Abstract

The major stratospheric sudden warming (SSW) event of January 2009 is analyzed using the Japan Meteorological Agency (JMA) Climate Data Assimilation System (JCDAS). This SSW event is characterized by the extraordinary predominance of the planetary-scale wave of zonal wavenumber 2 (wave 2). The total amount of the upward Eliassen–Palm (EP) flux for wave 2 was the strongest since the winter of 1978/79.

It is found that the remarkable development of the upper troposphere ridge over Alaska played important roles in the SSW in January 2009. During the first development stage, the ridge excited wave packets upward as well as eastward over around Alaska. The eastward-propagating packets intensified a trough over eastern Siberia, which led to the development of the planetary wave over eastern Siberia during the second development stage. The results of this study indicate that the pronounced wave-2 pattern observed in the stratosphere was brought about by accumulative effects of rather localized propagation of wave packets from the troposphere during the course of this SSW event rather than by the ubiquitous propagation of planetary-scale disturbances in the troposphere.

The features of the SSW in January 2009 are quite similar to those during the major stratospheric warming event in February 1989: both SSWs are characterized by the predominance of wave 2, the remarkable development of the upper troposphere ridge over around Alaska, and positive SSTs in the eastern part of the North Pacific corresponding to a La Niña condition.

1. Introduction

In the second half of January 2009, a prominent stratospheric sudden warming (SSW) occurred after a cold and undisturbed early winter. The warming (hereafter referred to as MSW09) was accompanied by clear splitting of the polar vortex due to unusual development of wave number 2 SSW events are described by an abrupt temperature warming of the polar stratosphere associated with breakdown of the cold polar vortex. They are generally classified into two categories on a degree of distortion of the polar vortex. Following the World Meteorological Organization (WMO) definition (WMO 1978, item 9.4, 35–36), a “minor warming” is identified when polar temperatures increase by 25 K or more within a week at any stratospheric level. In addition, if zonal-mean temperature increases in the polar region and if net zonal-mean zonal winds become easterly north of 60°N at 10 hPa or below, then the warming is classified as a “major warming.” On the basis of the definition, the MSW09 can be categorized to a major warming, as shown later.

On the other hand, there exist SSWs with polar vortex splitting, along with those without splitting. Using both National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) and 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data, Charlton and Polvani (2007) classified all major warmings occurring in the analysis period into a “vortex displacement” type and a “vortex split” type. According to their criteria, the MSW09 could be classified as the vortex split type.

The occurrence of SSWs is significantly affected by various external forcings such as the quasi-biennial oscillation (QBO), the 11-yr solar cycle, and sea surface temperature (SST) anomalies. These forcings appear to influence on the variability of the extratropical stratosphere (see, e.g., Labitzke and van Loon 1999; Baldwin et al. 2001). A possible relationship between the SSW and the equatorial QBO has been investigated by several authors (e.g., Holton and Tan 1980; Naito et al. 2003). When the QBO is in the easterly phase, the stratospheric polar vortex is weaker, warmer, and more disturbed during winters. SSW events tend to occur more frequently in the easterly phase (Labitzke 1982). The sunspot cycle may affect the stratospheric polar vortex. Labitzke and collaborators (e.g., Labitzke 1987; Labitzke and van Loon 1992; Labitzke et al. 2006) found a clear tendency that major midwinter SSWs easily occur during solar maximum during the westerly phase of the QBO.

El Niño–Southern Oscillation (ENSO) also has the possibility to modulate the extratropical circulation in the stratosphere. Taguchi and Hartmann (2006) showed in their modeling study that the El Niño condition favors wave-1 SSWs, while wave-2 events occur more frequently in the La Niña condition. Manzini et al. (2006) and Ineson and Scaife (2009) also argued for the distinct emergence of a stratospheric response to El Niño events and enhanced tropospheric driving of quasi-stationary wave 1.

Recent advanced studies on troposphere–stratosphere coupling reveal the possibility that SSWs significantly influence the tropospheric circulation. Baldwin and Dunkerton (2001) showed that anomalous stratospheric events precede shifts in the probability distributions of the location of storm tracks and extreme values of the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO). They suggested that these stratospheric harbingers may be used as a predictor of tropospheric weather regimes. Scaife and Knight (2008) also showed surface climate impact from the sudden warming of 2005/06. They showed that stratospheric sudden warmings have a larger impact on land temperatures than on sea surface conditions. Kuroda (2008) revealed the time evolution of the Eulerian meridional circulation after the occurrence of SSWs. Nakagawa and Yamazaki (2006) classified past SSW events into two categories in which anomalies propagate into the troposphere or not. Their composites revealed that planetary waves propagate more poleward in warm events and more equatorward in cold events. They also showed that the upward wave-2 propagation is enhanced in the warm events but weakened in the cold events. The predictability of SSWs was examined by Mukougawa et al. (2005) and Hirooka et al. (2007).

There exist some observational studies focused on the wave-2 warming (e.g., Finger and Teweles 1964; Fairlie and O’Neill 1988). However, the mechanism of the wave-2 warming was not clearly shown in these preceding studies. The purpose of this paper is to present the results of analyzing the atmospheric circulation not only in the stratosphere but also in the troposphere during the MSW09 and to clarify the mechanism of the upward wave-2 propagation during MSW09. In section 2, we describe the data and analysis methods. We present the features of MSW09 in section 3 and discuss the effects of ENSO, QBO, and solar activity, comparing with results from preceding studies, in sections 3 and 4. We offer concluding remarks in the final section.

2. Data and analysis methods

This study is based on the 6-hourly reanalysis datasets produced by both the Japan Meteorological Agency (JMA) Climate Data Assimilation System (JCDAS) and the Japanese 25-yr Reanalysis (JRA-25) Project. The JCDAS, which is used for near-real-time climate monitoring, succeeds the system of the JRA-25 Project. The JRA-25 covers 26 years from 1979 to 2004, and the JCDAS data are used for the period from January 2005 to April 2009. The global model used in JRA-25 has a spectral resolution equivalent to a horizontal grid size of around 120 km (T106) and 40 vertical layers with the top level at 0.4 hPa. Details of JRA-25 are available in Onogi et al. (2007). Daily climatological means are calculated for 1979–2004 and then smoothed using a 60-day low-pass filter (Duchon 1979). We also use the Centennial In Situ Observation-Based Estimates (COBE) dataset of variability of sea surface temperature and marine meteorological variables analyzed globally on the basis of an optimum interpolation technique with longitude and latitude resolutions of 1° × 1° (Ishii et al. 2005).

A wave activity flux that indicates a propagating packet of planetary waves in the three-dimensional space is calculated following Plumb (1985) in this study. It is quite useful to analyze passages where planetary waves propagate from the troposphere to the stratosphere in three-dimensional space. The definition of the wave activity flux Fs by Plumb (1985) on the sphere is represented in log-pressure coordinates as follows:

 
formula

where p is pressure (hPa)/1000 hPa, and ϕ and λ are latitude and longitude, respectively. The prime denotes small perturbations to a steady zonal flow. The streamfunction, the earth’s rotation rate, the radius of the earth, and the buoyancy frequency are given by ψ, Ω, a, and N, respectively.

In the zonal-mean circulation (the meridional plane), the Eliassen–Palm flux (EP flux) is proper to diagnose the eddy forcing (e.g., Andrews and Mclntyre 1976; Andrews et al. 1987). The vector of the EP flux represents the direction of wave energy propagation in the zonal-mean field and its horizontal and vertical components are related to eddy momentum flux and eddy heat flux, respectively. In addition, the total eddy forcing can be represented by the divergence (westerly acceleration) and convergence (westerly deceleration) of EP flux.

In this study, an extended refractive index is also used (Karoly 1983; Nishii and Nakamura 2004) to discuss the behavior of a wave packet. Generally, it tends to be refracted toward high values of this index. A band of maxima of the extended refractive index represents a localized waveguide of those waves if the band is associated with a westerly jet. Karoly’s definition of the extended refractive index Ks is expressed in log-pressure coordinates as follows:

 
formula

where U = (U, V) denotes a horizontal basic flow, f0 is the Coriolis parameter, N is buoyancy frequency, and H0 is scale height in a basic flow; Q signifies quasigeostrophic potential vorticity (PV) of the basic flow, and H is the horizontal gradient operator. Following Nishii and Nakamura (2004), we neglect the second term and third terms in the parentheses in (2) because vertical variations in N are small except in the immediate vicinity of the discontinuity at the tropopause. For the calculation of the extended refractive index, the grid points are spatially smoothed using a weighted average; the weights are 1.0 at the center point and 0.5 at the adjacent grid points, respectively. Eventually, the extended refractive index is nondimensionalized by multiplying it by acosϕ.

3. Results

Figure 1 shows the seasonal march of zonal-mean temperatures and zonal winds at 10 hPa during the winter of 2008/09. In early January, the axis of the polar night jet is located around 65°N and then shifts northward as decreasing maxima of westerlies in mid-January (Fig. 1b). After 18 January, temperatures rapidly increase and attain a maximum higher than 265 K on 23 January over the polar region north of 60°N (Fig. 1a). On 21 January, the polar night jet more quickly decelerates around 75°N. A zonal wind reversal occurs in the midlatitudes and spreads from 50°N to the North Pole and a peak easterly wind of −30 m s−1 is found on 28 January. Note that the northward shift of the polar night jet occurs prior to the zonal wind reversal. Such a feature has been frequently observed in past SSWs (Limpasuvan et al. 2004; Nishii et al. 2009). There exists a 5-day time lag between the peak of temperature increase and that of zonal wind reversal.

Fig. 1.

Latitude–time cross sections of (a) zonal-mean temperatures and (b) zonal-mean zonal winds at 10 hPa during the winter of 2008/09. Contour intervals are 5 K for the temperature and 10 m s−1 for the zonal wind, respectively.

Fig. 1.

Latitude–time cross sections of (a) zonal-mean temperatures and (b) zonal-mean zonal winds at 10 hPa during the winter of 2008/09. Contour intervals are 5 K for the temperature and 10 m s−1 for the zonal wind, respectively.

Figures 2a and 2b illustrate time–height cross sections of zonal-mean temperatures averaged over 75°–90°N and zonal-mean zonal winds at 60°N, respectively, along with deviations from the climatological means. In early January, temperatures in the stratosphere are colder than the climatological mean and westerly winds are stronger; the polar vortex was undisturbed and well developed. In the first half of January, the westerly wind gradually accelerates from the lower stratosphere to the troposphere. In mid-January 2009 (the beginning of the MSW09), the abrupt deceleration of zonal wind occurs above 5 hPa accompanied by the rapid temperature increase in heights from 20 to 2 hPa. In this stage, the zonal wind reversal is restricted above the middle stratosphere. However, the polar vortex splitting occurs clearly in late January (shown later) and the concomitant zonal wind reversal dominates the overall stratosphere over the polar region north of 60°N during the period from late January to early February. In addition, a poleward increase of the temperature is clearly observed over the polar region in late January (Fig. 1a). Hence, the MSW09 can be recognized as a major warming based on the WMO definition as described in the beginning of section 1. Consequently, the reversal of the stratospheric circulation is continued until the end of February 2009.

Fig. 2.

Time–height cross sections of (a) zonal-mean temperatures averaged over 75°–90°N and (b) zonal-mean zonal winds at 60°N during the winter of 2008/09. Contour intervals are 10 K for the temperature and 10 m s−1 for the zonal wind, respectively. Shading shows deviations from the climatological means in units of K and m s−1, respectively. Climatological means are calculated for the period from 1979 to 2004.

Fig. 2.

Time–height cross sections of (a) zonal-mean temperatures averaged over 75°–90°N and (b) zonal-mean zonal winds at 60°N during the winter of 2008/09. Contour intervals are 10 K for the temperature and 10 m s−1 for the zonal wind, respectively. Shading shows deviations from the climatological means in units of K and m s−1, respectively. Climatological means are calculated for the period from 1979 to 2004.

Here, we mention the atmospheric circulation during the MSW09. Figure 3 shows the distribution of 10-hPa geopotential heights for six successive 3-day means in January 2009. At the beginning of the MSW09 (Fig. 3a), the center of the polar night vortex is located over the North Pole, while both a trough over North America and the Aleutian high begin to develop. During the period 15–17 January (Fig. 3b), another high over western Europe and a trough over central Siberia also begin to develop. During 18–20 January (Fig. 3c), the polar vortex is distorted toward both the Labrador Peninsula and central Siberia. During 21–23 January (Fig. 3d), the trough over central Siberia and the Aleutian high further develop, and the polar vortex begins to split. During 24–26 January (Fig. 3e), one of the polar vortices shifts southward toward the north of Great Lakes and the high over western Europe shifts westward. Consequently, we can find a quadrupole structure (i.e., an unusual development of wave 2 during the MSW09). During 27–29 January (Fig. 3f), the two highs merge into a predominant anticyclone centered over the North Pole and the splitting vortices are further weakened, leading to the circulation reversal.

Fig. 3.

The 10-hPa geopotential heights for six successive 3-day means in January 2009. The contour interval is 240 m.

Fig. 3.

The 10-hPa geopotential heights for six successive 3-day means in January 2009. The contour interval is 240 m.

Now, we compare the upward EP flux for waves 1 and 2 in the Northern Hemisphere winters. Figure 4 plots interannual variations of vertical component of the EP flux (upward EP flux) averaged over 30°–90°N at 100 hPa in boreal winters [December–February (DJF)] for waves 1 and 2, along with the ratio of wave 2 to the sum of waves 1 and 2, during the period from 1979/80 to 2008/09. Initially, we find that the upward EP flux for wave 2 during the winter 2008/09 shows the highest value among the past 31 winters. Moreover, we have only three winters (1984/85, 1988/89, and 2008/09) during the period in which the ratios of the wave-2 contribution to the sum of wave 1 and 2 exceed 60% (the line graph in Fig. 4). In particular, the ratio during the winter 2008/09 surpasses 75%.

Fig. 4.

Interannual variations of upward EP fluxes (×104 kg s−2) at 100 hPa averaged over 30°–90°N in boreal winters (DJF) for waves 1 (white bars) and 2 (gray bars), along with the ratio of wave 2 to the sum of waves 1 and 2 (in the line graph). Arrows above line graph indicate that the ratios exceed 60% (for three winters: 1984/85, 1988/89, and 2008/09). It is noted that because the JRA-25 dataset starts in January 1979, the upward EP flux in the winter of 1978/79 is averaged only for January–February 1979.

Fig. 4.

Interannual variations of upward EP fluxes (×104 kg s−2) at 100 hPa averaged over 30°–90°N in boreal winters (DJF) for waves 1 (white bars) and 2 (gray bars), along with the ratio of wave 2 to the sum of waves 1 and 2 (in the line graph). Arrows above line graph indicate that the ratios exceed 60% (for three winters: 1984/85, 1988/89, and 2008/09). It is noted that because the JRA-25 dataset starts in January 1979, the upward EP flux in the winter of 1978/79 is averaged only for January–February 1979.

We have investigated features of all SSW events occurring in boreal midwinter during the past 31 seasons using the JRA-25 and the JCDAS data and have found that there are 22 major SSW events as shown in Table 1; the periods of major SSW events in Table 1 are basically consistent with Table 1 of Charlton and Polvani (2007). Note that we have examined which zonal wavenumbers primarily contribute to SSW events and found that most of them are caused by wave 1, whereas Charlton and Polvani (2007) classified SSW events in terms of “split” or “displacement” of the polar vortex. In addition, it is interesting that the SSW events caused by only wave 2 occur during La Niña events, except for the SSW in February 1979. Note that the SSW in February 1979 occurred under a “preconditioned” situation after pronounced minor warmings due to wave 1 (e.g., Labitzke 1981).

Table 1.

SSWs identified in JRA-25 and JCDAS datasets based on WMO definition, along with primarily contributing wavenumbers (WN) that are judged by the magnitude of upward EP fluxes at 100 hPa for waves 1, 2, and 3. EL and LA denote El Niño and La Niña in the oceanographic conditions, respectively.

SSWs identified in JRA-25 and JCDAS datasets based on WMO definition, along with primarily contributing wavenumbers (WN) that are judged by the magnitude of upward EP fluxes at 100 hPa for waves 1, 2, and 3. EL and LA denote El Niño and La Niña in the oceanographic conditions, respectively.
SSWs identified in JRA-25 and JCDAS datasets based on WMO definition, along with primarily contributing wavenumbers (WN) that are judged by the magnitude of upward EP fluxes at 100 hPa for waves 1, 2, and 3. EL and LA denote El Niño and La Niña in the oceanographic conditions, respectively.

Hence, we examine seasonal marches in the winters during La Niña events with the occurrence of the wave-2 SSWs. Figure 5 shows time evolutions of upward EP fluxes for each wavenumber averaged over 30°–90°N at 100 hPa in the three winters of (a) 2008/09, (b) 1988/89 and (c) 1984/85. In mid-January 2009 (Fig. 5a), upward EP flux of wave 2 sharply begins to increase and peaks on 19 January (indicated by a gray arrow in Fig. 5a) and has a second peak on around 25 January, while the enhancement of wave 1 and 3 is hardly observed. As a result, wave 2 is very dominant in planetary wave activity during the MSW09. It is also found that both the temperature increase and the zonal wind reversal occur just after the first peak of the upward propagation of wave 2, appearing with the northward shift of the westerly jet (Fig. 1).

Fig. 5.

Time series of upward EP fluxes (×105 kg s−2) averaged over 30°–90°N at 100 hPa in the winters of (a) 2008/09, (b) 1988/89, and (c) 1984/85. Upward EP fluxes of waves 1, 2, and 3 are denoted by thin solid, thick solid, and short dashed lines, respectively.

Fig. 5.

Time series of upward EP fluxes (×105 kg s−2) averaged over 30°–90°N at 100 hPa in the winters of (a) 2008/09, (b) 1988/89, and (c) 1984/85. Upward EP fluxes of waves 1, 2, and 3 are denoted by thin solid, thick solid, and short dashed lines, respectively.

The SSW events in both 1988/89 and 1984/85 are also classified into typical wave-2 warmings by Krüger et al. (2005). During the MSW09 (Fig. 5a), the upward propagation of wave 2 sharply peaks, compared with those in other two winters. The upward EP flux has a maximum of 2.7 × 105 kg s−2 during the MSW09, whereas it has a maximum of just 1.5 × 105 kg s−2 during the other SSW events.

Next, we examine the upward propagation of the planetary wave from the troposphere to the stratosphere in more detail on the basis of successive 3-day means in January 2009. The left-hand panels in Fig. 6 illustrate longitude–height cross sections of geopotential height deviations from the zonal mean and Plumb’s wave activity flux averaged over 55°–65°N; this latitudinal zone corresponds to a key region where the vertical propagation of planetary waves from the upper troposphere to the lower stratosphere mainly occurs. The right-hand panels in Fig. 6 show latitude–height cross sections of zonal-mean zonal wind and the EP flux.

Fig. 6.

(left) Longitude–height cross sections of geopotential height deviations from the zonal mean (contours) and Plumb’s wave activity fluxes (vectors) averaged over 55°–65°N. (right) Latitude–height cross sections of zonal-mean zonal wind (contours) and EP fluxes (vectors) for successive 3-day means. Contour intervals in heights and in zonal winds are 60 m and 5 m s−1, respectively. Plumb’s wave activity flux vectors are in units of m2 s−2; EP flux vectors are in units of kg s−2. The left and right vector scales at the lower-right corner of the panels denote vectors below and above 100 hPa, respectively. Dates are (a),(b) 9–11, (c),(d) 10–12, (e),(f) 11–13, (g),(h) 12–14, (i),(j) 15–17, (k),(l) 18–20, (m),(n) 21–23, (o),(p) 24–26, and (q),(r) 27–29 January.

Fig. 6.

(left) Longitude–height cross sections of geopotential height deviations from the zonal mean (contours) and Plumb’s wave activity fluxes (vectors) averaged over 55°–65°N. (right) Latitude–height cross sections of zonal-mean zonal wind (contours) and EP fluxes (vectors) for successive 3-day means. Contour intervals in heights and in zonal winds are 60 m and 5 m s−1, respectively. Plumb’s wave activity flux vectors are in units of m2 s−2; EP flux vectors are in units of kg s−2. The left and right vector scales at the lower-right corner of the panels denote vectors below and above 100 hPa, respectively. Dates are (a),(b) 9–11, (c),(d) 10–12, (e),(f) 11–13, (g),(h) 12–14, (i),(j) 15–17, (k),(l) 18–20, (m),(n) 21–23, (o),(p) 24–26, and (q),(r) 27–29 January.

In the period of 9–11 January (Fig. 6a), a wave-2 pattern has not been established throughout the troposphere and stratosphere. Most of the wave packets in the upper troposphere propagate rather horizontally and equatorward around 60°N (Fig. 6b); neither clear upward wave propagation nor development of wave 2 is seen in the stratosphere (Fig. 6a).

During the period 10–12 January and the period 11–13 January, the ridge over Alaska starts to develop in the region from the midtroposphere to the lower stratosphere with a maximum at 300 hPa (Figs. 6c,e), and wave packets begin to propagate from the ridge over Alaska to the trough over eastern Canada not only in the upper troposphere but also in the lower stratosphere. In the meridional plane (Figs. 6d,f), the direction of wave packets changes to a more upward region around at 60°N from the troposphere to the lower stratosphere.

During 12–14 January, the ridge over Alaska clearly develops (Fig. 6g) and wave-2 anomalies begin to emerge. This ridge has a vertically deep structure and extends in heights above 100 hPa. Moreover, it is connected with a large-scale ridge over the Aleutian in the middle stratosphere. Actually, a westward phase tilt with increasing height becomes clear in the region from 180°W to 30°W and the baroclinic structure is intensified over Alaska. In the meridional plane (Fig. 6h), wave packets in the middle and upper stratospheres propagate upward as well as equatorward along with the lower-latitude flank of the polar night jet, and then converge around 45°N in the upper stratosphere.

During the periods of 15–17 and 18–20 January, upper troposphere ridges located over the Date Line and Alaska develop further, and wave packets propagate eastward throughout the stratosphere (Figs. 6i,k). Consequently, during 18–20 January (Fig. 6k), wave 2 develops extraordinarily. At the same time, in the meridional planes (Figs. 6j,l), the planetary wave continues to propagate equatorward in the upper stratosphere, and the polar night jet is clearly decelerated in the midlatitudes in the upper stratosphere.

Subsequently, during 21–23 January, easterly winds appear above 5 hPa, while wave packets propagate in their directions, changing poleward in the lower stratosphere (Fig. 6n). In the zonal plane, the upper troposphere ridge over Alaska declines and the centers of the wave shift downward to the middle stratosphere (Fig. 6m).

During 24–26 January (Fig. 6o), wave propagation is restricted below 10 hPa because the easterly wind region migrates downward (Fig. 6p), which prevents the wave from propagating upward in the easterly region. In addition, the westward phase tilt with height becomes obscure over the Western Hemisphere; thus, the planetary wave has a barotropic structure and propagates only in the horizontal direction. On the other hand, over the Eastern Hemisphere, the westward phase tilt with height and upward wave propagation are still clear, accompanied by the trough development in the upper troposphere over eastern Siberia, which keeps the wave structure baroclinic there.

Finally, during 27–29 January, the centers of the wave shift farther downward, which is consistent with the migration of the easterly wind and the distribution of the wave propagation (Figs. 6q,r).

From these results, we find that the pronounced wave-2 pattern in the stratosphere was brought by cumulative effects of rather localized propagation of wave packets from the troposphere during the course of this SSW event, not by the ubiquitous propagation of planetary-scale disturbances in the troposphere. The features of the propagation could be understood as follows: Planetary waves in the real atmosphere have a longitudinally nonuniform structure due to the longitudinally nonuniform basic flow, even though the waves still hold nearly sinusoidal structure at least to a limited longitudinal extent under a Wentzel–Kramers–Brillouin–Jeffries (WKBJ) condition. In such a situation, the waves would be accompanied by localized wave activity fluxes rather than by ubiquitous ones, leading to localized strengthening of troughs and/or ridges corresponding to planetary-scale disturbances. In this case, the wave-2 pattern seems to be slightly distorted by superimposed smaller-scale waves that are able to exist owing to the nonuniform basic flow; this small distortion contributes to the localized wave propagation as the wave packets.

To confirm the progress of upward and eastward propagations of the wave packets, Fig. 7 illustrates longitude–time cross sections of vertical (WAFz) and zonal (WAFx) components of Plumb’s wave activity flux. At 100 hPa, a maximum of WAFz is found over the region from 150°W to 120°W, which corresponds to the upper edge of the upper-tropospheric ridge over Alaska during 14–21 January. Another maximum of WAFz can be found around 120°E during 20–30 January. At 50 hPa (Fig. 7c), a maximum of WAFz is located at around 90°W, which indicates an eastward shift of the maximum of WAFz at 100 hPa (Fig. 7a). From Fig. 7, the MSW09 can be divided into two development stages: The first stage is the period 14–21 January and the second is 20–30 January. During the first development stage, the wave packets emanating from the Alaska area propagate upward, whereas during the second development stage those emanating from over eastern Siberia propagate upward.

Fig. 7.

Longitude–time cross sections of WAFz and WAFx at 100 and 50 hPa in January 2009: (a) WAFz at 100 hPa, (b) WAFx at 100 hPa, (c) WAFz at 50 hPa and (d) WAFx at 50 hPa. Contour intervals are 0.02 m2 s−2 for WAFz and 5 m2 s−2 for WAFz. WAFz and WAFz are calculated for the disturbances with zonal wavenumbers less than or equal to 6.

Fig. 7.

Longitude–time cross sections of WAFz and WAFx at 100 and 50 hPa in January 2009: (a) WAFz at 100 hPa, (b) WAFx at 100 hPa, (c) WAFz at 50 hPa and (d) WAFx at 50 hPa. Contour intervals are 0.02 m2 s−2 for WAFz and 5 m2 s−2 for WAFz. WAFz and WAFz are calculated for the disturbances with zonal wavenumbers less than or equal to 6.

As for zonal components (Figs. 7b,d), maxima of WAFx are found in a region from 120°W to 60°W and a region around 120°E, whose locations are consistent with those of WAFz. Moreover, in mid-January, the strong wave activity area (say, above 10 m2 s−2) can be found from 60°W to 60°E at both 100 and 50 hPa. The large WAFx indicates that the planetary waves propagate not only upward but also eastward in the stratosphere.

As already shown in Figs. 6e,g,i, the two-way wave packets emanate from the ridge over Alaska in the upper troposphere as well as in the lower stratosphere. To see the behavior of the former wave packet in detail, corresponding horizontal propagation patterns in the upper troposphere (250 hPa) are shown in Fig. 8 in terms of Plumb’s wave activity flux. Remarkable wave packets emanate from the ridge over Alaska and propagate from North America to the North Atlantic in mid-January (Fig. 8a). Then the wave packets propagate along the subtropical jet (Fig. 8b) and reach a trough at the east of Lake Baikal and strengthen it (Fig. 8c). The development of this upper troposphere trough tends to strengthen the westward tilt with increasing height in the lower stratosphere. Because only the westward-tilting planetary wave can propagate upward, the trough over central Siberia, which has baroclinic structure, can emanate the planetary wave packet upward, leading to the polar vortex split. In that mean, the horizontal propagation of the Rossby wave from the Alaska region in the upper troposphere indirectly results in the MSW09. Hence, the upper troposphere ridge over Alaska plays an important role directly in the upward propagation of wave packets from the Alaska region and indirectly in the upward propagation of wave 2 from the Siberian region through the eastward Rossby wave propagation in the upper troposphere during the MSW09.

Fig. 8.

Horizontal propagations of Rossby wave packets at 250 hPa for three selected 3-day means: (a) 15–17, (b) 18–20, and (c) 21–23 Jan 2009. The contour interval of geopotential heights is 120 m. Vectors show Plumb’s wave activity flux (vectors) in units of m2 s−2. A scale of vectors is shown at the lower right corner.

Fig. 8.

Horizontal propagations of Rossby wave packets at 250 hPa for three selected 3-day means: (a) 15–17, (b) 18–20, and (c) 21–23 Jan 2009. The contour interval of geopotential heights is 120 m. Vectors show Plumb’s wave activity flux (vectors) in units of m2 s−2. A scale of vectors is shown at the lower right corner.

4. Discussion

As described in the previous section, the MSW09 was a pronounced wave-2 warming resulting from the record-breaking upward EP flux of wave 2. In this section, we carefully consider such unusual features of the MSW09 and also compare with past similar SSW events.

First, we discuss a formation mechanism of the wave 2 during the MSW09. As described in section 3, the wave packets were forced around the upper troposphere ridge over Alaska and eastern Siberia in the first and second development stages, respectively. Afterward, each wave packet propagated upward and eventually formed the pronounced wave-2 pattern in the stratosphere, leading to the occurrence of the MSW09.

To understand such a localized feature of the wave propagation, it is helpful to see the distribution of the extended refractive index Ks based on Eq. (2). Figure 9 shows longitude–height sections of the extended refractive index for quasi-stationary waves in zonally varying westerlies during the corresponding development stages (see also Nishii and Nakamura 2004). During the first development stage (Fig. 9a), a sharp waveguide is formed from the lower to the middle stratosphere around 60°W, which corresponds to the western part of the deep stratospheric trough over eastern Canada. Therefore, the wave packets associated with planetary-scale disturbances could propagate upward through the waveguide. Moreover, during the second development stage (Fig. 9b), another clear waveguide is formed in a region from 90°E to 130°E. The positions of these waveguides are consistent with the peaks of WAFz during the first and second development stages, respectively.

Fig. 9.

Longitude–height cross sections of extended refractive indices (Ks) averaged over 55°–65°N for (a) 14–21 and (b) 20–30 Jan 2009. Light and dark shadings show the areas where the equivalent zonal wavenumber exceeds 2 and 3, respectively. The blank region in (b) indicates the easterly region. The grid points are spatially smoothed in adjacent grid points using a weighted average. The extended refractive index is represented as the equivalent zonal wavenumber for this latitude circle (i.e., nondimensional value).

Fig. 9.

Longitude–height cross sections of extended refractive indices (Ks) averaged over 55°–65°N for (a) 14–21 and (b) 20–30 Jan 2009. Light and dark shadings show the areas where the equivalent zonal wavenumber exceeds 2 and 3, respectively. The blank region in (b) indicates the easterly region. The grid points are spatially smoothed in adjacent grid points using a weighted average. The extended refractive index is represented as the equivalent zonal wavenumber for this latitude circle (i.e., nondimensional value).

Second, we compare the characteristics of the MSW09 with past SSW events. As mentioned in section 3, in the cases of the warming in February 1989 (MSW89) and December 1984 (MSW84), the upward propagations of wave 2 were also dominant (Figs. 5b,c), and both the MSW89 and the MSW84 are also classified into a vortex split–type warming (Figs. 10b,c). In particular, development of an upper troposphere ridge over Alaska (Fig. 11b) is similar to that of the MSW89. During the MSW84, however, the upper troposphere ridge around Alaska is not so strong (Fig. 11c) compared to the others (Figs. 11a,b); however, the ridge over western Russia is rather strong. These features are different from those of the other two cases.

Fig. 10.

As in Fig. 9, but for 10-hPa geopotential heights for selected 3-day means for the three cases in which the wave-2 contribution exceeds 60% in the sum of waves 1 and 2: (a) 23–26 Jan 2009, (b) 19–21 Feb 1989, and (c) 27–31 Dec 1984. The contour interval is 240 m.

Fig. 10.

As in Fig. 9, but for 10-hPa geopotential heights for selected 3-day means for the three cases in which the wave-2 contribution exceeds 60% in the sum of waves 1 and 2: (a) 23–26 Jan 2009, (b) 19–21 Feb 1989, and (c) 27–31 Dec 1984. The contour interval is 240 m.

Fig. 11.

As in Fig. 10, but for horizontal propagations of Rossby waves at 250 hPa in the NH for selected 3-day means: (a) 15–17 Jan 2009, (b) 3–5 Feb 1989, and (c) 17–19 Dec 1984. The contour interval of geopotential heights is 60 m. Vectors show Plumb’s wave activity flux (vectors) in units of m2 s−2. A scale of vectors is shown at the lower right corner.

Fig. 11.

As in Fig. 10, but for horizontal propagations of Rossby waves at 250 hPa in the NH for selected 3-day means: (a) 15–17 Jan 2009, (b) 3–5 Feb 1989, and (c) 17–19 Dec 1984. The contour interval of geopotential heights is 60 m. Vectors show Plumb’s wave activity flux (vectors) in units of m2 s−2. A scale of vectors is shown at the lower right corner.

As shown in Fig. 12a, tropical SSTs in January 2009 are below normal in the central equatorial Pacific but above normal in the western part, which indicates that the oceanography in the tropics is in a weak La Niña condition. A “horseshoe pattern” of positive SST anomalies in association with this La Niña condition can be observed in the wide areas of the Pacific Ocean (thick dotted line in Fig. 12a). The oceanographic conditions in the tropics also correspond to the La Niña ones during the winter of 1988/89 and 1984/85. In addition, remarkable positive SST anomalies are observed in the eastern part of the North Pacific both in January 2009 and February 1989, but they are not so remarkable in December 1984. In fact, a surface low pressure system strikingly developed to the northwest of the remarkable positive SST anomalies, and a wave packet emanated from the surface low pressure toward the upper troposphere ridge over Alaska in mid-January 2009 (not shown). It is possible that the remarkable positive SST anomalies contributed to the development of the surface low pressure system by means of the moisture supply near the surface (Kuo et al. 1991; Martin and Oktin 2004). Thus, the positive SST anomalies are presumably related to the development of the upper troposphere ridge over Alaska. However, further studies are needed to confirm this issue both observationally and theoretically.

Fig. 12.

As in Fig. 10, but for monthly-mean SST anomalies from the climatological mean: (a) January 2009, (b) February 1989, and (c) December 1984. The contour interval is 0.5°C. The climatological mean is calculated for the period from 1970 to 2000. Thick dotted lines denote horseshoe patterns of positive SST anomalies in association with the La Niña condition.

Fig. 12.

As in Fig. 10, but for monthly-mean SST anomalies from the climatological mean: (a) January 2009, (b) February 1989, and (c) December 1984. The contour interval is 0.5°C. The climatological mean is calculated for the period from 1970 to 2000. Thick dotted lines denote horseshoe patterns of positive SST anomalies in association with the La Niña condition.

The effects of ENSO on SSWs have been investigated by numerical studies (e.g., Taguchi and Hartmann 2006; Manzini et al. 2006; Ineson and Scaife 2009). Taguchi and Hartmann (2006) showed that the El Niño condition favors wave-1 SSWs, whereas wave-2 events occur more frequently in the La Niña condition, under perpetual January conditions. Manzini et al. (2006) and Ineson and Scaife (2009) also showed general enhancement of the tropospheric forcing contributing to the stratospheric wave 1 in their numerical experiment. As mentioned in section 3, the MSW09 was a typical vortex split–type warming and wave 2 was dominant in the extratropical stratosphere. Because the winter of 2008/09 was classified as a weak La Niña condition as already described, the characteristics of the MSW09 dominated by wave 2 are qualitatively consistent with Taguchi and Hartmann (2006).

Third, we discuss the effects of the 11-yr solar cycle and the QBO on the MSW09. Labitzke (1987) and Labitzke et al. (2006) investigated the statistical relationship between the strength of the stratospheric polar vortex, the 11-yr solar cycle, and the QBO, using the NCEP–NCAR reanalysis and solar flux data. They showed that major midwinter SSWs tends to occur during solar maxima in the westerly phase of the QBO. The 2008/09 winter was classified as having a westerly phase and a minimum of solar activity. Thus, it is suggested that a relation proposed by Labitzke and her collaborators is not true in the present case (MSW09).

Finally, the Arctic Oscillation and North Atlantic Oscillation indices after the occurrence of the SSW show negative values at least during the second half of February (not shown), which is consistent with results of former studies (e.g., Baldwin and Dunkerton 2001; Scaife and Knight 2008; Kuroda 2008). Moreover, one of the split polar vortices in the stratosphere persists over the region from central to eastern Siberia, and a trough also persists over the same region in the troposphere (i.e., the equivalent barotropic structure). This may lead to the anomalous weather in the region from Siberia to East Asia. However, the effects of the MSW09 on the surface weather are beyond the scope of this paper.

5. Concluding remarks

As shown throughout our study, the MSW09 is underscored by the extraordinary predominance of wave 2. We found that the upward EP flux for wave-2 propagation from the troposphere to the stratosphere is the strongest since the winter of 1978/79. The MSW09 is classified into the vortex split type.

Moreover, the MSW09 is characterized by a remarkable upper troposphere ridge over Alaska that plays important roles directly in upward propagation of wave packets from the Alaska region in the first development stage and indirectly in the upward propagation of wave packets from the Siberian region in the second development stage, which result from the eastward propagation of wave packets in the upper troposphere emanating from the Alaska region. The results of our study suggest that the pronounced wave-2 pattern in the stratosphere resulted from cumulative effects of rather localized propagation of wave packets from the troposphere during the course of this SSW event, not from the propagation of ubiquitous planetary-scale disturbances in the troposphere.

We also found that the features of the MSW09 are quite similar to those during the MSW89 except that the amplitudes of planetary waves during the MSW09 were larger than those during the MSW89.

Acknowledgments

We thank two anonymous reviewers for their constructive comments on the manuscript. We also acknowledge stimulating discussions with Kunihiko Kodera, Yuhji Kuroda, Koji Yamazaki, Saburo Miyahara, Hisanori Itoh, Tomoko Ichimaru, and all the members in the Climate Prediction Division in JMA. The extended refractive index for stationary waves was calculated using routines originally developed by Hisashi Nakamura. This work was partly supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science.

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Footnotes

Corresponding author address: Yayoi Harada, Japan Meteorological Agency, 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan. Email: yayoi.harada@met.kishou.go.jp