1. Introduction
It is essential for understanding the wintertime climate system over East Asia and the northwestern Pacific to examine the variability of the Siberian high in its position and intensity. While many of the previous studies on the high focused on the circulation and heat budget in the lower troposphere associated with cold air outbreaks to the midlatitude Far East (e.g., Ding and Krishnamurti 1987; Boyle and Chen 1987; Ding 1990; Esbensen 1984; Clark et al. 1999), fewer studies examined the relation between the anomalous intensification of the surface high and upper-tropospheric circulation anomalies (Suda 1957; Joung and Hitchman 1982; Lau and Lau 1984; Hsu and Wallace 1985; Hsu 1987; Wu and Chan 1997). Those studies have indicated that a wavelike anomaly pattern is observed in the upper troposphere over the Eurasian continent associated with intraseasonal variability of the surface Siberian high.
Through a composite analysis for the 20 strongest events of the intraseasonal amplification of the surface Siberian high observed around its climatological center over 40 recent years, Takaya and Nakamura (2005, hereafter TN05) have shown that the intraseasonal amplification of the surface high is associated with a blocking ridge that forms from anomalies as a component of a Rossby wave packet propagating in the upper troposphere from the Euro-Atlantic sector into the Far East across the Eurasian continent. They call this type of the amplification events of the Siberian high as “wave-train (Atlantic-origin)” type. As discussed in detail in TN05, the vertical coupling between upper-level potential vorticity anomalies associated with the propagating wave packet and surface cold temperature anomalies is essential for the strong amplification of the surface cold high.
As Nakamura et al. (1997) suggested that the relative importance between an incoming quasi-stationary Rossby wave train and feedback forcing from migratory transient eddies in blocking formation can be rather sensitive to its position relative to a nearby storm track, it is our interest to examine whether an upper-level blocking anticyclone that accompanies the intraseasonal amplification of the surface high at any location over the Eurasian continent tends to form through the wave-train (Atlantic-origin) type evolution. In this short contribution, we show through our composite analysis applied to each grid point over the extratropical Eurasian continent that the blocking formation, including its precursory signals, is fundamentally different between the east and west of a climatological-mean polar-vortex trough over the Far East.
2. Data and analysis methods
a. Data
The data used in this study are the same as those in TN05, that is, twice-daily gridded fields of geopotential height, temperature, zonal and meridional wind components, and pressure velocity at the 12 standard pressure levels (100, 150, 200, 250, 300, 400, 500, 600, 700, 850, 925, and 1000 hPa) and at the lowest model level of σ = 0.995 just above the surface. We also use Ertel’s potential vorticity (PV) evaluated on the 330-K isentropic surface. These data are based on the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalyses for the period 1958–98 (Kalnay et al. 1996).
b. Compositing
To extract quasi-stationary, intraseasonal fluctuations associated with blocking formation over Siberia, a low-pass filter with a cutoff period of 8 days was applied to the data time series at each grid point, as in TN05. A local anomaly of a given variable for a particular time has been defined as its low-pass-filtered departure from the climatological-mean annual cycle at that location for the corresponding calendar day. The annual cycle has been obtained as the average of 31-day running-mean fields of the variable over the 40-yr period (1958–98). Compositing was then performed relative to the peak times (day 0) of the 20 strongest anticyclonic anomalies observed in 250-hPa geopotential height (Z250) around a given grid point over the 40 winter seasons. Every winter season is defined as a 150-day period from 16 November. TN05 and Nakamura et al. (1997) can be referred to for details on how the strongest events around a particular grid point of interest have been identified and also on specific manipulations applied in compositing the data fields in order for emphasizing systematic signals among the individual events.
c. Evaluation of transient eddy forcing
3. Results
a. Wave-train (Atlantic-origin) type
As a typical example of the wave-train (Atlantic-origin) type, TN05 showed the time evolution of 1000-hPa height (Z1000) anomalies composited for the 20 strongest surface anticyclonic anomalies around a target grid point (47°N, 90°E). In the composite time evolution, the surface high develops around the climatological center of the Siberian high, and those events tend to bring the strongest cold air outbreaks to the midlatitude Far East. Owing to the baroclinic structure of the anomalies, the center of the corresponding upper-tropospheric blocking ridge is shifted northwestward of the surface center of the anticyclonic anomalies (Fig. 3 of TN05). As typical time evolution of those upper-level blocking events, Fig. 1 shows composites of the 20 strongest blocking events that occurred around a target grid point (57°N, 80°E), the same location as the primary anticyclonic anomaly center in the Z250 composite for the 20 strongest surface events around (47°N, 90°E; Fig. 3 in TN05). The composited anomalous Z250 and associated wave-activity flux W for stationary Rossby waves, which is parallel to their local group velocity (Takaya and Nakamura 1997, 2001), both indicate that a prominent blocking ridge forms near the leading edge of a strong wave packet that emanates from the North Atlantic (right column of Fig. 1). The typical evolution of the upper-level blocking events shown in Fig. 1 is essentially the same as the upper-level blocking formation composited for the 20 strongest surface high amplification shown in Fig. 3 of TN05.
It should be noted, however, that while upper-level anticyclonic anomalies at the peak time of those upper-level events tend to be nearly twice as large in magnitude as their counterpart of the surface events, the associated anomalous surface high and surface cold anomalies are significantly weaker, especially downstream of the blocking ridge. Rather, the associated surface temperature anomalies are characterized by warm anomalies over central north Siberia. It is noteworthy that no preexisting cold anomalies are observed at the surface in the early amplification stage (day −4) of those upper-level events (middle panel of Fig. 1), which is in sharp contrast with the strong anticyclonic events at the surface (Fig. 3 in TN05). It is thus conjectured that preexisting surface cold anomalies over Siberia are an important precondition for the strong development of a cold surface high. In fact, as demonstrated in TN05, the interaction between an incoming stationary Rossby wave packet and preexisting surface cold anomalies is of critical importance in the pronounced amplification of the surface Siberian high.
Nakamura et al. (1997) showed that there are two types of blocking formation; one is mainly through transient eddy forcing and the other is associated mainly with an incoming quasi-stationary Rossby wave packet. It is our interest to assess how substantially anomalous transient eddy forcing contributes to the formation of the blocking ridge over Siberia. Figure 2c shows the composite evolution of the Z250 tendency for 2 days before the peak time due solely to the anomalous barotropic forcing from high-frequency transients (∂Z′250/∂t)HFT. Comparing it with the observed height tendency at day −2 (∂Z′250/∂t)OBS shown in Fig. 2b, one can recognize that (∂Z′250/∂t)HFT accounts only for 40% (30 versus 70 m day−1) of the observed amplification of the ridge, and the forcing is shifted upstream of the observed anticyclonic tendency. Furthermore, the statistical significance of the composited (∂Z′250/∂t)HFT is not as high as that of the observed height tendency, which implies that the forcing pattern is not particularly coherent from one event to another. These features are observed also in the corresponding composites for 4 days before the peak time (day −4). Noting that (∂Z′250/∂t)HFT can be regarded as an upper bound of the net forcing by transient eddies, we conclude that transient-eddy forcing is only of secondary importance in the development of the particular blocking ridge. Rather, low-frequency dynamics associated with the incoming quasi-stationary Rossby waves is likely the primary factor for the blocking formation over Siberia. In the composite evolution of upper-level PV field (not shown), a blocking formation is due to the amplification of anticyclonic anomalies in a region of the relatively weak westerlies on the equatorward flank of the subpolar jet. The subsequent breaking under the anticyclonic shear of the jet leads to the ejection of filamental high-PV air from the polar vortex, yielding a blocking-like dipole. This process is essentially the same as for the wintertime blocking formation over northern Europe shown by Nakamura et al. (1997), which can be regarded as local outward breaking of the polar vortex.
b. Pacific-origin type
Our examination of composite evolution of prominent upper-level anticyclonic anomaly events at every grid point over the Siberian continent has revealed another type of evolution typical for eastern Siberia that can be called “Pacific-origin” type. As a typical example of the Pacific-origin type, composite time evolution for the 20 strongest blocking events around a target grid point (67°N, 140°E), located to the north of the Sea of Okhotsk, is discussed in the following. In the total field of Z1000 at the peak time, the center of the Siberian high is shifted toward the northeast of its climatological-mean position (not shown). In the composite time evolution, no wave-packet propagation from upstream is evident during the amplification stage of the blocking (right column of Fig. 3). Rather, the wave-activity flux is confined within dipolelike anomalies that constitute the blocking. The blocking ridge forms as anticyclonic anomalies that have initially been located in the Aleutian region gradually extend westward into eastern Siberia, which may be related to the slowly retrogressing anomaly pattern pointed out by Kushnir (1987), Branstator (1987), and Lau and Nath (1999). This process may be viewed as local inward breaking of the polar vortex associated with blocking formation (Nakamura and Plumb 1994; Swanson 2000, 2001). A more apparent signature of the local inward breaking can be seen in the corresponding composite evolution of the total PV field at the 330-K surface. In the PV evolution shown in Fig. 4, low PV anomalies penetrate and then become isolated in the polar vortex, while high PV air associated with the climatological trough over the Far East becomes elongated to the south of the blocking ridge.
In the Pacific-origin type, barotropic forcing by migratory synoptic-scale eddies is substantial in the development and maintenance of the blocking ridge (Figs. 2d–f). A region of the anticyclonic eddy forcing, which is statistically significant, almost coincides with the anticyclonic anomalies that constitute the blocking dipole. Though regarded as an upper bound of the net forcing, the eddy barotropic anticyclonic forcing (50 m day−1) accounts for a major fraction of the observed height tendency (70 m day−1). Such a substantial contribution from migratory eddies to the blocking formation has been found in other blocking ridges that develop in the vicinity of the Pacific storm track (Nakamura and Wallace 1993; Nakamura et al. 1997).
The blocking anomalies are nearly equivalent barotropic, especially during their amplification stage, even near the surface where the anticyclonic anomalies are accompanied by warm anomalies (left and right columns of Fig. 3). The barotropic structure of the blocking is consistent with the major contribution of transient eddy feedback forcing to the blocking development. An exception is found to the north of the Tibetan Plateau, where cold anomalies form as the upper-level anticyclonic anomalies are anchored to the north of the Sea of Okhotsk, inducing anomalous northerlies that act to advect cold air of the background surface temperature field to the north of the Tibetan Plateau. After the peak time, the cold anticyclonic anomalies at the surface extend southward into the midlatitude Far East, where cyclonic anomalies still remain in the upper troposphere. Such baroclinic structure of the circulation anomalies over eastern China and Japan associated with the anomalous cold surface air is common to both the wave-train (Atlantic-origin) and Pacific-origin types. It can be verified through the PV inversion method used in TN05 that the interaction between the upper-tropospheric circulation anomalies and surface baroclinicity, as shown in TN05, is operative also in the evolution of the Pacific-origin type (not shown).
4. Summary
Our composite analysis has revealed two different types of the formation of upper-level blocking that accompanies intraseasonal amplification of the surface Siberian high, namely the wave-train (Atlantic-origin) and Pacific-origin types. Statistics summarized in Fig. 5 are based on composite anomaly evolution for the 20 strongest upper-level blocking episodes, as shown in Fig. 1, for each of the grid points over the extratropical Eurasian continent and the northwestern Pacific. Figure 5 shows clear geographical dependency of those two types of upper-level blocking formation, which appears to be related to climatological features of the upper-tropospheric wintertime circulation. The wave-train (Atlantic-origin) type characterized by propagation of a quasi-stationary Rossby wave packet over the Eurasian continent is found common over a vast area of central and western Siberia, located to the west of the climatological-mean trough over the Far East. At some locations, the main anticyclonic center associated with the wave-train (Atlantic-origin) type moves slowly eastward, as shown in Fig. 1. The blocking formation of this type that occurs under modest feedback forcing from transient eddies (Fig. 2) is thus primary through low-frequency dynamics associated with a propagating quasi-stationary Rossby waves, as pointed out by Nakamura (1994), Nakamura et al. (1997) and Swanson (2000). Meanwhile, the Pacific-origin type characterized by slow retrogression of the primary anticyclonic center from the North Pacific region with no apparent signature of any incoming quasi-stationary wave packet (Fig. 3) is found common over eastern Siberia located to the east of the climatological trough. The retrogression of anticyclonic anomalies, which may be related to the results of Kushnir (1987), Branstator (1987), and Lau and Nath (1999), occurs under strong feedback forcing from the Pacific storm track. A comparison between Fig. 2 in TN05 and Fig. 5 reveals that, although blocking formation of the Pacific-origin type tends to be followed by a cold air outbreak to the midlatitude Far East, the outbreak tends to be weaker than its counterpart that follows blocking formation of the wave-train type.
The geographical distinction between the two types of blocking formation evident in Fig. 5 can be interpreted as follows. As shown in Fig. 6, the tropospheric low-frequency variability in winter tends to be strongest both over the North Atlantic and the North Pacific. Each of those regions is located in the exits of an upper-level jet stream and associated storm track, and thus barotropic feedback forcing from migratory transient eddies on low-frequency anomalies is also strongest (Lau and Nath 1991). As apparent in Fig. 6, a belt of local maxima in the low-frequency variability extends from the North Atlantic downstream into the Eurasian continent, which corresponds to relatively large Z250 fluctuations associated with quasi-stationary Rossby wave packets (Blackmon et al. 1984) along a Rossby wave guide over Eurasia (Hsu and Lin 1992; Hoskins and Ambrizzi 1993). Located downstream of the North Atlantic maximum of the intraseasonal variability, the tropospheric circulation over western and central Siberia is under the strong influence of stationary Rossby wave packets emanating from the North Atlantic, and formation of a blocking ridge associated with such a wave packet as above will lead to the intraseasonal amplification of the surface Siberian high. The amplification can be pronounced in the presence of preexisting cold anomalies at the surface (TN05).
Presumably, a wave packet propagating over the Eurasian continent can hardly cross the persistent polar-vortex trough over the Far East in which the westerlies are very weak. Unlike in other portions of Siberia, therefore, the tropospheric circulation over eastern Siberia, located under or to the east of the climatological trough, is unlikely under the remote influence of the North Atlantic variability. Rather, the circulation over northeastern Siberia is under the stronger influence of the Pacific variability. In fact, blocking formation of the Pacific-origin type seems to be associated with the local inward breaking of the polar vortex under the strong feedback forcing from the Pacific storm track, which is triggered by the amplification of preexisting anticyclonic anomalies over the North Pacific. A belt of local maxima in the upper-tropospheric low-frequency variability that extends zonally across the Far Eastern trough of the polar vortex above the Arctic Sea (Fig. 6) may be a manifestation of such inward breaking as shown in Fig. 4.
Acknowledgments
We thank Dr. John Nielsen-Gammon and two anonymous reviewers for their valuable comments that have led to the improvement of our paper. We also appreciate valuable comments and suggestions provided by Drs. Yoshihisa Matsuda, Toshio Yamagata, Masahide Kimoto, and Ryuji Kimura of University of Tokyo. Discussions with Dr. Takeshi Enomoto of the Earth Simulator Center were also helpful.
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Composite time evolution for the 20 strongest blocking events at the 250-hPa level observed around a target grid point (57°N, 80°E) over 40 recent winter seasons. The compositing was performed relative to the peak time (day 0) for each of the events. (left) Low-pass-filtered Z1000 anomalies normalized by a factor of sin(45°N)/sin(lat), contoured every 40 m from ±20 m (dashed for negative values). The anomalies significant at the 95% confidence level are indicated with stippling. (middle) Low-pass-filtered temperature anomalies at the level of σ = 0.995 (contoured every 4 K from ±2 K; solid and dashed lines for cold and warm anomalies, respectively). Surface elevation over 1500 m is indicated with shading (also contoured every 1500 m). (right) Low-pass-filtered Z250 anomalies normalized as Z1000, contoured every 100 m from ±50 m with thick lines for Z250 (dashed for negative values). The horizontal component of a wave-activity flux W, defined by Takaya and Nakamura (1997, 2001), is superimposed with arrows whose scaling (unit: m2 s−2) is given near the lower-right panel. Heavy and light stippling signifies the flux convergence and divergence, respectively, whose magnitudes are greater than 1.5 × 10−5 m s−2. The flux divergence and convergence are also contoured every 3.0 × 10−5 m s−2 from 1.5 × 10−5 m s−2.
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
Composite time evolution for the 20 strongest blocking events at the 250-hPa level observed around a target grid point (57°N, 80°E) over 40 recent winter seasons. The compositing was performed relative to the peak time (day 0) for each of the events. (left) Low-pass-filtered Z1000 anomalies normalized by a factor of sin(45°N)/sin(lat), contoured every 40 m from ±20 m (dashed for negative values). The anomalies significant at the 95% confidence level are indicated with stippling. (middle) Low-pass-filtered temperature anomalies at the level of σ = 0.995 (contoured every 4 K from ±2 K; solid and dashed lines for cold and warm anomalies, respectively). Surface elevation over 1500 m is indicated with shading (also contoured every 1500 m). (right) Low-pass-filtered Z250 anomalies normalized as Z1000, contoured every 100 m from ±50 m with thick lines for Z250 (dashed for negative values). The horizontal component of a wave-activity flux W, defined by Takaya and Nakamura (1997, 2001), is superimposed with arrows whose scaling (unit: m2 s−2) is given near the lower-right panel. Heavy and light stippling signifies the flux convergence and divergence, respectively, whose magnitudes are greater than 1.5 × 10−5 m s−2. The flux divergence and convergence are also contoured every 3.0 × 10−5 m s−2 from 1.5 × 10−5 m s−2.
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
Composite time evolution for the 20 strongest blocking events at the 250-hPa level observed around a target grid point (57°N, 80°E) over 40 recent winter seasons. The compositing was performed relative to the peak time (day 0) for each of the events. (left) Low-pass-filtered Z1000 anomalies normalized by a factor of sin(45°N)/sin(lat), contoured every 40 m from ±20 m (dashed for negative values). The anomalies significant at the 95% confidence level are indicated with stippling. (middle) Low-pass-filtered temperature anomalies at the level of σ = 0.995 (contoured every 4 K from ±2 K; solid and dashed lines for cold and warm anomalies, respectively). Surface elevation over 1500 m is indicated with shading (also contoured every 1500 m). (right) Low-pass-filtered Z250 anomalies normalized as Z1000, contoured every 100 m from ±50 m with thick lines for Z250 (dashed for negative values). The horizontal component of a wave-activity flux W, defined by Takaya and Nakamura (1997, 2001), is superimposed with arrows whose scaling (unit: m2 s−2) is given near the lower-right panel. Heavy and light stippling signifies the flux convergence and divergence, respectively, whose magnitudes are greater than 1.5 × 10−5 m s−2. The flux divergence and convergence are also contoured every 3.0 × 10−5 m s−2 from 1.5 × 10−5 m s−2.
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
Composite time evolution of (a) low-pass-filtered Z250 anomalies, (b) observed time tendency in those anomalies, (∂Z′250/∂t)OBS, and (c) anomalous barotropic forcing by high-frequency transients represented as anomalous Z250 tendency, (∂Z′250/∂t)HFT, for 2 days before the peak time (day −2), based on the 20 strongest events of the upper-level blocking events observed around (57°N, 80°E) over 40 recent winter seasons composited for the days relative to the peak time (day 0) as indicated. The composited anomalies significant at the 95% confidence level are indicated with stippling. Contour lines are (left) ±50, ±100, ±150, . . . m; (middle and right) ±10, ±30, ±50, . . . m day−1. Dashed and solid lines signify cyclonic and anticyclonic anomalies (or forcing), respectively. (d)–(f) As in (a)–(c), respectively, but for the 20 strongest events of upper-level blocking events observed around (67°N, 140°E).
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
Composite time evolution of (a) low-pass-filtered Z250 anomalies, (b) observed time tendency in those anomalies, (∂Z′250/∂t)OBS, and (c) anomalous barotropic forcing by high-frequency transients represented as anomalous Z250 tendency, (∂Z′250/∂t)HFT, for 2 days before the peak time (day −2), based on the 20 strongest events of the upper-level blocking events observed around (57°N, 80°E) over 40 recent winter seasons composited for the days relative to the peak time (day 0) as indicated. The composited anomalies significant at the 95% confidence level are indicated with stippling. Contour lines are (left) ±50, ±100, ±150, . . . m; (middle and right) ±10, ±30, ±50, . . . m day−1. Dashed and solid lines signify cyclonic and anticyclonic anomalies (or forcing), respectively. (d)–(f) As in (a)–(c), respectively, but for the 20 strongest events of upper-level blocking events observed around (67°N, 140°E).
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
Composite time evolution of (a) low-pass-filtered Z250 anomalies, (b) observed time tendency in those anomalies, (∂Z′250/∂t)OBS, and (c) anomalous barotropic forcing by high-frequency transients represented as anomalous Z250 tendency, (∂Z′250/∂t)HFT, for 2 days before the peak time (day −2), based on the 20 strongest events of the upper-level blocking events observed around (57°N, 80°E) over 40 recent winter seasons composited for the days relative to the peak time (day 0) as indicated. The composited anomalies significant at the 95% confidence level are indicated with stippling. Contour lines are (left) ±50, ±100, ±150, . . . m; (middle and right) ±10, ±30, ±50, . . . m day−1. Dashed and solid lines signify cyclonic and anticyclonic anomalies (or forcing), respectively. (d)–(f) As in (a)–(c), respectively, but for the 20 strongest events of upper-level blocking events observed around (67°N, 140°E).
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
As in Fig. 1, but for the 20 strongest events of an upper-level blocking high around a target grid point (67°N, 140°E). Stippling for the flux divergence in right column is omitted to keep the clarity of the figures.
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
As in Fig. 1, but for the 20 strongest events of an upper-level blocking high around a target grid point (67°N, 140°E). Stippling for the flux divergence in right column is omitted to keep the clarity of the figures.
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
As in Fig. 1, but for the 20 strongest events of an upper-level blocking high around a target grid point (67°N, 140°E). Stippling for the flux divergence in right column is omitted to keep the clarity of the figures.
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
As in Fig. 3, but for low-pass-filtered total and anomalous Ertel’s potential vorticity (PV) at the 330-K isentropic surface for the 20 strongest events of an upper-level blocking high around 67°N, 140°E in its amplification stage and at the peak time. The total PV is denoted by contours for every 0.5 PVU (where 1 PVU = 10−6 m2 s−1 K kg−1) from 4.0 PVU (heavy lines for 6.0 PVU). Heavy and light stippling signifies negative (i.e., anticyclonic) and positive (i.e., cyclonic) PV anomalies, respectively, with light contours for every 0.4 PVU from ±0.4 PVU.
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
As in Fig. 3, but for low-pass-filtered total and anomalous Ertel’s potential vorticity (PV) at the 330-K isentropic surface for the 20 strongest events of an upper-level blocking high around 67°N, 140°E in its amplification stage and at the peak time. The total PV is denoted by contours for every 0.5 PVU (where 1 PVU = 10−6 m2 s−1 K kg−1) from 4.0 PVU (heavy lines for 6.0 PVU). Heavy and light stippling signifies negative (i.e., anticyclonic) and positive (i.e., cyclonic) PV anomalies, respectively, with light contours for every 0.4 PVU from ±0.4 PVU.
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
As in Fig. 3, but for low-pass-filtered total and anomalous Ertel’s potential vorticity (PV) at the 330-K isentropic surface for the 20 strongest events of an upper-level blocking high around 67°N, 140°E in its amplification stage and at the peak time. The total PV is denoted by contours for every 0.5 PVU (where 1 PVU = 10−6 m2 s−1 K kg−1) from 4.0 PVU (heavy lines for 6.0 PVU). Heavy and light stippling signifies negative (i.e., anticyclonic) and positive (i.e., cyclonic) PV anomalies, respectively, with light contours for every 0.4 PVU from ±0.4 PVU.
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
Geographical distribution of the wave-train (Atlantic-origin) and Pacific-origin types on the basis of upper-level blocking composites over the Eurasian continent and the northwestern Pacific Ocean. Each of the heavy lines signifies a propagation axis of a wave train at the 250-hPa level for the wave-train type. An end of the line with a closed triangle marks the position of a blocking ridge at its peak time, while the other end corresponds to the position of the strongest upstream cyclonic anomaly center associated with the wave train 4 days before the peak time. Each of the light lines signifies a migration path of the primary anticyclonic anomaly center at the 250-hPa level. An end of the line with a triangle corresponds to the anticyclonic center at its peak time, while the other end corresponds to the same center but 4 days before its peak time. Contour lines indicate the climatological-mean Ertel’s PV at the 330-K isentropic surface (contoured every 1 PVU from 2.5 PVU). The statistics shown in this figure are based on the 250-hPa composite anomaly evolution for the 20 strongest blocking events around each of the grid points over the Eurasian and northwestern Pacific regions, but those only for selected grid points are shown for clarity. The wave-train (Atlantic-origin) type is characterized by an incoming Rossby wave packet and, at some locations, slow eastward migration of the blocking anticyclonic center, whereas the Pacific-origin type is characterized by no signature of any incoming wave packet and by slow retrogression of the blocking center.
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
Geographical distribution of the wave-train (Atlantic-origin) and Pacific-origin types on the basis of upper-level blocking composites over the Eurasian continent and the northwestern Pacific Ocean. Each of the heavy lines signifies a propagation axis of a wave train at the 250-hPa level for the wave-train type. An end of the line with a closed triangle marks the position of a blocking ridge at its peak time, while the other end corresponds to the position of the strongest upstream cyclonic anomaly center associated with the wave train 4 days before the peak time. Each of the light lines signifies a migration path of the primary anticyclonic anomaly center at the 250-hPa level. An end of the line with a triangle corresponds to the anticyclonic center at its peak time, while the other end corresponds to the same center but 4 days before its peak time. Contour lines indicate the climatological-mean Ertel’s PV at the 330-K isentropic surface (contoured every 1 PVU from 2.5 PVU). The statistics shown in this figure are based on the 250-hPa composite anomaly evolution for the 20 strongest blocking events around each of the grid points over the Eurasian and northwestern Pacific regions, but those only for selected grid points are shown for clarity. The wave-train (Atlantic-origin) type is characterized by an incoming Rossby wave packet and, at some locations, slow eastward migration of the blocking anticyclonic center, whereas the Pacific-origin type is characterized by no signature of any incoming wave packet and by slow retrogression of the blocking center.
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
Geographical distribution of the wave-train (Atlantic-origin) and Pacific-origin types on the basis of upper-level blocking composites over the Eurasian continent and the northwestern Pacific Ocean. Each of the heavy lines signifies a propagation axis of a wave train at the 250-hPa level for the wave-train type. An end of the line with a closed triangle marks the position of a blocking ridge at its peak time, while the other end corresponds to the position of the strongest upstream cyclonic anomaly center associated with the wave train 4 days before the peak time. Each of the light lines signifies a migration path of the primary anticyclonic anomaly center at the 250-hPa level. An end of the line with a triangle corresponds to the anticyclonic center at its peak time, while the other end corresponds to the same center but 4 days before its peak time. Contour lines indicate the climatological-mean Ertel’s PV at the 330-K isentropic surface (contoured every 1 PVU from 2.5 PVU). The statistics shown in this figure are based on the 250-hPa composite anomaly evolution for the 20 strongest blocking events around each of the grid points over the Eurasian and northwestern Pacific regions, but those only for selected grid points are shown for clarity. The wave-train (Atlantic-origin) type is characterized by an incoming Rossby wave packet and, at some locations, slow eastward migration of the blocking anticyclonic center, whereas the Pacific-origin type is characterized by no signature of any incoming wave packet and by slow retrogression of the blocking center.
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
Local standard deviation of low-pass-filtered anomalies in (left) Z250 and (right) Z1000 for winter. Contour intervals are shown every 20 m for Z250 (heavy lines for 100 and 180 m), and every 10 m for Z1000 (heavy lines for 50 and 100 m). In the left panel, an arrow drawn from the North Atlantic into Eurasia indicates a typical waveguide for stationary Rossby wave trains (Blackmon et al. 1984), and another arrow drawn over the Bering straight indicates a typical direction of inward breaking of the polar vortex, as in Fig. 4.
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
Local standard deviation of low-pass-filtered anomalies in (left) Z250 and (right) Z1000 for winter. Contour intervals are shown every 20 m for Z250 (heavy lines for 100 and 180 m), and every 10 m for Z1000 (heavy lines for 50 and 100 m). In the left panel, an arrow drawn from the North Atlantic into Eurasia indicates a typical waveguide for stationary Rossby wave trains (Blackmon et al. 1984), and another arrow drawn over the Bering straight indicates a typical direction of inward breaking of the polar vortex, as in Fig. 4.
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
Local standard deviation of low-pass-filtered anomalies in (left) Z250 and (right) Z1000 for winter. Contour intervals are shown every 20 m for Z250 (heavy lines for 100 and 180 m), and every 10 m for Z1000 (heavy lines for 50 and 100 m). In the left panel, an arrow drawn from the North Atlantic into Eurasia indicates a typical waveguide for stationary Rossby wave trains (Blackmon et al. 1984), and another arrow drawn over the Bering straight indicates a typical direction of inward breaking of the polar vortex, as in Fig. 4.
Citation: Journal of the Atmospheric Sciences 62, 12; 10.1175/JAS3628.1
