Impact of the Scandinavian Pattern on Long-Lived Cold Surges over the South China Sea

Bo Pang aState Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

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Riyu Lu aState Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
bCollege of Earth and Planetary Sciences, University of the Chinese Academy of Sciences, Beijing, China

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Rongcai Ren aState Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
cKey Laboratory of Meteorological Disaster, Ministry of Education (KLME), Joint International Research Laboratory of Climate and Environment Change (ILCEC), Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD), Nanjing University of Information Science and Technology, Nanjing, China

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Abstract

This study investigates the influence of the Scandinavian (SCA) pattern on long-lived cold surges over the South China Sea (SCS). The results show that, different from the short-lived ones, the majority of long-lived cold surges over the SCS are preceded by a negative phase of the quasi-stationary SCA pattern in the extratropics, which is characterized as a primary cyclonic center over the Scandinavian Peninsula and two anticyclonic ones over the North Atlantic and central Siberia. This connection is mainly conducted through a continuous amplification of the high pressure anomalies over East Asia. On the other hand, the SCA-related anomalies also reveal identical responses as an increase in sea level pressure over East Asia and northerly flows over the SCS. Besides, the SCA pattern may influence the long-lived cold surges over the SCS by facilitating blocking occurrences through the extensive and quasi-stationary anticyclone over central Siberia. The present results have an implication for the extended weather forecast: long-lasting circulation anomalies, such as the SCA pattern, can affect long-lasting weather phenomena in the regions that are located remotely in both the zonal and meridional directions, such as long-lived cold surges over the SCS.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Bo Pang, pangbo@mail.iap.ac.cn

Abstract

This study investigates the influence of the Scandinavian (SCA) pattern on long-lived cold surges over the South China Sea (SCS). The results show that, different from the short-lived ones, the majority of long-lived cold surges over the SCS are preceded by a negative phase of the quasi-stationary SCA pattern in the extratropics, which is characterized as a primary cyclonic center over the Scandinavian Peninsula and two anticyclonic ones over the North Atlantic and central Siberia. This connection is mainly conducted through a continuous amplification of the high pressure anomalies over East Asia. On the other hand, the SCA-related anomalies also reveal identical responses as an increase in sea level pressure over East Asia and northerly flows over the SCS. Besides, the SCA pattern may influence the long-lived cold surges over the SCS by facilitating blocking occurrences through the extensive and quasi-stationary anticyclone over central Siberia. The present results have an implication for the extended weather forecast: long-lasting circulation anomalies, such as the SCA pattern, can affect long-lasting weather phenomena in the regions that are located remotely in both the zonal and meridional directions, such as long-lived cold surges over the SCS.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Bo Pang, pangbo@mail.iap.ac.cn

1. Introduction

Cold surges are episodic progressions of northerly winds, which occur frequently over the South China Sea (SCS) during the boreal winter (Ramage 1971; Chang et al. 1983; Lau and Chang 1987; Boyle and Chen 1992; Compo et al. 1999; Chan and Li 2004; Chang et al. 2011). When they penetrate into the tropics, cold surges play a key role in inducing heavy rainfall over Southeast Asia and active convection over the Maritime Continent (e.g., Johnson and Houze 1987; Slingo 1998; Chang et al. 2005; Wu et al. 2007; Tangang et al. 2008; Yokoi and Matsumoto 2008; Trilaksono et al. 2012; Chen et al. 2015; Pullen et al. 2015; Lim et al. 2017). Despite their usual characteristic as a transient disturbance, some cold surge events over the SCS can persist for a long period, even two weeks (Chang et al. 2006). However, most studies have focused on the synoptic features during cold surge outbreaks and less attention has been paid to their maintenance, even though the duration has more severe socioeconomic effects on the SCS and adjacent countries. For instance, Tangang et al. (2008) analyzed the role of northerly cold surges on the extreme floods over peninsular Malaysia lasting from mid-December 2006 to late January 2007, which was the worst occurrence in a century and resulted in 200 000 people being evacuated and USD 500 million of economic losses.

It is well documented that cold surges over the SCS are strongly affected by extratropical circulation and are directly driven by a southward extension of the Siberian high along its southeastern edge (Boyle 1986; Ding 1990; Wu and Chan 1995; Zhang et al. 1997; Lu and Chang 2009; Pang and Lu 2019). Our recent study indicated that distinct extratropical circulation anomalies are responsible for short-lived and long-lived cold surges (Pang et al. 2020). The long-lived cold surges are accompanied with frequent occurrence of Siberian blocking, which results in persistent high pressure anomalies over East Asia via continuous cold advection in the lower troposphere. However, the blocking frequency in the short-lived ones shows no obvious increase. This finding suggests a discrepancy in the extratropical circulations in inducing long- and short-lived cold surges over the SCS.

The atmospheric teleconnection patterns, in addition to blocking highs, are also the dominant low-frequency anomalies in extratropical circulation (Wallace and Gutzler 1981; Barnston and Livezey 1987). In particular, the Scandinavian (SCA) pattern, which comprises an anticyclonic center over the Scandinavian Peninsula and two cyclonic centers over North Atlantic and central Siberia for its positive phase, exerts an extensive influence on weather over the Eurasian continent (Bueh and Nakamura 2007). For instance, the positive phase of SCA pattern, featuring anticyclones over the Scandinavian Peninsula, can induce vast cooling from western Siberia to Lake Baikal due to the prevailing northerly anomalies (Bueh and Nakamura 2007; Sohn et al. 2011; Liu et al. 2014; Yu et al. 2017). The impacts of the SCA pattern might extend into farther remote regions, and this pattern is argued to be responsible for the lasting low temperature and heavy snowfall over southern China in January 2008 (Wen et al. 2009; Zhou et al. 2009; Bueh et al. 2011). Considering that one of the active centers of the SCA pattern is over central Siberia, it is reasonable to hypothesize that the SCA pattern may be responsible for the persistence of cold surges over the SCS through modulating the blocking frequency over central Siberia.

This study aims to investigate the influence of the SCA pattern on long-lived cold surges over the SCS and conduct the analysis from the view of both cold surge events and SCA events. The rest of this paper is arranged as follows. Section 2 describes the data, indices, and methods. Section 3 explores the preceding circulation anomalies related to the long-lived cold surges. Section 4 further analyzes the following circulation anomalies resulted from the SCA pattern. Section 5 is devoted to a summary and discussion.

2. Data, indices, and methods

The data used in this study are obtained from the European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim; Dee et al. 2011) covering the period from 1979 to 2018. This dataset is obtained at a horizontal resolution of 0.75° × 0.75° and temporal resolution of 6 h. All the 6-h data are converted to daily-mean values and the anomalies are calculated by removing the 40-yr mean of the calendar day from the raw data. The boreal winter is defined as periods from November to February (NDJF) with omission of 29 February in leap years.

The cold surges over the SCS are identified based on a low-level wind index and a sea level pressure (SLP) index. The low-level wind index is computed as 925-hPa meridional winds averaged over 110°–117.5°E along 15°N, and the SLP index is defined as area-averaged SLP anomalies over the domain 15°–45°N, 100°–120°E (blue box in Fig. 3a) weighted by the cosine of the latitude. A cold surge day is counted when the wind index is below a threshold (−8.5 m s−1) of −0.75 standard deviations (σ) from the winter mean and also the SLP index is positive. This method is identical to previous ones used, for example, by Chang et al. (2005) and Lim et al. (2017). The major factor distinguishing this from these previous definitions is the average regions for the SLP index: 18°–22°N, 105°–122°E is used in Lim et al. (2017) but 15°–45°N, 100°–120°E in this study, where high pressure anomalies are dominant by compositing all cold surges (Pang and Lu 2019; their Fig. 2a). Furthermore, continuous surge days are merged into one cold surge event. Accordingly, 307 cold surge events are obtained, and the first occurrence day is defined as day 0.

Figure 1 shows the numbers of cold surge events with different durations. The general distribution is identical to Pang et al. (2020; their Fig. 1), but the number of occurrences decreases due to the limitation of the SLP index, which excludes northerly surges induced by tropical disturbances. As in their criteria, cold surge events longer than 5 days are classified as long-lived events (red bars; 33 events) while the others are considered short-lived ones (gray bars; 274 events). On average, the long-lived events persist nearly 9 days, whereas the short-lived ones are less than 3 days.

Fig. 1.
Fig. 1.

Occurrences of cold surge events with different durations. Red and gray bars refer to the long- and short-lived events, respectively.

Citation: Journal of Climate 35, 6; 10.1175/JCLI-D-21-0607.1

The SCA index is defined according to Wang and Tan (2020). First, rotated empirical orthogonal functions (REOFs) are conducted on monthly 300-hPa geopotential height (Z) anomalies over the domain 20°–87.5°N, 60°W–150°E during the boreal winter. Here, the field was weighted by the square root of the cosine of the latitude. The SCA pattern is identified as the third REOF pattern with an explained variance of 10.7% (Fig. 2). Besides, the principal component shows a significant correlation (r = 0.8; p < 0.01) with the time series obtained from Climate Prediction Center/National Oceanic and Atmospheric Administration (CPC/NOAA; ftp://ftp.cpc.ncep.noaa.gov/wd52dg/data/indices/scand_index.tim), which performed a larger domain in the REOFs (20°–90°N). Here, the significance of correlation is estimated by a two-tailed Student’s t test. Then, the daily SCA index is calculated by projecting daily 300-hPa Z anomalies onto the SCA pattern and standardized for the boreal winter afterward. Finally, a SCA event is detected when the daily SCA index exceeds 1.0σ for at least 4 consecutive days and the interval for two adjacent SCA events should be larger than 15 days as the e-folding time scale is about two weeks. The reference (day 0) of a SCA event is defined when the SCA index reaches its strongest amplitude.

Fig. 2.
Fig. 2.

The third rotated empirical orthogonal function (REOF) on geopotential height (Z; m; interval: 15 m) anomalies at 300 hPa over the Eurasian sector (20°–87.5°N, 60°W–150°E) during the boreal winter. Red and solid (blue and dashed) contours indicate positive (negative) anomalies, and zero contours are omitted.

Citation: Journal of Climate 35, 6; 10.1175/JCLI-D-21-0607.1

Atmospheric blocking is identified by a two-dimensional index based on the reversal of the daily 500-hPa Z meridional gradient from south (GHGS) and north (GHGN) following Davini et al. (2012):
GHGS(λ,ϕ)=Z(λ,ϕ)Z(λ,ϕS)ϕϕS>0,
GHGN(λ,ϕ)=Z(λ,ϕN)Z(λ,ϕ)ϕNϕ<10 m (° latitude)1,
in which λ and ϕ are the longitude and latitude that range from 0° to 360° and from 30° to 75°N, respectively; ϕs = ϕ − 15°, ϕN = ϕ + 15°. An instantaneous blocking at a fixed grid point (λ, ϕ) is identified when both the above criteria are satisfied. Blocking frequency is calculated as the proportion of days when instantaneous blocking is detected.

Time-lag composites are performed to analyze the temporal and spatial evolutions of relevant circulation. To assess statistical significance, a Monte Carlo bootstrap resampling is performed 1000 times on randomly selected subsamples for each grid point at the composite days (Efron and Tibshirani 1993). The result is regarded as significant at 95% confident level if zero lies outside the 2.5–97.5 percentiles of these resampled values. We also examined the significance by a two-tailed Student’s t test, adjusted for the effective degree of freedom following Zwiers and von Storch (1995), and obtained similar results. A two-dimensional wave activity flux is used to measure the horizontal propagation of wave packets based on circulation anomalies relevant to the SCA pattern, which is parallel to the group velocity of a stationary Rossby wave, following Takaya and Nakamura (1997, 2001).

3. Preceding circulation anomalies associated with the long-lived cold surges over the SCS

Figure 3 compares the composites of circulation at day 0 relevant to the short- and long-lived cold surge events. A remarkable difference is found in the spatial extent of anomalous circulation. The anomalies for the short-lived events are mainly dominant over East Asia, featuring high pressure anomalies over eastern China in the lower troposphere (Fig. 3a), an intensified Siberian ridge, and a deepened East Asian trough in the upper troposphere (Fig. 3c). The results are similar to the composites for all cold surges in Pang and Lu (2019; their Fig. 2), indicating the major contribution of the short-lived events. However, for the long-lived cold surges, circulation anomalies are widely spread across the whole Eurasian continent. In the lower troposphere, high pressure anomalies over East Asia, which result in northerlies over the SCS directly, are much stronger and more extensive than the short-lived ones (Fig. 3b). Apart from that, pronounced cyclonic anomalies are found over northern Europe, which are absent in the composite of the short-lived cold surges. Meanwhile, the upper-tropospheric circulation is manifested as wavelike anomalies stretching from North Atlantic to East Asia (Fig. 3d), which resembles the negative phase of the SCA pattern (SCA; Bueh and Nakamura 2007). In comparison, the SCA index at day 0 distinguishes between the short- (−0.2) and long-lived (−0.8) events at 99% confidence level. Therefore, the result indicates an obvious discrepancy in extratropical circulation between the short- and long-lived cold surges and implies a crucial role of the SCA pattern in inducing the long-lived cold surges.

Fig. 3.
Fig. 3.

Composites of (a),(b) sea level pressure (SLP; contours; hPa; interval: 2 hPa) and 925-hPa horizontal wind (vectors; m s−1) anomalies; (c),(d) 300-hPa Z anomalies (contours; m; interval: 25 m) and horizontal wave activity flux (WAF; vectors; m2 s−2); and (e),(f) 2-m temperature (T2m; contours; K) anomalies at day 0 for the (left) short-lived and (right) long-lived cold surge events. Contours significant at the 95% confidence level are shaded and vectors are shown as thick and black when they are greater than one and significant in at least one direction. The box in (a) refers to the region used to define SLP index and the ones in (e) and (f) are used to define the T2m index.

Citation: Journal of Climate 35, 6; 10.1175/JCLI-D-21-0607.1

Additionally, the surface temperature anomalies for both the short- and long-lived cold surges feature cooling over East Asia and warming over Siberia, corresponding to the southward thermal advection in the lower troposphere (Figs. 3e,f). However, the cooling over East Asia is stronger and more extensive for the long-lived cold surges, implying a more severe impact of the persisting events. The difference in surface temperature is attributed to the distinct northerly anomalies along the coastal East Asia, which extend farther for the long-lived events (Figs. 3a,b). Moreover, the larger range of northerly anomalies in the lower troposphere may be related to the stronger and broader positive height anomalies in the upper troposphere over central Siberia, whereas the strengthening of East Asian trough, indicated by negative height anomalies over East Asia, shows a similar intensity (Figs. 3c,d).

Figure 4 shows the temporal evolutions of circulation indices relevant to the onset of cold surge events. Here, the T2m index is defined as area-averaged surface temperature anomalies over the domain 15°–45°N, 100°–130°E (boxes in Figs. 3e,f). For the short-lived events, the rapid increase of northerlies is contributed to SLP rising over East Asia, but the amplitude of SCA index is much weaker and insignificant (Fig. 4a), consistent with Fig. 3c. However, for the long-lived cold surges, it indicates that the continuous northerlies over the SCS are associated with not only an increased SLP over East Asia, but also a successive SCA pattern (Fig. 4b). As a reference, the wind index intensifies rapidly at day 0 and maintains its amplitude below threshold until day +7. However, the extratropical circulation indices are enhanced ahead of the wind index by approximately 1–2 days. On the one hand, the SLP index stays positive significantly from day −2 to day +7, which has been reported as a direct cause of persistent northerlies over the SCS. On the other hand, the SCA index also remains negative continuously from day −2 to day +7 as well. Additionally, the intensity of the SCA index is significantly (p < 0.05) stronger for the long-lived events (−0.7) than the short-lived one (−0.1) averaged over the prominent period from day −2 to day +7, suggesting its contribution to the long-lived cold surges over the SCS. As a result, the cooling over East Asia shows a great distinction in duration between the short- and long-lived cold surges. Unlike a transient drop of surface temperature for the short-lived events, the long-lived one displays a successive and prominent cooling from day −1 to day +6, implying a lasting impact on East Asia. The averaged amplitudes of T2m index are distinguished between the short-lived (−0.2) and long-lived (−0.8) cold surge events at 99% confidence level over the prominent period from day −1 to day +6.

Fig. 4.
Fig. 4.

Temporal evolutions of the normalized wind index (bars), T2m index (purple curves), SLP index (red curves), and SCA index (blue curves) relevant to the (a) short-lived and (b) long-lived cold surge events. Shaded bars and thick curves represent values significant at the 95% confidence level compared with the winter mean.

Citation: Journal of Climate 35, 6; 10.1175/JCLI-D-21-0607.1

Figure 5 displays scatterplots of the SLP index and SCA index averaged from day −2 to day +7. In general, most of the long-lived cold surges (26/33; 79%) show a positive SLP index and a negative SCA index, illustrated by red dots in the lower-right quadrant (Fig. 5b), whereas there is no such strong tendency in the SCA index for the short-lived ones (Fig. 5a). It is revealed that most of the long-lived cold surges are dominated by high pressure anomalies over East Asia and SCA anomalies across Eurasia in the upper troposphere, as shown in Figs. 3b and 3d. Moreover, the SCA indices for the 26 negative cases are relatively strong as the amplitudes of half cases exceed −1.0σ, whereas the values of the seven positive SCA cases are all but one weaker than +1.0σ. Thus, the SCA pattern appears in the most long-lived cold surge events, and hereafter the composites are performed on the 26 SCA cases to highlight the relationship between SCA pattern and long-lived cold surges.

Fig. 5.
Fig. 5.

Scatterplots of normalized SLP index vs SCA index averaged from day −2 to day +7 for (a) short-lived and (b) long-lived cold surge events.

Citation: Journal of Climate 35, 6; 10.1175/JCLI-D-21-0607.1

Figure 6 shows the lead–lag composites of anomalous SLP and 925-hPa wind anomalies for the 26 long-lived cold surges with negative SCA index. The result shows a prominent impact of SCA anomalies on lower-tropospheric circulation over East Asia that further leads to long-lasting cold surges. The persistent northerlies over the SCS are driven by a successive intensification and southward movement of the Siberian high, as mentioned in previous work (Pang et al. 2020). However, the lower-level anomalies can be traced back to disturbances upstream, which are formed as anticyclones over North Atlantic and cyclones over northern Europe at day −6 (Fig. 6a). Then, the SLP anomalies are enhanced and further evolve into tripolar anomalies with a primary cyclonic center over the Scandinavian Peninsula from day −3 (Fig. 6b). With the development of the SCA pattern, it favors the amplification of high pressure anomalies over East Asia, in company with northeasterly flows along the southeastern flank from day 0 onward (Fig. 6c). As a result, the sustained SCA anomalies are responsible for the persistent intensification of SLP over East Asia and northerlies over the SCS (Figs. 6d–f). Besides, the seven cases with a positive SCA index were also examined, and it was found that the results show a great diversity in the pattern of anomalous SLP (not shown).

Fig. 6.
Fig. 6.

Composite evolutions of SLP (contours; hPa; interval: 3 hPa) and 925-hPa horizontal wind (vectors; m s−1) anomalies relative to the onset of long-lived cold surge events with the SCA pattern. Solid (dashed) contours indicate positive (negative) values, and zero contours are omitted. Contours significant at the 95% confidence level are shaded, and vectors are shown when they are greater than one and significant in at least one direction.

Citation: Journal of Climate 35, 6; 10.1175/JCLI-D-21-0607.1

Figure 7 demonstrates the results of 300-hPa Z anomalies and corresponding horizontal wave activity flux. The upper-level circulation features wavelike anomalies, highly consistent with the evolution of lower-tropospheric circulations shown in Fig. 6. The wavelike disturbance emanates from the North Atlantic (region A; Fig. 7a) and develops to a full SCA pattern from day −3 (Fig. 7b). It is worth to note that the SCA anomalies are confined with three centers located over North Atlantic (region B), the Scandinavian Peninsula (region C), and central Siberia (region D), respectively, and are manifested as a quasi-stationary wave train, which affects following circulation by propagating continuous wave activity flux eastward (Fig. 7c). Furthermore, the result also shows that the SCA pattern is responsible for the persistent anticyclones over central Siberia from day −3 to day +6 (Figs. 7b–e), indicating more frequent occurrence of blocking highs that have been found to be conducive to the long-lived cold surges (Pang et al. 2020). Previous results have documented the influence of blocking on cold events over East Asia (e.g., Lu and Chang 2009; Luo et al. 2016a,b; Park et al. 2020). Statistical analysis shows that, among the total 26 cases, 23 events are accompanied with blockings over 90°–150°E when the anticyclones over central Siberia are prominent.

Fig. 7.
Fig. 7.

Composite evolutions of 300-hPa Z anomalies (contours; m; interval: 40 m) and horizontal WAF (vectors; m2 s−2) relative to the onset of long-lived cold surge events with the SCA pattern. Solid (dashed) contours indicate positive (negative) values, and zero contours are omitted. Contours significant at the 95% confidence level are shaded and vectors are shown when they are greater than one. The major centers are labeled as letters A–E and connected by purple lines in (c).

Citation: Journal of Climate 35, 6; 10.1175/JCLI-D-21-0607.1

Figure 8 delineates the spatial and temporal evolution of 300- and 925-hPa Z anomalies along the pathways marked in Fig. 7c. The result highlights the propagating feature of the SCA pattern and its further influence on the lower-tropospheric circulation. It is evident that the ridge–trough–ridge pattern evolves into a Rossby wave train and maintains the anomalies successively in the upper troposphere, which establishes a linkage between North Atlantic and East Asia. Moreover, the low-level anomalies almost overlap with the upper-level ones in the upstream areas (A–C) but are shifted southeastward by nearly a quarter wavelength in the downstream areas (D and E). This suggests that the downstream part of the SCA pattern exhibits baroclinic structure, which favors the growth of the surface Siberian high (Takaya and Nakamura 2005a). Additionally, the SCA pattern tends to be quasi-stationary with small range of zonal movement. This is quite different from the synoptic wave packets associated with cold events over East Asia (e.g., Takaya and Nakamura 2005b; Park et al. 2014, 2015) or negative cold surge events over the SCS (Pang and Lu 2019), which show rapid eastward movement across the Eurasian continent.

Fig. 8.
Fig. 8.

Hovmöller diagram of Z anomalies at 300 hPa (contours; m; interval: 60 m) and 925 hPa (shading; interval: 15 m) along the pathways shown in Fig. 7c. Solid (dashed) contours indicate positive (negative) anomalies, and zero contours are omitted. Shadings significant at the 95% confidence level are stippled.

Citation: Journal of Climate 35, 6; 10.1175/JCLI-D-21-0607.1

4. Influence of the SCA pattern on the following circulation anomalies

Results in the previous section explored the preceding circulation anomalies composited by the long-lived cold surge events and found an evident precursor of the SCA pattern. In this section, the analysis will be conducted from the perspective of the SCA event in order to figure out its effect on following circulation over East Asia. Based on the definition in section 2, there are 65 SCA events during the studied periods.

Figure 9 shows the composites of anomalous circulation associated with the SCA events. It reveals a prominent impact of the SCA pattern on circulation downstream. In the upper troposphere, initial disturbances are found over the North Atlantic (Fig. 9a) and the tripolar anomalies are established afterward as a primary cyclone over the Scandinavian Peninsula and two weaker anticyclones over the North Atlantic and central Siberia, respectively (Fig. 9b). In addition, the SCA pattern is able to maintain anticyclones over central Siberia by continuous eastward propagation of wave activity fluxes, suggesting its positive influence on the occurrence of Siberian blockings (Figs. 9b–e). Accompanying that, the lower-tropospheric anomalies develop synchronously and exhibit equivalent barotropic structure in the upstream centers (Fig. 9f). Influenced by the upper-tropospheric anticyclones over central Siberia, there are successive high pressure anomalies over East Asia and northeasterly anomalies over the SCS from day 0 onward, suggesting the possible outbreak and maintenance of cold surges (Figs. 9h–j). The results composited by the SCA events resemble those associated with the long-lived cold surge events in the previous section (Figs. 6 and 7), confirming a reliable linkage between the SCA pattern and cold surge over the SCS. Therefore, it is inferred that the SCA pattern in the upper troposphere conducts its influence on cold surges over the SCS through enhancing the blocking frequency over central Siberia.

Fig. 9.
Fig. 9.

Composite evolutions of (a)–(e) 300-hPa Z anomalies (contours; m; interval: 40 m) and horizontal WAF (vectors; m2 s−2), and (f)–(j) SLP (contours; hPa; interval: 3 hPa) and 925-hPa horizontal wind (vectors; m s−1) anomalies relative to the peak of SCA events. Solid (dashed) contours indicate positive (negative) values, and zero contours are omitted. Contours significant at the 95% confidence level are shaded and vectors are shown when they are greater than one and significant in at least one direction.

Citation: Journal of Climate 35, 6; 10.1175/JCLI-D-21-0607.1

Figure 10 shows the temporal evolutions of three circulation indices relevant to the peak of SCA events. The results further reveal a prominent impact of the SCA pattern on enhancing SLP over East Asia and northerlies over the SCS. The SCA index is significantly different from the winter mean during the periods from day −7 to day +7, with a peak value of −2.3 at day 0. As a result, persistent intensification is found in both the SLP index and wind index, which are pronounced from day −3 to day +4 and from day −2 to day +7, respectively. The successive developments of the SLP index and wind index indicate a continuous increase of high pressure over East Asia and northerly flows over the SCS, which suggests the possible occurrence of long-lived cold surges. Specifically, both indices are remarkably stronger in the SCA events than their climatological means at a 95% confidence level when averaged over their prominent periods. In addition, there is a strong tendency of being positive in SLP index (75%) or negative in the wind index (82%) among the total 65 SCA events. More importantly, 63% of the SCA events (41/65) feature persistent high pressure anomalies over East Asia and northerly anomalies over the SCS, consistent with the results in Figs. 9f–j. Hereafter, the 41 cases are denoted as SCA events accompanied with lasting cold surges (SCA-CS), while the other 24 cases are as those without cold surge (SCA-NCS).

Fig. 10.
Fig. 10.

Temporal evolutions of normalized wind index (black), SLP index (red), and SCA index (blue) relative to the peak of SCA events. Thick segments indicate values significant at the 95% confidence level.

Citation: Journal of Climate 35, 6; 10.1175/JCLI-D-21-0607.1

Figure 11 demonstrates the blocking frequency anomalies associated with the SCA events. The results are composited from day −3 to day +6 when the anticyclonic anomalies over central Siberia are prominent (Figs. 9b–e). It reveals a prominent and high frequency of Siberian blocking for the SCA events compared to the climatological mean. Specifically, the blocking frequency anomalies stay negative in the northern Eurasian continent, but positive from Lake Baikal to the Sea of Okhotsk (Fig. 11a), corresponding to the cyclone and anticyclone associated with the SCA pattern (Fig. 9). Besides, the extent of frequent blocking for the SCA events here (115°–150°E) is smaller than that of the long-lived cold surges (90°–150°E; Pang et al. 2020; their Fig. 6b). The difference arises from the range of negative height anomalies, which extend to western Siberia for the SCA events (Fig. 9) but stay to the west of Ural Mountain for the long-lived cold surges events (Fig. 7). The frequent occurrence of blocking is further confirmed by estimating the probability density function (PDF) of anomalous frequency averaged over the domain 50°–65°N, 115°–150°E by the Monte Carlo bootstrapping technique (Efron and Tibshirani 1993). The PDF is performed by randomly resampling the blocking frequency of the SCA events and winter days for 10 000 times, respectively. The PDF of SCA events is well separated from that of winter means at the 95% confidence level (Fig. 11b). Furthermore, the occurrence of large-scaled blocking is also checked if instantaneous blocking occurs for at least a continuous 15° of longitude (Davini et al. 2012). The frequency of large-scale blocking in the SCA events (24%) is also higher than the climatological mean (16%). Considering the influence of Siberian blocking on long-lived cold surges, it is indicated that frequent and persistent blocking over central Siberia plays an intermediate role in the process of the SCA affecting long-lived cold surges over the SCS.

Fig. 11.
Fig. 11.

(a) Composite of blocking frequency anomalies (contours; %; interval: 2%) from day −3 to day +6 for the SCA events. Solid (dashed) contours indicate positive (negative) values, and zero contours are omitted. Values significant at the 95% confidence level are shaded. (b) Probability density functions (PDFs; curves; %) and medians (vertical solid lines) of area-averaged blocking frequency anomalies estimate from 10 000 bootstrapped samples for the SCA events (red) and winter mean (black) over the domain 50°–65°N, 115°–150°E shown as a rectangle in (a). Vertical dashed lines indicate the 2.5% and 97.5% confidence levels.

Citation: Journal of Climate 35, 6; 10.1175/JCLI-D-21-0607.1

Figure 12 further compares the anomalous circulation between the SCA-CS and SCA-NCS events. Here, the results are composited from day −3 to day +6, as in Fig. 11. The comparison indicates distinct influences on East Asia. Both types are characterized as a tripole pattern in the upper troposphere, but the anomalies for the SCA-CS (Fig. 12a) are stronger than the SCA-NCS events (Fig. 12b). The distinct strength of SCA pattern occurs from day 0 onward, as the difference of SCA index is significant (p < 0.05) from day 0 to day +6 (not shown). In particular, for the SCA-CS events, a wide range of positive height anomaly extends from the Caspian Sea to the Sea of Okhotsk, which is more intensified and persistent than the SCA-NCS ones. This implies that the frequent blockings shown in Fig. 11 are more likely to be related to the SCA-CS events. Actually, the frequency of large-scale blocking is higher in the SCA-CS (25%) than the SCA-NCS (21%). As the baroclinicity over central Siberia is responsible for the growth of Siberian high, the height anomalies induce pronounced high pressure over East Asia and northerlies over the SCS in the lower troposphere for the SCA-CS events (Fig. 12c), which are absent for the SCA-NCS events (Fig. 12d). Together with the southward thermal advection, there are prominent cold anomalies over the south of China, the Indo-China Peninsula, and the surrounding seas (Fig. 12e). However, the SCA-NCS events are unable to result in equivalent cooling over East Asia because of the lack of northerly flow (Fig. 12f). Moreover, the warm anomalies are more intensified and widespread from the Ural Mountains to Northeast Asia for the SCA-CS type, which are related to the stronger and more extensive cyclones over northern Europe.

Fig. 12.
Fig. 12.

Composites of (a),(b) 300-hPa Z anomalies (m; interval: 40 m), (c),(d) SLP (contours; hPa; interval: 2 hPa) and 925-hPa horizontal wind (vectors; m s−1) anomalies, and (e),(f) T2m anomalies (K; interval: 2 K) averaged from day −3 to day +6 for the SCA events accompanied (left) with and (right) without cold surges. Solid (dashed) contours indicate positive (negative) values, and zero contours are omitted. Contours significant at the 95% confidence level are shaded, and vectors are shown when they are greater than one and significant in at least one direction.

Citation: Journal of Climate 35, 6; 10.1175/JCLI-D-21-0607.1

5. Summary and discussion

This study investigates the impact of SCA pattern on long-lived cold surge events over the SCS. On the one hand, the preceding circulation anomalies contributing to the long-lived cold surges are examined. It is revealed that extratropical circulation for the long-lived cold surges is dominated by a SCA pattern, which is absent in the short-lived ones. The anomalies for the long-lived events extend expansively across the Eurasian continent, with a primary cyclonic center over the Scandinavian Peninsula but two subcenters of anticyclones over North Atlantic and central Siberia, respectively. Further analysis reveals that the SCA index stays negative prominently in the majority of long-lived cold surge events, with much stronger average amplitude than the short-lived ones and climatological mean. In addition, the lead–lag composites indicate that the SCA pattern is established and maintained as a quasi-stationary Rossby wave train before the outbreak of long-lived cold surges. Accompanying the eastward propagation of wave activity fluxes, the SCA anomalies manage to sustain persistent anticyclones over central Siberia, which are found to be linked to more frequent occurrence of blocking highs (Pang et al. 2020). As a result, the maintenance of the SCA pattern is responsible for a successive intensification of SLP over East Asia and further results in continuous northerlies over the SCS.

On the other hand, the consequent impact of the SCA pattern on circulations downstream is examined by compositing the SCA events. It turns out that, accompanying the tripolar pattern in the upper troposphere, positive Z anomalies are active over central Siberia, where frequent occurrence of blocking is found to affect long-lived cold surges over the SCS (Pang et al. 2020). Further analysis reveals that blocking frequency is indeed high over 115°–150°E for the SCA events, which shows significant difference with the climatological mean. In addition, after the peak of SCA events, the lower-tropospheric circulation features successive SLP rising over East Asia and northerly anomalies over the SCS, suggesting the role of SCA events in causing persistent cold surges. More specifically, the SLP index and wind index are found to be persistently pronounced from day −3 to day +4 and from day −2 to day +7, respectively. Among the total 65 SCA events, 41 cases feature continuous intensification of both the SLP index and wind index. In conjunction with that, a prominent and long-lasting cooling is found over the south of China, the Indo-China Peninsula, and surrounding seas, which is absent in the other 24 SCA events. The results indicate that the majority of SCA events are associated with the occurrence of long-lasting cold surges over the SCS. Moreover, the 41 SCA events followed by cold surges tend to be characterized as stronger and more persistent anticyclones over the central Siberia, compared with the other 24 events without cold surges, suggesting more frequent blockings. Therefore, the SCA anomalies are responsible for the persistent cold surges over the SCS by modulating blocking frequency over central Siberia.

This study indicated that the quasi-stationary SCA anomalies are important to the following influence on long-lived cold surges over the South China Sea. Therefore, a better understanding of the formation and maintenance of the SCA may be helpful for improving the extended prediction of the long-lived cold surges over the South China Sea. Previous studies suggested that potential factors for the formation and maintenance of the SCA include the forcing of high-frequency transient eddies (Bueh and Nakamura 2007), low-frequency oscillations of basic flows (Nakamura et al. 1987; Kim et al. 2021), or the interaction between the SCA pattern and stratospheric polar vortex (Lee et al. 2020; Wang and Tan 2020). In addition, this study focused on the extratropical circulations responsible for the long-lived cold surges. However, cold surges over the SCS may also be modulated by tropical factors such as the Madden–Julian oscillation (Chang et al. 2005; Jeong et al. 2005; He et al. 2011; Lim et al. 2017; Abdillah et al. 2018, 2021). The possible impacts of tropical factors on the long-lived cold surges require further investigation. On the other hand, cold surges over the SCS, in turn, can modulate the Madden–Julian oscillation activity (Chen et al. 2017; Pang et al. 2018), and the long-lived cold surges may exert more significant impacts on the tropical anomalies, which also requires further investigation.

Acknowledgments.

The authors appreciate the editor and three anonymous reviewers for their constructive and detailed comments, which greatly improved the presentation. This research was jointly sponsored by the National Natural Science Foundation of China (Grants 41721004 and 42075052).

Data availability statement.

The ERA-Interim reanalysis data are available online at https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era-interim.

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Save
  • Abdillah, M. R., Y. Kanno, and T. Iwasaki, 2018: Tropical–extratropical interactions associated with East Asian cold air outbreaks. Part II: Intra-seasonal variation. J. Climate, 31, 473490, https://doi.org/10.1175/JCLI-D-17-0147.1.

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  • Abdillah, M. R., Y. Kanno, T. Iwasaki, and J. Matsumoto, 2021: Cold surge pathways in East Asia and their tropical impacts. J. Climate, 34, 157170, https://doi.org/10.1175/JCLI-D-20-0552.1.

    • Crossref
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  • Barnston, A. G., and R. E. Livezey, 1987: Classification, seasonality, and persistence of low-frequency atmospheric circulation patterns. Mon. Wea. Rev., 115, 10831126, https://doi.org/10.1175/1520-0493(1987)115<1083:CSAPOL>2.0.CO;2.

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  • Boyle, J., and T. Chen, 1992: Synoptic aspects of the winter East Asian monsoon. Monsoon Meteorology, C.-P. Zhang and T. N. Krishnamurti, Eds., Oxford University Press, 125160.

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    • Export Citation
  • Bueh, C., and H. Nakamura, 2007: Scandinavian pattern and its climatic impact. Quart. J. Roy. Meteor. Soc., 133, 21172131, https://doi.org/10.1002/qj.173.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 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, 273280, https://doi.org/10.1002/asl.333.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Chan, J. C., and C. Y. Li, 2004: The East Asian winter monsoon. East Asian Monsoon, C.-P. Chang, Ed., World Scientific, 54106.

  • Chang, C.-P., J. E. Milard, and G. T. J. Chen, 1983: Gravitational character of cold surges during winter MONEX. Mon. Wea. Rev., 111, 293307, https://doi.org/10.1175/1520-0493(1983)111<0293:GCOCSD>2.0.CO;2.

    • Crossref
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