1. Introduction
A cutoff low (COL) is a closed, cold, deep system settled in the middle–upper troposphere and evolves from the complete separation of the westerly trough after its deepening to the low latitudes. East Asia, North America, and Europe are the three most frequent regions of global COL activity (Nieto et al. 2005). The Northeast China cold vortex (NCCV) is a persistent and quasi-stationary COL that occurs over Northeast China and the adjacent areas. It has been revealed that the NCCV exerts pronounced influences on weather and climate anomalies such as low temperature and cold damage, heavy rainfall and floods, reducing grain production over Northeast China and even the whole of China (Sun and An 2001; Sun et al. 2002; Wang et al. 2010; Hu et al. 2011; Li et al. 2014; Xie and Bueh 2012; Liu et al. 2015; Fang et al. 2016, 2021a,b; Lian et al. 2016; Gao and Gao 2018; Gang et al. 2019; Niu et al. 2021; Zhu et al. 2022).
The NCCV activity presents significant seasonal variation, with the most frequent occurrence during the early boreal summer (Sun et al. 1994; Zhang and Li 2009; Yang et al. 2021), which has profound impact on regional climate. For example, He et al. (2007) found that the frequent activity of the NCCV has an accumulative effect on the monthly temperature and precipitation anomalies. Liu et al. (2017) suggested that the NCCV-related precipitation accounts for 62.5% of the total precipitation in Northeast China, indicating a significant cumulative effect. Moreover, the NCCV can transport the cold air southward and thereby affect the first rainy season in South China (Miao et al. 2006a,b) and mei-yu precipitation (He et al. 2007; Zhang and Li 2009; Wang et al. 2010; Hu et al. 2011; Xie and Bueh 2012; Liu et al. 2012). Given the crucial impact of frequent NCCV activities on weather and climate, the NCCV has been regarded as an important predictor of temperature and precipitation in North China (Liu et al. 2019; Wang et al. 2022).
It has been demonstrated that the NCCV activity is collectively influenced by atmospheric internal dynamics and land–sea thermal conditions, including the western Pacific subtropical high (Liang et al. 2009), blocking high (Liang et al. 2009; Liu et al. 2016, 2017; Xie and Bueh 2017), Northern Hemisphere annular mode (Liu et al. 2002; He et al. 2006; Miao et al. 2006a,b), wave–current interaction (Fu and Sun 2012; Xia et al. 2012; Li et al. 2015; Nie et al. 2022), low-frequency Rossby wave activity (Lian et al. 2010), land surface thermal anomaly over West Asia (Chen et al. 2018; Wang et al. 2018), North Atlantic sea surface temperature triple pattern (Fang et al. 2018), and tropical North Atlantic and northwest Pacific sea surface temperature anomalies (Lu et al. 2020). Besides the impacts of interannual variabilities (Fang et al. 2018, 2020; Lu et al. 2020), the NCCV can be significantly modulated by intraseasonal variability (ISV). For example, Liu et al. (2012) found that the NCCV shows evident features on an intraseasonal time scale and has a phase-locked relationship with the summer precipitation anomaly in eastern China, which plays an important role in modulating the position of the rain belt. Xie and Bueh (2017) revealed that the dynamics of interaction between the NCCV and the blocking high on an intraseasonal time scale significantly affects the climate in Northeast China, where the formation and maintenance of the blocking high are significantly modulated by ISV (Simmons et al. 1983; Takaya and Nakamura 2005; Schneidereit et al. 2012; Yang and Li 2017b, 2020).
The Madden–Julian oscillation (MJO) is the primary pattern of ISV in the tropics (Madden and Julian 1971, 1972). The Rossby wave induced by the MJO can significantly alter and regulate the extratropical circulation in the Northern Hemisphere (Matthews et al. 2004; Cassou 2008; Hamill and Kiladis 2014). However, ISV not only exists in the tropics but also in the mid–high latitudes. It propagates southwest in geopotential height during summer (Yang et al. 2013a) and eastward in air temperature during winter (Yang et al. 2014; Yang and Li 2016b). The anomalous circulation associated with ISV at mid–high latitudes plays a crucial role in the summer drought and flood events in the Yangtze–Huaihe River basin (Mao and Wu 2005; Han et al. 2006; Yang et al. 2013b). In summer, the ISV from the mid–high latitudes propagates southward and encounters the ISV propagated from the low latitudes, generating a strong interaction center in the Yangtze River basin and thereby favoring heavy rainfall therein (Wang and Ding 2008). The ISV circulation in the mid–high latitudes can also directly affect the persistent precipitation in South China by emanating Rossby wave energy along the low-frequency teleconnection wave train (Gao et al. 2013; Miao et al. 2017). The ISV signal revealed in the above studies mainly shows two distinct periods of 10–30 and 30–60 days.
Previous studies have shown that ISV has an important role in convection (Yang et al. 2010), the South Asia high (Yang and Li 2016a), diabatic heating (Wang and Duan 2015; Yang and Li 2017a), the Aleutian low (Zhang and Ren 2017), and some other mid–high-latitude circulation systems. Less attention has been paid to the impact of ISV at mid–high latitudes on the climate in Northeast China. It has been suggested that the modulation of the NCCV on weather and climate in Northeast China is strongly related to the variabilities on the intraseasonal time scale. However, the influence of ISV at mid–high latitudes on the NCCV activity still remains unclear. This study aims to reveal the effects of ISV at mid–high latitudes on the NCCV activity, which is helpful to understand the formation and maintenance of the NCCV and provides implications for the extended-range forecast. Based on empirical orthogonal function (EOF) analysis of a single variable of geopotential height or air temperature, previous studies revealed that the major ISV modes at mid–high latitudes show completely different characteristics of propagation between winter and summer. Since the NCCV is a cold low pressure system that can be modulated by thermodynamic and dynamic processes, multivariable EOF (MVEOF) analysis is adopted in this study to extract the independent thermal–pressure coupled modes associated with ISV at mid–high latitudes. Details of this method are described in section 2. The remainder of this paper is organized as follows. Data and method are introduced in section 2. Section 3 presents the climatological features of the NCCV, and section 4 shows the influences of ISV on the occurrence frequency of the NCCV. In section 5, the dynamic mechanism is discussed based on the geopotential height and air temperature tendency budgets. The summary and discussion are presented in section 6.
2. Data and method
a. Reanalysis data and statistical method
The 6-hourly reanalysis data used in this study are obtained from the fifth major global reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis of the global climate (ERA5) (Hersbach et al. 2020), including geopotential height, air temperature, and horizontal winds from 1 January 1979 to 31 December 2020, with a horizontal resolution of 0.25° × 0.25° (latitude/longitude). The climatological mean is calculated by averaging the daily variables over 1981–2010 and then smoothed by retaining the zeroth to third Fourier harmonics. Long-term linear trend in the time series has been removed to focus on the ISV. All analyses are carried out from late spring to early summer [i.e., from April to July (AMJJ)] at the 500-hPa isobaric level.
The Lanczos bandpass filter with 41 weights is adapted to extract the variability of the target time scale; a value of 41 weights is a common use that can ensure the filtering effect without losing too much data length (Duchon 1979; Ren et al. 2012). Since the NCCV is a cold low pressure system that can be modulated by thermodynamic and dynamic processes, MVEOF (Wang 1992; Wang et al. 2008) is applied on a set of geopotential height and air temperature data to capture the thermal–pressure coupled modes associated with ISV at mid–high latitudes. In this approach, the geopotential height and air temperature are examined in a single temporal sequence, and a covariance matrix is constructed by considering the sequence of the two variables as one time step. The spatial pattern for each MVEOF mode contains two sequential patterns representing the thermal–pressure coupled mode of the geopotential height and air temperature that share the same explained percentage variance and time series with their corresponding principal component (PC). Each MVEOF spatial pattern is illustrated by linear regressions of the geopotential height and air temperature anomalies onto the MVEOF time coefficient. North’s rule of thumb (North et al. 1982) is used to evaluate the eigenvalue separation of MVEOF. The statistical significance of composite, regression, and correlation analyses is assessed using the two-tailed Student’s t test.
b. Identification of the NCCV
The NCCV is a synoptic low pressure circulation system at 500 hPa, which is accompanied by cold air with obvious cold trough activity. An NCCV objective identification procedure proposed by Fang et al. (2021a) is applied to the 6-hourly synoptic chart to get the NCCV information, including date, center position, and intensity. The main steps are as follows:
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Step 1—Tracing the equigeopotential height line: Trace the equigeopotential height lines within the range of 500–600 dagpm on the 500-hPa isobaric level at an interval of 4 dagpm every 6 h and output their longitude and latitude values.
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Step 2—Sifting the equigeopotential height line: Using the results of step 1, the closed equigeopotential height lines within 30°–80°N, 85°–150°E are sifted out.
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Step 3—Identifying the NCCV center: The center of the innermost equigeopotential height circle of the same vortex is defined as the NCCV center, and the average values of all the longitudes and latitudes of the innermost equigeopotential height circle are identified as the center longitude and center latitude, respectively.
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Step 4—Identifying NCCV duration: If there are NCCV centers in two adjacent times and the distance between the two NCCV centers is less than 800 km, it is regarded as the same NCCV system. The duration of the NCCV sifted out in step 3 is counted, and the NCCV system whose duration is greater than or equal to 72 h will be screened out and outputted.
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Step 5—Verifying the rationality: Checking sequentially whether the identified NCCV process accompanies cold air temperatures and matches the time and geographical location of the precipitation process in Northeast China.
Since NCCV is identified from 6-hourly data, the daily average intensity and the latitude and longitude of the innermost equigeopotential height circle are defined as daily NCCV intensity and location.
c. Geopotential height and air temperature budget diagnosis
d. Horizontal wave activity flux
3. Climatic features of the NCCV
According to the objective identification of the NCCV system, the monthly distribution of the total days of NCCV occurrence is displayed in Fig. 1a. One of the main reasons why we focus on NCCV in early summer is that the occurrence frequency of NCCV is highest from April to July. During these months, influences of the western Pacific subtropical high and the subtropical monsoonal circulation are usually confined in southern China, and thus the anomalous atmospheric circulation from the mid–high latitudes is relatively more important for the NCCV activity. Moreover, we further counted the total number of NCCVs with different durations and intensities in early summer, as shown in Fig. 1b. Since the NCCV is a synoptic system, the number of NCCVs lasting for 3–4 days accounts for more than 80% of the total number. Only a few NCCVs can last for more than 7 days. However, there seems to be no direct connection between the duration of the NCCV and its intensity. The average intensity of NCCVs with a duration of fewer than 6 days is around 540 dagpm, and the mean intensity of NCCVs with a duration of more than 7 days is even weaker.
(a) Monthly distribution of occurrence frequency in terms of total NCCV days during 1979–2020 (the red bars highlight the frequencies in early summer months). (b) Scatterplot of mean intensities of NCCVs against durations in early summer. The number in parentheses on the x axis indicates the number of NCCVs. Blue thick dots denote the average intensities of total NCCVs with different durations.
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
To investigate the spatial features of NCCV occurrence frequency, we calculated the total days of NCCV for each 2.5° latitude/longitude grid box over the cold vortex activity area (30°–80°N, 85°–150°E) in the early summer, as shown in Fig. 2a. It is clear that there are two maximum centers of NCCV occurrence frequency to the south and north of 60°N, locating at near 65° and 50°N, respectively. The magnitude of the southern maximum center is greater than that of the northern one, indicating the frequent impact of NCVVs on the climate in Northeast China. In terms of intensity, it may be questionable to directly compare the intensity of NCCVs at different latitudes, because the mean geopotential height is larger in the south than that in the north, which interferes with the results in Fig. 1b, but it is feasible to compare them at the same latitude. As displayed in Fig. 2b, the intensity of the NCCV in the high occurrence frequency region near 50°N is relatively stronger than that at the same latitude, which is consistent with the maximum center of occurrence frequency shown in Fig. 2a.
(a) Spatial distribution of total days of the NCCV center in early summer that is counted for each 2.5° latitude/longitude grid box. (b) As in (a), but for the NCCV intensity (dgpm).
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
4. Modulation of ISV on the occurrence frequency of the NCCV
a. Main period affecting the NCCV in the early summer
As a synoptic system, NCCV activity essentially is the formation and decay of the COL, and its occurrence may be discontinuous in time. According to the objective procedure of the NCCV identification, we can calculate the NCCV intensity in days when NCCV exists during AMJJ in each year and then construct the time series of the NCCV intensity index by splicing the intensity records according to time. Figure 3 shows the spatial distribution of temporal correlation coefficients between the time series of the constructed NCCV intensity index and the associated simultaneous 500-hPa circulation anomaly, whose length is about 2573 days. It is clear that the intensity of the NCCV is significantly correlated with the local geopotential height over Northeast Asia (black solid box in Fig. 3) accompanied by an anomalous cyclone, where the negative correlation pattern of air temperature slightly shifts westward compared to the geopotential height pattern (red dashed box in Fig. 3). In addition, positive correlations are also found along the Ural Mountains north to Novaya Zemlya, indicating the important role of the upstream Ural anticyclone and Arctic circulation anomaly in the formation of the NCCV.
Spatial distribution of correlation coefficients between time series of the NCCV intensity index (multiplied by −1.0) and the associated 500-hPa geopotential height anomaly (shading), air temperature (contours with an interval of 0.02), and horizontal winds (vectors; shown are those exceeding the 95% significance level based on the two-sided Student’s t test) in early summer. The black and yellow dots denote significant correlation coefficients of geopotential height anomaly and air temperature, respectively, above the 95% confidence level based on the two-sided Student’s t test.
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
Figure 4 shows the power spectra of regional, mean 500-hPa geopotential height and air temperature anomalies over the aforementioned key regions during AMJJ. It is demonstrated that if we connect all the AMJJ series together, the spectra of both the geopotential height and air temperature for the key regions show significant peaks that exceed the red noise test within 10–60 days. The average of spectra for each year AMJJ suggests a unimodal pattern with the peak at about 40 days. Due to the year-to-year spread, the mean spectra at the quasi-biweekly time scale (10–20 days) is relatively smooth, while those at the intraseasonal time scale are ubiquitous in each year, which results in a clear 20–60-day period in the mean spectra, namely, the main intraseasonal period affecting the NCCV in the early summer. We did not focus in this study on the quasi-biweekly oscillation of 10–20 days in the mid–high latitudes.
(a),(c) Power spectra of all the AMJJ and (b),(d) mean power spectra of the AMJJ regional mean 500-hPa (top) geopotential height over the black solid boxes in Fig. 3 and (bottom) air temperature anomalies over the red dashed boxes in Fig. 3 for each year from 1979 to 2020. The red lines represent the red-noise spectra, with the shadings in (b) and (d) denoting spectra spread as two standard deviations about the mean spectra (blue) and red-noise spectra (red).
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
b. Evolution of major ISV modes and the associated impact on the occurrence frequency of NCCVs
Since the NCCV in early summer is closely related to the ISV of 20–60 days, MVEOF analysis is applied to the 20–60-day-filtered 500-hPa geopotential height and air temperature anomalies over the Eurasia region of 30°–80°N, 0°–150°E in early summer to extract the main ISV thermal–pressure coupled modes associated with the NCCV. Figure 5 shows the spatial patterns of the first and second MVEOF modes (denoted as MVEOF1 and MVEOF2) and the lead–lag correlation coefficients of their PC time series (denoted as PC1 and PC2). The two MVEOF modes explain 17.2% and 14.5% of the total variance, respectively. One of the notable spatial features of the two MVEOF modes is the zonal dipole pattern. The MVEOF1 shows a dipole pattern in geopotential height and air temperature with negative anomalies over the Ural Mountains but positive anomalies over Lake Baikal. Meanwhile, the MVEOF2 also shows a dipole pattern in geopotential height and air temperature with negative anomalies over Scandinavia but with strong positive anomalies to the east of the Ural Mountains.
(a) Spatial pattern of the first MVEOF mode of 20–60-day-filtered 500-hPa air temperature (shading; K) and geopotential height anomalies (contours with an interval of 4 gpm) over the Eurasia region of 30°–80°N, 0°–150°E in early summer. (b) As in (a), but for the second MVEOF mode. Percentages of variance explained by each mode are given at the top left. (c) Lead–lag correlation coefficients between PC1 and PC2. A negative sign on the x axis indicates that PC1 leads PC2.
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
It can be seen from Fig. 5c that when PC1 leads (lags) PC2 by 7 days (about a quarter cycle), the correlation coefficient reaches its maximum (minimum), with the absolute value exceeding 0.4. In addition, according to North’s rule of thumb, the first and second MVEOF modes cannot be significantly separated. The significant lead–lag correlation between PC1 and PC2 and the nonindependence of each MVEOF mode indicate that MVEOF1 and MVEOF2 represent the spatial distribution of two different phases in the same intraseasonal cycle. The positive and negative centers of MVEOF2 can be regarded as a result of the westward propagation of the corresponding centers in MVEOF1.
To verify whether the MVEOF1 and MVEOF2 are two different phases of the same cycle, Fig. 6 shows the composite evolution of a 500-hPa atmospheric circulation anomaly on different lags associated with the PC1 and PC2, where the composite pattern is obtained by positive phase minus negative phase with the absolute value of PC peaks and valleys greater than 1.0 standard deviation. Evidently, the geopotential height and air temperature anomalies are coupled together with a slight zonal shift. Both MVEOF1 and MVEOF2 present a westward propagation of the zonal dipole pattern. On day −6, the positive and negative anomalies in MVEOF1 are located at the west and east of the Urals, respectively, accompanied by an anomalous anticyclone and cyclone therein. Then, the two anomalous centers move westward with weakening in the positive centers but strengthening in the negative centers. Another positive anomaly and the associated anticyclonic circulation are generated near Lake Baikal. On day 0, the MVEOF1 reaches its peak and the previous positive anomaly in the western Ural Mountains moves to the Atlantic Ocean, leaving the Ural Mountains region controlled by a strong negative anomaly. The positive anomaly and the associated anticyclonic circulation over Lake Baikal attain their maximum magnitude. After day 0, the negative and positive anomalies and their associated circulations gradually weaken and move westward.
Composite patterns (positive phase minus negative phase) of 500-hPa horizontal winds (vectors; shown for those exceeding the 95% significance level based on the two-sided Student’s t test; m s−1), air temperature (shading; °C), and geopotential height (contours with an interval of 4 dagpm) anomalies associated with (left) PC1 and (right) PC2 from day −6 to day +6.
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
The evolution from day −6 to day 0 in MVEOF2 is quite similar to that of MVEOF1 from day 0 to day 6. The anomalous cyclonic circulation in the Ural Mountains and anticyclonic circulation over Lake Baikal of MVEOF2 are gradually strengthened from day −6 to day 0. After day 0, these anomalies continually move westward and weaken gradually. On day +6, the negative anomaly enters the Atlantic Ocean, and a positive geopotential height anomaly and the associated anticyclonic circulation prevail in the Ural Mountain region. These results suggest that MVEOF1 and MVEOF2 are two different phases of the same cycle of ISV that propagates westward. Northeast China is mainly controlled by the enhanced Baikal anticyclone in the developing stage of MVEOF1. After day 0, with the Baikal anticyclone gradually weakened and moved westward, a new cyclonic circulation and negative geopotential height and air temperature anomalies generate in the southeast part of Northeast China, which is evident in the evolution of MVEOF2. Meanwhile, anomalous anticyclonic circulation with negative geopotential height and air temperature anomalies gradually develops in Northeast China. In the decay stage of MVEOF2, as the previous Baikal anticyclone gradually controls the Ural Mountains area, the new anomalous cyclone enters the Lake Baikal region and completes the phase transition.
To find out the impact of the evolution of major ISV modes on NCCV occurrence frequency, we calculated the total days of NCCV activity when PC1 and PC2 reach the peak and valley, as shown in Fig. 7. During the developing stage of MVEOF1, the occurrence frequency of NCCV in the positive phase is close to that in the negative phase. In the decay stage, the occurrence frequency of NCCV increases notably in the positive phase but decreases in the negative phase. It indicates that the positive phase of MVEOF1 acts to promote the NCCV genesis, while the negative phase tends to suppress the NCCV genesis. For the MVEOF2, the NCCV genesis is promoted in the positive phase of the developing stage, but it is suppressed in its negative phase. In the decay stage of MVEOF2, the occurrence frequency of NCCV in both phases is similar.
(a) Temporal evolution of the composite normalized PC1 index (dotted lines) and occurrence frequency in terms of total NCCV days (bars) in positive (red) and negative (blue) EOF1 phases from day −8 to day 8. (b) As in (a), but for MVEOF2.
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
Since the evolution of the circulation and the corresponding impact on the occurrence frequency of NCCV in the decay stage of the MVEOF1 is quite similar to that in the developing stage of the MVEOF2, it is reasonable to speculate that the MVEOF1 and MVEOF2 are the modes of two different phases in a same cycle of ISV. Therefore, we divided the ISV cycle into eight phases following Matthews (2000) (see appendix A for details). Figure 8 shows the spatial distribution of NCCV occurrence frequency in the eight ISV phases with amplitudes greater than 1.0. For the first and second phases, there are two main centers of NCCV occurrence frequency near 65°N, 105°E and 50°N, 125°E, respectively. However, the activity region and the total frequency are relatively small. For the third and fourth phases, NCCV mainly appears south of 55°N and its active region moves from the northwest to the southeast, but the total frequency is still small. After phase 5, NCCV appears to be active in the whole region. NCCV mainly occurs over 65°N, 105°E and 50°N, 125°E and increases significantly. It is clear that NCCV occurrence is much more frequent in phases 5–8 around 65°N, 105°E than that in phases 1–4. Overall, the NCCV in phases 5–8 is more frequent and shows a broader active area than that in phases 1–4.
Composite phase evolution of the spatial distribution of the total NCCV days counted for each 2.5° latitude/longitude grid box. Phase numbers are given at the top left of each panel.
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
It can be seen that the occurrence frequency of NCCV is regulated by ISV from mid–high latitudes. Thus, we further focus on the evolution of ISV circulation in this section. Figure 9 gives the composite phase evolution of 500-hPa circulation anomalies, which shows a zonally distributed positive–negative dipole pattern over the Eurasian mid–high latitudes in the first phase. As mentioned before, the geopotential height and air temperature anomalies are coupled together with a slight zonal shift. Positive geopotential height and air temperature anomalies and an anomalous anticyclone circulation can be found in the Ural Mountains. To the east of the anticyclone, negative geopotential height and air temperature anomalies appear over Lake Baikal. For the second phase, the negative anomaly and the associated cyclone strengthen and propagate westward, while positive anomaly prevails over all of Europe. In phase 3, the negative anomaly is further strengthened, and the positive anomaly begins to weaken. In phase 4, the positive anomaly moves westward to the North Atlantic, and the negative anomaly strengthens remarkably and prevails in the Ural Mountains. The evolution from phase 1 to phase 4 acts as the westward-propagated zonal dipole pattern at mid–high latitudes over Eurasia. Particularly, there are new positive geopotential height and air temperature anomalies generated in northeast China, which gradually strengthen and move to the northwest, and eventually replace the negative anomaly over the Lake Baikal region. The zonal dipole pattern from phase 5 to phase 8 is opposite to that of phases 1–4, which completely evolves into the zonally distributed negative–positive pattern. Note that the positive geopotential height and air temperature anomalies in the active region of NCCV gradually strengthen in phases 1–4, while negative ones intensify from phases 5–8. This process suggests the key role of ISV in modulating the occurrence frequency of NCCV.
Composite phase evolution of 500-hPa horizontal winds (vectors; shown for those exceeding the 95% significance level based on the two-sided Student’s t test; m s−1), air temperature (shading; K), and geopotential height (contours with an interval of 2 dagpm) anomalies. Phase numbers are given at the top left of each panel.
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
c. Propagation of Rossby waves associated with ISV phase evolution
The above results suggest that the genesis of NCCV is closely related to the westward-propagating ISV. It can be found in Fig. 9 that a new system generally forms over the Lake Baikal region, moves to the Ural region, and subsequently reaches its peak intensity, and finally weakens and moves westward into the Atlantic Ocean. To explore the possible cause for the evolution, we examined the phase evolution of two-dimensional WAFs and their divergence at 500 hPa, as shown in Fig. 10. During the westward propagation of ISV, the WAF mainly propagates eastward. The positive and negative flux divergences are steadily located to the west of the Ural Mountains and west of Lake Baikal, respectively, with rare movement. This suggests that the excited wave energy mainly disperses eastward and gradually accumulates to the west of Lake Baikal. It does not mean the ISV propagation must be eastward. The Rossby wave velocity (Rossby 1940) is closely related to wavelength and latitude. Since the intensity of zonal basic flow is weaker in early summer than that in winter, and the wavelength of ISV is about 120°–150° of longitude (more than 10 000 km), it is reasonable for ISV to propagate westward. The accumulated energy near Lake Baikal may be an important factor in the excitation of ISV perturbation but needs to be further studied.
Composite phase evolution of the intraseasonal geopotential height anomalies at 500 hPa (contours with an interval of 2 dagpm; zero contours are omitted) and associated horizontal WAF (vectors; shown for those exceeding the 95% significance level based on the two-sided Student’s t test; m2 s−2) and WAF divergence (shading; m s−2). Phase numbers are given at the top left of each panel.
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
5. The possible driving mechanism: Result from the geopotential height and air temperature tendency budgets
Figure 11a shows the composited occurrence frequency in terms of total NCCV days in each ISV phase. It is clear that the occurrence frequency of NCCVs in phases 1–4 is much less than that in phases 5–8, indicating the suppression effects of ISV in phases 1–4 and the facilitation influences in phases 5–8 on the NCCV activity. To further analyze the characteristics of intraseasonal perturbation related to the NCCV, we calculated the regional mean 500-hPa geopotential height anomaly (Z′) and its change rate (∂Z′/∂t) in the key region of NCCV occurrence frequency of the black solid box in Fig. 3, as shown in Fig. 11b. It can be seen that positive geopotential height anomalies occur in phases 3–7, while negative anomalies are found in phases 1, 2, and 8. However, positive change rates are found from phases 1–4, while change rates in phases 5–8 are negative. Similarly, the regional mean 500-hPa air temperature anomaly (T′) and its change rate (∂T′/∂t) in the key region of the red dashed box in Fig. 3 is also calculated, as shown in Fig. 11c. Positive temperature anomalies occur in phases 3–6, while negative anomalies occur in phases 1–2 and 7–8. However, positive change rates are found from phases 1–4, while change rates in phases 5–8 are negative. Hence, the occurrence frequency of NCCV is closely related to the change rates of the key regional geopotential height and air temperature anomalies in the eight ISV phases. The positive change rate appearing over Northeast China acts to inhibit the generation of NCCV, and the occurrence frequency tends to decrease. When the change rate is negative, the generation of NCCV would be promoted, resulting in an obvious increase in the occurrence frequency of NCCV.
(a) Phase composite of occurrence frequency in terms of total NCCV days. (b) Regional mean 500-hPa geopotential height anomaly (Z′; red bars; gpm) and its rate of change (∂Z′/∂t; blue bars; gpm day−1) in each phase with ISV amplitudes greater than 1.0 over the key region of NCCV occurrence frequency in the black solid box in Fig. 3. (c) Regional mean 500-hPa air temperature anomaly (T′; red bars; K) and its rate of change (∂T′/∂t; blue bars; K day−1) in each phase with ISV amplitudes greater than 1.0 over the key region of NCCV occurrence frequency in the red dashed box in Fig. 3.
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
Since the ISV at mid–high latitudes can modulate the occurrence frequency of NCCV, we focus on how ISV affects Northeast China during its westward propagation in this section. As demonstrated in Fig. 11, the change rates of geopotential height and air temperature are the key to the genesis of NCCVs. Here, we chose the second and seventh phases as the references to diagnose the relative contribution of the four terms in the geopotential height tendency equation and the three terms in the air temperature tendency equation, owing to the strongest change rate of geopotential height, as shown in Fig. 12 (also see appendix B for the full budget of geopotential height tendency). Both terms A and B are consistent with the sign of change rate of geopotential height, but the magnitude of term B is obviously stronger than that of term A. The contributions of both terms C and D are negative. For the change rate of air temperature, only term E shows a positive contribution with a large magnitude. The sign of terms F and G is opposite to the change rate, and the magnitude of term G is also extremely small. It is indicated that term B and term E are the main contributors to the geopotential height and air temperature tendencies of the NCCV region, respectively. The diagnostic highlights the important role of the transports of mean vorticity and mean air temperature by anomalous atmospheric circulation.
Contribution of each term for 500-hPa (a),(c) geopotential height tendency (gpm day−1) in Eq. (3) and (b),(d) air temperature tendency (K day−1) in Eq. (4) at (top) phase 2 and (bottom) phase 7. A, B, C, and D denote the right-side four terms in Eq. (3). E, F, and G denote the right-side three terms in Eq. (4).
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
Moreover, scale decomposition is used to diagnose the major items at the right of Eqs. (6) and (7) contributing to the mean vorticity and mean air temperature transport, as shown in Fig. 13. By examining the evolution of eight phases, it is found that the terms B2 [refers to
(a) Contribution of interaction between low-frequency component (B1; red bars), intraseasonal oscillation (B2; green bars), synoptic-scale disturbances (B3; blue bars), and mean flow in the term B (gpm day−1) for eight phases based on Eq. (6). (b) As in (a), but for the term E (K day−1) for eight phases based on Eq. (7).
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
Figure 14 displays the evolution of the circulation anomaly on the intraseasonal time scale, mean vorticity, and the induced geopotential height tendency in the eight phases. Notable negative mean vorticity can be found to the north of the Tibetan Plateau, while positive mean vorticity prevails to the south of the Tibetan Plateau and over North China to the Sea of Japan. The distribution of the mean vorticity is attributed to the mean westerly wind influenced by the large topography of the Tibetan Plateau, which generally induces an anticyclonic shear to the north and cyclonic shear to the south, thus resulting in the negative and positive mean vorticity to the north and south of the Tibetan Plateau. The area of North China to the Sea of Japan is controlled by the mean East Asian trough, which is the main reason for the positive mean vorticity there. During the westward propagation of ISV, the evolution from phases 1 to 4 reflects the development of the anomalous cyclone over Lake Baikal and its westward movement to the Ural Mountains with a new anomalous anticyclone generated in phase 4. The key region of NCCV (see the blue dashed box in Fig. 14) is basically controlled by the anomalous intraseasonal southwesterly wind in phases 1–4, which transports the negative mean vorticity from the north side of the Tibetan Plateau to the northeast and contributes to the positive geopotential height tendency and thereby restrains the genesis of the NCCV. From phases 5 to 8, as the anomalous cyclone weaken and move westward into the North Atlantic, the newly formed anomalous anticyclone over Lake Baikal continually moves westward. The key region of the NCCV is basically controlled by the anomalous northeasterly wind, which is consistent with the direction of the mean vorticity gradient. It induces negative geopotential high tendency in the key region and thus promotes the genesis of the NCCV. Particularly, in phase 8, the key region is generally impacted by anomalous southerly winds. These transport the mean positive vorticity from the North China and the Sea of Japan to the key region, which further contributes to the negative geopotential height tendency and thereby promotes the NCCV genesis.
Composite phase evolution of intraseasonal winds (
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
The evolution of circulation, mean air temperature, and the induced air temperature tendency in the eight phases is also displayed in Fig. 15. The distribution of mean air temperature is warm in the south and cold in the north. An anomalous southerly wind transports the warm mean air temperature from low latitude to high latitude, forms a warm advection, and thereby contributes to the positive air temperature tendency. The opposite is also true. It needs to be noted that in phases 4 and 8, the west and east sides of the key region are influenced by the anomalous wind from different directions. Thus, the induced, net air temperature tendency is dependent on the intensity of temperature advection from both directions. Observations show that in phase 4 (phase 8), the strength of the anomalous southwesterly (northeasterly) in the west side of the key region is much stronger than that of the anomalous northwesterly (southeasterly) in the east side, resulting in a net positive (negative) air temperature tendency. The negative air temperature tendency in phases 6–8 is always stronger in the west side than that in the east side, which may explain why the NCCV occurs much more frequently in phases 5–8 around 65°N, 105°E than that in phases 1–4 (Fig. 8).
Composite phase evolution of intraseasonal winds (
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
Through examining the zeroth-order geopotential height and air temperature tendency equations, we proposed a plausible schematic summarization of how ISV modulates the environmental background atmospheric circulation anomaly associated with the occurrence frequency of NCCV, as shown in Fig. 16. The tendency of the geopotential height and air temperature anomalies over the key NCCV activity region are primarily attributed to the advection of mean vorticity and mean air temperature by the westward-propagating intraseasonal flow, which dominates the occurrence frequency of the NCCV. Other small first-order terms in the equations are artificially ignored but may also affect the occurrence frequency of NCCVs, which needs further research.
Schematic diagram for the influence mechanism of ISV in modulating the NCCV.
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
6. Summary and discussion
This study investigates the modulation of ISV at mid–high latitudes in the occurrence frequency of NCCVs in early summer, through statistical analysis and thermodynamic diagnostics. According to an objective identification procedure, the NCCV occurs most frequently from April to July. Both the geopotential height and air temperature over the NCCV active area exhibit a mean period of statistically significant 20–60 days, suggesting that the intrinsic dynamics affecting the NCCV is on the intraseasonal time scale.
MVEOF and composite analysis reveal that the leading mode of ISV at mid–high latitudes features pronounced westward propagation of the zonal dipole pattern. ISV perturbation is generally generated over the Lake Baikal region, where the intraseasonal wave energy accumulates. The occurrence of NCCVs in the decaying stage of MVEOF1 and the developing stage of MVEOF2 is evidently more frequent than that in other periods. By dividing the ISV cycle into eight different phases, it is illustrated by phase composite that the phase evolution of ISV actually reflects the alternation of the Ural Mountains area anticyclone (cyclone) and Lake Baikal area cyclone (anticyclone) in their westward propagation. The NCCV genesis is promoted in phases 5–8 and has a relatively larger activity region than that in phases 1–4. The positive tendencies of geopotential height and air temperature in phases 1–4 act to suppress the NCCV activity, while the negative geopotential height tendency in phases 5–8 tends to facilitate the NCCV activity.
The geopotential height and air temperature tendency budgets were used to investigate the mechanism of ISV at mid–high latitudes in modulating the occurrence frequency of NCCV. Through scale decomposition, it is found that the change rate of the geopotential height and air temperature anomalies over the key NCCV activity region are primarily attributed to the advection of mean vorticity and mean air temperature by the anomalous intraseasonal flow, which is associated with the westward propagating ISV. When an anomalous cyclone moves from Lake Baikal to the Ural Mountains, the anomalous intraseasonal southwesterly transports the negative mean vorticity from the north side of the Tibetan Plateau to Northeast China, and transports the warm mean air temperature from low latitude to high latitude, which leads to positive geopotential height and air temperature tendencies, thereby suppressing the NCCV genesis. The reverse is also true.
This study demonstrates that ISV at mid–high latitudes is a dominant intrinsic dynamic affecting the NCCV. However, the tropics are another important source of ISV and can significantly alter and regulate the extratropical circulation through Rossby wave transport (Matthews et al. 2004; Cassou 2008; Hamill and Kiladis 2014). For example, Henderson et al. (2016) and Henderson and Maloney (2018) quantitatively studied the influence of different MJO phases on the occurrence frequency of a winter blocking high in the Northern Hemisphere and found that the formation of a blocking high is promoted in phases 7 and 8 but restrained in phases 3 and 4. Kim et al. (2020) shows the lagged impact of tropical ISV on the East Asian cold surface temperature anomalies in the winter. By using the thermodynamic equation, it is shown that both meridional temperature advection linked with the MJO-related enhanced convection over the western North Pacific and the adiabatic cooling term associated with the MJO-related suppressed convective anomaly in the tropical Indian Ocean play key roles in modulating the East Asian cooling tendency. Our study is somewhat limited by composite analysis of reanalysis data, which cannot purely isolate the effect of mid–high-latitude ISV on the NCCV. Some idealized experiments (i.e., Goss and Feldstein 2018) will be our next focus to examine the NCCV activity. Besides, recent studies have revealed that synoptic eddy feedback is extremely important to the formation of mid–high-latitude systems over Eurasia, including the blocking high (Takaya and Nakamura 2005; Luo et al. 2016) and East Asian trough (Song et al. 2016). Our result shows less importance of synoptic eddy feedback to the genesis of NCCV compared to that of ISV. This may be because the synoptic eddy feedback contributes more to ISV itself rather than directly to the NCCV in the modulation of ISV. It is also possible that the contribution of synoptic eddy feedback is submerged in terms D and G of the geopotential height and air temperature tendency equations because we only considered the combined effects from all time scales. Moreover, NCCV presents significant differences in its meridional position (Fang et al. 2020), and its activity paths have distinct impacts on local weather and climate (Fang et al. 2021a,b). Our study cannot yet explain such important issues. Further validation and numerical experiments need to be carried out to reveal these questions and detailed mechanisms in future work.
Acknowledgments.
This work was jointly supported by the National Natural Science Foundation of China (Grants 42088101, 41905062, and 42105046) and The Startup Foundation for Introducing Talent of NUIST (Grant 2021R079).
Data availability statement.
The data presented in this study are openly available ECMWF at https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-pressure-levels?tab=overview as cited in Hersbach et al. (2020).
APPENDIX A
Phase Division Based on PC1 and PC2
Schematic diagram of (a) phase angle and (b) actual evolution of PC1 and PC2 corresponding to ISV.
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
APPENDIX B
Full Budget of the Geopotential Height Tendency Equation
Contribution of each term (gpm day−1) at phase 2 and phase 7.
Citation: Journal of Climate 36, 12; 10.1175/JCLI-D-22-0691.1
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