1. Introduction
The teleconnections are usually defined as the simultaneous correlations between the planetary-scale circulation anomalies [
1,
2]. These patterns could exercise an extensive influence on global weather and climate. Hence, there were a large number of studies aimed to explore the formation mechanisms and spatio-temporal variability of teleconnections for the past 30 years [
3,
4,
5]. The Western Pacific pattern (WP) is one of the main teleconnections over the North Pacific. During the winter, when WP is in the positive (negative) phase, the pattern is characterized by the dipole-like atmospheric circulation anomalies with a negative (positive) active center located on the Okhotsk Sea and another positive (negative) one situated around the subtropical western North Pacific [
6]. In consequence, the temperature anomalies above (below) normal conditions are observed over almost all regions of East Asia [
7]. The Arctic Oscillation (AO) is associated with the sea level pressure (SLP) field and has two oscillation centers at the Arctic and the mid-latitudes, respectively [
8,
9]. This pattern has a dramatic effect on the Siberian High (SH) and East Asian winter monsoon (EAWM) thereby controlling the climate variability over the middle–high latitude regions within East Asia. The El Niño-Southern Oscillation (ENSO) is known as the most influential oscillation for the global climate fluctuation, which is identified with abnormal warm or cold SST in the central or eastern Pacific. Zhang et al. [
10] proposed that the ENSO could affect the strength of EAWM via low-level atmospheric circulation around the maritime continent. Wang et al. [
11] stressed that the ENSO combined with Pacific Decadal Oscillation (PDO) has a substantial impact on dry–wet variations over various regions. However, since the eastern Pacific ENSO (EP) transformed into the central Pacific ENSO (CP) after the mid-1970s [
12], the effect of ENSO on the climate variations over East Asia has pronouncedly decreased in the past several decades [
2].
In recent years, a large body of studies has revealed the close relationship between the sea surface temperature (SST) and some teleconnections, which triggers climate variations on the regional scale Liu et al. [
13] showed that the spatial structures and temporal variability of three Eurasian teleconnections. They found that the conventional Eurasian pattern (EU) is primarily induced by the SST anomalies over North Atlantic whereas the Scandinavian pattern (SCAND) is highly related to the SST anomalies around the tropical and southern Indian Ocean SST anomalies. When the conventional EU pattern is in the positive phase, the cold weather would prevail on the Yangtze River basin, Inner Mongolia and Northeast China. In comparison, the positive phase of the SCAND pattern is mainly associated with the cold anomalies over northern Eurasia and East Asia. Sun et al. [
14] considered that the SST anomalies over the tropical Pacific and Indian Oceans are responsible for the interdecadal fluctuation of the Pacific–Japan teleconnection pattern (PJ) in the previous winter and spring. The fluctuation further enhances the interdecadal variations of summer precipitation in the Yangtze–Huaihe River valley of China. In addition, Sun et al. [
15] stated that a seesaw pattern responds to the SST anomalies in extratropical North Pacific and the pattern drives the precipitation to increase or decrease over East Asia and the western North Pacific. Above all, it is important to figure out the SST variability and its association with teleconnections and climate changes.
The frequencies and intensities of extreme events have been increasing dramatically in East Asia over the past several decades. The extreme climate events, such as snow disasters, heat waves, flood and drought pose a severe threat to people’s lives and property. Scientists from various fields have paid their attention to the possible mechanisms of climate extremes over East Asia. Historically, numerous studies have exhibited that climate variations are associated with large-scale circulations [
2,
9,
16,
17]. In particular, the WP, AO and ENSO have an enormous effect on the strength and intensity of EAWM and then modulate the climate fluctuations over East Asia. Despite the fact that there are many papers that used the three patterns to explain the abnormal climate variations, few studies have focused on the relative importance among these patterns for the extreme temperature events within East Asia. Hence, the main aim of this study is to present a comprehensive understanding of the relationship between the cold extremes and the WP, AO and ENSO patterns.
The paper is organized as follows: the data and methodology are described in
Section 2; the influence of teleconnections on the extreme cold events over East Asia is shown in
Section 3; the summary and discussions are presented in
Section 4.
4. Summary and Discussions
The effect of the large-scale teleconnection patterns on the temperature variability within East Asia during the boreal wintertime from 1979 to 2017 was examined in this study. The positive and negative phases were defined as when the index values falling at the upper and lower 25% of the indices distribution, respectively. In general, the WP and AO patterns could cause the pronounced warm and cold Tmin anomalies over almost the whole of East Asia. However, the Tmin anomalies in the ENSO events have characteristics of regional differences. The result was consistent with the previous findings [
2,
12,
23,
24].
A Monte Carlo method was used to identify quantitatively the relationship between these patterns and daily as well as monthly cold extremes. We found that the extreme days were primarily related to the negative phase of three patterns. Overall, the ENSO cycle exhibited a relatively weak effect on the cold extremes over East Asia in comparison with the WP and AO pattern. Wang and He [
12] noted that the correlation between the Niño 3.4 index and the surface air temperature at 2 m over East Asia during the winter has reduced after the mid-70s. Li et al. [
25] documented that the relationship between the EAWM and AO has strengthened since the 1980s. Moreover, Linkin and Nigam [
26] considered that the North Pacific Oscillation pattern (NPO) in the sea level pressure field was related to the WP pattern in the mid-troposphere field. Yeh et al. [
27] stated that the NPO was significantly enhanced after 1988. Those studies could explain the relatively weak effect of ENSO compared to WP and AO patterns. The spatial distribution of monthly cold extremes resembled that of daily cases but with a more scattered and smaller range because of the smaller sample size [
20]. In particular, the extreme months were more significantly associated with the large-scale teleconnections due to that these patterns usually have a longer life cycle [
2].
Sun et al. [
15] pointed out that the turbulent heat flux (both sensible and latent heat fluxes) is strongly correlated with the SST variability over the southwestern North Pacific. This phenomenon implied that the warm SST anomalies at mid-latitudes could release energy into the atmosphere, which would augment (decrease) the temperature contrast at middle latitudes vs high latitudes (middle latitudes vs low latitudes). Correspondingly, the westerlies over the middle–high latitudes were stronger (weaker) than normal states based on the physical process of thermal wind. In consequence, the high-latitude (middle-latitude) region within East Asia was governed by an anticyclonic (cyclonic) circulation. In our study, the cold SST anomalies over the western North Pacific in the negative phase of WP and AO were favorable to the stability of cyclone at mid-latitudes. Hence, widespread negative Tmin anomalies and extremes appeared over most of the region within East Asia. Zhang, Sumi and Kimoto [
10] showed that the anomalous SST pattern for an El Niño event was evident cold near the western Pacific Warm Pool. The cold SST anomalies around the equator suppressed the convective motion and enhanced subsidence of air due to the strong static stability of the lower layers of the atmosphere. Also, the cold SST anomalies could induce the equatorial Rossby wave. These factors contributed to the formation of a low-level anticyclone over the northern West Pacific together. Our study also exhibited a similar result but for the circulation system at the 500 hPa level. Besides, the dipole structure for SST anomalies in the West Pacific increased the temperature gradient at middle latitudes vs high latitudes and reduced the temperature gradient at middle latitudes vs low latitudes. This SST pattern could also favor to the sustainability of anticyclone. As a result, the coast of East Asia experienced relatively warm weather. However, the characteristics of the dipole for SST anomalies were not evident and have less effect on the persistence of cyclone in a La Niña event. That caused the asymmetry of climate states between the warm and cold ENSO events [
28].
In addition, the composite analysis involved in Z500 and SST anomalies at individual regions on different time scales was performed to investigate the formation mechanism of Tmin anomalies. We found that the extreme days could be triggered by the synoptic-scale climate events with a smaller area, shorter duration and higher degree randomness other than teleconnections [
21]. The phenomenon was indicative of that the teleconnections in combination with the synoptic-scale climate events determined together on the occurrence of the extreme days. Additionally, the subtropical and tropical areas were more subject to the synoptic-scale climate systems, while the large-scale recurrent patterns seemed to play a more important role in cold extremes in extratropical regions. Especially, since the teleconnections were more influential on the Tmin variability at the monthly time scale, the relationship between the large-scale recurrent patterns and extreme months was more prominent than extreme days.
The composite of SST anomalies at location primarily affected by the WP pattern was distinct from that in
Figure 5b, while it has some commonalities with the Shanxi case that not subjected to any patterns involved in this study. Similarly, the case impacted by both WP and ENSO patterns exhibited the Z500 composite with a WP-event and SST composite with a La Niña event. Li and Wettstein [
29] demonstrated that WP is a mid-latitude eddy-driven pattern. Tanaka et al. [
30] claimed that the WP was maintained by available potential energy originated from the climatological-mean flow. In our study, the ambiguous connection between the WP pattern and SST anomalies maybe result from that the WP mainly is generated by the internal atmospheric mechanisms but less related to the oceanic process. In contrast, the ENSO cycle tended to reflect the oceanic states rather than atmospheric circulations [
20]. This also led the ENSO cycle to present a less effect on the cold extremes over East Asia compared to the WP and AO pattern. On the other hand, recent studies reported that the ENSO events include two different types, EP (eastern-Pacific) and CP (central-Pacific). The CP ENSO was characterized by the SST anomalies in the tropical central Pacific, which evidently differed from the EP ENSO. The distinction of spatial distribution in SST anomalies for two types of ENSO could result in the occurrence of completely different climate events [
31,
32,
33,
34]. For instance, Feng et al. [
31] found that the EP ENSO favored the positive precipitation anomalies over South China and negative precipitation anomalies around the Philippines and Indonesia during the wintertime, whereas the CP ENSO was related to the relatively weak dry and further northward anomalies over the Philippines. Hence, not classifying for the ENSO event may also cause its relatively weak association with the Tmin extremes in our work.
The study presented here has given insight into the relationship between the large-scale teleconnections and the cold extremes over East Asia during the boreal winter. However, the air–sea interaction mechanism involved in our paper is relatively rough and simple because of the uncertainty and complexity in the coupled systems. Hence, the further exploration of the role of the atmospheric and oceanic processes on climate variability will be our subject of forthcoming research.