Relative contributions of synoptic and intraseasonal variations to strong cold events over eastern China

Abstract

The present study investigates the relative roles of intraseasonal oscillations (ISOs) and synoptic variations in strong cold events over eastern China during the boreal winter. The ISOs and synoptic variations explain about 55% and 20% of the total area-mean temperature anomaly in eastern China, respectively. The advection of synoptic winds on synoptic temperature gradients has a leading contribution to the temperature decrease before the cold events and thus the synoptic variations are important in determining the time of peak cold anomalies. The ISOs have a larger role in sustaining the cold events. The height anomalies associated with ISOs and synoptic variations are manifested as Rossby wave trains propagating along the polar front jet over the Eurasian continent before the cold events. They both contribute to the deepening of the East Asian trough and the development of cold events. Compared to the ISO wave train, the synoptic wave train has a smaller spatial scale and moves faster. There are obvious intraseasonal signals in the stratosphere about 1 week before the cold events over eastern China. Large negative height anomalies associated with the weakening of the polar vortex are observed over the North Atlantic. These anomalies move eastwards and propagate downwards after reaching the west coast of Europe. The downward moving stratospheric signal triggers height anomalies in the troposphere over the entrance region of the polar front jet. Then the anomalies propagate towards East Asia along the wave train, contributing to the intensification of the Siberian high and the East Asian trough and the occurrence of cold events over eastern China.

1 Introduction

Eastern China is often subjected to impacts of cold events with large-scale low temperature during the boreal winter. A strong cold event affected eastern China in January 2008, accompanied by extremely low temperature, strong snowstorms and icing conditions, which caused severe damages and substantial economical loses (e.g., Wen et al. 2009; Zhou et al. 2009). During January 20–25 of 2016, a strong cold event struck eastern China with excessive cold anomaly especially over southern China. This cold event broke the low temperature records in many areas over East Asia and at least 85 people died in Taiwan (http://edition.cnn.com/2016/01/25/asia/asia-cold- weather-travel-disruption/index.html). Understanding the factors and processes of the occurrence of cold events over eastern China can help us to improve the prediction of and prepare for future cold events.

The occurrence of cold events over eastern China is closely related to the mid-latitude circulation systems, including the Siberian high, the Aleutian low, the East Asian trough, and upper troposphere jets (e.g., Ding and Krishnamurti 1987; Zhang et al. 1997; Wang et al. 2009; Takaya and Nakamura 2013). The status of the Siberian high and the East Asian trough, two key systems for cold air activity over East Asia, is associated with the Rossby wave trains propagating along the polar front jet (Takaya and Nakamura 2005b). The wave trains propagating along the subtropical jet may influence the occurrence of the strong cold events as well (Wen et al. 2009; Zhou et al. 2009). The Arctic Oscillation (AO) may affect the cold events over eastern China by modulating the Siberian high, the East Asian trough, and the polar front jet (Gong et al. 2001; Wu and Wang 2002; Jeong and Ho 2005; Park et al. 2010).

There is indication of signals in the stratosphere that could propagate downward and influence the tropospheric circulation by modulating the phase of the AO (Baldwin and Dunkerton 2001; Baldwin et al. 2003; Wang and Chen 2010). A weak stratospheric polar vortex is followed by the development of cold anomalies over the Eurasian continent (Kolstad et al. 2010). The regional coupling between the stratosphere and the troposphere over East Asia during the life cycle of the weak stratospheric vortex can influence the strength of the East Asian trough, and thus the activity of cold events (Woo et al. 2015).

The intraseasonal oscillation (ISO) is not only observed over the tropics (Madden and Julian 1971, 1972), but also appears over the extratropical regions (Anderson and Rosen 1983; Krishnamurti and Gadgil 1985). Yang and Li (2016) analyzed the intraseasonal variations of air temperature over the mid- and high-latitude Eurasian region during the boreal winter. The intraseasonal temperature disturbances move southeastward due to the advection of the temperature anomaly by the mean flow (Yang and Li 2016). The intraseasonal variations of temperature over East Asia are associated with the ISO of the AO index (Yao et al. 2016). The intraseasonal variation of the East Asian trough is closely related to the intraseasonal variation of the Siberian high, and they can influence the boreal winter climate over East Asia (Takaya and Nakamura 2005a, b; Song et al. 2016).

Previous studies have revealed the influences of the ISOs and synoptic variations on the occurrence of cold events over eastern China during the boreal winter. However, there have been no investigations about the relative contributions of the synoptic variations and the ISOs to the strong cold events over eastern China. Besides, the ISOs and synoptic variations that affect the cold events may have different sources. Clarifying the respective contributions to the cold events and the sources of the synoptic and intraseasonal variations are relevant to the prediction of cold events. In the present study, we attempt to identify the relative importance of synoptic and intraseasonal components of temperature anomalies in different stages of strong cold events over eastern China. We compare the horizontal and vertical structures of synoptic and intraseasonal circulation perturbations to reveal their sources and evolution processes. Our analysis unravels an obvious intraseasonal signal in the stratosphere before the cold events over eastern China.

The rest sections are organized as follows. The dataset and methodology are described in Sect. 2. Section 3 documents the relative contributions of the synoptic and intraseasonal components to the temperature anomalies of large-scale strong cold events and the roles of the ISOs in sustaining the long duration of cold events over eastern China during the boreal winter. Section 4 examines and compares the horizontal and vertical structures of the ISOs and synoptic variations. In Sect. 5, the signals in the stratosphere and the processes of their influences on the cold events are analyzed. In Sect. 6, a summary is presented along with some discussions.

2 Data and methodology

The dataset used in this study is the National Centers for Environmental Prediction (NCEP)-Department of Energy (DOE) Reanalysis 2 product from the NOAA/OAR/ESRL PSD (Kanamitsu et al. 2002). The variables including daily geopotential height, meridional and zonal winds, and vertical motion at different pressure levels as well as surface air temperature at 2 m and surface winds at 10 m. The horizontal resolution is 2.5° × 2.5° with 17 vertical layers extending from 1000- to 10-hPa for variables at the pressure levels. The surface temperature and winds are on the T62 Gaussian grid. The NCEP-DOE reanalysis product is available since January 1979.

The present study extracts the ISO, synoptic variation, and low frequency variation from the original field using the Butterworth band-pass (10–90-day), high-pass (shorter than 10-day), and low-pass (longer than 90-day) filters. We employ a composite analysis to obtain the common features of cold events during 1979/80 to 2015/16 winters. The cold events are determined when the surface air temperature anomaly averaged in the region of 20°N–40°N and 100°E–120°E exceeds −5 °C, and the day when the negative temperature anomaly reaches the maximum is taken as reference (lag 0 day). The threshold of −5 °C is about 1.5 standard deviation of the area-mean daily temperature in this region. Total 61 cold events are identified during the whole period (1979–2016) (Table 1). Among the 61 cold events, the longest cold event lasts 25 days, and the shortest cold event is 5 days long. Most of the cold events are sustained for 10–15 days. The mean duration of 61 events is 12 days. The significance of composite anomalies is estimated based on the Student t test.

Table 1 The years and lag 0 day of the cold events

In order to illustrate the horizontal and vertical propagation of Rossby wave train, we use the Rossby wave activity flux which is independent of wave phase and parallel to the local group velocity on a zonally varying basic flow (Plumb 1985; Takaya and Nakamura 2001). The three dimensional wave activity fluxes are written as:

$$W=\frac{{{p_0}}}{{2\left| {\vec {V}} \right|}}\left\{ {\begin{array}{*{20}{c}} {\bar {u}\left( {{{v^{\prime}}^2} - \psi ^{\prime}{{v^{\prime}}_x}} \right)+\bar {v}\left( { - u^{\prime}v^{\prime}+\psi ^{\prime}{{u^{\prime}}_x}} \right)} \\ {\bar {u}\left( { - u^{\prime}v^{\prime}+\psi ^{\prime}{{u^{\prime}}_x}} \right)+\bar {v}\left( {{{u^{\prime}}^2}+\psi ^{\prime}{{u^{\prime}}_y}} \right)} \\ {\frac{{{f_0}R}}{{{N^2}{H_0}}}\left[ {\bar {u}\left( {v^{\prime}T^{\prime} - \psi ^{\prime}{{T^{\prime}}_x}} \right)+\bar {v}\left( { - u^{\prime}T^{\prime} - \psi ^{\prime}{{T^{\prime}}_y}} \right)} \right]} \end{array}} \right\}$$

where $\vec {V}=\left( {\bar {u},\bar {v}} \right)$ is the winter mean horizontal wind averaged over the 1979–2010 period, $V'=(u',v')$ is the perturbation horizontal geostrophic wind velocity, $\psi '$ is the perturbation geostrophic stream function, ${f_0}$ is the Coriolis parameter at 45°N, $R$ is the gas constant of dry air, ${N^2}$ is the square of the buoyancy frequency, ${p_0}$ is the pressure divided by 1000 hPa, $T'$ is the perturbation air temperature, and ${H_0}$ is the scale height. Subscripts $x$ and $y$ denote partial derivatives in zonal and meridional directions, respectively. Prime denotes the value with the climatological mean removed.

Previous studies have indicated a dominant role of horizontal advection to the temperature change before the cold events (e.g., Yang and Li 2016; Song and Wu 2017). Here, we evaluate the relative contributions of the ISOs and synoptic variations in the temperature advection term. The contributions of other terms are not considered in the present analysis. The temperature tendency equation is written as:

$$\frac{{\partial T}}{{\partial t}}= - \overrightarrow V \cdot \nabla T - {\left( {\frac{p}{{{p_0}}}} \right)^{\frac{R}{{{C_p}}}}}\omega \frac{{\partial \theta }}{{\partial p}}+Q$$

One variable (A) can be divided into three terms:

$$A=\bar {A}+A^{\prime}+A^{\prime\prime}$$

Here, the overbar denotes the low-frequency component (periods longer than 90-day) (as the background state), the prime denotes the intraseasonal component (periods of 10–90-day), and the double prime denotes the synoptic component (periods shorter than 10-day). So, the advection term in the temperature tendency equation is decomposed as:

$$\begin{aligned} \vec{V}\cdot\nabla T & = \left( {\bar{\vec{V}} + \vec{V}^{\prime } + \vec{V}^{{\prime \prime }} } \right)\cdot\nabla (\bar{T} + T^{\prime } + T^{{\prime \prime }} ) \\ & = \left( {\bar{u}\frac{{\partial \bar{T}}}{{\partial x}} + \bar{u}\frac{{\partial T^{\prime } }}{{\partial x}} + \bar{u}\frac{{\partial T^{{\prime \prime }} }}{{\partial x}}} \right) + \left( {\bar{v}\frac{{\partial \bar{T}}}{{\partial y}} + \bar{v}\frac{{\partial T^{\prime } }}{{\partial y}} + \bar{v}\frac{{\partial T^{{\prime \prime }} }}{{\partial y}}} \right) \\ &\quad + \left( {u^{\prime } \frac{{\partial \bar{T}}}{{\partial x}} + u^{\prime } \frac{{\partial T^{\prime } }}{{\partial x}} + u^{\prime } \frac{{\partial T^{{\prime \prime }} }}{{\partial x}}} \right) + \left( {v^{\prime } \frac{{\partial \bar{T}}}{{\partial y}} + v^{\prime } \frac{{\partial T^{\prime } }}{{\partial y}} + v^{\prime } \frac{{\partial T^{{\prime \prime }} }}{{\partial y}}} \right) \\ &\quad + \left( {u^{{\prime \prime }} \frac{{\partial \bar{T}}}{{\partial x}} + u^{{\prime \prime }} \frac{{\partial T^{\prime } }}{{\partial x}} + u^{{\prime \prime }} \frac{{\partial T^{{\prime \prime }} }}{{\partial x}}} \right) + \left( {v^{{\prime \prime }} \frac{{\partial \bar{T}}}{{\partial y}} + v^{{\prime \prime }} \frac{{\partial T^{\prime } }}{{\partial y}} + v^{{\prime \prime }} \frac{{\partial T^{{\prime \prime }} }}{{\partial y}}} \right) \\ \end{aligned}$$

To diagnose the contributions of the ISOs and synoptic variations to the temperature advection term, we divide the advection terms of the temperature tendency equation into three parts, i.e., the term associated with the ISOs only, the term associated with the synoptic variations only, and the term of mixture associated with both the ISOs and synoptic variations:

$$\begin{gathered} u^{\prime}\frac{{\partial T^{\prime}}}{{\partial x}}+u^{\prime}\frac{{\partial \bar {T}}}{{\partial x}}+\bar {u}\frac{{\partial T^{\prime}}}{{\partial x}}+v^{\prime}\frac{{\partial T^{\prime}}}{{\partial y}}+v^{\prime}\frac{{\partial \bar {T}}}{{\partial y}}+\bar {v}\frac{{\partial T^{\prime}}}{{\partial y}}(ISO) \\ u^{\prime\prime}\frac{{\partial T^{\prime\prime}}}{{\partial x}}+u^{\prime\prime}\frac{{\partial \bar {T}}}{{\partial x}}+\bar {u}\frac{{\partial T^{\prime\prime}}}{{\partial x}}+v^{\prime\prime}\frac{{\partial T^{\prime\prime}}}{{\partial y}}+v^{\prime\prime}\frac{{\partial \bar {T}}}{{\partial y}}+\bar {v}\frac{{\partial T^{\prime\prime}}}{{\partial y}}(\rm synoptic) \\ u^{\prime}\frac{{\partial T^{\prime\prime}}}{{\partial x}}+u^{\prime\prime}\frac{{\partial T^{\prime}}}{{\partial x}}+v^{\prime}\frac{{\partial T^{\prime\prime}}}{{\partial y}}+v^{\prime\prime}\frac{{\partial T^{\prime}}}{{\partial y}}(\rm mixture) \\ \end{gathered}$$

3 The contributions of the ISOs and the synoptic variations to the cold events

In this section, we compare the original, intraseasonal, and synoptic components of composite anomalies for the 61 identified cold events. The purpose is to examine the relative contributions of the ISOs and the synoptic variations to surface air temperature variations during the cold events over eastern China. We first analyze the spatial–temporal evolutions of surface air temperature anomalies and then compare specifically surface air temperature anomalies over eastern China and the magnitude of the temperature change due to different components.

During cold events, the main body of cold anomalies moves southeastward from lag −2 to lag 0 day (Fig. 1a–c). The negative temperature anomalies over −3 °C reach 20°N on lag −1 day (Fig. 1b). Later, the temperature anomalies decrease (Fig. 1d). The pattern of the temperature anomalies due to the ISOs is very similar to that of the original anomalies during the developing stage of cold events (Fig. 1e–g). This indicates an important contribution of ISOs to the total temperature anomalies. The distribution of temperature anomalies due to synoptic variations displays notable differences (Fig. 1i–l). The cold anomaly moves southward from the region south of Lake Baikal (Fig. 1i–k). The cold anomalies reach southern China on lag 0 day (Fig. 1l). The magnitude of synoptic temperature anomalies is much smaller than that of intraseasonal anomalies. In addition, the synoptic temperature anomalies appear to move faster and display smaller spatial scales.

Fig. 1

Composite surface air temperature anomalies (ºC) on lag days of a 2, b 1, c 0, d 2 of cold events; e–h are the ISO parts; i–l the synoptic parts. Black dots indicate anomalies significant the 95% confidence level

The spatial–temporal evolution of surface air temperature anomalies is closely associated with that of surface wind anomalies. An anomalous anticyclone is located around Lake Baikal on lag −2 day (Fig. 2a). The anomalous anticyclone moves southeastward on the following days, accompanied by strong anomalous northeasterly winds along the southeast flank of the anticyclone (Fig. 2a–c). The large anomalous northerly winds over eastern China indicate the southward intrusion of cold air (Fig. 1a–c). On lag 2 day, the wind anomalies weaken quickly (Fig. 2d). The surface wind anomalies due to ISOs display a spatial–temporal evolution similar to that of total wind anomalies but with a weaker intensity (Fig. 2e–h). The surface wind anomalies due to synoptic variations are weaker over the land (Fig. 2i–l) compared to those intraseasonal anomalies. The northerly wind anomalies over eastern China intensify and move southward from lag −2 to lag 0 day.

Fig. 2

The same as Fig. 1, but for surface wind anomaly (vector). Black vectors indicate anomalies significant at the 95% confidence level

We calculate the regional mean temperature anomalies of different time scales in order to evaluate the contributions of the ISOs and the synoptic variations quantitatively. The total temperature anomaly over eastern China drops from −1 °C on lag −5 day to −6.5 °C on lag 0 day (Fig. 3). The intraseasonal temperature anomaly drops from 0 °C to −3.6 °C. The synoptic temperature anomaly starts dropping from lag −3 day and it reaches the maximum of −1.3 °C on lag 0 day. The low-frequency temperature anomaly has a value of −1.3 °C. The temperature anomaly associated with the ISOs explains about 55% of the total anomaly at the peak, indicating the importance of the ISOs to the occurrence and strength of the cold events. The temperature anomaly related to the synoptic variations explains about 20% of the total magnitude of temperature anomaly. The ISOs and the synoptic variations together explain about 75% of the total peak temperature anomaly. Although the magnitude of the synoptic temperature anomaly is smaller compared to that of intraseasonal temperature anomaly, the rate of temperature change due to the synoptic variations is comparable or even larger than that due to the ISOs. This indicates that the synoptic variation is important to determine when the total cold anomaly peaks during the cold events over eastern China.

Fig. 3

Time evolution of surface air temperature anomalies (ºC) for the original temperature anomaly (black), the synoptic part (green), the ISO part (red), and the low-frequency part (blue) during the lag −5 to lag 5 days of cold events

The evolution of the surface air temperature anomaly tendency (calculated using central differential) and the contributions of grouped advection terms are presented in Fig. 4. The temperature anomaly tendency drops largely from lag −4 day and it reaches the largest negative value on lag −1 day. The tendency turns to positive after lag 0 day, indicating the recovery of temperature from cold anomaly. The three groups of advection terms evolve in a consistent manner. At the time of peak temperature anomaly tendency (lag −1 day), the sum of the three parts explains over 90% of the total temperature tendency. This indicates a dominant contribution of temperature advection to cold events. The advection term associated only with the ISOs evolves slowly, and maintains a negative value larger than the other two terms from lag 0 to lag 3 day. This suggests the role of the ISOs in the sustaining the duration of cold events.

Fig. 4

Time evolution of surface air temperature anomaly tendency (°C/day) and different parts of the advection term during lag −5 to lag 5 days of cold events

The advection term associated with the synoptic variations is larger than the ISO and mixture terms on lag −2 and lag −1 days. This indicates that the synoptic variations are important to the temperature decrease before the cold events. Among the advection terms associated only with the synoptic variations on lag −1 day, the $- u^{{\prime \prime }} \frac{{\partial T^{{\prime \prime }} }}{{\partial x}}$ and $- v^{{\prime \prime }} \frac{{\partial T^{{\prime \prime }} }}{{\partial y}}$ are the largest (Fig. 5). This suggests a large role of synoptic wind and temperature anomalies in the occurrence of strong cold events over eastern China (Figs. 1j, 2j). In addition, advection terms associated with intraseasonal wind and temperature anomalies as well as associated with synoptic wind and intraseasonal temperature anomalies make large contributions to the temperature tendency on lag −1 day.

Fig. 5

Different parts of the advection term on lag −1 day of cold events

4 The structures of the ISOs and synoptic variations

In this section, we compare the horizontal and vertical structures and temporal evolutions of the ISOs and the synoptic variations associated with strong cold events over eastern China. The differences of temporal evolutions help to identify the sources of the ISOs and the synoptic variations that affect the cold events. This may unravel some precursory signals for the occurrence of cold events. We first compare the horizontal structures and then the vertical structures between the ISOs and the synoptic variations.

The intraseasonal height anomalies are manifested as a Rossby wave train propagating along the polar front jet over the mid-high latitudes of Eurasian continent (Fig. 6). The anomalous ridge over western Siberia intensifies and moves southeastward with the development and propagation of the Rossby wave train. The negative height anomalies over East Asia indicate the deepening of the East Asian trough, which leads to the intrusion of cold events to eastern China.

Fig. 6

Composite geopotential height (gpm) (contour) and ISO geopotential height anomalies (gpm) (shading) at 500-hPa on lag days of −4 (a), −3 (b), −2 (c), −1 (d), 0 (e), 1 (f) of cold events. Green lines indicate the cross section along which Fig. 8 is plotted. Grey dots in the figures indicate anomalies significant the 95% confidence level. The contour level is 100 gpm

The synoptic height anomalies display an obvious wave train pattern along the polar front jet as well (Fig. 7). In comparison, the magnitude of height anomalies is obviously smaller and the wave length is apparently shorter compared to the ISO wave train. The synoptic wave train also moves faster. The negative height anomalies move from northwest of Lake Baikal on lag −4 day to Korea on lag 0 day, contributing to the deepening of the East Asian trough. This feature agrees with the development and rebuilding of the East Asian trough associated with the short wave trains over the Eurasian continent (Song et al. 2016).

Fig. 7

The same as Fig. 6, but for synoptic parts. Green lines indicate the cross section along which Fig. 9 is plotted

From the vertical cross section along the wave train, the intraseasonal height anomalies have a baroclinic structure over the region east of 50°E (Fig. 8). This baroclinic structure corresponds to the intraseasonal strengthening of the East Asian trough (Song et al. 2016). The cold anomaly and negative height anomaly in the stratosphere are large and significant over the region west of 100°E. The negative height anomaly in the stratosphere extends downward to the troposphere west of 50°E before lag −3 day (Fig. 8a–d). Then, the signal retreats upward (Fig. 8e–f). Following the downward extension of negative height anomalies from the stratosphere, the positive height anomalies over the Ural region intensify, which is accompanied by a similar intensification of downstream East Asian trough (Fig. 6). This suggests a probable influence of the stratospheric processes on the cold events through modulating intraseasonal anomalies over Europe. We will discuss the stratospheric impacts in detail in the following section.

Fig. 8

Vertical cross section along the wave train path (green line in Fig. 6) of the ISO geopotential height (contour, interval is 20 gpm) and temperature (shading, °C) anomalies on lag days of −6 (a), −5 (b), −4 (c), −3 (d), −2 (e), −1 (f), 0 (g), 1 (h) of cold events

The synoptic height anomalies feature baroclinic structure as well (Fig. 9). The geopotential height anomalies tilt westward and the temperature anomalies tilt eastward, it is corresponding to the baroclinic growth structure (Park et al. 2013). Before lag −2 day, there are positive height anomalies around 120°E in the mid-upper troposphere (Fig. 9a–c), indicating a weaker East Asian trough. When the negative anomalies accompanying the propagation of the short wave train reach this region after lag −3 day (Fig. 9d, e), the East Asian trough deepens, leading to the southward intrusion of cold air over eastern China. A pronounced difference from the intraseasonal anomalies is that there are no systematic synoptic temperature and height anomalies in the stratosphere.

Fig. 9

The same as Fig. 8, but for synoptic parts with the contour interval of 10 gpm

The Rossby wave flux is observed at upper-level over the Eurasian continent in association with the ISOs (Fig. 10). The wave train emerges from the negative height anomalies over Scandinavian Peninsula on lag −7 day (Fig. 10a). The development of the negative height anomalies coincides with the downward propagation of stratospheric signal in Fig. 8. The correspondence indicates a probable influence of stratosphere on the source of the wave train in the troposphere. The wave train trapped in the polar front jet strengthens and propagates southeastward in the following days (Fig. 10b–h). With the accumulation of wave energy, the negative anomalies over East Asia develop and intensify. The results suggest that the deepening of the East Asian trough is caused by the upstream Rossby wave propagation, and in turn it contributes to the southward intrusion of cold air over eastern China (Song et al. 2016).

Fig. 10

Geopotential height anomalies at 300-hPa (gpm) (shading) and wave activity flux in association with the ISOs (unit: m²/s²) (scale at right-bottom of the forth panel) (vector) on lag days of −7 (a), −6 (b), −5 (c), −4 (d), −3 (e), −2 (f), −1 (g) and 0 (h) of cold events. Black dots in the figures indicate anomalies significant at the 95% confidence level

5 The stratospheric signals

In the previous section, obvious intraseasonal anomalies over Europe appear to be linked to stratospheric height anomalies (Fig. 8), indicating that the ISO wave train may be induced by stratospheric process. Previous studies have revealed the stratospheric influence upon the occurrence of cold events over East Asia (Jeong et al. 2006; Wang and Chen 2010; Woo et al. 2015). Jeong et al. (2006) detected strong negative potential vorticity in the stratosphere over Eurasian continent about 1 week before the occurrence of cold surges over East Asia. It induces upper tropospheric disturbances, which leads to the intensification of the Siberian high and the East Asian trough (Jeong et al. 2006). The downward propagation of positive height anomalies corresponding to a weak stratospheric polar vortex in Jeong et al. (2006) occurs over the Ural region, which is different from the present study. Woo et al. (2015) identified that with the weakening of the stratospheric polar vortex, negative height anomalies propagate downward from stratosphere to troposphere over East Asia, leading directly to the deepening of the East Asian trough. Their comparative analysis showed that the deepening of the East Asian trough can occur without accompanying intensification of the anticyclone over the Ural region. Note that the cold events in Woo et al. (2015) are confined to the region north of 45°N, which is different from the present study.

In order to examine the contribution of ISOs in the stratosphere to the cold events, we use the polar vortex activity as an indicator for the intensity of the polar vortex. Following Kolstad et al. (2010) and Woo et al. (2015), the vortex strength index is defined as $- {Z_p}$, where ${Z_p}=\sum {\left( {Z^{\prime}\cos \varphi } \right)/\sum {\cos \varphi ,Z^{\prime}=Z - \bar {Z}} }$, $Z$ is the geopotential height, $\bar {Z}$ is the climatological mean, $\varphi$ is the latitude, and the sum is performed on the region north of 65°N (Kolstad et al. 2010). According to the above definition, a positive index denotes a strong polar vortex.

The development of the cold anomalies over Eurasian continent is associated with a weakening of the stratospheric polar vortex. During the cold events, the polar vortex index starts to decrease from lag −11 day (Fig. 11). The index reaches the lowest value on lag −6 day, and then the index recovers. This indicates that the weakening of the polar vortex occurs about 10 days before the cold events over eastern China. The evolution of the ISO polar vortex index is at the same pace as the original index, and its amplitude is about 60% of the original index. The synoptic component of the polar vortex index is small during the period. This suggests that the change in the polar vortex is dominated by the intraseasonal time scale variations.

Fig. 11

Time evolution of polar vortex index during lag −13 to lag 5 days of cold events. The black line is the original index, the red line is the ISO part, and the green line is the synoptic part

The intraseasonal height anomalies at 50-hPa on lag −7, lag −5 and lag −3 days are presented in Fig. 12. The large negative height anomalies are centered over the northern Atlantic Ocean on lag −7 day, indicating a shift of the polar vortex away from the polar region that leads to the weakening of the polar vortex (Fig. 11). On the following days, the negative height anomalies continue to move southeastward. On lag −3 day, the center is situated over northwestern edge of Eurasian continent (Fig. 12c). This location corresponds to the negative height anomalies over the region west of 40°E at 500- and 300-hPa in Figs. 6b and 8e. Thus, we infer that the tropospheric anomalies are triggered by the downward propagation of the stratospheric anomalies.

Fig. 12

Composite ISO geopotential height anomalies (unit: gpm) at 50-hPa on lag −7 (a), −5 (b), −3 (c) days of cold events

The above inference is validated by latitude-height cross sections of height and height anomalies, and wave activity flux along 0°-40°E presented in Fig. 13. The negative height anomalies in the polar region start moving equatorward and downward from lag −7 day (Fig. 13a), the negative anomalies reach 300-hPa on lag −6 day (Fig. 13b), accompanied by downward wave fluxes, indicating the downward propagation of energy from stratosphere to troposphere. After the negative height anomalies reach the troposphere, the wave train propagating along the polar front jet is established after lag −7 day (Fig. 8a). On the following days, the negative anomalies continue their downward propagation, and the downward wave fluxes become stronger (Fig. 13c–e). When the negative anomalies reach the lower troposphere, the vertical structure turns to be barotropic over the mid-high latitudes. On lag −6 day (Fig. 13b), upward wave fluxes emerge over 70°N. In the following days, upward wave fluxes intensify (Fig. 13c–e). This is followed by the weakening of downward wave fluxes (Fig. 13d–f).

Fig. 13

Vertical cross section of 0°–40°E mean of the ISO geopotential height (contour, interval: 30, unit: gpm) and temperature (shading, unit: °C) anomalies and wave activity fluxes (vector, scale at bottom-right) on lag days of −7 (a), −6 (b), −5 (c), −4 (d), −3 (e), −2 (f) of cold events. Vertical component of the wave activity fluxes has been multiplied by 100

The above results suggest the following sequence of the stratospheric influence on the cold events over eastern China. The stratospheric anomalies over the west Europe appear from the displacement and weakening of the polar vortex (Mitchell et al. 2012). Then, the stratospheric anomalies propagate downward to the troposphere, which triggers the height anomalies over the entrance region of the polar front jet. The anomalies disperse towards East Asia along the waveguide as a Rossby wave train. This leads to the intensification of the Siberian high and the East Asian trough and consequently the intrusion of cold air into eastern China (Takaya and Nakamura 2005a, b; Song et al. 2016).

The weakening of the polar vortex in our study is featured by vortex displacement, similar to Mitchell et al. (2012). Differently, the weak polar vortex in our study could influence the occurrence of cold events over East Asia through the downward propagation of stratospheric signal and triggering of the mid-latitude Rossby wave train in the troposphere. The location of the downward propagation of stratospheric signal in our study is more westward compared to Jeong et al. (2006). Unlike the regional downward propagation of stratospheric anomalies associated with the weak polar vortex over East Asia in Woo et al. (2015), the downward propagation associated with the weak polar vortex in our study is observed over Scandinavian Peninsula which is the entrance region of the polar front jet. The stratospheric signal then induces upper tropospheric disturbances and leads to the Rossby wave train propagation towards East Asia. Another important feature to note that the stratospheric signal is detected only on the intraseasonal time scale.

6 Summary and discussions

This study documents relative contributions of the ISOs and the synoptic variations to strong cold events over eastern China during the boreal winter. The cold events over eastern China are defined when the surface air temperature anomaly in the region of 20°N–40°N and 100°E–120°E exceeds −5 °C. The day when the negative temperature anomaly reaches the maximum is taken as reference (lag 0 day). 61 cold events are identified during the whole analysis period (1979–2016). The daily variables are filtered to obtain the ISOs (periods within 10–90-day), synoptic variations (periods shorter than 10-day), and the low-frequency (periods longer than 90-day).

Both the ISOs and the synoptic variations contribute to the development of cold events over eastern China. The ISOs explain about 55% of the total area-mean temperature anomaly over the region of 20°N–40°N and 100°E–120°E, and the evolution of the ISOs is of quite consistent manner with the total anomaly. The synoptic variations contribute about 20% of the total area-mean temperature anomaly. The movement of the synoptic anomalies is faster than the ISO and the total anomalies. However, the synoptic variations are important to determine when the total cold anomaly peaks with a major contribution from the advection of synoptic winds of synoptic temperature gradients. From the diagnosis of temperature tendency, the ISO advection terms have a larger contribution to the sustaining of the cold events.

The height anomalies associated with ISOs over the Eurasian continent during the cold events are characterized as a Rossby wave train propagating along the polar front jet. The southeastward propagation and strengthening of the intraseasonal wave train contribute to the deepening of the East Asian trough and the development of cold events. The vertical structure of the intraseasonal wave train is baroclinic, which is corresponding to the intraseasonal strengthening of the East Asian trough. The synoptic variations associated with the development of the cold events are manifested as shorter and faster moving Rossby wave train with a baroclinic vertical structure and the negative synoptic height anomalies over East Asia contribute to the deepening of the East Asian trough.

The emergence of the Rossby wave train associated with ISOs during cold events over eastern China is closely related to the stratospheric process. Before the occurrence of cold events, the polar vortex weakens, the negative height anomaly related to the weak polar vortex shifts towards the North Atlantic and then it moves eastwards. When the negative height anomalies reach Scandinavian Peninsula, the signals propagate downward to the troposphere, which leads to the development of tropospheric height anomalies in the entrance region of the polar front jet. Then the wave train is established, and the anomalous energy disperses towards East Asia along the waveguide, which leads to the intensification of the Siberian high and the East Asian trough and consequently the occurrence of cold events over eastern China.

The present study investigates the impacts of mid- and high-latitude ISOs and synoptic variations upon the occurrence of cold events over eastern China. Previous studies have shown that the tropical ISOs, for example, the Madden-Julian Oscillation (MJO) may contribute to the cold events over East Asia (Jeong et al. 2005; Park et al. 2010; He et al. 2011). The MJO is just part of the tropical ISOs that extend from 10-day to 90-day periods. The contributions of these tropical ISOs to the occurrence of cold events over East Asia need further discussion. Previous studies paid much attention to the sole influence of MJOs upon the cold events over East Asia. We wonder if MJO could interact with the mid- or high-latitude ISOs in the happening of cold events activity over East Asia. Further studies are needed to explore the individual and combined effects of tropical and extratropical ISOs on cold events over East Asia.