The Role of Warm North Atlantic SST in the Formation of Positive Height Anomalies over the Ural Mountains during January 2008

Transcription

1 ADVANCES IN ATMOSPHERIC SCIENCES, VOL. 28, NO. 2, 2011, The Role of Warm North Atlantic SST in the Formation of Positive Height Anomalies over the Ural Mountains during January 2008 HAN Zhe 1,3 ( ), LI Shuanglin 2 ( ), and MU Mu 1,4 ( ) 1 State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing Nansen-Zhu International Research Centre and Key Laboratory of Regional Climate-Environment Research for Temperate East Asia, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing Graduate University of Chinese Academy of Sciences, Beijing Key Laboratory of Ocean Circulation and Wave, Institute of Oceanology, Chinese Academy of Sciences, Qingdao (Received 16 April 2010; revised 22 July 2010) ABSTRACT The most severe snowstorm and freezing-rain event in the past 50 years hit central and southern China in January One of the main reasons for the anomalous climate event was the occurrence of atmospheric circulation anomalies over middle and high latitudes, particularly the persistent blocking that occurred over the Ural Mountains. Along with atmospheric anomalies, a strong La Niña event in the Pacific and warm sea surface temperature anomalies (SSTAs) in the North Atlantic were the most significant in the lower boundary. Since a brief analysis suggests that La Niña exerts no significant impact on the Urals, the key point of focus in this study is on the role of the warmer SSTAs in the North Atlantic. Based on an observational composite, North Atlantic SSTAs pattern when the height anomaly over the Urals is strongly positive is found similar to that in January 2008, but no significant SSTAs occurred elsewhere, such as the Pacific. Using an atmospheric general circulation model, ECHAM5, the impact of North Atlantic SSTAs on the extratropical atmosphere circulation in the event was investigated. The results show that the warm SSTAs strengthened the blocking high over the Urals, through anomalous transient eddies. The consistency between the study model and the observational composite indicates that the warm SSTAs in the North Atlantic were indeed an important factor in the formation of the snowstorm disaster of January Key words: blocking, severe snow, North Atlantic SSTA, transient eddy Citation: Han, Z., S. L. Li, and M. Mu, 2011: The role of warm North Atlantic SST in the formation of positive height anomalies over the Ural Mountains during January Adv. Atmos. Sci., 28(2), , doi: /s Introduction In January 2008, a severe snowstorm disaster hit central and southern China, causing serious economic losses and a calamity for the country. Its occurrence can be attributed to a combination of several atmospheric circulation anomalies. First, the western Pacific subtropical high extended westward and northward relative to the norm, which, along with the enhanced trough over the southern Tibetan Plateau Corresponding author: HAN Zhe, hanzhe@mail.iap.ac.cn China National Committee for International Association of Meteorology and Atmospheric Sciences (IAMAS), Institute of Atmospheric Physics (IAP) and Science Press and Springer-Verlag Berlin Heidelberg 2011

2 NO. 2 HAN ET AL. 247 (at 85 E, the so-called plateau southern-branch trough ), brought more moisture to South China. Second, the frequent occurrence and long persistence of blocking over the Urals favored a cold-air pile-up in the north which then broke out toward the south. Third, the deeper East Asian grand trough near the western Pacific coast shaped and guided cold-air along a path (the so-called eastern path ) reaching farther south, including South China, more easily (Tao, 1957). The cold air from the north met and converged in South China with the warm and moist air from the South China Sea and the Bay of Bengal, creating the disastrous weather event. The Urals, one of three regions with the most frequent occurrence of persistent circulation anomalies in boreal winter (Dole and Gordon, 1983), exerts strong influence on the East Asian winter monsoon (Ye et al., 1962). Thus, for the January 2008 case, understanding the formation of the blocking over the Urals has utmost importance in understanding the formation of this anomalous climate event. Concurrent with the disastrous snowstorm event, significant sea surface temperature anomalies (SSTAs) emerged. In addition to the strong La Niña event in the tropical Pacific, the North Atlantic was anomalously warm, with the SSTAs two to three times the monthly standard deviation (Fu et al., 2008). The composite and statistical analyses (Fu et al., 2008; Wen et al., 2009) indicated no significant correlation between the circulation anomaly over the Urals and the La Niña event. Whether the warm North Atlantic played a role in the formation of the anomalous blocking over the Urals is an outstanding question for investigation. Previous studies indicate that extratropical SSTAs may act as a forcing mechanism in the overlying atmosphere, potentially enhancing the persistence and predictability of atmospheric anomaly (Kushnir et al., 2002); extratropical SSTAs are large heat-containing anomalies in the upper ocean (during winter). Thus, SSTAs may be critical factors in short-term forecasting. Particularly, several studies have suggested that the circulation anomalies over the Urals, which comprise one part of a wave-train like height anomaly chain originating from the upstream North Atlantic (Palmer and Sun, 1985; Li, 2004), are further linked to North Atlantic SSTAs (Palmer and Sun, 1985; Peng et al., 1995; Kushnir and Held, 1996; Li, 2004). However, much uncertainty about the pattern and the amplitude of the SSTA impacts exists in different models. Li (2004) used a version of NCEP s operational seasonal forecast model with T42 horizontal resolution and conducted two sets of ensemble runs, one with climatological SST and the other with the Northwest Atlantic SSTAs added to the climatological SST. Li s (2004) results indicated that the warm SSTAs in the northwestern Atlantic can induce positive height anomalies over the Urals in early winter (from October to December) through the direct diabatic heating and indirect transient eddy forcing. The fact that the model, overall, reproduces the observed atmospheric intrinsic variability argues the reasonability of his results. Whether the January 2008 snowstorm is such a case remains unclear because the atmospheric responses depend on the background circulation, which may depend on the model and seasonal shifts (Peng et al., 1995). The latter has been confirmed by the difference of atmospheric responses between the early winter (October December) and the late winter (February April) to the same forcing (Li, 2004). In our experiment, the atmospheric responses between the early winter and January also have an apparent discrepancy. In order to understand the January 2008 event, the atmospheric response to the North Atlantic SSTA in January must be specifically studied. In fact, many recent studies have been devoted to the purpose. Gu et al. (2008) found a positive correlation between the North Atlantic SSTA (30 50 N, W) and the height anomaly over the Urals and Siberia. In contrast, Chang et al. (2008) found that the correlation between the SSTA (45 N, 30 W) and the height anomaly over the Urals was not significant. Fu et al. (2008) speculated that the North Atlantic SSTA may have contributed to the formation of the positive height anomaly over the Urals, considering the similarity of the SSTA in January 2008 to that used for study in Li (2004). Zong et al. (2008) and Liu et al. (2008) studied the same issue by two different atmospheric general circulation models (AGCMs), IAP-T42L19 and IAP9L CoLM, separately. The observed positive height anomaly over the Yenisei was captured by both models when forced with the North Atlantic SSTAs, but not for the Urals. Particularly, the simulated response was even opposite to the observation data in the former model over the Urals. These results suggest a considerate uncertainty about the role of North Atlantic SSTAs in blocking over the Urals. Thus, it substantiates similar studies by using another AGCM. The AGCM used in our study is ECHAM5 which is the new Hamburg version of ECMWF model (Roeckner et al., 2003, 2006). Since a validation suggests that the model captures observed atmospheric intrinsic variability well. Hopefully, the present study can not only give an explanation for the height anomaly over the Urals in January 2008 but can also provide a validation for the previous results in Li (2004), Liu et al. (2008), and Zong et al. (2008). The remainder of the article is organized as follows. In section 2, the background circulations leading to the

3 248 THE ROLE OF WARM NORTH ATLANTIC SST IN JANUARY 2008 VOL Observation analyses Fig. 1. (a) The 500-hPa geopotential height anomaly (units: 10 gpm), (b) 850-hPa horizontal wind anomaly (units: m s 1 ) in January 2008, (c) Time-latitude cross section of 500-hPa geopotential height anomaly averaged over N (units: gpm). The shaded areas represent the regions where the geopotential height anomalies are >50 gpm. snowstorm are analyzed. Then, the North Atlantic SSTAs associated with positive height anomalies over the Urals are given using composite analysis. The oceanic heat status in December 2007 is then discussed briefly. In section 3, the atmospheric model and experimental design are introduced. In section 4, as a validation for the model used, simulated intrinsic variabilities are first compared with the observation data. The modeled responses are then displayed, and the mechanism for the maintenance of the responses is explored. Finally, a discussion and conclusions are given. The observed daily mean geopotential height and horizontal wind data, and monthly mean sensible heat net flux and latent heat net flux were obtained from the NCEP NCAR (National Center for Atmospheric Research) reanalysis (Kalnay et al., 1996). The monthly SST dataset was obtained from the Met Office Hadley Centre (Rayner et al., 2003). In January 2008, an excess of snowfall primarily occurred in central and southern China. One contributing factor was the strong anticyclone anomaly that extended from Europe to Asia, centering near the Urals (Fig. 1a). The northwesterly wind east of the Urals caused cold air to accumulate in northern Asia and then intruded into central and southern China between 100 E and 120 E (Fig. 1b). From the evolution of the height anomaly between 57.5 N and 62.5 N (Fig. 1c), a positive height anomaly remained around the Urals for almost the entire month of January and moved eastward slowly to 120 E, which favored the cold air invading China. The positive height anomaly over the Urals held for such a long period of time that it is easy to link it with the persistent SSTAs. Previous studies have also suggested that it was linked to the North Atlantic SSTAs. In order to investigate this linkage in January, as did Li (2004), the background circulation associated with positive height anomaly over the Urals was analyzed. First, a key point, (60 N, 60 E), was selected to represent the Urals, and a positive anomaly month was defined when the normalized height value at the key point was >1. Following this criterion, 15 January months with positive anomalies were selected from observation data. The composite was defined as the average of all of the selected cases. Figure 2a displays a wave-train-like height anomaly chain with a positive anomaly over the Urals, which indicates that the anomaly over the Urals may be linked with the anomaly over the North Atlantic. From the distribution of the composite of horizontal wind (Fig. 2b), strong northerly wind can be found over North China, which is similar to the situation in January Figure 2c shows the composite of SSTAs over North Atlantic for all of the selected cases, and significant warm SSTAs can be seen. The distribution of observed SSTAs (Fig. 3a) in December 2007, one month prior to the snowstorm, is similar to the composite (Fig. 2c). This suggests that the association of the observed SSTA in the north Atlantic with the positive height anomaly over the Urals in January 2008 is not unique; it is also reflected in historic records. No significant relationship occurs elsewhere, such as the Pacific (not shown). Our attention in this study

4 NO. 2 HAN ET AL. 249 Fig. 2. (a) Composite of 500-hPa geopotential height anomaly (units: gpm), (b) 850-hPa horizontal wind anomaly, and (c) SSTA (units: C) when the Ural has a positive anomaly, there are 15 Ural positive anomaly months in January during The shaded areas indicate significance at the 5% level. mainly focused on the influences of the North Atlantic SSTAs. Figure 3a also suggests that the North Atlantic SSTAs in December 2007 was very strong, since it was two to three times the climatological standard deviation (SDV hereafter) in December (Fig. 3b). Shown in Fig. 3c is surface atmosphere ocean heatflux anomaly, which reflects atmosphere ocean interactivity. From it more active atmosphere ocean interaction in the North Atlantic can be seen compared with other regions, which means that the North Atlantic may have been feeding back on the atmosphere. To quantify this influence, we used sensitivity experiments in an AGCM by forcing it with the North Atlantic SSTAs in December 2007, one month prior to the snowstorm. 3. Model and design of experiment The AGCM used here was the fifth version developed by the Max Plank Institute for Meteorology (ECHAM5), with T42 horizontal resolution and 19 vertical levels. The model was developed from the

5 250 THE ROLE OF WARM NORTH ATLANTIC SST IN JANUARY 2008 VOL. 28 Fig. 3. (a) The SST anomalies (units: C), dark (light) shading represents the region where the SSTA is less (more) than 0.5 C (1 C), (b) The SST anomaly standard deviation in December calculated from 1971 to 2000 (units: C), the shaded areas represent the region where the SDV is >0.5 C, (c) the surface atmosphere ocean heat fluxes anomaly (sensible heat net flux added latent heat net flux, units: W m 2 ) in December spectral weather prediction model of the European Centre for Medium Range Weather Forecasts. More detailed description can be found in Roeckner et al. (2003, 2006). The North Atlantic SSTA (30 60 N) was extracted from the observed anomaly pattern relative to the climatological monthly mean. Two pairs of experiments were conducted, one with the climatological seasonal cycle of SST (the control runs), and the other with the North Atlantic SSTA (30 60 N) added to the climatological SST (SSTA runs). The model was integrated for 20 years, and the first year was discarded as the spin-up period. Thus, only the remaining 19 years were used for analysis. 4. Modeled response and its maintenance 4.1 Model s intrinsic variability As discussed in the introduction, the SSTA-forced responses in the different models may disagree, and possibly due to model deficiencies in simulating the observed climate and intrinsic variability. Many researchers have noted that mid-latitude SSTAs usually only shift the distribution and change the amplitude of existing internal modes rather than inducing any new mode responses, and that the atmospheric response to mid-latitude SSTAs share the same dynamic mechanism with the internal low-frequency modes (Peng and Whitaker, 1999; Ting and Peng, 1995; Branstator,

6 NO. 2 HAN ET AL ). Therefore, only models that reproduce intrinsic low-frequency and storm-track variability similar to the observation may realistically simulate mid-latitude SSTA influences. Figure 4 shows the comparison between the observational intrinsic variability and the model s intrinsic variability. The amplitude and distribution of SDV of 500-hPa geopotential height in the model (Fig. 4a) are similar to those of the observation (Fig. 4b), although the amplitude in the model is a little larger. Figure 4c and 4d display the comparison of the leading EOF in the model and the observation. Evidently, both exhibit a wave-train like chain from the North Atlantic to the Urals. The value in the model is also bigger than in the observations, suggesting that the responses forced by the mid-latitude SSTA may be slightly exaggerated. From Figs. 4e and 4f, the amplitude and distribution of storm track in the model coincide with the observation data. Here, a poor man s filter was used to isolate the synoptic components with periods <9 days, which was not significantly different from a 61-point filter (Li, 2004). The latter is often used to obtain the component with time scales of 3 10 days. The above analyses illustrate that the model was able to reproduce the observed wave-like low-frequency variability and the storm track from the North Atlantic to the Urals. This suggests that, to a large extent, the modeled atmospheric response to mid-latitude SSTA was able to reflect the realistic situation in the observations. 4.2 Atmospheric responses to the midlatitude SSTA Figure 5a displays the difference of 500-hPa geopotential heights between the SSTA and the control runs in January. The modeled atmospheric responses resemble the observations in January 2008 over the domain stretching from the northern Atlantic through Europe to western Asia. Particularly, the model bears a positive height response over the Urals, which is similar to the observed anomaly, although the maximum center of modeled 500-hPa geopotential height response over the Urals shifted slightly to the west (Fig. 1a). The modeled 850-hPa horizontal wind response also resembled the observed anomalies over the extratropical regions (Figs. 5b and 1b). Particularly, an anticyclone circulation anomaly was significant in southeastern China in the model, which was similar to the enhanced subtropical high in the observations. The mechanism for the response over the Urals deserves further study. 4.3 Maintenance of response Synoptic eddies forcing plays an important role in the persistence of the blocking (Trenberth, 1986; Wu et al., 1995; Lu, 2001; Luo et al., 2002; Luo, 2005). So what about the January 2008 case? To illustrate the role of synoptic eddies in the blocking. First, the effect of synoptic eddy forcing on largescale anomaly was analyzed using Luo s theory (2005). From the large-scale vorticity feedback induced by synoptic eddies, it was easy to determine whether the lowfrequency anomaly was driven by transient synoptic eddies. Then, the local E P vector and its divergence were diagnosed (Trenberth, 1986). The role of transient eddies on zonal and meridional wind is discussed separately. In Luo s theory (2005), the role of transient synoptic eddies was determined using the formula: V ξ ) P, where ξ is the relative vorticity, V is the horizontal wind vector, subscript P denotes a large-scale component, and the primes denote a synoptic transient quantity. For example, (V ξ ) P have a negative anomaly where there is a positive height anomaly if the large-scale anomaly is driven by transient eddies. In Fig. 6a, there is a positive distribution of (V ξ ) P between 45 N and 60 N over the North Atlantic, which leads a negative height anomaly. With the positive height anomaly between 30 N and 45 N that was also intrigued by transient eddies, the positive phase of NAO can be explained. The effect of transient eddies on NAO was also clear and significant in the model (Fig. 6b). (V ξ ) P was guided by negative component in the north branch of the blocking (north 55 N) in January 2008 and benefitted the positive height anomaly, while the positive component in the south branch benefitted the negative height anomaly (Fig. 6a). Negative (V ξ ) P was obvious north of 45 N over Europe and Asia in the model, which showed the indirect effect of North Atlantic SSTAs on the positive height anomaly over the Urals. Local E P flux was then analyzed. The westerly accelerated where the E u diverged and decelerated where the E u converged, while the southerly accelerated where E v diverged and decelerated where E v converged. The local E P is expressed as: [ ] 1 ) E u = (v 2 2 u 2, u v, f v Φ z S [ ] E v = u v, 1 ) (v 2 2 u 2 Φ z = RT, f u Φ z S (1)

7 252 THE ROLE OF WARM NORTH ATLANTIC SST IN JANUARY 2008 VOL. 28 Fig. 4. Left panels are for ECHAM5, and right panels are for the observations. The SDV (a, b) (The shaded areas represent the region where the SDV is more than 80 gpm) and leading EOF (c, d) of 500-hPa monthly geopotential height in January, the domain for EOF analysis is ( N 90 W 90 E), the variance explained by the leading EOF is 45% for the model and 33% for the observations. The mean of SDV of transient eddies in January for (e, f). (Units: gpm.) Here R is the gas constant, S is the static stability. The time means are indicated by overbars and synoptic transient quantity by primes. In the article, only the horizontal components which indicate transient vorticity forcing are analyzed. The relative group velocity C g V was parallel to E u, while C g was also the velocity of wave-energy propagation. In Fig. 6c, the thick line represents a 500-hPa geopotential height, and the thin line is the divergence of E v. Two divergence centers of E v upstream of the blocking acted to accelerate southerly, while the convergence of E v north of the blocking accelerated northerly. All of these conditions favored the maintenance of the blocking. Compared to Fig. 6b, the two divergences in the model were similar to the observations. Figure 6c suggests that the transient eddy favors the westerly acceleration over the North Atlantic; it explains the positive phase of NAO in the observations. The divergences of E u extended from southeast to northeast over upstream blocking, which caused the westerly deceleration and bifurcation into two branches, thus favoring the maintenance of the blocking. Compared to

8 NO. 2 HAN ET AL. 253 Fig. 5. (a) The model s 500-hPa geopotential height (units: 10 gpm), (b) 850-hPa horizontal wind (units: m s 1 ) response in January. (c) The model s 500-hPa geopotential height (units: 10 gpm) response during October December. The shaded areas indicate significance at the 10% level. Fig. 6c, although the divergences of E u occurred over upstream blocking in the model (Fig. 6d) was small compared to the convergence around it, the pattern of the divergence of E u resembled that shown by the observations. From this analysis, the importance of the role of transient eddies in the formation of low-frequency anomalies can be seen. When the direction of propagation was determined, the original source of the transient eddies which drove the large-scale circulation over the Urals was made clear. Compared to the climatology, the activity of transient eddies over the North Atlantic and Europe was found to be reinforced, while the activity over the Urals was found to be weakened in January 2008 (Fig. 7a). As mentioned above, the relative group velocity C g V was parallel to E u. This suggests that the velocity of wave energy propagated eastward, where both horizontal wind V and E u had eastward components. The shaded areas in Fig. 7 represent the region with eastward wave-energy propagation, which indicates strong interactivity between transient eddies and low-frequency anomalies from the North Atlantic to the Urals. The transient eddies induced by the SSTA exhibited a similar distribution from the North Atlantic to the Urals. This illustrates that the transient eddies played a significant role in the maintenance of the blocking over the Urals in January Hence the transient-eddy anomalies induced by the North Atlantic SSTA were an important mechanism for the positive geopotential height anomaly over

9 254 THE ROLE OF WARM NORTH ATLANTIC SST IN JANUARY 2008 VOL. 28 Fig. 6. (a) The thick line is 500-hPa geopotential height (units: gpm), shaded areas are (V ξ ) P (units: s 2 ). (b) (V ξ ) P simulated by the model (units: s 2 ). (c) The thick line is 500-hPa geopotential height (units: gpm), and the thick line is divergence of E v (units: m s 2 ) in January 2008, dark (light) shading represents the region where the divergence of E v is more (less) than m s 2 ( m s 2 ). (d) The divergence of model simulated E v (units: m s 2 ), dark (light) shading represents the region where the divergence of E v is more (less) than m s 2 ( m s 2 ). (e) Same as (c), but for observational E u. (f) Same as (d), but for E u simulated by the model. the Urals. 5. Summary and discussion Previous studies suggested that the frequency and persistence of blocking over the Ural Mountains played an important role in the formation of a severe snowstorm in southern China in January 2008 and that the warm North Atlantic SSTA was a substantial feature at the lower boundary prior to the event. Whether the warm North Atlantic played a role in the formation of positive height anomalies over the Urals was investigated in the study. First, based on historical records, we analyzed the middle and high latitudinal background circulation associated with the positive height anomalies over the Ural Mountains, and we illustrated a wave-train like height chain linking the Urals to the North Atlantic. Meanwhile, the composite of horizontal winds showed strong northerly wind in North China that resembled the situation in January The spatial pattern of the SSTA prior to the snowstorm (December 2007) resembled the historical SSTA composite in North Atlantic. In the composite, significant SSTAs did not occur elsewhere, such as the Pacific. The role of the North Atlantic SSTA in the formation of the anomaly in the Urals was primarily investigated. The modeled atmospheric responses to the North Atlantic SSTA resembled the observed circulation anomalies over the domain from the North Atlantic to the Urals; however, the positive height anomaly over Siberia was not captured. Then (V ξ ) P and the local E P flux were examined in order to understand how the mechanism of the responses maintained over time was. Significant energy transformation from the transient eddies to the low-frequency variability existed over the domain from the north Atlantic to the Urals, and the transient-eddy forcing favored the maintenance of the blocking over the Urals. The situations in the model and the observation data were similar over upstream

10 NO. 2 HAN ET AL. 255 Fig. 7. (a) The difference of the activity of transient eddies between January 2008 and climate, (b) The activity of transient eddy anomalies simulated by the model. Shading indicates the wave energy propagate eastward. Fig. 8. The 850-hPa horizontal wind responses in January to the central and eastern Pacific negative SSTA in December The shaded areas indicate significance at the 10% level. (units: m s 1 ) blocking. The resemblance of the present results to Li (2004) suggests that the forcing from the North Atlantic on the positive height anomalies over the Urals is robust, since a different model was used and a somewhat different season (middle winter vs. early winter) was concerned. On the other hand, the atmospheric responses were different between January (Fig. 4b) and early winter (October December; Fig. 4c), supporting the finding that the atmospheric responses depend on the background circulation. Possibly because of this, the response over the Urals between this study and Li s (2004) experiments were similar, although the forcing methods were different. This study extends Li s (2004) result and suggests that the impact of the North Atlantic on the Urals occurs not only in early winter but also in middle winter (January). This result is important for improving the short-term climate prediction of East Asian winter monsoon; the SSTA tends to persist longer than atmospheric anomalies. Whether and to what extent the strong La Niña event affected the Urals is a question that remains to be answered. One composite analyses (not shown) indicated no significant correlation between them. To further verify this, we conducted an additional experiment with the same model using the observed SSTA over the central and eastern Pacific (December 2007); the results display a cyclone anomaly response over the Urals in January (Fig. 8), contrary to the positive anomaly in the observations and in the North Atlantic SSTA experiments. Consistent with the composite and statistical analyses of other studies (Fu et al., 2008; Wen et al., 2009), the relationship between them is not significant. This suggests no significant influence on the Urals from the La Niña event, although we speculate that La Niña may have played a substantial role in the occurrence of the snowstorm. As mentioned in the introduction, the atmospheric circulation anomalies leading to the occurrence of the January 2008 snowstorm include several aspects, and the positive height anomaly over the Urals is only one of them. Thus, the present result is insufficient, albeit helpful, for completely understanding the snowstorm. In January 2008, the strong anomalous southwesterly over the Bay of Bengal and the subtropical western Pacific high were also crucial, and the storm s formation is also unclear. These aspects of this storm are topics for future research. Acknowledgements. The authors are grateful to the two anonymous reviewers for their helpful critiques and suggestions. This study was jointly supported by the National Key Basic Research Program of China (Grant No. 2009CB421401) and the Special Program of China Mete-

11 256 THE ROLE OF WARM NORTH ATLANTIC SST IN JANUARY 2008 VOL. 28 orological Administration (Grant No. GYHY ) and is contributing to the Norwegian Research Council project East Asian DecCen (Grant No ). REFERENCES Branstator, G., 1995: Organization of storm track anomalies by recurring low-frequency circulation anomalies. J. Atmos. Sci., 52, Chang, R., Q. Y. Zhang, and J. B. Peng, 2008: Response of the atmospheric circulation with heavy snow over southern China to the sea surface temperature anomaly in the key areas. Climatic and Environmental Research, 13(4), (in Chinese) Dole, R. M., and N. D. Gordon, 1983: Persistent anomalies of the extratropical northern hemisphere wintertime circulation: Geographical distribution and regional persistence characteristics. Mon. Wea. Rev., 111, Fu, J. J., S. L. Li, and Y. M. Wang, 2008: Influence of prior thermal state of global oceans on the formation of the disastrous snow storm in January Climatic and Environmental Research, 13(4), (in Chinese) Gu, L., K. Wei, and R. H. Huang, 2008: Severe disaster of Blizzard, freezing rain, and low temperature in January 2008 in China and its association with the anomalies of East Asian monsoon system. Climatic and Environmental Research, 13(4), (in Chinese) Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40- year reanalysis project. Bull. Amer. Meteor. Soc., 77, Kushnir, Y., and I. M. Held, 1996: Equilibrium atmospheric response to North Atlantic SST anomalies. J. Climate, 9, Kushnir, Y., W. A. Robinson, I. Blade, N. M. J. Hall, S. Peng, and R. Sutton, 2002: Atmospheric GCM response to extratropical SST anomalies: Synthesis and evaluation. J. Climate, 15(16), Li, S. L., 2004: Influence of t he Northwest Atlantic SST anomaly on the circulation over t he Ural Mountains. J. Meteor. Soc. Japan, 82(4), Liu, S. F., H. Chen, and Z. H. Lin, 2008: Numerical simulation of the impact of sea surface temperature anomalies on the climate anomalies over China in January Climatic and Environmental Research, 13(4), (in Chinese) Lu, R. Y., 2001: Eddies during the blocking maintenance over the northeastern Asia in summer. Chinese J. Atmos. Sci., 25(3), (in Chinese) Luo, D. H., 2005: A barotropic envelope Rossby soliton model for block eddy interaction. Part I: Effect of topography. J. Atmos. Sci., 62, Luo, D. H., J. P. Li, and F. Huang, 2002: Life cycles of blocking flows associated with synoptic-scale eddies: Observed results and numerical experiments. Adv. Atmos. Sci. 19(4), Palmer, T. N., and Z. Sun, 1985: A modeling and observational study of the relationship between sea surface temperature in the Northwest Atlantic and the atmospheric general circulation. Quart. J. Roy. Meteor. Soc., 111, Peng, S., and J. S. Whitaker, 1999: Mechanism determining the atmospheric response to midlatitude SST anomalies. J. Climate, 12, Peng, S., L. A. Mysak, H. Ritchie, J. Derome, and B. Dugas, 1995: The differences between early and midwinter atmospheric responses to sea surface temperature anomalies in the northwest Atlantic. J. Climate, 8, Rayner, N. A., D. E. Parker, E. B. Horton, C. K. Folland, L. V. Alexander, D. P. Rowell, E. C. Kent, and A. Kaplan, 2003: Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. J. Geophys. Res, 108(D14), Roeckner, E, and Coauthors, 2003 Atmospheric General Circulation Model ECHAM5: Part I. Tech. Rep. 349, Max-Planck-Institute for Meteorology, 140pp. Roeckner, E., R. Brokopf, M. Esch, M. Giorgetta, S. Hagemann, and L. Kornblueh, 2006: Sensitivity of simulated climate to horizontal and vertical resolution in the ECHAM5 atmosphere model. J. Climate, 19(16), Tao, S. Y., 1957: Study on East Asia cold waves activity. Handbook of Short-term Forecast, Central Weather Bureau 220pp. Ting, M., and S. Peng, 1995: Dynamics of the early and middle winter atmospheric responses to the Northwest Atlantic SST anomalies. J. Climate, 8, Trenberth, K. E., 1986: An assessment of the impact of transient eddies on the zonal flow during a blocking episode using localized Eliassen Palm flux diagnostics. J. Atmos. Sci., 43, Wen, M., S. Yang, A. Kumar, and P. Zhang, 2009: An analysis of the large-scale climate anomalies associated with the snowstorms affecting China in January Mon. Wea. Rev., 137, Wu, G. X., H. Liu., F. Chen, Y. C. Zhao, and Y. Lu, 1995: Wave-meanflow interaction and formation of blocking-persistent anomalous weather in China in the summer of Acta Meteorological Sinica, 9(2), Ye, D. Z., S. Y. Tao, and B. Z. Zhu, 1962: The Study on Wintertime Blocking Pattern of the Northern Hemisphere. Science Press, 220pp. (in Chinese) Zong, H. F., Q. Y. Zhang, C. L. Bueh, L. R. Ji, and L. T. Chen, 2008: Numerical Simulation of possible impacts of Kuroshio and north Atlantic sea surface temperature anomalies on the South China snow disaster in January Climatic and Environmental Research, 13(4), (in Chinese)