Snowfall over central-eastern China and Asian atmospheric cold source in January

Transcription

1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 32: (2012) Published online 21 March 2011 in Wiley Online Library (wileyonlinelibrary.com) DOI: /joc.2318 Snowfall over central-eastern China and Asian atmospheric cold source in January Sulan Nan a and Ping Zhao b,c * a Chinese Academy of Meteorological Sciences, Beijing, China b State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China c National Meteorological Information Centre, Beijing, China ABSTRACT: In January 2008, a severe snowstorm disaster occurred in central-eastern China. Using the monthly means from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data set and the observations from surface stations of China for the period , we statistically investigate the relationship between the January snowstorm weather over central-eastern China and the synchronous atmospheric thermal condition over the Asian continent. The results show that the extreme snowfall weather over central-eastern China is closely associated with the Asian atmospheric cold source (AACS) in January. When AACS is weak (corresponding to a higher AACS value), the heavy snowfall weather appears in central-eastern China. The values of both AACS and snowfall over central-eastern China in 2008 are most anomalous during the recent 30 years. This link may be explained by the atmospheric circulation well. Under a weak AACS (with a higher AACS value), a 500-mb anomalous low covers the mid-low latitudes of Asia, accompanying an anomalous high over the eastern coasts of East Asia. Accordingly, the southerly wind anomalies between the anomalous low and high prevail in the lower troposphere over central-eastern China, with the northerly wind anomalies prevail over this region at the surface, which also strengthen upward motion over central-eastern China. The southerly wind anomalies transport more water vapour into central-eastern China from both the South China Sea and the Bay of Bengal. These anomalies in the atmospheric circulation are responsible for the formation of the heavy snowfall weather in central-eastern China. Compared to both the El Niño-Southern Oscillation and Arctic Oscillation during winter, the thermal condition over the Asian continent has a closer relationship with the occurrence of the heavy snowfall weather over central-eastern China. Copyright 2011 Royal Meteorological Society KEY WORDS snowstorm over central-eastern China; Asian atmospheric cold source; East Asian winter monsoon Received 21 October 2009; Revised 9 December 2010; Accepted 10 February Introduction A severe snow disaster occurred in central-eastern China, North China, and the eastern part of Northwest China in January 2008, leading to the collapse of electric network and the severe damage of highway and railway transportation over these regions and an economic loss above billion Chinese Yuan. Therefore, many meteorologists have paid much attention to the causes for this disaster. Studying disastrous snowstorms has extensively received attention for a long term. Some progress has been made in understanding the links between snowstorm disasters and atmospheric circulation systems and the associated physical mechanisms. For example, the snowstorms occurring in North America and Europe are usually associated with the activities of fronts and cyclones at the mid latitudes of these regions (e.g. Braham, 1983; Ulbrich et al., 2001). Sanders (1986) investigated an effect of large-scale frontogenetical forcing and moist symmetric * Correspondence to: Ping Zhao, National Meteorological Information Centre, 46 Zhongguancun Nandajie, Beijing , China. zhaop@cma.gov.cn instability on a snowstorm process over the New England on 5 6 December Tayanç (1998) addressed that the blizzard over the eastern Mediterranean and Balkan regions in March 1987 may be related to the formation of the extensive blocking over northern Europe, the substantial amplification of the planetary-scale waves and the strong cyclogenesis over the eastern Mediterranean and Balkan region. Laird et al. (2003) emphasized the sensitivity of the lake-effect snowstorms in the United States to the local wind, temperature, moisture, and stability. Moreover, Ninomiya (1991) examined an effect of the development of a polar low near the eastern coast of the Asian continent to a Japanese snowstorm process on 9 11 December The persistent snowstorm weather often occurs under the background of a unique climate anomaly. Michanel and Smith (1994) pointed out that the blizzard occurring over the eastern part of the United States is related to El Niño-Southern Oscillation (ENSO). The winter atmospheric circulation in East Asia is characterized by the local strong lower-tropospheric northerly or northeasterly wind (e.g. Ramage, 1968; Chang et al., 1980), namely the East Asian winter monsoon (EAWM). Because the EAWM anomalies are related to atmospheric Copyright 2011 Royal Meteorological Society

2 SNOWFALL OVER CENTRAL-EASTERN CHINA AND ASIAN ATMOSPHERIC COLD SOURCE 889 intrinsic dynamic and thermal processes and external forcing, many studies paid attention to the EAWM circulation anomalies and the associated mechanisms. For example, the Arctic Oscillation (AO) may modulate EAWM through affecting the Siberian high (Wu and Wang, 2002). The EAWM anomalies are also usually accompanied by variations of upper-tropospheric westerly jet streams in the mid latitudes of Eurasia (Lau and Li, 1984; Zhang et al., 1997; Yang et al., 2002). When the propagation of quasi-stationary planetary waves into the stratosphere in the high latitudes of the Northern Hemisphere becomes weak, its propagation into the upper troposphere strengthens in the lower latitudes, which may lead to the weakening of the Siberian high and thus the weakening of the EAWM (Chen et al., 2008). Moreover, the land surface conditions on the mid-high latitudes of Eurasia and the tropical Pacific sea surface temperature (SST) anomalies are also related to the EAWM (Li, 1988; Chen and Sun, 2003). Snowstorms in eastern China often occur under the background of the EAWM circulation anomalies. Under such a background, low-level cold surges frequently invade into China along with the breakdown of the Siberia-Mongolia high (Tao, 1959). Shi et al. (2008) and Ji et al. (2008) showed that the snowstorm weather over central-eastern China in January 2008 was related to the strengthening of both the western Pacific subtropical high and India-Burma trough, and the southwesterly flow in front of the active India-Burma trough transports more moisture into central-eastern China, providing a favourable condition of water vapour for the occurrence of the snowstorm weather. Wen et al. (2009) investigated a link between this persistent snowstorm and jet stream anomalies over the Middle East. Hong and Li (2009) emphasized the combined effect of the intraseasonal oscillation heating over the Asian Maritime Continent and ENSO on initiating and maintaining the northerly anomaly over Southeast Asia in February after the snowstorm weather. In spite of the progress in understanding the EAWM variability and snowstorms over eastern China, some questions remain unanswered. For example, LaSW (2008) indicated that SST in the equatorial eastern Pacific and the AO might be the important factors influencing the persistent snowstorm over central-eastern China in January However, Gu et al. (2008) showed that the occurrence of this snowstorm disaster could not well be explained by the La Niña event. Moreover, Yang and Li (2008) addressed that the AO is also not closely related to this snowstorm weather. These results imply some differences in understanding the mechanism responsible for the formation of the January snowstorm weather over central-eastern China. Because the Asian atmospheric circulation anomalies are closely related to atmospheric thermal conditions over the Asian continent (Ye and Gao, 1979; Li and Yanai, 1996; Wu et al., 1997) and the EAWM intensity is modulated by the heating gradient between the Siberian region and the equatorial western Pacific (Ramage, 1968; Chang et al., 1980; Ding and Krishnamurti, 1987), some studies examined relationships between the Asian atmospheric thermal condition and the EAWM. For example, Zhao and Chen (2001) investigated relationships between the Tibetan winter cold source and the atmospheric circulation over the Asian-Pacific region. They addressed that during a weaker winter Asian cold source, a 500-mb anomalous cyclonic circulation usually appears in the mid-low latitudes of Asia and there is a stronger India- Burma trough. This anomalous pattern in atmospheric circulation is similar to that associated with the persistent snowstorm over central-eastern China in January Moreover, Bao et al. (2010) investigated a relationship between the Tibetan Plateau climate and this extreme snowstorm event with an atmospheric general circulation model. They concluded that a warmer Tibetan Plateau (TP) provided a favourable atmospheric circulation condition for the occurrence of this snowstorm. Then, does a change in the Asian thermal condition affect the snowstorm disaster over central-eastern China? If yes, what physical processes are responsible for the formation of the snowstorm weather? With these questions in mind, the objective of this work is therefore to investigate relationships of the atmospheric thermal condition over the Asian continent with the snowfall weather over central-eastern China during January and the associated physical processes. The rest of this article is organized as follows. In Section 2, we describe the main features of data sets and analysis methods. In Section 3, we study variability of the extreme snowfall over central-eastern China and analyse its relationships with the atmospheric cold source over Asia. Finally, conclusions and discussions are provided in Section Data and methods We utilize observations of the number of snowfall days (NSDs) at 527 surface stations in China (Figure 1(a)) from the National Meteorological Information Centre of China Meteorological Administration and the monthly data from the NCEP-NCAR reanalysis (Kalnay et al., 1996) for the period The monthly mean SST from the HadISST data set (Rayner et al., 2003) with a horizontal resolution of 1 in latitude and longitude is also used in this study. The AO index is from the Climate Prediction Center of NOAA s National Weather Service. To increase the reliability of our results obtained using the NCEP-NCAR reanalysis, we repeated some analyses using the monthly data from the European Centre for Medium-Range Weather Forecast (ECMWF) ERA-40 data set for (Uppala et al., 2005). Following the previous study (Yanai et al., 1992; Zhao and Chen, 2001), for an air column, the atmospheric apparent heat source/sink (<Q 1 >)isdefinedas <Q 1 >= SH + R net + LP

3 890 S. NAN AND P. ZHAO (a) is used to assess the statistical significance. Unless specified, the 90% confidence level is used to measure a significant signal. (b) Figure 1. (a) Distribution of surface meteorological stations in China, in which the small box indicates central-eastern China ( E/ N). (b) The anomaly of NSD (unit: day) in January 2008 from the climatology ( ). In (b), the shaded areas are 9days. where SH is the sensible heat flux at surface, R net the net radiation absorbed by the air column, and LH the latent heat produced by precipitation condensation. Because <Q 1 > is usually negative in winter in continents, the atmosphere shows a cold-source feature, which is called the atmospheric cold source (Yanai et al., 1992; Zhao and Chen, 2001). In this study, we use the downward shortwave radiation flux, the upward longwave radiation flux, the upward solar radiation flux at tropopause, the SH, the net longwave radiation, the net shortwave radiation at the surface, and the precipitation rate from the NCEP reanalysis data set to calculate <Q 1 >. Moreover, the surface thermal radiation, the surface solar radiation, the top solar radiation, the top thermal radiation, the surface sensible heat flux, the convective precipitation, and the stratiform precipitation (large-scale precipitation) from the ERA-40 reanalysis data set are also used to calculate <Q 1 >. The analysis methods employed in this study include correlation and composite analyses. The Student s t-test 3. Relationship between snowfall over central-eastern China and Asian atmospheric heating 3.1. Snowfall and associated atmospheric circulation and heating in January 2008 It is necessary to briefly review the snowfall weather over central-eastern China in January 2008 and the associated atmospheric circulation and atmospheric cold source before studying the climatological features of the snowfall weather over central-eastern China and associated physical processes during Because NSD may reflect both precipitation and temperature information, we select NSD as an indicator of the snowstorm weather. Figure 1(b) is the anomaly of NSD in January 2008 from the climatological ( ) mean value. Positive NSD values cover the region to the south of 40 N (including central-eastern China, North China, and the eastern part of Northwest China) and the large-scale NSD value exceeding 9 days mainly appears over centraleastern China (east of 105 E) between 25 N and 35 N, with the central value of 12 days in the middle and lower valleys of the Yangtze River. It is evident that NSD over these regions is much bigger in January 2008 than its climatological mean value. Moreover, some scatted centres of 9 days also appear over northwestern China (north of 35 N and west of 105 E). The anomaly of atmospheric circulation directly caused this snowstorm weather in Figure 2(a) shows the climatology of 500-mb geopotential height during and the anomaly of the 500-mb height in January 2008 from the climatology. In the figure, there was a large-scale anomalous low at the mid-low latitudes of the Asian continent. Because an extratropical ridge appears over Asia on the climatological map, this anomalous low indicates a weaker ridge in the mid-low latitudes of Asia during January To the north of the anomalous low, there were positive anomalies, indicating a weaker Arctic low over Eurasia. Meanwhile, positive anomalies also appeared over the eastern coasts of East Asia where the southern part of a long-wave trough over East Asia and the northern part of the western North Pacific subtropical ridge often appear, suggesting the weakening of the trough and the northwestward subtropical ridge relative to the climatology. Corresponding to the variations in the 500-mb height field at the mid-low latitudes of the Asian-western Pacific region, some pronounced changes are also observed in the lower troposphere and at the surface. At 850- mb (Figure 2(b)), negative geopotential height anomalies appeared over Asia to the south of 30 N, with their centres below 10 gpm, while positive geopotential height anomalies appeared over the eastern coasts of East Asia. In the wind field (Figure 2(b)), there was an anomalous cyclonic circulation around the TP, indicating a

4 SNOWFALL OVER CENTRAL-EASTERN CHINA AND ASIAN ATMOSPHERIC COLD SOURCE 891 Figure 2. (a) The climatology of January 500-mb geopotential height for (shade; unit: gpm) and the anomaly of January 500-mb geopotential height in 2008 (contour; unit: gpm) from the climatology. (b) The anomalies of January 850-mb geopotential height (shade; unit: gpm) and horizontal winds (vector; unit: m s 1 ) in (c) The anomalies of January surface wind (vector; unit: m s 1 ) and surface pressure (contour; unit: Pa) in In (b) and (c), the thick line is the 1500-m topographic contour. stronger India-Burma trough, and there was an anomalous anticyclone over the eastern coasts of East Asia, with its circulation centre appearing near 120 E/45 N. The anomalous southerly winds to the southwest of the anomalous anticyclone centre and from the Bay of Bengal prevailed over eastern China, transporting water vapour into central-eastern China. Corresponding to the positive anomalies of the 500- and 850-mb geopotential heights over the eastern coasts of East Asia, there existed a positive anomaly of surface pressure, with its centre appearing at 120 E/45 N (Figure 2(c)). To the south of the positive pressure anomalies, anomalous northerly or northeasterly winds prevailed over central-eastern China south of 40 N, which favoured the invasion of cold air masses into central-eastern China. This vertical structure with warm and wet air masses in the lower troposphere and the underlying cold masses was favourable for the persistent freezing rain and snowfall over central-eastern China. Then, what reasons may be responsible for the variations of the India-Burma trough and the trough over the eastern coasts of East Asia? Following the previous studies (Ramage, 1968; Chang et al., 1980; Zhao and Chen, 2001), we examine the atmospheric thermal condition over the Asian continent. Figure 3 shows the anomaly of <Q 1 > in January Positive anomalies of <Q 1 > covered

5 892 S. NAN AND P. ZHAO most of the Asian hinterland between 20 N and 50 N and between 80 E and 100 E, with the centres of 150 W m 2 near 97.5 E/ N, 100 W m 2 near E/40 N and 65 E/32.5 N, and 200 W m 2 near 75 E/ N, indicating a weaker atmospheric cold source over these regions relative to the climatological mean value. Then, does this relationship of the extreme snowfall weather over central-eastern China with the atmospheric circulations in the mid-low latitudes of the Asian-Pacific region and <Q 1 > over the Asian continent occur in the other years? In the following section, we expand the above analyses to the period Variations of snowfall over central-eastern China and Asian atmospheric heating Figure 4(a) shows the standard deviation of <Q 1 > during Over the Asian continent, the standard deviation of <Q 1 > exceeding 60 W m 2 appears in the region E/25 50 N. These features are also observed from the result of the ERA-40 reanalysis (shown in Section 4). Comparing with Figure 3, it is seen that the Asian positive anomalies of <Q 1 > in January 2008 correspond to the large standard deviation of <Q 1 >. Referring to the distributions of the Asian large standard deviation shown in Figure 4(a) and <Q 1 > in January 2008 shown in Figure 3, the regionally ( E/25 50 N) averaged <Q 1 > is defined as the Asian atmospheric cold source (AACS) index in January. Figure 4(b) shows the correlation between the January AACS index and the simultaneous <Q 1 > at each grid point during It appears that the significant positive correlation covers a broad area in the middle latitudes from the Mediterranean Sea to East Asia, with a maximum correlation coefficient of 0.6 appearing between 30 N and 40 N. Clearly, the AACS index well represents the variability of AACS at the mid-low latitudes of the Asian continent. More calculation shows that the temporally averaged value of the AACS index over is 80.6 Wm 2, with a standard deviation of 10.8 W m 2. Figure 5(a) shows the time series of the normalized January AACS index. In the figure, the Figure 3. The anomaly of January <Q 1 > (unit: W m 2 ) in 2008 from the climatology ( ). Figure 4. (a) The standard deviation of January <Q 1 > (unit: Wm 2 ) during , in which the contours 5, 15, 30, 50, 80, and 120 W m 2 are plotted and the box indicates the region ( E/25 50 N) defining the AACS index. (b) Correlation coefficient between the January AACS index and the synchronous <Q 1 > at each grid point during , in which the light and heavy shaded areas are significant at the 90% confidence level for the negative and positive values, respectively. AACS index exhibits the remarkable interannual variability, with the biggest value of 50.4 W m 2 in 1969 and the smallest value of W m 2 in In 2008, this index is most anomalous during the recent 30 years. Referring to the position of the large NSD value exceeding 9 days over central-eastern China in January 2008, we use the regionally averaged NSD value over E/25 35 N (including 177 stations in Figure 1(a)) as an index of NSD over central-eastern China (hereafter referred to as the NSD-CEC index). Figure 5(b) shows the time series of the January normalized NSD-CEC index. The climatological ( ) mean value of the NSD-CEC index is 4.3 days, with its standard deviation of 2.1 days. The January NSD-CEC index also exhibits the remarkably interannual variability, with the biggest value of 11.2 days in 1977 and the smallest value of 1.2 days in In 2008, the NSD- CEC is also most anomalous during the recent 30 years. A correlation analysis shows that the correlation coefficient between the AACS and NSD-CEC indices is 0.56

6 SNOWFALL OVER CENTRAL-EASTERN CHINA AND ASIAN ATMOSPHERIC COLD SOURCE 893 Figure 5. Time series of the normalized AACS index from the NCEP-NCAR reanalysis (a) and the normalized NSD-CEC index for (b). for the period (significantly at the 99.9% confidence level), greater than those between the AACS index and the synchronous precipitation (0.38, significant at the 99% confidence level) over central-eastern China ( E/25 35 N) and between the AACS index and the synchronous temperature ( 0.32, significant at the 98% confidence level). Although the formation of snowfall is greatly affected by both more water vapour and lower air temperature, snowfall is not equivalent to precipitation or air temperature. For example, when more (less) water vapour is accompanied by higher (lower) air temperature, it is difficult to form snowfall. To assess the robustness of the link between snowfall over centraleastern China and AACS, we slightly adjusted the regions where the NSD-CEC and AACS indices are defined, respectively. The results show that the time curves of these two indices do not show a substantial change, which suggests the robustness of this link. Moreover, in the seven highest NSD-CEC-index years (1956, 1969, 1974, 1977, 1984, 1993, and 2008) with their normalized values greater than one standard deviation, there are five years (1956, 1969, 1974, 1977, and 2008) with a positive anomaly of the AACS index (Table I). In the nine low NSD-CEC-index years (1963, 1965, 1975, 1986, 1987, 1992, 1999, 2002, and 2007) with their values less than one negative standard deviation, there are seven years (1963, 1975, 1986, 1987, 1992, 2002, and 2007) with a negative anomaly of the AACS index. These results further support a close relationship between AACS and the extreme snowfall weather over central-eastern China in January Snowfall over central-eastern China and East Asian atmospheric circulation associated with AACS To detect variations in the atmospheric circulation associated with the interannual variability of the January AACS Table I. Years with the extreme high and low normalized January NSD-CEC indices beyond one standard deviation (σ ) during and the corresponding normalized AACS, Niño 3 4 SST and AO indices during January. Years NSD-CEC AACS Niño 3.4 SST AO NSD-CEC σ NSD-CEC σ from a composite analysis, we select nine high and low AACS-index years on the basis of the January AACS index (displayed in Figure 5(a)). They are 1953, 1954, 1957, 1969, 1970, 1972, 1973, 1977, and 2008 for the high AACS-index years, called the high AACS-index cases, and 1963, 1976, 1979, 1982, 1986, 1987, 1990, 2003, and 2007 for the low AACS-index years, called the low AACS-index cases. The composite patterns of NSD in the high and low AACS-index cases are constructed (Figure 6). In general, the patterns in the high and low AACS-index cases

7 894 S. NAN AND P. ZHAO Figure 6. (a) The composite patterns of NSD (unit: day) for the high AACS-index cases. (b) Same as in (a), but for low AACS-index cases. (c) Same as in (a), but for the difference between (a) and (b), in which the shaded areas are at the 90% confidence level. are similar to that of the climatological mean. However, for the high AACS-index cases (Figure 6(a)), the NSD exceeding 6 days dominates most of central-eastern China (east of 100 E) between 25 N and 35 N, with the central value exceeding 9 days along the middle and lower valleys of the Yangtze River. For the low AACSindex cases (Figure 6(b)), NSD decreases remarkably. The NSD around 3 days dominates most of centraleastern China between 25 N and 35 N. Furthermore, the differences in NSD between the high and low AACSindex cases are greater than 4 days over the Yangtze River and Yellow River valleys, with a maximum difference of 8 days (Figure 6(c)). It is evident that when the AACS index is higher (lower), NSD is remarkably greater (smaller) over most of central-eastern China. Figure 7(a) shows the composite difference of 500-mb geopotential height between the high and low AACSindex cases. In the figure, negative anomalies below 60 gpm appear over Eurasia to the south of 50 N, indicating a weaker ridge at the mid-low latitudes of Asia on the climatological mean map, while the positive height anomalies appear in the high latitudes and Arctic region of Europe. Meanwhile, the positive anomalies also appear over the eastern coasts of East Asia, indicating the weakening of the East Asian long-wave trough and the northwestward subtropical western Pacific ridge relative to the climatology. Corresponding to the changes of 500-mb geopotential height over the Asian-western Pacific, in the 850-mb horizontal wind field (Figure 7(b)), an anomalous cyclonic circulation surrounds TP, while an anomalous anticyclone appears over Northeast Asia. Accordingly, the anomalous southwesterly to the southeast of the anomalous cyclonic circulation in the midlow latitudes of Asia and the southeasterly winds to the southwest of the anomalous anticyclonic circulation over Northeast Asia prevail over the mid-low latitudes of East Asia. At the surface (Figure 7(c)), anomalous northeasterly winds to the south of the anomalous anticyclonic circulation over Northeast Asia prevail over central-eastern China to the south of 40 N. These anomalous features of the atmospheric circulation between the high and low AACS-index cases favours the formation of the persistent snowstorm weather over central-eastern China, similar to those in January 2008 (shown in Figure 2). This similarity shows that the relationship among the extreme snowfall weather over central-eastern China, the atmospheric circulations over the Asian-Pacific region, and <Q 1 > over the Asian continent in 2008 does occur in the other years. Figure 8(a) further shows the composite difference in the integrated water vapour flux from the surface to 600-mb between the high and low AACS-index cases. It appears that the strong anomalous water vapour flux towards central-eastern China comes mainly from two passages. One originates from the South China Sea and then flows into central-eastern China. The other comes from the Bay of Bengal, and the southwesterly wind anomalies in front of the strengthened Indian- Burma trough (in Figure 7) transport more water vapour from the Bay of Bengal into eastern China. Figure 8(b) further shows the composite difference in the water vapour flux divergence between the high and low AACSindex cases. In the figure, central-eastern China to

8 SNOWFALL OVER CENTRAL-EASTERN CHINA AND ASIAN ATMOSPHERIC COLD SOURCE 895 Figure 7. (a) Composite differences of 500-mb geopotential height (unit: gpm) between the high and low AACS-index cases. (b) Same as in (a), but for 850-mb horizontal winds (unit: m s 1 ). (c) Same as in (a), but for surface wind (vector; unit: m s 1 ) and surface pressure (contour and shade; unit: Pa). In (a) and (c), the light and heavy shaded areas are significant at the 90% confidence level for negative and positive differences, respectively. the south of 40 N is covered by negative anomalies of the divergence, with the centre of 80 g cm 2 s 1 near E/25 N, indicating the enhanced convergence or reduced divergence of the water vapour flux in the lower troposphere over central-eastern China under a high AACS-index condition and vice versa. These strengthened transports of water vapour towards centraleastern China provide a favourable condition for the local heavy snowfall. Figure 9 shows the composite difference of vertical p-velocity along N. It is seen from the figure that significant negative anomalies appear to the east of 105 E, with a maximum magnitude of Pa s 1, indicating the strengthening of upward motion or the weakening of downward motion under a higher AACS-index condition. Thus, when the AACS index is higher, a variation in the atmospheric circulation over East Asia may strengthen upward motion or weaken downward motion over central-eastern China, favouring an increase in the local snowfall. In brief, AACS is closely related to the anomalous Asian atmospheric circulations. When the AACS index is higher, the 500-mb ridge at the mid-low latitudes of Asia is weaker, with a large-scale anomalous low over these regions, while the East Asian long-wave trough weakens and the subtropical high over the western North Pacific moves northwestward, with an anomalous high over the eastern coasts of East Asia. Accordingly, lower tropospheric anomalous southwesterly or southeasterly winds between the

9 896 S. NAN AND P. ZHAO Figure 8. Composite differences of water vapour flux (a; unit: 10 5 gcm 1 s 1 ) in the lower troposphere (from the surface to 600 mb) and its divergence (b; unit: 10 g cm 2 s 1 ) between the high and low AACS-index cases. The shaded areas are significant at the 90% confidence level. 4. Conclusions and discussions Figure 9. Longitude-height cross section of composite difference of vertical p-velocity (unit: 10 2 Pa s 1 ) along N between the high and low AACS-index cases. The light and heavy shaded areas are significant at the 90% confidence level for negative and positive differences, respectively. The black shaded area is for the topography. anomalous low and high prevail over eastern China with underlying anomalous northerly winds prevailing over the region, which strengthens the tropospheric upward motion over central-eastern China and the transport of low-level water vapour towards centraleastern China. These anomalies finally contribute to the occurrence of snowstorm weather over central-eastern China. Using the monthly mean NCEP-NCAR reanalysis data set and the monthly temperature, precipitation, and snowfall data from surface stations of China for the period , we have investigated the features of snowfall weather over central-eastern China, the atmospheric circulation over Asia-Pacific region, and the atmospheric cold source over the Asian continent in January 2008 and also examined the relationship between the January AACS and the synchronous snowfall over central-eastern China for the period The result shows that in January 2008, a severe snowstorm disaster occurred in central-eastern China and the NSDs over the region (NSD-CEC) were much bigger compared to the climatology. Meanwhile, AACS is much weak (with a high AACS-index value) relative to the climatology. Both the AACS and NSD-CEC in January 2008 are most anomalous during the recent 30 years. This relationship between the AACS and NSD-CEC is also observed for the period and there is a significantly positive correlation between AACS and NSD-CEC, namely that when the AACS index is higher (lower), the NSD value over central-eastern China is bigger (smaller). When AACS is weak, a 500-mb anomalous low covers the mid-low latitudes of Asia, accompanying an anomalous high over Northeast Asia. Under such an anomalous circulation pattern, anomalous southerly winds prevail in the lower troposphere over East Asia, while anomalous

10 SNOWFALL OVER CENTRAL-EASTERN CHINA AND ASIAN ATMOSPHERIC COLD SOURCE 897 (a) (b) Figure 10. (a) and (b) are same as in Figures 4a and 5a, respectively, but from the ERA-40 reanalysis. The shaded areas are 15 W m 2. northerly winds prevail at the surface over eastern China. This vertical feature of the anomalous atmospheric circulations is responsible for the persistent snowfall over central-eastern China. The anomalous southerly winds also favour the transports of water vapour towards eastern China from the South China Sea and the Bay of Bengal. Meanwhile, the anomalous upward motion occurs over central-eastern China. Thus, these atmospheric circulation anomalies provide a large-scale atmospheric circulation background and a favourable vapour condition for more snowfall over central-eastern China. Moreover, the atmospheric circulation anomalies associated with a higher AACS index are also consistent with those in January Therefore, the AACS index may be used as an indicator of the extremely heavy snowfall weather over central-eastern China. To verify the relationship between AACS and snowfall over central-eastern China and increase the reliability of the results obtained from the NCEP-NCAR reanalysis, we use the ERA-40 reanalysis data to repeat the above analyses. Figure 10(a) shows the standard deviation of <Q 1 > from the ERA-40 reanalysis. It is seen that high values exceeding 15 W m 2 occur over West Asia and South Asia. Although the standard deviation of <Q 1 > in the ERA-40 data is generally less than that in the NCEP-NCAR data, their distribution patterns are similar. In the same way, the AACS index is also calculated using the ERA-40 data (Figure 10(b)). There is a significant positive correlation of 0.69 between the NCEP-NCAR and ERA-40 <Q 1 > values. For the high AACS-index cases (1969, 1972, 1973, 1974, 1977, and 1991) from the ERA-40 data, NSD exceeding 9 days dominates central-eastern China (Figure 11(a)), while for the low AACS-index cases (1959, 1960, 1963, 1966, 1986, and 1987), NSD around 3 days appears over central-eastern China (Figure 11(b)). Compared to the low AACS-index cases, there are more snowfall days over central-eastern China in the high AACS-index cases (Figure 11(c)). These anomalous features are consistent with those from the NCEP-NCAR reanalysis (shown in Figure 6(c)). Figure 12(a) shows the composite difference of 500- mb geopotential height between the high and low ERA-40 AACS-index cases. In the figure, an anomalous low/an anomalous high appear over the mid-low latitudes of Asia/the eastern coasts of East Asia, indicating a weaker ridge over mid-low latitudes of Asia/a northwestward subtropical high over the western North Pacific and a weak East Asian long-wave trough. In the lower troposphere, anomalous southerly winds prevail over eastern China (Figure 12(b)). These results are also consistent with those from the NCEP-NCAR reanalysis, further increasing the reliability of out results. As mentioned in Section 1, the effects of ENSO and AO on the snowstorm weather are still an open question (e.g. Gu et al., 2008; LaSW, 2008; Yang and Li, 2008). Is snowfall over central-eastern China associated with ENSO and AO during ? Table I shows the

11 898 S. NAN AND P. ZHAO Figure 11. Same as in Figure 6, but for the AACS-index cases from the ERA-40 reanalysis. Figure 12. (a) and (b) are same as in Figure 7(a) and (b), respectively, but from the ERA-40 reanalysis. years with the extremely high or low January NSD-CEC index beyond one standard deviation during , the corresponding normalized SST index over the Niño 3 4 region ( W/5 S 5 N) used to indicate variability of ENSO and the corresponding normalized AO index. In seven years with a high NSD-CEC index, there are four years with a La Niña phase and four years with a positive AO index. In nine years with a low NSD- CEC index, there are six years with a La Niña phase and five years with a positive AO index. Although the heaviest snowstorm occurred over central-eastern China in both 1977 and 2008, there is an opposite sign in the Niño 3 4 or AO index between these two years. This weak relationship of the NSD-CEC index with the Niño

12 SNOWFALL OVER CENTRAL-EASTERN CHINA AND ASIAN ATMOSPHERIC COLD SOURCE or AO index is also supported by a correlation analysis. The correlation coefficient is 0.13 between the NSD-CEC and Niño 3 4 indices during and 0.03 between the NSD-CEC and AO indices, not significant at the 90% confidence level. These results imply that compared with the AACS index, the ENSO and AO indices are weaker in reflecting the variability of the January snowfall weather over central-eastern China. Acknowledgements We thank the Climate Diagnostic Center/NOAA for providing the NCEP-NCAR reanalysis data and UK Meteorological Office, Hadley Centre, for providing monthly mean HadISST data on their homepages. We also thank the Climate Prediction Center in NOAA/National Weather Service for providing AO index. The ECMWF ERA-40 data used in this study are obtained from the ECMWF data server. 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