Large-Scale Circulation Features Typical of Wintertime Extensive and Persistent Low Temperature Events in China

ATMOSPHERIC AND OCEANIC SCIENCE LETTERS, 2011, VOL. 4, NO. 4, 235 241 Large-Scale Circulation Features Typical of Wintertime Extensive and Persistent Low Temperature Events in China BUEH Cholaw 1, 2, FU Xian-Yue 1, and XIE Zuo-Wei 1 1 International Center for Climate and Environment Sciences (ICCES), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China 2 State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China Received 22 April 2011; revised 27 May 2011; accepted 27 May 2011; published 16 July 2011 Abstract A pair of northeast-southwest-tilted midtropospheric ridges and troughs on the continental scale was observed to be the key circulation feature common among wintertime extensive and persistent low temperature events (EPLTE) in China. During the persistence of such anomalous circulations, the split flow over the inner Asian continent and the influent flow over the southeastern coast of China correspond well to the expanded and amplified Siberian high with tightened sea level pressure gradients and hence, a strong, cold advection over southeastern China. The western Pacific subtropical high tends to expand northward during the early stages of most EPLTEs. Keywords: extensive and persistent low temperature event, Siberian high, tilted ridge and trough Citation: Bueh, C., X.-Y. Fu, and Z.-W. Xie, 2011: Large-scale circulation features typical of wintertime extensive and persistent low temperature events in China, Atmos. Oceanic Sci. Lett., 4, 235 241. 1 Introduction Persistent low temperature events covering extensive regions of China have occurred frequently in the last several years and have aroused extensive public attention and scientific interest. For instance, during mid-late January 2008, southern China experienced persistent low temperatures and freezing rain (or snowstorms) at record levels not seen since 1950 (Tao and Wei, 2008). These events caused an enormous amount of damage to life and property in China. This long-lasting disastrous event has been extensively investigated by studying relevant typical features and related formation mechanisms (Tao and Wei, 2008; Wen et al., 2009; Zhou et al., 2009; Bueh et al., 2011). To gain deeper insight into past persistent events similar to the one in January 2008, Peng and Bueh (2011) put forward a definition for extensive and persistent low temperature events (EPLTEs). They have since documented 52 such events in China from 1951 to 2008 using the observed daily-temperatures from over 756 meteorological stations in China. Thus, their study offers a historical context in which common features of such events can be captured. This context in turn improves our under- Corresponding author: BUEH Cholaw, bueh@lasg.iap.ac.cn standing of the related dynamic processes involved in these persistent low temperature events. In examining the persistent low temperature event of January 2008, it has been noticed that the primary Asian trough in the mid-troposphere tends to be tilted with a northeast-southwest orientation, which constitutes a splitflow configuration over the Asian continent (Wen et al., 2009; Zhou et al., 2009; Bueh et al., 2011). However, the following two questions have not yet been answered. Is such a feature of tilted troughs common during persistent low temperature events? How is the tilted trough positioned relative to the upstream circulation anomalies? It is well known that the subtropical western Pacific high (WPSH) at 500 hpa expanded northward during the persistent low temperature event of January 2008. Wen et al. (2009) proposed that the corresponding La Niña event possibly contributed to this northward expansion, although He et al. (2008) pointed out that the La Niña event is not favorable for the northward expansion of the WPSH. Bueh et al. (2011) proposed that the Rossby wave propagation from the upstream regions was favorable for the northward expansion of the WPSH. However, the previous studies raised two additional questions. Were previous persistent low temperature events always accompanied by a northward expansion of the WPSH? How is the anomalous WPSH positioned relative to the primary Asian trough? This study focuses on the common features typical of EPLTEs and tries to answer the four questions posed above. 2 Data and methods The EPLTEs analyzed in this study were adopted from the work of Peng and Bueh (2011). Based on the daily temperature data from China, They identified 52 EPLTEs defined according to the following three criteria: (1) a station is defined as an extreme cold station (ECS) if the observed temperature is lower than its 10th percentile threshold; (2) an extensive low temperature event is present if the approximated area occupied by the ECS is more than 10% of the total area of China; and (3) an EPLTE is present if the extensive low temperature event lasts for at least eight days. Their identification has also been examined by the homogenized observational data in China archived by Li and Yan (2010). In this study, we chose 38 EPLTEs defined using an even stricter constraint

236 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 4 that the extensive low temperature event must last for at least 10 days (Table 1). The 10-day constraint was chosen for the reason that the EPLTEs in this study have been recognized as events on the extended-range time scale (10 30 days), and 10 days is simply the lower bound of this time scale. For reference, the starting day and the day when the area occupied by the ECS was largest in an EPLTE are called day 0 and day L, respectively. Table 1 Timing information for the 38 EPLTEs. The events are listed in chronological order. The persistent periods, duration (days), and dates of day L are listed in the second, third, and fourth columns, respectively. No. Period Duration Day L 1 3 March 1954 to 14 March 1954 12 5 March 1954 2 1 December 1954 to 16 December 1954 16 9 December 1954 3 26 December 1954 to 17 January 1955 23 6 January 1955 4 7 December 1956 to 25 December 1956 19 15 December 1956 5 5 February 1957 to 19 February 1957 15 11 February 1957 6 5 March 1957 to 16 March 1957 12 14 March 1957 7 17 December 1959 to 26 December 1959 10 25 December 1959 8 22 November 1960 to 1 December 1960 10 26 November 1960 9 20 November 1962 to 3 December 1962 14 28 November 1962 10 8 February 1964 to 27 February 1964 20 21 February 1964 11 20 December 1966 to 17 January 1967 29 27 December 1966 12 26 November 1967 to 15 December 1967 20 30 November 1967 13 30 January 1968 to 22 February 1968 24 7 February 1968 14 27 January 1969 to 7 February 1969 12 4 February 1969 15 13 February 1969 to 4 March 1969 20 21 February 1969 16 25 February 1970 to 25 March 1970 29 17 March 1970 17 21 November 1970 to 30 November 1970 10 29 November 1970 18 27 January 1971 to 7 February 1971 12 29 January 1971 19 27 February 1971 to 14 March 1971 16 7 March 1971 20 3 December 1974 to 21 December 1974 19 14 December 1974 21 7 December 1975 to 23 December 1975 17 12 December 1975 22 10 November 1976 to 27 November 1976 18 14 November 1976 23 25 December 1976 to 15 January 1977 22 28 December 1976 24 26 January 1977 to 10 February 1977 16 30 January 1977 25 9 February 1978 to 18 February 1978 10 15 February 1978 26 10 November 1979 to 29 November 1979 20 18 November 1979 27 29 January 1980 to 9 February 1980 12 5 February 1980 28 1 November 1981 to 10 November 1981 10 7 November 1981 29 19 January 1984 to 10 February 1984 23 6 February 1984 30 16 December 1984 to 30 December 1984 15 24 December 1984 31 4 March 1985 to 21 March 1985 18 9 March 1985 32 6 December 1985 to 17 December 1985 12 11 December 1985 33 26 November 1987 to 7 December 1987 12 30 November 1987 34 27 February 1988 to 8 March 1988 10 3 March 1988 35 16 March 1992 to 27 March 1992 12 17 March 1992 36 14 January 1993 to 24 January 1993 11 16 January 1993 37 24 January 2000 to 2 February 2000 10 31 January 2000 38 14 January 2008 to 15 February 2008 33 1 February 2008

NO. 4 BUEH ET AL.: EXTENSIVE AND PERSISTENT LOW TEMPERATURE EVENTS 237 The data used in this study were daily meteorological field data based on reanalysis by the US National Centers for Environmental Prediction (NCEP) and the US National Center for Atmospheric Research (NCAR) (Kalnay et al., 1996). We adopted the definitions of the El Niño and La Niña episodes released by the Climate Prediction Center (CPC) of the NCEP on the basis of the Oceanic Niño Index (ONI). The ONI is defined as three-month running-mean sea surface temperature (SST) departures in the Niño 3.4 region on the basis of the Extended Reconstructed SST (ERSST.v3b) updated by the National Oceanic and Atmospheric Administration (NOAA). To analyze the key circulation features of the EPLTEs, we adopted the following strategy. First, to capture the common feature of most EPLTEs, compositing was performed at one or two key time points, i.e., at day 0 and/or day L, and the significance was tested by the Student s t-test. Then, to illustrate any discrepancies of the circulation features among the EPLTEs, all EPLTEs have beenclassified into five clusters according to the spatial distribution of the ECSs, as in Peng and Bueh (2011), and the key circulation feature of each cluster has been analyzed in the composite or typical case. With the 10-day constraint, Cluster 1 (C1, 22 members) is the largest among all clusters and is characterized by a nationwide distribution of ECSs for each member. Cluster 2 (C2) is the second largest cluster (six members), and its ECSs were distributed mainly over northwestern China and to the south of the Yangtze River basin. Cluster 3 (C3, five members) is characterized by the ECSs over the region to the south of the Yellow River. For Cluster 4 (C4, five members), the ECSs were primarily distributed over the eastern part of China. Cluster 5 (C5) is the smallest cluster (three members), with the ECSs located only over northeastern and northern China. 3 Results 3.1 Siberian high The composite surface air temperatures (SATs), sea level pressures (SLPs), and the related anomalies for the 38 EPLTEs (Table 1) are shown in Fig. 1. The SAT anomalies at day L are characterized by the North Warm/ South Cold pattern with the node line at approximately 50 N (Fig. 1a). This pattern was found to be a key feature of the SAT pattern during 10 31 January 2008 (Zhou et al., 2009; Bueh et al., 2011). As seen in Figs. 1b and 1c, the Siberian High (SH) at day L was strengthened and expanded over the Asian continent with the composited SLP anomaly rising to 12 hpa at the center of the climatological SH. Moreover, the isobars for the 1020 hpa and 1030 hpa (thick solid lines) cover a considerably larger area than their climatological counterparts (thick dashed lines). In fact, compared with the situation at day L, the SH at day 0 (Fig. 1d) is amplified in the same broad extent, as its amplitude is even larger, with a maximum SLP anomaly of 17 hpa. Therefore, the long duration of the amplified and expanded SH is one of the key features common to EPLTEs. Note that this characteristic of the EPLTE is distinct from the typical feature of the SH for the cold waves that frequently attack East Asia. In the latter case, the SH is often amplified over Central Siberia and subsequently spills out toward East Asia (Ding and Krishnamurti, 1987; Takaya and Nakamura, 2005). Figure 2 shows the horizontal SAT advection at 1000 hpa in the EPLTEs and in the climate mean state. Climatologically, the East Asian winter monsoon is characterized by a dominant cold advection along the coastal areas and the neighboring seas of East Asia, as shown in Fig. 2a. This temperature advection pattern is still main- Figure 1 (a) Composite SAT (K) anomalies at day L for the 38 EPLTEs. (b) The same as in (a) except for the total SLP (hpa) values. The two bold lines represent isobars of 1020 hpa and 1030 hpa. (c) The same as in (b) except for SLP anomalies. (d) The same as in (c) except for SLP anomalies at day 0. The isolines are drawn for every 2 K, 4 hpa, 2 hpa, and 2 hpa in (a) (d), respectively. Negative values are dashed and zero lines are eliminated. Shading is applied where the significance level exceeds 95%. The black shading in (a) (d) denotes a topography greater than 3000 m.

238 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 4 Figure 2 (a) Multi-year (1951 2008) mean winter (November to March) horizontal temperature advection (K d 1 ) at 1000 hpa. (b) The same as in (a) except for the composite horizontal temperature advection at day L for the 38 EPLTEs. (c) The same as in (b) except for the composite difference between those at day L and at day 0. The isolines are drawn for every 2 K d 1 in (a) (c). Negative values are dashed, and zero lines are eliminated. Shading is applied as in Fig. 1. tained in the EPLTE at day L (Fig. 2b), but the amplitude is strikingly amplified, and the area of coverage is expanded inland. In particular, the strong cold advection dominated central and southern China at this time, consistent with the expanded and long lasting SH (Figs. 1b and c). The difference between temperature advections at day L and day 0 (Fig. 2c) still reflects the persistent and amplifying cold advections over middle and southern China with a slow southeastward migration. Such a persistent cold advection feature is quite different from the southeastward outbreak feature of cold advections for the frequent cold waves over East Asia (Ding and Krishnamurti, 1987; Takaya and Nakamura, 2005). 3.2 Large-scale tilted ridge/trough and WPSH The composited 500 hpa geopotential height fields and the corresponding anomalies in the EPLTEs are displayed in Fig. 3. For the climate mean state, a broad trough exists over East Asia/northwestern Pacific at 500 hpa, and its stationary eddy (departure from zonal-mean) field shows a wave-like structure in the mid-latitudes (not shown). In sharp contrast, as seen in Figs. 3a and 3b, a pair of northeast-southwest elongated positive and negative anomalies over the Asian continent (right panels) show features of an anticyclonic wave-breaking event (Tyrlis and Hoskins, 2008a, b) coinciding with a pair of northeast-southwest tilted ridges and troughs (left panels). Compared with those in Fig. 3a, the height anomalies in Fig. 3b are shown to be in a more zonally elongated structure and are displaced slightly southeastward, forming an effective influent configuration over the southeastern coast of China. Therefore, the key feature of the mid-troposphere flow common for most EPLTEs is a pair of northeastsouthwest-tilted ridges and troughs over the Asia/North Pacific region, which show a marked split flow over the inner continent and influent flow over the southeastern coast of China. For the largest cluster, C1, the mid-tropospheric tilted ridge and trough (Fig. 3c) are almost identical to those in Figs. 3a and 3b, which verifies again that most EPLTEs share this common feature. For the remaining clusters (Figs. 3d g), the tilted ridges and troughs are still the key circulation features in the mid-troposphere, though they vary slightly in strength and longitudinal position. For the EPLTE in January/February 2008 in particular, the tilted trough over the Asian continent was merged with the trough over the Bay of Bengal, showing a broad meridional extent (Fig. 3d). After examining all 38 EPLTEs, it was found that, for the major duration of an EPLTE, the tilted trough basically maintains its zonal extent at day 0 (not shown). However, the zonal extents of the tilted troughs vary considerably among the EPLTEs. After visual inspection, it was found that the zonal extent of the tilted troughs was often associated with the strength and position of the WPSHs. Therefore, it may be anticipated that distinct features of EPLTEs can be distinguished by the zonal extents of their tilted troughs, keeping in mind that the tilted trough is an essential feature of EPLTEs. On the basis of this finding, we categorized the EPLTEs into two main groups according to the longitudinal position of the eastern end of their tilted troughs at day 0 as follows. If the tilted trough of an EPLTE extends to the east of 160 E, the EPLTE is defined as having a long tilted trough; otherwise, it is referred to as having a short tilted trough. Twenty-one (fifteen) EPLTEs were characterized by a short (long) tilted trough (Table 2), and the other two EPLTEs exhibited the cut-off low characteristics over East Asia (not shown). The 500 hpa geopotential height fields and the corresponding anomalies at day 0 for the two main categories are displayed in Fig. 4. In the EPLTEs with short tilted troughs, the troughs were embedded between the primary tilted ridges over northern Asia and the North Pacific ridges with the eastern ends only reaching northern Japan (Figs. 4a and 4b). In contrast, the long tilted troughs, as shown in Figs. 4c and 4d, extend to the east of the date line, and the accompanying primary ridges at the northwestern sides also show a broad extent. Note that the circulation anomalies over the northern and northwestern Pacific in Fig. 4b bear a resemblance to those in the La Niña years, while the anomalies shown in Fig. 4d are

NO. 4 BUEH ET AL.: EXTENSIVE AND PERSISTENT LOW TEMPERATURE EVENTS 239 Figure 3 The geopotential height fields (m, left column) and the corresponding height anomalies (m, right column): (a) Composite for all 38 EPLTEs at day 0, (b) As in (a), but at day L, (c) Composite for C1 at day L, (d) Typical case (14 January 15 February 2008) of C2 at day L, (e) As in (d), except for C3 (27 February March 1971) at day L, (f) As in (d), except for C4 (16 27 March 1992), and (g) As in (d), except for C5 (21 30 November 1970). Contours are drawn for every 50 m in left panels and every 30 m in right panels. Negative values are dashed, and zero lines are eliminated. Shading is applied as in Fig. 1. similar to those in the El Niño years (He et al., 2008). Indeed, as listed in Table 2, twelve (seven) out of twentyone (fifteen) events with short (long) tilted troughs occurred in La Niña (El Niño) years as indicated by ONI. In addition, the 500 hpa geopotential height fields at day L vary slightly from those at day 0 (not shown). Specifically, the large-scale ridge and trough over the Asian continent moved slightly southeastward. The meridional extents of the WPSHs at day 0 of the EPLTEs, as represented by the contours of 5860 m at 500 hpa, are shown in Fig. 5. Compared with the winter (November to March) climatological positions (thick dotted black line), the northern edges of the WPSHs were located northward in most EPLTEs (twenty-six of thirty-six events), as shown in Fig. 5a. As the ECSs increased afterward, the WPSHs also gradually retreated southward and, at day L, were closer to the climatological positions (not shown). In other words, the WPSHs tended to expand northward in the early stages of an EPLTE. The individual WPSHs show a more consistent behavior of northward expansion in the long tilted trough EPLTEs (Fig. 5b) than in those with short tilted troughs (Fig. 5c). Specifically, in twelve out of all fifteen EPLTEs with long tilted troughs, the WPSHs expanded northward with an amplitude similar to winter climatological conditions (Fig. 5b). In contrast, the amplitudes of the WPSHs shown in Fig. 5c are

240 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 4 Table 2 Oceanic Niño Index (ONI) for the two categories of EPLTEs with short and long tilted troughs. The numbers listed are the same as in Table 1. The ONI values in shallow (bold) indicate that the tropical Pacific Ocean in the relevant months is under La Niña (El Niño) conditions. No. ONI Tilted troughs 2 1.1 Short 3 1.1/ 1.0 Long 4 0.8 Short 9 0.7/ 0.7 Long 13 0.7/ 0.9 Short 14 1.0/1.0 Long 15 1.0/0.9 Long 17 0.9 Short 18 1.3/ 1.3 Short 19 1.3/ 1.1 Short 20 0.7 Short 21 1.7 Short 22 0.8 Long 23 0.7/0.6 Long 24 0.6/0.5 Long 30 1.1 Short 31 0.7 Short 33 1.3/1.1 Long 35 1.5 Long 37 1.6/ 1.4 Short 38 1.4/ 1.4 Short considerably weakened, with the contours at 5860 m having a large variation in all cases. Moreover, as apparent in Table 2, approximately half of the EPLTEs with long tilted troughs occurred during El Niño years, which is consistent with the finding of He et al. (2008) that WPSHs tend to expand northward in El Niño years. 4 Discussion This study focuses on the key features common to wintertime EPLTEs in China. In its most extensive area of extreme low temperatures, the event is marked by a pair of northeast-southwest-tilted ridges and troughs in the mid-troposphere on the continental scale. Meanwhile, the splitting flow over the inner Asian continent and the influent flow over the southeastern coast of China correspond well with the expanded and amplified SHs with tightened SLP gradients and strong cold advections over southeastern China. During the early stages of the EPLTEs, the WPSHs tend to expand northward, and events with long tilted troughs show this behavior more consistently. Wang et al. (2009) examined various important and distinct roles played by the tilted East Asian major troughs in winter monsoon circulations; however, they focused on properties on the interannual time scale, whereas the present study analyzes the crucial role of EPLTEs on the medium and extended range time scales. Another difference between these studies is that the tilted Asian troughs discussed in Wang et al. (2009) were located near the East Asian coast, whereas in the present study, the tilted troughs were located in the inner continent of Asia. What kinds of internal atmospheric processes or external forces can trigger continental-scale tilted ridges and Figure 4 (a) The composite 500 hpa geopotential height field (m) for EPLTEs with short tilted troughs. (b) The same as in (a) except for height anomalies. (c) The same as in (a) except for EPLTEs with long tilted troughs. (d) The same as in (b) except for EPLTEs with long tilted troughs. The isolines are drawn for every 40 m in (a) and (c) and for every 20 m in (b) and (d). Negative values are dashed, and zero lines are eliminated. Shading is applied as in Fig. 1.

NO. 4 BUEH ET AL.: EXTENSIVE AND PERSISTENT LOW TEMPERATURE EVENTS 241 Technologies R&D Program of China (Grant No. 2009BAC51B02). References Figure 5 (a) Contours of 5860 m in the 500 hpa geopotential height fields at day 0 for 36 EPLTEs. (b) The same as in (a) except for the 15 EPLTEs with long tilted troughs. (c) The same as in (a) except for the 21 EPLTEs with short tilted troughs. The black bold dashed contours represent the multi-year (1951 2008) mean five-month (November to March) averaged contours of 5860 m, and the black bold solid contours represent the 5860 m isolines for the composite 500 hpa geopotential height fields. The contours in red (5860 m) indicate the WPSHs that expanded northward, and the contours in blue (5860 m) indicate the WPSHs that did not expand northward. The contours in green denote the contours of 5840 m for the WPSHs, in which the contours of 5860 m were absent. troughs in association with EPLTEs? Why do these tilted ridges and troughs persist and cause long-lasting low temperature events? These questions require further investigation. Acknowledgements. The authors are grateful to the two anonymous reviewers constructive suggestions, which improved the manuscript substantially. This work was supported by the National Key Bueh, C., N. Shi, and Z. Xie, 2011: Large-scale circulation anomalies associated with persistent low temperature over Southern China in January 2008, Atmos. Sci. Lett., 12(3), 273 280, doi:10.1002/asl.333. Ding, Y. H., and T. N. Krishnamurti, 1987: Heat budget of the Siberian high and the winter monsoon, Mon. Wea. Rev., 115, 2428 2449. He, X., Y. Ding, and J. He, 2008: Response characteristics of the East Asian winter monsoon to ENSO events, Chinese J. Atmos. Sci. (in Chinese), 32, 335 344. Kalnay, M. E., M. Kanamitsu, R. Kistler, et al., 1996: The NCEP/ NCAR 40-year reanalysis project, Bull. Amer. Meteor. Soc., 77, 437 471. Li, Z., and Z. Yan, 2010: Homogenized China daily mean/ maximum/minimum temperature series 1960 2008, Atmos. Oceanic Sci. Lett., 2(4), 237 243. Peng, J., and C. Bueh, 2011: Definition and classification of wintertime extensive persistent low temperature events in China, Atmos. Oceanic Sci. Lett., in press. Takaya, K., and H. Nakamura, 2005: Mechanisms of intraseasonal amplification of the cold Siberian high, J. Atmos. Sci., 62, 4423 4440. Tao, S., and J. Wei, 2008: Severe snow and freezing rain in January 2008 in Southern China, Climatic Environ. Res. (in Chinese), 13, 337 350. Tyrlis, E., and B. J. Hoskins, 2008a: Aspects of a Northern Hemisphere atmospheric blocking Climatology, J. Atmos. Sci., 65(5), 1638 1652. Tyrlis, E., and B. J. Hoskins, 2008b: The morphology of Northern Hemisphere blocking, J. Atmos. Sci., 65(5), 1653 1665. Wang, L., W. Chen, W. Zhou, et al., 2009: Interannual variations of East Asian trough axis at 500 hpa and its association with the East Asian winter monsoon pathway, J. Climate, 22, 600 614. Wen, M., S. Yang, A. Kumar, et al., 2009: An analysis of the large scale climate anomalies associated with the snowstorms affecting China in January 2008, Mon. Wea. Rev., 137, 1111 1131. Zhou, W., J. C. L. Chan, W. Chen, et al., 2009: Synoptic-scale controls of persistent low temperature and icy weather over Southern China in January 2008, Mon. Wea. Rev., 137, 3978 3991.