Characteristics of the atmospheric circulation in April 2020
Figure 2 illustrates the 200-, 500-, and 850 hPa geopotential heights, 200 hPa wave activity flux, and blocking frequency anomalies for April 2020 using the European Centre for Medium-Range Weather Forecasts Re-analysis 5 (ERA5). In the east-west (horizontal) direction, the anticyclonic circulation anomalies appeared over western/central Europe (45–60 °N, 5 °W–15 °E) and Siberia (50–70 °N, 80–120 °E) near Lake Baikal. The cyclonic circulations occurred over northwest/central Russia (50–70 °N, 30–60 °E) and the East Sea (25–40 °N, 125–145 °E) with alternating signs (i.e., in a ridge-trough-ridge-trough pattern). Besides, the location of maximum intensity for the anomalous ridge over Siberia/Russian Far East and the location of minimum intensity for the anomalous trough over the East Sea/northwest Pacific slightly tilted westward with height in the troposphere, in which the vertical structure shows the growth of baroclinic disturbance. This atmospheric circulation structure is analogous to a wave train pattern. The wave activity flux was calculated using Takaya and Nakamura14 method to estimate the Rossby wave propagation (Fig. 2a green vectors). The wave-flux vector started from western Europe and seemed to proceed towards the marginal northwest Pacific, coincided with the ridge-trough-ridge-trough pattern.
A series of anticyclonic and cyclonic circulation anomalies exhibited over northeast Eurasia in the north-south (meridional) direction. The anomalous ridge over Siberia/Russian far East had a grossly equivalent barotropic vertical structure, but the anomalous trough over the East Sea/northwest Pacific had an apparent westward tilt of about 10° in height. These barotropic ridge and baroclinic trough systems are comparable to a blocking pattern. To identify the blocking occurrence, the blocking days were estimated using the hybrid atmospheric blocking method15,16, which combines each advantage of the Dole–Gordon index17 and the Tibaldi–Molteni index18. The blocking days over Siberia during April 2020 increased by an area average of 10.6 days than climatology of 1982–2019 (11.6 days up to a maximum of 23 days in 2020 and 1 day in climatology; Supplementary Fig. S2 and Fig. 2b purple contours). These results suggest that the atmospheric structure associated with the cold condition in April 2020 might be attributed to a mixed type of wave train and blocking.
To determine how different the atmospheric circulation related to cold condition in April 2020 compared with normal years, we plotted a scatter diagram of the dipole pattern (ordinate) under the wave train (abscissa) and blocking (maker) indices (Fig. 3). Here, the normal years mean only 17 years occurred the Siberian blocking out of 39 years (in April from 1982 to 2019). The dipole atmospheric circulation index is considered as the cold condition with the northerly flow in northeast Asia. The wave train and blocking indices are potential causes of cold condition (see indices details in the Methods section). During the 17 years, the dipole circulation index showed a positive linear relationship with the wave train index (correlation coefficient = 0.6, p = 0.01). The blocking indices with large magnitudes are linked to large wave train and dipole indices as observed in 1997, 2007, 2011, and 2020. In particular, the wave train index in April 2020 was the same as that in 2007 at 3.9, which are the strongest years; however, the dipole index value differed from 4.7 in 2020 and 3 in 2007. This larger dipole index in 2020 seems to reflect the extreme blocking index of 11.6 in 2020 that is the strongest for 39 years, compared with the 6 in 2007. The result reveals that this exceptional Siberian blocking could play a crucial role in modulating the dipole atmospheric circulation over Siberia and the East Sea in concert with the wave train pattern.
Possible Causes For The Cold Condition In April 2020
A question arises as to which forcing can form the atmospheric circulation in April 2020 over Eurasia. To this end, we performed a numerical experiment using the linear baroclinic model (LBM) that is capable of diagnosing the steady-state atmospheric dynamical response to prescribed forcing19–21. The experiment was forced with the robust positive vorticity profile for the northwest/central Russia region from the ERA5 (black box in Fig. 4a and Supplementary Fig. S3) based on the result of wave activity flux. The simulated upper-, mid-, and low-level geopotential height anomalies for near steady-state (averaged over 16–20 days) are plotted in Fig. 4. All tropospheric geopotential height anomalies for the vorticity forcing exhibited horizontally and meridionally alternating positive and negative signs over Eurasia. The spatial correlation between simulated and observed 200 hPa geopotential height was approximately 0.71 over 25–80 °N and 0–150 °E, which was significant at the 99% confidence level. The result indicates that the numerical experiment reasonably reproduces the observed April 2020 of atmospheric circulation. On the other hand, the location of positive height anomalies in western Europe was not well simulated in the model compared to the reanalysis (Figs. 2 and 4), presumably relating to the single effect of north Atlantic or tropics forcing or the complex impact of them20,22,23.
It is noteworthy that the atmospheric circulation anomalies in the horizontal direction have an arc-shaped wave pattern (Fig. 4). The ray tracing method was applied to identify a path of the Rossby wave propagation. The calculated rays for zonal wavenumber 2 matched well with the centers of upper-tropospheric geopotential height anomalies (navy lines and yellow squares in Fig. 4a), implying the vorticity forcing at northwest/central Russia could generate the teleconnection pattern toward the marginal northwest Pacific. This wave energy propagation was possible since the modified Rossby wave theory considers the effect of basic zonal and meridional wind19,24−26(Fig. 5a and Supplementary Fig. S4). In climatology, the meridional vorticity gradient showed a positive value across 40–60 °N and 30–120 °E (Fig. 5a), which was proportional to the polar front westerly27,28 and the northerly wind over Ural-Siberia (Supplementary Fig. S4). Along the westerly and northerly background flows that are included in the dispersion relationship for the barotropic Rossby wave, the stationary wave penetrated southeastward into the East Sea.
This wave train pattern is analogous to the East Atlantic/Western Russia (EAWR) teleconnection. The EAWR pattern is one of the most dominant atmospheric teleconnections that affect the weather and climate in Eurasian throughout the year29–34. The positive phase of the EAWR pattern has positive height anomalies over Europe and northern China and negative over the central North Atlantic and north of the Caspian Sea, and vice versa. External and internal forcing could generate and maintain atmospheric wave trains in mid-latitude19,29,32,33,35,36. As external forcing factor, anomalous boundary condition such as North Atlantic Sea surface temperature and North American/Eurasian snow cover could induce the upper-level divergence wind, acting as the Rossby wave source for the EAWR pattern32,33,35. Also, as internal dynamics, the interaction between background westerly and synoptic-scale transient eddies could drive the atmospheric Rossby wave energy propagation from the North Atlantic29,33. The spatial map of 200 hPa geopotential height anomalies was regressed against the EAWR index for 1982–2020 (Supplementary Fig. S5). This height regression field with the EAWR index showed a consistent pattern to the observation of the geopotential height anomaly in April 2020 (Fig. 2a). The EAWR index exhibited a strong positive correlation coefficient of 0.89 with the wave train index (defined the index in this study) for 39 years, suggesting that the EAWR pattern accounts for approximately 79% of the year-to-year variance of wave train patterns.
In the meridional direction, the barotropic high-pressure anomaly over Siberia appeared with the baroclinic low-pressure anomaly over the East Sea, similar to the observed blocking pattern (Fig. 4). The atmospheric blocking system was closely connected with the wind speed variation in upper-level troposphere. The 200 hPa zonal winds showed the strong negative anomaly (i. e., easterly anomaly wind) over the Mongolia–northeast China in April 2020 compared with climatology, but strong positive anomaly over the northern Russia and southern China (Fig. 5b). The wind structure of positive-negative-positive anomalies near 120 °E corresponded with anomalous anticyclone and cyclone over Siberia and the East Sea, respectively. Furthermore, the zonal wind anomaly exhibited a wave-like pattern over 40–80 °N, resembling the distribution of blocking frequency anomaly (Fig. 5b and Supplementary Fig. S2c): anomalous easterly/westerly wind region coincided with reduced/increased blocking frequency area. This result indicates that the Siberian blocking is relevant to the Rossby wave train pattern over Eurasia9,14,37,38. As shown in Fig. 4a, the Rossby wave train transferred the wave energy from upstream of Europe to downstream of Asia; this convergence of wave activity flux can reinforce and rebuild the anticyclonic circulation over Siberia; thus, likely creating a more intense and persistent blocking.
Summary And Discussion
This study elucidates that the causes of the dipole atmospheric circulation over Siberia and the East Sea associated with the cold condition in April 2020 in northeast Asia. The anomalous ridge and trough structure were a mixed pattern of stationary Rossby wave train and blocking. The wave train with alternating anticyclonic and cyclonic circulations across Eurasia was induced by the vorticity forcing of northwest/central Russia and propagated from western Europe to the East Sea. The climatological polar front jet and northerly wind allowed the wave energy to propagate toward the southeast. This wave pattern almost corresponds to the EAWR teleconnection pattern, one of the influential atmospheric circulations in Eurasian weather. In addition, the blocking frequency increased by approximately ten times over Siberia in April 2020 than that of climatology. This Siberia region of increased blocking days nearly paralleled the weaker westerly wind in April 2020 over Mongolia–northeast China. The change in blocking frequency distribution was related to the wavy zonal flow change across the high latitude, similar to the Rossby wave response9,14,37,38. The propagating/breaking Rossby wave might reestablish and re-strengthen the blocking high.
The reason that the Siberian blocking frequency in April 2020 was the strongest over the past 39 years remains still unclear. Many previous studies demonstrated that the Siberian blocking tends to occur in stronger and longer cold condition over East Asia during the negative AO period in winter10,11,39−42. However, no relationship between blocking and AO was identified in April 2020; the AO index was positive (0.93), while the blocking index had the highest value of 11.6 days. For the winter and early spring of 2020 (January–March) in the last four decades, the positive AO accompanied by the exceptional stratospheric polar vortex was extraordinarily robust at 2.8343. This record low-frequency variability enabled maintaining the pattern of the cold Arctic and warm Eurasia until March 2020 through the strengthened westerly flow. Then, the AO index became weaker in April and more neutral in May (-0.03) than during January–March, coinciding with the change in westerly circulation surrounding the Arctic (Fig. 5b). We suggest a link between the blocking over Siberia and the AO phase transition from an extreme positive value in January–March to a weak negative value in May (from 2.83 to -0.03). Thus, further study is warranted to clarify the relationship between the blocking and AO phase transition in springtime.
Our study highlights that extreme Siberian blocking and strong EAWR pattern influenced the dipole atmospheric circulation causing the cold condition in northeast Asia in April 2020. This cold condition seriously harmed agricultural and marine ecosystems over northeastern Asia during the peak growth period, such as fruit trees blossom, farm sprout, and phytoplankton spring bloom6 (
www.cma.gov.cn and
www.mafra.go.kr). Therefore, the current results will be helpful for better understanding the extreme weather or climate over northeastern Asia in April and its damage to the land-ocean ecosystems.