urals blocking high. The wave energy in the downstream low-value system mainly featured intense convergence, but the energy dispersion to the downstream of the Sea of Okhotsk region weakened gradually, which was not beneficial to the maintenance and development of the Okhotsk blocking. Thus, the Okhotsk blocking weakened and the double-blocking situation faded until its collapse.5 Conclusions and discussion
By identifying the Eurasian blocking high-pressure system objectively and automatically, we statistically analyzed the climatological characteristics of the Ural Mountains–Sea of Okhotsk double-blocking processes over the summer months of 1948–2009, before and after an abrupt change in 1977, and then dynamically diagnosed the possible causes for such blocking flow activities. Our conclusions are as follows.
(1) The Urals–Okhotsk double-blocking activity experienced a significant abrupt change around 1977. After the abrupt change, the number of days of double-blocking activity was reduced greatly, but its intensity became much stronger; especially, the northern boundary of the Okhotsk blocking high extended northward noticeably. The zonal spacing between the Urals blocking high and the Okhotsk high became smaller, but the meridional positions of their activity ranges did not change much. The IPV analysis results before and after the abrupt change verified the above conclusion.
(2) The low-level wave energy from the upstream region of the Ural Mountains fueled the double-blocking activities, and then dispersed downstream via the Urals blocking, benefiting the formation and development of the downstream vortex. Meanwhile, the long-term maintenance of the Okhotsk blocking was also an important factor for the maintenance of the vortex. The energy propagation characteristics from the Ural Mountains to the Sea of Okhotsk altered from a mid–low-latitude spread before the abrupt change to a high-latitude spread after the change. On days without double-blocking highs, regardless of the abrupt change, all wave energies were conveyed from high to low latitudes and from low to high levels, and the intensity of convergence and divergence was also weak.
(3) Case analysis suggested that the Urals–Okhotsk double-blocking activity modulated the development and maintenance of the Northeast China cold vortex. The existence of a double-blocking high was conducive to the development and maintenance of the Northeast China cold vortex and in turn, the stable and sustained Northeast China cold vortex helped promote the maintenance of the Urals–Okhotsk blocking high pressure.
In short, an examination of the wave energy propagation can help to better explain the energy propagation characteristics of the establishment, development, and dissipation of Urals–Okhotsk blocking high situations. However, some problems remain, which need to be discussed further. For example, given the abrupt change of double-blocking activity around 1977, what might be its relationship to the known climate change event of 1976–77 (in association with the abrupt and obvious deepening of the Aleutian low)? Is it a temporal coincidence or an inherent mechanism? Does the double-blocking flow pattern reflect a climatic feature or a kind of low frequency variability? Actually, it is difficult to reveal the differences in double-blocking activity before and after the abrupt change through analysis of a single case only. In addition, how do the Urals blocking and Okhotsk blocking interact with one another? All these problems remain to be discussed in future research.
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