Unusual major stratospheric sudden warming event from January to February 2009
1. Introduction
A major stratospheric sudden warming (SSW) event began in January 2009.

This reportoutlines analysis of the atmospheric circulation during the event using the Japan Meteorological
Agency (JMA) Climate Data Assimilation System (JCDAS).
In winter, JMA’s Climate Prediction Division (CPD) monitors stratospheric circulation in the
Northern Hemisphere. When an SSW occurs, JMA begins to issue stratospheric alerts (known as
STRAT-ALERT reports) via the Global Telecommunication System (GTS).

According to theWorld Meteorological Organization (WMO) definition, a minor SSW is identified when polar
temperatures increase by 25 K or more within a week at any stratospheric level.

If the zonal mean

temperature increases in the polar region and net zonal mean zonal winds become easterly to thenorth of 60˚N at 10 hPa or below, the event is classified as a major SSW.
2. Data and Methods
JMA used s i x - h o u r l y reanalysis datasets produced from both the Japanese 25-year Reanalysis
Project (JRA-25) and JCDAS, which succeeds the system of the JRA-25 project for near-real-time
climate monitoring.

The global model used in JRA-25 has a spectral resolution equivalent to ahorizontal grid size of around 120 km (T106) and 40 vertical layers, with the top level at 0.4 hPa.
The details of JRA-25, which covers the 26-year period from 1979 to 2004, can be referred to inOnogi et al. (2007).

In this study, the daily climatological means were calculated from 1979 to2004 and smoothed using a 60-day low-pass filter (Duchon, 1979).
Wave activity flux indicates the direction of a Rossby wave packet propagation inthree-dimensional space, and these values were calculated based on Plumb (1985). The results arequite useful in analyzing the passage of Rossby waves propagating from the troposphere to thestratosphere.
In the zonal mean field, Eliassen Palm flux (EP flux) is used to diagnose eddy forcing of thebasic flow as well as to provide a direct view of the westerly momentum transport processes (Palmer,1982).

EP flux also represents the direction of an eddy’s energy propagation in the zonal mean

field, and its horizontal and vertical components represent meridional westward momentum
transport and heat transport by eddies, respectively.

In addition, the total eddy forcing of the basic

flow can be represented by the divergence (i.e., the acceleration of westerly winds) and convergence(the deceleration of westerly winds) of EP flux.
3. Results
The zonal wind speed and temperature at the 10-hPa level in the zonal mean field during thewinter of 2008/2009 are shown in Fig. 1.
located at around 65°N.

In early January, the axis of the polar night jet was

The air temperature in the polar region was around 200 K. Subsequently, apoleward shift of the polar night jet was observed along with a rapid increase in air temperature,
which exceeded 265 K at the 10-hPa level by mid-January, and the CPD began to issue
STRAT-ALERT reports for a minor SSW.

Then, zonal winds reversed from westerlies to1

easterlies in the mid-latitudes, and easterlies
expanded from 50°N to the North Pole.
Figures 2a and 2b show time-height crosssections of zonal mean temperature and its
anomalies averaged over 75° – 90°N and ofzonal mean zonal wind and its anomalies at60°N, respectively.

At the beginning of thismajor SSW event, the increase in temperatureand the reversed zonal wind direction werelimitedtothestratosphere.

areaabove

themiddle

The polar vortex split clearly

into two in late January (shown later), and thereversed zonal wind direction was observed
throughout the stratosphere to the north of 60°N
in early February.

Accordingly, this event wasidentified as a major SSW in line with WMO’sclassification.

A

poleward

increase

intemperature was clearly observed to the north of60°N (Fig. 1).

Consequently, the reverse ofFigure 1. Latitude-time cross section of 10-hPa zonal

zonal mean zonal winds continued until the end

mean zonal wind (contours) and temperatureof February 2009.

(shaded). The contour interval is 10 ms .

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Figure 2c shows a time series of the verticalcomponent of EP flux averaged over 30° – 90°N at the 100-hPa level.

In early January, upward

propagation of the planetary wave was below the climatological mean.

In mid-January, the verticalcomponent of EP flux for the planetary wave of zonal wave number 2 began to sharply increase, andpeaked on 19 January.

Conversely, only a slight increase was observed in the vertical componentof EP flux for zonal waves 1 and 3, which indicates that the energy of zonal wave number 2 wasdominant among planetary waves during MSSWJ09. Figure 2d shows a time-height cross sectionof the vertical component of EP flux averaged over 30° – 90°N for the planetary wave of zonal wave
number 2.

In mid-January, the vertical component of EP flux began to increase in the troposphere.

It also increased in the stratosphere within a few days and peaked within a week. This means that it
took less than a week for the energy of the planetary wave of zonal wave number 2 to propagate
upward from the troposphere to the stratosphere.

After the peak in the stratosphere, the region inwhich the vertical component of EP flux exceeded 4 m2s-2 gradually descended toward the lower
stratosphere from late January to early February, and disappeared throughout the stratosphere in thesecond half of February. Comparing Figs. 2a and 2b with Fig. 2c, it can be seen that both thetemperature increase and the easterly winds were observed just after the peak of the upward
propagation of the planetary waves for zonal wave number 2, and that the strong upward
propagation was generally limited to the westerly region.
Figure 3 shows the annual variability of each zonal wave number to the sum of zonal waves 1and 2 in the winter total amount of the vertical component of EP flux at the 100-hPa level averaged

2

Figure 2. (a) Time-height cross sections of zonal mean temperature (contours) and its deviation from theclimatological mean (shaded) averaged over 75° – 90°N, (b) time-height cross sections of zonal
mean zonal wind (contours) and its deviation from the climatological mean (shaded) at 60°N, (c) timeseries of the vertical component of EP flux averaged over 30° – 90°N at the 100-hPa level, and (d)time-height cross sections of the vertical component of EP flux for zonal wave number 2 averaged-1

over 30° – 90°N. The contour intervals are (a) 10 K and (b) 5 ms , respectively. The red bars in (c)denote the vertical component of EP flux for whole zonal wave numbers. The purple, light-green andlight-blue lines denote the vertical components of EP flux for zonal waves 1, 2 and 3, respectively.
The broken line in (c) denotes the climatological mean for the vertical component of EP flux for whole
2 -2

zonal wave numbers. The unit of the vertical component for EP flux in (c) and (d) is m s .

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90%

300
250

EPFZ100 WN2
EPFZ100 WN1RATIO WN2/(WN1+WN2)

80%
70%
60%

200

50%

150

40%
30%

100

20%

50

10%

00%

1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008

Figure 3. Annual variability in the total vertical component of EP flux at the 100-hPa level averaged over 30°– 90°N during boreal winter (Dec. – Feb.). The yellow and light-blue bars denote the ratio of the2 -2

seasonal total amounts of the vertical component of EP flux (m s ) for zonal waves 1 and 2,
respectively. The line in the graph denotes the values for the ratio of zonal wave number 2 to the sumof zonal waves 1 and 2.

over 30° – 90°N.

The ratios of the vertical component of EP flux for zonal wave number 2 haveexceeded 60% only in the three winters of 1984/85, 1988/89 and 2008/09. In addition, it isnoteworthy that the ratio for the planetary wave of zonal wave number 2 during winter 2008/09 was
the highest among the past 30 winters. This was extraordinary because the normalized value of thedeviation from its climatological mean exceeded 4 σ.

According to Charlton and Polvani as well as

the results of CPD/JMA analysis, 22 cases of major sudden stratospheric warming in boreal
mid-winter have been analyzed over the past 31 winters.

We investigated the zonal wave numberthat contributed most to SSW events using this past data, and found that the majority were caused by
the planetary wave of zonal wave number 1.

It can be concluded that MSSWJ09 provides the mostprominent discovery in terms of the energy of the planetary wave of zonal wave number 2.
Here, we outline atmospheric circulation during the MSSWJ09 event.

Figure 4 shows 10-hPa

geopotential heights and their deviation from the zonal mean (top panels) and longitude-height crosssections of geopotential height deviations, temperature deviations and Plumb's wave activity fluxes
averaged over 50° – 70°N (bottom panels) for selected five-day means.

During the period prior toMSSWJ09 (Fig. 4a), the polar vortex was located over the North Pole.

A westward tilt with height

was found only over Europe, where only a weak upward propagation of the planetary wave was seen.
During the first developmental stage (Fig. 4b), geopotential height deviations corresponding to thedevelopment of a planetary wave of zonal wave number 2 became clear, and the polar vortex began
to split into two vortices. One of these was located over the Taimyr Peninsula, and the other waslocated over Baffin Island.

The geopotential height field in the stratosphere clearly shows thewestward tilt with height, which indicates the upward propagation of wave activity.

Remarkable

planetary wave packets emanated from a ridge over the Alaska Peninsula and propagated over theNorth Atlantic in the stratosphere. Part of these planetary wave packets propagated along both thetropospheric polar front jet and the subtropical jet, reaching and strengthening a trough over central

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(a)

(b)

(c)

(d)

Figure 4. (Top) 10-hPa geopotential height (contours) and deviations from the zonal mean (shaded), and(bottom) longitude-height cross sections of height deviations from the zonal mean (contours) and
Plumb's wave activity flux (vectors) in the stratosphere from 100 to 1 hPa averaged over 50° – 70°N
for selected five-day means. The contour intervals are 240 m (top panels) and 60 m (bottom2 -2panels). The unit of Plumb's wave activity flux is m s . (a), (b), (c) and (d) denote the means for 6 –10 January, 14 – 18 January, 22 – 26 January and 30 January – 3 February, respectively.

5Siberia (not shown).

Moreover, these wave packets propagated upward into the stratosphere,

although they were restricted to the lower stratosphere during the second developmental stage (Fig.
4c).

The Aleutian high developed in the stratosphere, and the two polar vortices shifted

southwestward.

The development of the Aleutian high was observed below the 10-hPa level, but

the westward tilt of the geopotential height deviation was not clear.

It can be said that the planetary

wave packets emanating from the trough over central Siberia appeared to be crucial for the polar
vortex split. During the mature stage (Fig. 4d), the two polar vortices completely split into two,and the upward propagation of the planetary wave packets weakened. The two vortices gradually
lost strength until mid-February.
Here, we describe the characteristics of atmospheric circulation in the troposphere during thefirst developmental stage of SSW09. Figure 5 shows (a) 250-hPa Plumb's wave activity flux,
geopotential height and its deviation from the zonal mean, (b) longitude-height cross section of
Plumb's wave activity flux and geopotential height deviation from the zonal mean for 12 – 16January.

A ridge developed remarkably over Alaska in the upper troposphere (Fig. 5a) due to wave

packets emanating from a synoptic-scale disturbance in the lower troposphere in the North Pacific(not shown).

(a)

(b)

Figure 5. (a) 250-hPa Plumb's wave activity flux (vectors), geopotential height (contours) and its deviationfrom the climatological mean (shaded), and (b) longitude-height cross section of geopotential heightdeviations from the zonal mean (contours), temperature deviations from the zonal mean (shaded)and Plumb's wave activity flux (vector) averaged over 50° – 70°N in the troposphere and the lower
stratosphere from 1,000 to 50 hPa averaged for 12 – 16 January 2009. The contour intervals in (a)2 -2and (b) are 60 m and 10 m, respectively. The unit of Plumb's wave activity flux is m s .

4. Summary
The major characteristics of this SSW event can be summarized as follows:First, the planetary wave of zonal wave number 2 was dominant during this SSW.

We foundthat the total amount of energy propagated by this planetary wave from the troposphere to thestratosphere was the strongest since winter 1979/80.

6This was extraordinary because thenormalized value of the deviation from its climatological mean exceeded 4 σ.

This SSW represents

the most prominent discovery in terms of the energy of the planetary wave of zonal wave number 2.
Second, the development of a ridge over Alaska played an important role in the development of
the planetary wave, as this ridge emanated strong Rossby wave packets toward the eastern part ofNorth America and was related to the intensification of the tropospheric polar front jet at thebeginning of this SSW.

Moreover, during the first developmental stage of MSSWJ09, part of thestrong planetary wave packets directly propagated from the troposphere to the stratosphere, as
shown in the bottom panels of Fig. 4b (from 100 to 1 hPa) and Fig. 5b (from 1,000 to 50 hPa).
During the second developmental stage of MSSWJ09, part of the wave packets emanating from theridge over Alaska propagated along the sub-tropical jet in the upper troposphere and moved upward
toward the stratosphere over central Siberia.

These wave packets were crucial for the polar vortex

split.
References
Duchon, C. E., 1979: Lanczos filtering in one and two dimensions. J. Applied Meteor., 18,1016-1022.
Onogi, K., J. Tsutsui, H. Koide, M. Sakamoto, S. Kobayashi, H. Hatsushika, T. Matsumoto, N.
Yamazaki, H. Kamahori, K. Takahashi, S. Kadokura, K. Wada, K. Kato, R. Oyama, T. Ose,N. Mannoji and R. Taira, 2007: The JRA-25 Reanalysis. J. Meteor. Soc. Japan, 85, 369-432.
Palmer, T. N., 1982: Properties of the Eliassen-Palm flux for planetary scale motions. J. Atmos. Sci.,
39, 992-997.
Plumb, R. A., 1985: On the three dimensional propagation of stationary waves. J. Atmos. Sci., 42,217-229.

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