Annales
Geophysicae
Open Access
Ann. Geophys., 31, 1397–1415, 2013
www.ann-geophys.net/31/1397/2013/
doi:10.5194/angeo-31-1397-2013
© Author(s) 2013. CC Attribution 3.0 License.
The impact of planetary waves on the latitudinal displacement of
sudden stratospheric warmings
V. Matthias1 , P. Hoffmann1 , A. Manson2 , C. Meek2 , G. Stober1 , P. Brown3 , and M. Rapp4,*
1 Leibniz-Institute
2 Institute
of Atmospheric Physics at the Rostock University, Schloss-Str. 6, 18225 Kühlungsborn, Germany
of Space and Atmospheric Studies, University of Saskatchewan, 116 Science Place, Saskatoon, Sask. S7N5E2,
Canada
3 Canada Research Chair in Meteor Science, Department of Physics and Astronomy, University of Western Ontario, London,
Ontario N6A 3K7, Canada
4 Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
* also at: Meteorologisches Institut München, Ludwig-Maximilian Universität München, Munich, Germany
Correspondence to: V. Matthias (matthias@iap-kborn.de)
Received: 18 April 2013 – Revised: 19 June 2013 – Accepted: 20 June 2013 – Published: 9 August 2013
Abstract. The Northern Hemispheric winter is disturbed by
wards. These differences between the normal SSW in 2006
large scale variability mainly caused by Planetary Waves
and the displaced events in 2009, 2010 and 2012 are linked
(PWs), which interact with the mean flow and thus result
to an increased PW activity between 30◦ N and 50◦ N and the
in Sudden Stratospheric Warmings (SSWs). The effects of
changed stationary wave flux in the stratosphere around the
a SSW on the middle atmosphere are an increase of stratodisplaced events compared to 2006.
spheric and a simultaneous decrease of mesospheric temperKeywords. Meteorology and atmospheric dynamics (midature as well as a wind reversal to westward wind from the
dle atmosphere dynamics; waves and tides)
mesosphere to the stratosphere. In most cases these disturbances are strongest at polar latitudes, get weaker toward
the south and vanish at mid-latitudes around 50◦ to 60◦ N
as for example during the winter 2005/06. However, other
1 Introduction
events like in 2009, 2010 and 2012 show a similar or even
Geoscientiic
Geoscientiic
stronger westward wind at mid- than at polar latitudes either
Sudden Stratospheric Warmings (SSWs) are known as exin the mesosphere or in the stratosphere during the SSW. This
ceptional polar vertical coupling processes during winter, afstudy uses local meteor and MF-radar measurements, global
fecting all atmospheric layers. They are caused by an upward
satellite observations from the Microwave Limb Sounder
propagation of Planetary Waves (PWs) and their interaction
(MLS) and assimilated model data from MERRA (Modernwith the mean flow (for details see Matsuno, 1971, and AnERA Retrospective analysis for research and Applications).
drews et al., 1987, Chapt. 6). SSWs can be classified into 3
We compare differences in the latitudinal structure of the
Geoscientiic
different types (Labitzke and Naujokat,
2000): major, minor
Geoscientiic
zonal wind, temperature and PW activity between
a “norand Canadian warmings. This classification is based on the
mal” event, where the event in 2006 was chosen represenresponse of the zonal mean zonal wind (weakening, revertatively, and the latitudinal displaced events in 2009, 2010
sal) at 60◦ N and the temperature gradient between 60◦ and
and 2012. A continuous westward wind band between the
90◦ N, both at 10 hPa. A large number of studies describe
pole and 20◦ N is observed during the displaced events. Furthe individual response of SSWs on the middle atmosphere
thermore, distinctive temperature differences at mid-latitudes
regarding the dynamical and thermal structure, especially of
occur before the displaced warmings compared to 2006 as
the record warming in 2009, e.g. Manney et al. (2009), Kuriwell as a southward extended stratospheric warming afterhara et al. (2010) and Shepherd et al. (2009).
Published by Copernicus Publications on behalf of the European Geosciences Union.
1398
In connection with SSWs, a weakening or reversal of the
dominating eastward zonal winds to summerly westward
winds in the Mesosphere/Lower Thermosphere (MLT) region has first been observed by Gregory and Manson (1975).
This effect is more pronounced at high northern latitudes
(e.g. Manson et al., 2006) than at southern or mid-latitudes.
To illuminate the contribution of PWs on the MLT during
SSWs, Coy et al. (2011) used a data assimilation system covering the 0 to 90 km altitude range to investigate the temporal
development of PWs during the record breaking SSW 2009
and their interaction with the mean flow. Summarising their
results, they found a transient non-stationary wave 2 propagating rapidly from the troposphere into the upper mesosphere, where it dissipated and produced easterly mean-flow
accelerations which intensified the SSW.
More general statements about the main characteristics
of SSWs in the tropo- and stratosphere are made by Charlton and Polvani (2007). They compared the composite analysis of vortex displacement and splitting events between
1958 and 2002 from reanalysis data and found differences
in the seasonal distribution as well as in the tropospheric and
stratospheric structure. Multi-year observations are used by
Matthias et al. (2012) to characterise the average behaviour
of SSW-related wave activity in the MLT region. From a
composite analysis of 5 major warmings between 1999 and
2010 they found a strong 10-day wave (period: 8–12 d) simultaneous with the warming and a weaker 16-day (period:
12–20 d) wave before.
The effects of SSWs like zonal wind reversal, temperature increase/ decrease and elevated stratopause are strongest
at polar latitudes and get weaker toward the south in most
cases, see for example Limpasuvan et al. (2004), Hoffmann
et al. (2002, 2007) and Manney et al. (2007). However, some
stratospheric warming events occur similarly strong or even
stronger at mid- than at high latitudes. During the DYANA
campaign in 1990, for example, Cevolani (1991) and Singer
et al. (1994) found a strong perturbation of the zonal wind
between the upper stratosphere and lower thermosphere at
mid-latitudes which was in some cases similarly strong compared to higher latitudes (see Singer et al., 1994). A more
current event was studied by Stober et al. (2012) where a
stronger wind reversal was observed at mid- than at high latitudes during the SSW event of 2010. This mid-latitudinal
wind reversal in 2010 was also observed in MF radar winds
by Chen et al. (2012) over Langfang (39◦ N, 166◦ E).
Fritz and Soules (1970) were the first who found temperature anomalies in the tropical stratosphere during the SSW of
1970 with the help of global satellite data. More recently observations of stratospheric and mesospheric tropical anomalies during the winter 2004/05 were made by Shepherd et al.
(2007). A composite analysis of reanalysis data by Kodera
(2006) also shows a clear effect of SSWs on the equatorial
stratospheric temperature. Therefore, SSWs affect not only
mid- and high latitudes, but can also affect the circulation at
low latitudes.
Ann. Geophys., 31, 1397–1415, 2013
V. Matthias et al.: Latitudinal displacement of SSWs
Strong mid-latitudinal dynamical disturbances occur not
in the majority of SSWs. An obvious question is: under
which circumstances are such effects observable at mid- and
lower latitudes? To address this question we compare the latitudinal and altitudinal variability of zonal wind reversal and
temperature changes for different SSW events. Such an analysis was partly made by Chen et al. (2012) for the SSW in
2010. These authors considered MLS gradient winds and investigated their latitudinal structure. However, they did not
offer a possible explanation for this phenomenon.
Since the main reason for SSWs are upward and poleward
propagating PWs interacting with the mean flow (see Matsuno, 1971; Andrews et al., 1987) it is tempting to speculate that latitudinal differences in the PW activity might be
one reason for the increased mid-low-latitudinal SSW effects
during some events. In this work we will therefore investigate
the latitudinal variability of the zonal wind reversal and of the
temperature changes related to the PW activity.
In detail this article deals with the question: how does the
PW behaviour affect the latitudinal expansion of a SSW? We
therefore compare 3 SSW events in 2009, 2010 and 2012,
where the zonal wind reversal reaches down to lower latitudes, with the SSW of 2006, where the zonal wind reversal
is strongest at the pole and weakens towards mid-latitudes
but does not occur at lower latitudes, as the “normal” case.
We investigate the zonal wind reversal at different latitudes
and altitudes with the help of MF- and meteor radar measurements at different latitudinal and longitudinal locations
and assimilated model data from MERRA (Modern-ERA
Retrospective analysis for research and Applications). Furthermore we use global temperature and geopotential height
data from the Microwave Limb Sounder (MLS) for northern
hemispheric temperature variations and for an estimation of
PWs characteristics.
Note that there are only very few studies considering the
latitudinal extension of circulation changes during and after SSWs. Most of them are based on model simulations,
e.g. carried out with the WACCM model (De La Torre
et al., 2012; Limpasuvan et al., 2012) or with the Japanese
T213L256GCM (Tomikawa et al., 2012). With WACCM,
an enhanced effect of SSW on the circulation at latitudes
south of 40◦ N has been found in connection with splitting
events. Additionally, Tomikawa et al. (2012) and Limpasuvan et al. (2012) used their simulation to estimate the
latitudinal-pressure dependence of the Eliassen-Palm flux
and its divergence for all wave numbers which shows an enhancement during and after the simulated major warmings.
In contrast to the above mentioned model simulations, we
use observational radar and satellite data together with assimilated MERRA data during four SSW events in this study.
Section 2 provides an overview of the used instruments
and model data as well as data analysis methods. The comparison of the zonal wind, temperature and PW activity at
different latitudes from radar and satellite measurements and
www.ann-geophys.net/31/1397/2013/
V. Matthias et al.: Latitudinal displacement of SSWs
1399
Table 1. Technical details of Meteor radar systems at Tavistock (CMOR), Juliusruh, Andenes and Eureka.
Tavistock
(CMOR)
(43◦ N, 81◦ W)
Frequency
Power
PRF
Coherent integ.
Height range
Sampling resol.
Wind analysis
Observation since
17.45, 29.85,
38.15 MHz
6 kW
(per frequency)
532
1
70–120 km
3 km
DBS
1999–today
Juliusruh
Andenes
Eureka
(55◦ N, 13◦ E)
(69◦ N, 16◦ E)
(80◦ N, 86◦ W)
32.55 MHz
32.55 MHz
32.55 MHz
12 kW
18 kW
12 kW
2114
4
80–100 km
2 km
DBS
2007–today
2094
4
80–100 km
2 km
DBS
2001–today
2094
4
80–100 km
2 km
DBS
2007–today
model data of the considered years is shown in Sect. 3 and
discussed in Sect. 4. The results are summarised in Sect. 5.
2
2.1
Table 2. Technical details of MF-radar systems at Saskatoon and
Juliusruh.
Experimental data and methods
Radar measurements
For this study 4 Meteor Radars (MR) located at Andenes
(69◦ N, 16◦ E), Juliusruh (55◦ N, 13◦ E), Eureka (80◦ N,
86◦ W) and at Tavistock (43◦ N, 81◦ W), named Canadian
Meteor Orbit Radar (CMOR) as well as 2 Medium Frequency radars (MF-radar), located at Juliusruh and Saskatoon (52◦ N, 107◦ W) are used to investigate the latitudinal differences of zonal wind, mesospheric temperature and
planetary wave activity between mid and polar latitudes. An
overview of the radar locations is given in Fig. 1. Note that in
Andenes a MR and MF-radar are colocated, but in this study
only MR data is used. A short description of the MF-radar
and afterwards of the MR follows.
Basic parameters of all MF-radars used in this study are
summarised in Table 2.
The MF-radar at Juliusruh operates at a frequency of
3.17 MHz with a peak power of 128 kW. Thirteen interconnected narrow-beam cross dipoles (arranged as a Mills
Cross) transmit radio wave pulses of 4 km length and ∼ 15◦
width. The reception of the atmospheric signal occurs by four
crossed horizontal dipoles close to the Mills Cross. This system has been measuring wind continuously since 2003 and
is an enhancement of the MF-radar system which operated
at the same place with slightly different characteristics from
1990 to spring 2003. For more information about the development of both MF-radar systems at Juliusruh and their features see Keuer et al. (2007).
The Saskatoon MF-radar operates at a frequency of
2.22 MHz with a peak power of 20 kW. It consists of four
spaced receiving arrays and a transmitter antenna with a full
beam width of ∼ 15◦ . Wind measurements have been conducted since 1978 between 60 and 110 km with a height reswww.ann-geophys.net/31/1397/2013/
Radar
Frequency
Peak power
Beam width
Height range
Sampling resolutuion
Wind analysis
Observation since
Saskatoon
(52◦ N, 107◦ W)
Juliusruh
(55◦ N, 13◦ E)
MF
2.22 MHz
20 kW
∼ 15◦
60 – 110 km
3 km
FCA
1978–today
MF
3.17 MHz
128 kW
∼ 15◦
70 – 94 km
2 km
FCA
1990–today
olution of 3 km. A detailed description of the MF-radar at
Saskatoon can be found in Meek and Manson (1987).
All-sky meteor radars employ one antenna for transmission and a five-antenna interferometer for reception. This
provides a range resolution of 2 km and an angular resolution of 2◦ for meteor location. The basic construction and
functionality of the MRs used in this study is nearly identical
to the system originally described in Hocking et al. (2001). A
summary of the characteristics of all MRs used in this study
is given in Table 1.
In this study, we also investigate the day-to-day variability
of the mesospheric temperature estimated from meteor fading decay times at the peak of the meteor layer at around
90 km. Temperatures are derived by the combination of altitude variations in the meteor decay time and an empirical
model of the mean temperature gradient at the peak altitude
of the meteor layer (for details see Singer et al., 2003; Hocking et al., 2004; Stober et al., 2012).
The diurnal, semidiurnal and terdiurnal tides, which are
obtained from least-squares fits of hourly mean winds for 4day intervals shifted by one day, were removed from the prevailing wind for our wind analysis. The estimation of PWs
results from a wavelet analysis (Torrence and Compo, 1998).
Ann. Geophys., 31, 1397–1415, 2013
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V. Matthias et al.: Latitudinal displacement of SSWs
Fig. 1. Map of MR and MF-radar stations used in this study.
The calculation of the wavelet transform Wn (s) is conducted
as described in Matthias et al. (2012).
2.2
MLS measurements
For a global view of the temperatures and for the estimation of the wavenumber and period of PWs during SSWs,
temperature and geopotential height (GPH) data from the
Microwave Limb Sounder (MLS) are used. MLS is a limb
scanning emission microwave radiometer on the NASA Aura
satellite (Waters et al., 2006; Livesey et al., 2007). Aura was
launched on 15 July 2004 into a sun-synchronous polar orbit
at 705 km altitude with a 98◦ inclination. The MLS instrument scans the limb in the forward direction of the orbital
plane which gives a global coverage from 82◦ S to 82◦ N
on each orbit. The useful height range of temperature data
is approximately 8 to 97 km (316–0.001 hPa) with a vertical
resolution of ∼ 4 km in the stratosphere and ∼ 14 km at the
mesopause determined by the full width at half maximum
(FWHM) of the averaging kernels (Livesey et al., 2007).
GPH and temperature have the same height range and vertical resolution because they are linked through hydrostatic
balance and gas law. Comparison of MLS measurements
with pre-validated satellite observations show a bias of −2
to 2 K in the troposphere and stratosphere and a cold bias
of −4 . . . −9 K in the mesosphere for temperature measurements. GPH observations have a bias of 50 to 150 m in the
troposphere and stratosphere and up to −450 m at 0.001 hPa
(see Froidevaux et al., 2006, and Schwartz et al., 2008).
Here we use data from the level 2 version 2.2 data product. We removed poor data by screening methodologies described by Livesey et al. (2007). The geometric altitudes are
estimated from the pressure levels as follows:
h = −7 · ln(p/1000), where h is the altitude in km and p the
pressure in hPa. Note that there is a difference between geometric and geopotential heights especially in the mesosphere.
However, for studies of PWs and considering the altitudinal
resolution of MLS in the mesosphere, this difference is not
relevant.
To estimate the period and wavenumber of a PW, a twodimensional least-squares method for spectral analysis of
space-time series is used following the procedure described
in Wu et al. (1995). The basic function is given by
yi = A cos(2π(f ti − sλi )) + B sin(2π(f ti − sλi ))
Ann. Geophys., 31, 1397–1415, 2013
(1)
where A and B are the parameters to be fitted and where f
is the frequency, s is the wavenumber, ti is the time and λi
and yi are longitude and GPH, respectively. We note that this
method has its limits on the one hand to distinguish between
particular superimposed waves as discussed by Pancheva
et al. (2009) and on the other hand we have to consider possible aliasing effects as discussed by Tunbridge et al. (2011).
Results should therefore be interpreted carefully.
For the determination of the latitudinal and altitudinal expansion of a PW with a given wavenumber and period the following calculation is made for 5◦ steps and at every pressure
level. The maximum amplitude of a sliding window of reasonable length (4 times the length of the considered wave’s
period) within a given time interval over a 5◦ latitudinal band
centred around the considered latitude is calculated by using
Eq. (1). Frequency/wavenumber spectra of PWs from MLS
mainly show aliasing effects. These effects are discussed by
Meek and Manson (2009), Tunbridge et al. (2011) and McDonald et al. (2011) for example. These aliasing effects can
be neglected though, since they are mostly weaker side lobes
of a “true” wave and we use the maximum amplitude in this
study.
2.3
MERRA
For the investigation of the spatial extent of the zonal wind
reversal in the stratosphere and mesosphere during the SSWs
between 2006 and 2012 considered here, we use the assimilated model data from MERRA from NASA. The analysed fields of MERRA on model levels with a native grid of
1◦
2◦
2 × 3 and a 6 h temporal resolution are used. The vertical
range of this MERRA product is 985 to 0.01 hPa, i.e. from
the surface to approximately 80 km. For further information
on MERRA see Rienecker et al. (2011) and for a file specification of MERRA products see Lucchesi (2012). Comparison of MERRA with other reanalysis products and satellite
measurements shows a good agreement in the stratosphere
(Rienecker et al., 2011; Yoo et al., 2013). However, MERRA
temperatures have a cold bias of 5 K above 1 hPa compared to
MLS temperatures (Rienecker et al., 2011). Thus, MERRA
data in the lower mesosphere have to be considered carefully.
3
Results
An outstanding effect of a SSW besides stratospheric warming and mesospheric cooling, is the wind reversal in the
strato- and mesosphere. Figure 2 shows the zonal wind at Andenes (69◦ N, 16◦ E) and Juliusruh (55◦ N, 13◦ E) at 85 km
from MR and at 49 km from MERRA data centred on the
central day for the SSWs in 2006, 2009, 2010 and 2012. Note
that the MR at Juliusruh started to operate at a later time in
2006. Thus the zonal wind data for 2006 are substituted by
the MF-radar data also located at Juliusruh. The central day
for major SSWs is defined following Labitzke and Naujokat
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V. Matthias et al.: Latitudinal displacement of SSWs
1401
Fig. 2. Zonal wind at Andenes (black) and Juliusruh (red) at 85 km (left) from MR and 49 km (right) from MERRA data centred around the
central day (black dashed line) of the respective SSW of 2006, 2009, 2010 and 2012. Terdiurnal, semidiurnal and diurnal tides were removed
for the MR data.
(2000) as the day where the zonal mean zonal wind reverses
(ū < 0) at 60◦ N at 10 hPa and the temperature gradient between 60 and 90◦ N has its local maximum (as in Matthias
et al., 2012). The central days of the major events considered here are 21 January 2006, 22 January 2009 and 28 January 2010. The central day of the minor warming 2012 is
defined as the day where the zonal mean zonal wind at 60◦ N
and at 10 hPa has its minimum, i.e. the central day is 17 January 2012.
The mesospheric wind reversal from eastward to westward wind at 85 km during the SSW of 2006 occurs before
the central day at Andenes and at the central day at Juliusruh whereas the maximum of the westward wind at polar
is stronger than at mid-latitudes around the central day. Afterwards, Andenes shows a strong and rapid increase of the
eastward wind with no significant wave activity. In contrast
to Andenes occurs at Juliusruh at mesospheric altitudes a
weaker eastward wind with a strong wave activity. The zonal
wind at 49 km at both locations reverses slightly before the
www.ann-geophys.net/31/1397/2013/
central day, but first at Juliusruh and a short time later at Andenes. Similarly to mesospheric altitudes the westward wind
at polar appears stronger than at mid-latitudes. In contrast to
mesospheric altitudes the eastward wind occurs stronger at
Juliusruh after the SSW than at Andenes, but shows again
a strong wave-like behaviour at mid-latitudes while at Andenes no significant wave activity is presented. This is what
we consider to be a “normal” SSW with a typical latitudinal
behaviour. The SSW of 2006 is representative for all “normal” events. In the following, we will therefore use this event
for comparison with the other events considered here.
In contrast to 2006, the events in 2009, 2010 and 2012
show a simultaneous or even earlier wind reversal with a similar strong or even stronger westward wind at Juliusruh than
at Andenes at 85 km. The wave activity increased at 85 km
in contrast to 2006 before and after the SSW at Juliusruh and
at Andenes except for 2012 where no wave-like behaviour
is considered after the SSW at Juliusruh. The onset of the
wind reversal at 49 km varies with time, and the following
Ann. Geophys., 31, 1397–1415, 2013
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V. Matthias et al.: Latitudinal displacement of SSWs
Fig. 3. Zonal wind at Eureka (black) and Saskatoon (red) at 85 km (left) from the respective radar data and 49 km (right) from MERRA data
centred around the central day (black dashed line) of the SSW of 2009. Terdiurnal, semidiurnal and diurnal tides were removed from the MR
and MF-radar data.
westward wind is usually as in 2006, weaker at Juliusruh
than at Andenes, whereas in 2009 the westward wind appears stronger at Juliusruh than at Andenes. Also the wave
activity after the events varies from year to year. While the
wave activity is strong at Juliusruh and Andenes after the
warming in 2009, the stratosphere is stable with no significant wave activity at both locations in 2010. The wave activity at 49 km was increased at Andenes after the event in
2009 but no significant wave activity occurred at Juliusruh.
Note that zonal wind observations at both locations, i.e. Andenes and Juliusruh, are representative for the zonal wind in
the Eastern Hemisphere.
As an example for the Western Hemisphere, similar to
Fig. 2, Fig. 3 shows the zonal wind at Eureka (80◦ N, 86◦ W)
and Saskatoon (52◦ N, 107◦ W) at 85 km from MR (Eureka)
and MF-radar (Saskatoon) and at 49 km from MERRA data
centred on the central day for the SSW in 2009. Similar to
the Eastern Hemisphere (Andenes and Juliusruh) reverses the
zonal wind in 2009 simultaneously at Eureka and Saskatoon
at 85 km with a stronger and longer lasting westward wind
at Eureka than at Saskatoon. Afterwards, there is a rapid and
strong increase of the eastward wind with no wave-like behaviour at Eureka and a weaker eastward wind with a wavelike behaviour at Saskatoon. In contrast to the Eastern Hemisphere, the wind reversal occurs first at Eureka and 3 days
later at Saskatoon at 49 km in 2009 and the westward wind
has similar strength at both locations. Like in the Eastern
Hemisphere the zonal wind has shown a wave-like behaviour
at Eureka and Saskatoon after the SSW.
These results show differences in the latitudinal behaviour
of the zonal wind between 2006 and the other three years
considered in this study. While in 2006 the westward wind
during the wind reversal occurs stronger at pole than at midlatitudes it appears similar strong or even stronger at midlatitudes than at polar latitudes especially in the mesosphere
during the events in 2009, 2010 and 2012. Also the latitudinal dependence of the onset of the wind reversal differs
from year-to-year. Note that besides latitudinal also longitudinal differences occur in the local measurements of the
Ann. Geophys., 31, 1397–1415, 2013
zonal wind. The longitudinal dependence of SSWs is not the
main focus in this paper, but will be a matter of future investigations.
These local measurements indicate an unusual latitudinal
behaviour of the SSW in 2009, 2010 and 2012 in comparison with the “normal” warming in 2006. For a more global
view on the zonal wind Fig. 4 represents the zonal mean
zonal wind from MERRA as a function of latitude and height
5 days before, at the corresponding central day, and 5 days
after the central day of the SSWs of 2006, 2009, 2010 and
2012. Five days before the central day, all events are characterised by a weak wind reversal at the pole in the stratoand mesosphere whereas these reversals are separated by an
eastward wind around 50 to 60 km except for 2009. In 2009
a strong eastward wind at high and mid-latitudes appears five
days before the central day of the record warming.
On the central day the wind reverses from polar Mesosphere and Upper Stratosphere (hereafter: MUS) to stratospheric mid-latitudes in 2006. Hence, there is no continuous
westward wind band between the pole and lower latitudes.
In contrast to 2006, the other events in 2009, 2010 and 2012
show this continuous westward wind band only between the
pole and lower latitudes and the wind reversal reaches from
polar MUS to the lower latitude stratosphere around 20◦ N.
Five days after the central day the wind reversal of the
SSW of 2006 looks very similar to that on the central day
but has moved downwards. This downward movement or
downward progression is also observable in 2009 and 2010
in which the eastward wind in 2010 already dominates the
MUS. The polar latitudes in 2012 show a strong eastward
wind five days after the central day from stratosphere to
mesosphere, i.e. the wind reverses back from westward to
eastward. However, at mid and lower latitudes the stratosphere still shows westward wind as a result of the previous
wind reversal. It seems like the wind reversal in this particular year is breaking down from the polar mesosphere to the
lower latitude stratosphere. Note that there is a dependence
from the selected central day, especially in 2010 where after
a short wind reversal around 28 January a second one occurs
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V. Matthias et al.: Latitudinal displacement of SSWs
1403
Fig. 4. Zonal mean zonal wind 5 days before, at and 5 days after the central day of the year 2006, 2009, 2010 and 2012 as a function of
latitude and height from MERRA data.
which lasts much longer. Chen et al. (2012) discussed the
definition of the central day for 2010 and rescheduled the
central day on 2 February 2010.
Apart from the zonal wind characteristics around SSWs,
the latitudinal and altitudinal temperature structure must also
be considered. Figures 5a and 5b show the daily mean zonal
mean MLS temperature at 20, 40, 59 and 81 km as a function
of time and latitude for 2006 and 2009 in Fig. 5a and for
2010 and 2012 in Fig. 5b. The white dashed line marks the
central day of the corresponding SSW. In the following, each
altitude from bottom to top will be described separately for
all years, i.e. Figs. 5a and 5b will be considered together for
each height.
In 2006, 2009 and 2010 a long lasting warming occurs
from pole to mid-latitudes at 20 km after the central day.
While in 2009 and 2010 this warming reaches down to
around 50◦ N, in 2006, with a typical polar behaviour, it is
observable only down to 60◦ N. Note that in 2010 the warming did not start directly after the central day but ∼ 15 days
later. The cause for this time shift might be the temporal development of the zonal wind in 2010 as discussed before. The
rescheduled central day by Chen et al. (2012) on 2 February 2010 is in a good agreement with the warming at 20 km
www.ann-geophys.net/31/1397/2013/
observed here. During the minor warming of 2012 a warming also occurs after the central day at 20 km, but in contrast
to those in 2006, 2009 and 2010 it is observed between 50
and 70◦ N and not at the pole. This extraordinary behaviour
will be further investigated in the discussion (see Sect. 4).
At 40 km warmings occur during all events considered
in this study. The temperature peaks around the central day
from pole to ∼ 60◦ N during every SSW. After these warmings a temperature increase occurs between 20◦ and 40◦ N.
This indicates an equatorward progression of the warming in
the stratosphere around 40 km which can be influenced from
the mean meridional residual circulation.
However, there is no latitudinal difference of the temperature during the warming between the typical polar behaviour
in 2006 and the more mid-low-latitudinal behaviour in 2009,
2010 and 2012. But the comparison of the temperature at
mid-latitudes around 50◦ N at 40 km before the warmings
shows much higher temperatures in 2006 than in the other
years considered here. While in 2006 the cold polar temperatures before the warming reach down to 60◦ N, they are also
observed down to 40◦ and 50◦ N during the SSWs of 2009,
2010 and 2012.
Ann. Geophys., 31, 1397–1415, 2013
1404
V. Matthias et al.: Latitudinal displacement of SSWs
Fig. 5a. Zonal mean temperature at 20, 40 , 59 and 81 km from MLS for the winter 2005/06 and 2008/09. The vertical white dashed line
marks the central day of the respective SSW and the horizonal black dashed line at 60◦ N is used for help of orientation.
The mesospheric temperature at 59 km shows a polar cooling after the corresponding central days which varies in
length of time and strength. At the same time of the maximum of the polar cooling a warming around 50◦ N occurs
and spreads out to the pole with time. The polar and midlatitude mesospheric temperatures before the SSWs are very
variable due to increased PW activity and more stable afterwards due to the decreased PW activity after SSWs (see for
example Matthias et al., 2012).
The mesospheric cooling at 80 km is more narrow during
all considered events than at 59 km and occurs around the
central day and not afterwards as it is the case at 59 km. After this cooling a strong warming occurs at polar latitudes
whereas this warming appears weaker in 2010 and 2012 than
in 2006 and 2009. The polar mesosphere before the SSW in
all cases is very variable. This can be also attributed to the increased PW activity. During the SSW of 2006 a strong warmAnn. Geophys., 31, 1397–1415, 2013
ing occurs between 40◦ and 55◦ N at 80 km simultaneous to
the polar cooling. Similar observations are obtained in 2010
and 2012 whereas the warming in 2010 appears much weaker
and in 2012 slightly before the central day. It seems that this
phenomenon occurs only during vortex displacement events
and not during splitting events like in 2009 where the mesosphere shows cold temperatures at all latitudes during the
SSW.
Independent of the the latitudinal variations of the temperature we found a downward progression from mesosphere
to stratosphere during all events. Such a downward movement was previously mentioned in connection with the zonal
wind reversal. Here, the mesospheric cooling first occurs at
81 km and then moves downward to the lower mesosphere
around 59 km where it also lasts much longer. This downward movement can also be continued to stratospheric altitudes around 40 km where after the warming the temperature
www.ann-geophys.net/31/1397/2013/
V. Matthias et al.: Latitudinal displacement of SSWs
1405
Fig. 5b. Same as Fig. 5a just for the winter 2009/10 and 2011/12.
again decreases to a typical polar stratospheric level. However, this cooling occurs later as in the lower mesosphere at
59 km and thus there is a downward movement of the mesospheric cooling to stratospheric heights. A similar behaviour
can be observed in the stratosphere. The warming at 40 km
occurs around the central day while the warming at 20 km appears afterwards and lasts much longer. Note that this downward movement of the cooling/warming during SSWs is consistent with the downward progression of the wind reversal
as discussed by Hoffmann et al. (2007) and found in the composite analysis of Matthias et al. (2012).
Summarising the temperature characteristics around
SSWs it is to be said that we did not find a continuous band
of warm/cold temperatures in the stratosphere/mesosphere
between the pole and lower latitudes as in the zonal wind.
Nevertheless, the exceptional SSWs in 2009, 2010 and 2012
(with a continuous westward wind band between pole and
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lower latitudes) show differences in temperatures between
35◦ and 60◦ N in the strato- and mesosphere before and after
the central day of the warmings compared to the polar dominated event in 2006. Besides the stratospheric equatorward
movement of the warming, we also observed a downward
progression of the stratospheric warming and mesospheric
cooling during all events.
The previously mentioned mid-latitudinal mesospheric
warming that occurs in the zonal mean MLS temperature
data during vortex displacement events will be considered
more closely in the following.
Figure 6 shows the relative temperature variations from
meteor radar data at approximately 90 km for the years 2009,
2010 and 2012 from radar stations at Andenes (69◦ N, 16◦ E)
and Juliusruh (55◦ N, 13◦ E) for the Eastern Hemisphere
and Tavistock (Canadian Meteor Orbit Radar, short: CMOR,
(43◦ N, 81◦ W)) for the Western Hemisphere. Note that the
Ann. Geophys., 31, 1397–1415, 2013
1406
Fig. 6. Relative temperature from MR at ∼90 km at Andenes,
Juliusruh and Tavistock (CMOR) centred around the central day of
the respective SSW. The black dashed line marks the central day.
year 2006 is missing because the MR was only installed at
Juliusruh later in the year. In addition, there is no meteor
radar at Saskatoon so that we use the CMOR data instead as
a substitution for mid-latitudinal Western Hemisphere measurements. The relative temperature is centred on the central
day of the respective warming which is marked as a black
dashed line. Note that meteor radar temperatures depend on
the assumption of an empirical temperature gradient model.
Therefore, we subtracted the mean temperature of the observation period from each temperature profile and concentrate
on the day-to-day variability and on the tendency of each single temperature curve in this study.
During the record warming of 2009 all 3 locations show a
temperature decrease around the warming as it was observed
in the zonal mean temperatures from MLS in Figs. 5a and
5b. In 2010 there is a strong cooling at Andenes (polar latitudes) while a weak cooling is observed at Juliusruh (midlatitudes). However, the western hemispheric mid-latitudinal
CMOR radar shows no significant cooling in connection
with the SSW. The minor warming in 2012 splits the hemispheres. While the polar and the mid-latitudinal temperature
decrease in the Eastern Hemisphere, the mesospheric temperature in the mid-latitudinal Western Hemisphere increases.
Thus Fig. 6 indicates that the mesospheric mid-latitudinal
Ann. Geophys., 31, 1397–1415, 2013
V. Matthias et al.: Latitudinal displacement of SSWs
warming in Figs. 5a and 5b depends on the longitudinal location.
To understand the differences between local measurements and zonal mean temperature observations at mesospheric mid-latitudes during SSWs, Fig. 7 shows the projection of MLS temperatures at 81 km at the corresponding central day for the events considered in this study. The
white points mark the location of the local measurements in
Fig. 6. Note that the meteor temperatures of Fig. 6 are observed around 90 km and the MLS temperatures at 81 km
with a vertical resolution of 10 km. So there is an altitudinal discrepancy that should be regarded. During all events
considered here Andenes and Juliusruh lie in the cold part
of the global temperature pattern which leads to the decreasing temperatures of Fig. 6. In contrast, the CMOR radar is
mostly located at the much warmer part of the temperature
pattern with the exception of 2009 where it is located between the cold and the warm part of the global temperature
pattern. Thus the measured temperatures strongly depend on
their location relative to the global temperature pattern. From
this it follows that zonal mean values should be considered
very critically, especially for comparison with local measurements. A possible reason for the mesospheric mid-latitudinal
warming is discussed in Sect. 4.
Our hypothesis is that the reason for the continuous westward wind band from the pole to lower latitudes and the temperature changes at mid- and lower latitudes is the increased
PW activity at the same latitudes. Therefore, the next Fig. 8
shows the wavelet spectrum of the meridional wind at 85 km
for the winter 2008/09 at the different considered locations.
The vertical black dashed line marks the central day of the
SSW in 2009. The dominating waves around the warming
at all locations except for Saskatoon are a 10-day (period:
8–12 d) and/or a 16-day wave (period: 12–20 d) as also mentioned by Matthias et al. (2012). Another wave that occurs
around the warming is a 6-day wave (period: 5–7 d). Beside
these waves also 2- and 3-day waves occur but their direct
relation to SSWs is beyond the scope of this paper.
With the help of MLS geopotential height data, we found
that all waves in all years have a wavenumber between −1
and 1, i.e. westward or eastward propagating (not shown, see
also Matthias et al., 2012). Our main interest focuses on the
latitudinal behaviour of PWs responsible for the latitudinal
variability of SSW effects. Therefore, Fig. 9 shows the amplitude of the 6-day, 10-day and 16-day wave with wavenumbers between −1 and 1 as a function of latitude and height for
the four considered winters from MLS geopotential height
data. The amplitude is calculated as the maximum amplitude
of a sliding window of 24 days for the 6-day wave, of 40 days
for the 10-day wave and of 70 days for the 16-day wave at
each latitude and height between day 335 of the previous year
and day 60 of the actual year. The 6-day wave has its maximum at polar latitudes in the MUS and extends as far as south
as 50◦ N in every year. However, the 10-day wave shows an
increased activity to as far south as 30◦ N except for 2006, i.e.
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V. Matthias et al.: Latitudinal displacement of SSWs
1407
Fig. 7. Projection of the MLS temperature data at 81 km over the Northern Hemisphere of the central day of the respective SSW, i.e. each
projection shows the same day as in Fig. 6 the dashed line for the respective year. The white points mark the local meteor temperature
measurements for (A) Tavistock, (B) Andenes and (C) Juliusruh.
the SSW without the continuous westward wind band from
the pole to the subtropics.
Maximum amplitudes are found at the pole in all considered winters. However, in 2009 for example, there is a second
smaller maximum of the 10-day wave activity at 50◦ N which
reaches also down to 30◦ N. The 10-day wave activity in
2010 and 2012 is very similar. Both years show an increased
wave activity in the MUS from the pole as far as south as
30◦ N. The 16-day wave shows a strong increased activity
between 40◦ and 80◦ N in 2010 while in 2009 and 2012 the
16-day wave occurs only at polar latitudes. In 2006 the 16day wave also appears at polar and mid-latitudes down to
50◦ N but is very weak compared to the 6- and 10-day wave
during this winter and therefore less important for this warming. Thus the transient PWs show an increased wave activity
between 30◦ and 50◦ N around the exceptional warmings that
does not occur in the polar dominated year 2006.
It is well accepted that the temporal development of stationary waves is responsible for the occurrence of SSW
(Charlton and Polvani, 2007). Figure 10 shows the latitudinal structure of the amplitude of the stationary wave 1 of the
SSWs considered in this study expect for 2009 which was a
www.ann-geophys.net/31/1397/2013/
splitting event (see Manney et al., 2009), and therefore the
dominating stationary wave 2 is presented. The amplitude of
the stationary geopotential height wave from MLS data is
shown as a function of latitude and height. The left column
displays the amplitude 5 days before the central day, the middle one at the central day and the right column 5 days after
the central day.
Five days before every central day considered in this study
an increased stationary wave 1 activity occurs from the pole
to around 45◦ to 50◦ N. Only the maximum amplitude of the
stationary wave 2 in 2009 shows an increased activity from
35◦ to 75◦ N and not at the pole like in the other events. At the
central day, the normal polar dominated SSW in 2006 shows
an increased activity again from pole to mid-latitudes while
in the other three events a clear increased activity down to
30◦ N is observed. Five days after the central day the amplitudes decrease in every event. At this point the warming in
2006 also shows an increased activity in the lower latitude
mesosphere but with a much weaker amplitude compared
to that before and at the central day. The other events basically show the same behaviour as at the central day but with
weaker amplitudes. Only in 2010 a third maximum occurs
Ann. Geophys., 31, 1397–1415, 2013
1408
Fig. 8. Wavelet spectrum of the meridional wind at 85 km at different locations for winter 2008/09 from the respective MF and MR
systems. The black dashed line marks the central day of the SSW
and the white dashed lines at the edge represent the cone of influence.
above the other two in the upper mesosphere/lower thermosphere. Thus transient and stationary PWs show an increased
wave activity between 30◦ and 50◦ N during the displaced
warmings in 2009, 2010 and 2012.
To understand the differences in the stationary wave activity between the events considered here, we examine the
three-dimensional wave activity fluxes for quasi-geostrophic
stationary waves following Plumb (1985, Eq. 7.1). The wave
flux vector F is a three-dimensional vector depending on the
longitude λ, the latitude ϕ and on the height z.
Figure 11a) shows the wave flux activity vectors from
MERRA as a function of longitude and latitude averaged between 25 km and 50 km for a 5 day mean after the correAnn. Geophys., 31, 1397–1415, 2013
V. Matthias et al.: Latitudinal displacement of SSWs
sponding central day of each SSW considered in this study.
The coloured background represents the flux divergence, i.e.
red coloured regions are sources of stationary PW flux and
blue coloured regions are sinks. During the SSW of 2006, the
flux vectors indicate wave 1 structure between 60◦ N and the
pole. Below 60◦ N between 50◦ E and 50◦ W the flux vectors
are equatorward directed but decrease rapidly below 30◦ N.
Around the zero meridian between 60 and 80◦ N occurs a
big source of wave flux with 2 smaller arms between 40 and
60◦ N. Sources and sinks alternate with a light eastward shift
from pole to 20◦ N.
The vortex splitting event 2009 shows a wave 2 structure
symmetric around 60◦ N with an equatorward flux around
100◦ E and 100◦ W. This is also the region where the sources
and sinks alternate equatorward with a light eastward shift,
but there is an additionally longitudinal variation.
The flux vector structures of the events in 2010 and 2012
are very similar whereas the intensity is stronger in 2010
than in 2012. Both events show two stripe pattern of equatorward movement. The weaker one occurs between 40◦ N and
80◦ N and between 100◦ W and the zero meridian. The second stronger stripe pattern is shifted parallel to the first one
and occurs between 70◦ N and 30◦ N and between 50◦ W and
90◦ E. These stripe pattern are also visible in the alternation
of the sources and sinks which goes as before equatorward
with a light eastward shift and are stronger in 2010 than in
2012 too. In comparison to 2006 these equatorward fluxes are
stronger and reach from polar to subtropical latitudes which
is in contrast to the downward flux in 2006 which starts at
60◦ N.
Figure 11b shows the zonally averaged wave flux activity
vectors as a function of latitude and height for a 30 day mean
before the corresponding central day of 2006, 2009, 2010 and
2012. All events studied here show strictly poleward flux until 40◦ N which passes into a strictly upward flux around the
pole. Only the vortex splitting event in 2009 shows a poleward flux almost until 80◦ N and therefore passes much later
into the upward flux.
We summarise that the differences in the zonal wind and
temperature behaviour between the normal polar dominated
SSW in 2006 and the southward displaced SSWs in 2009,
2010 and 2012 are accompanied by the unusual increased
PW activity (stationary and transient) at latitudes between
30◦ N and 50◦ N and the changed stratospheric dynamics
during the three exceptional SSWs.
4
Discussion
In the following the impact of the latitudinal behaviour of
PWs on the latitudinal variability of SSW effects like the
zonal wind reversal and temperature changes will be discussed.
This study shows 3 exceptional SSW events with respect to
their latitudinal structure within a short period (2009–2012).
www.ann-geophys.net/31/1397/2013/
V. Matthias et al.: Latitudinal displacement of SSWs
1409
Fig. 9. Amplitude of the 6-day (period: 5–7 d), 10-day (period: 8–12 d) and 16-day (period: 12–20 d) wave from MLS geopotential. Amplitude
is calculated by the maximum of a sliding window of 24/40/70 days shifted by one day between day 335 of the previous year and day 60 of
the actual year. The dashed line at 60◦ N is used for the help of orientation.
That these three events are exceptions shows the composite analysis of 39 major and minor warmings between 1958
and 2001 from NCEP-NCAR reanalysis data of Limpasuvan et al. (2004). They found no evidence for a continuous
westward wind band between the pole and 20◦ N in the average behaviour of a warming in the lower stratosphere up to
32 km. This polar activity of SSW effects without a continuous westward wind band from the pole to lower latitudes is
also observed in case studies for individual events, as for example in Hoffmann et al. (2007) and especially in Mukhtarov
et al. (2007). Therefore, the events studied here are exceptional even if they occur in a temporally short interval.
Our observations of a continuous westward wind band
from the pole to the subtropics and an occasionally stronger
westward wind at mid- than at polar latitudes are corroborated by case studies of the SSW in 2010 by Chen et al.
(2012) using MLS gradient winds and by Stober et al. (2012)
using local radar measurements at 55◦ N. However, composite analysis of Charlton and Polvani (2007) with NCEPNCAR and ECMWF re-analysis data show no continuous
westward wind band between 20◦ N and the pole neither during vortex displacement events nor during splitting events.
Only the vortex splitting events show an increased PW acwww.ann-geophys.net/31/1397/2013/
tivity and westward wind down to 30◦ N which is consistent with the 2009 splitting event considered in this study. A
possible explanation for the continuous westward wind band
during the events in 2009, 2010 and 2012 is given in Fig. 11a.
The stationary wave flux vectors, where the continuous wind
band occurs, show an equatorward movement from polar to
subtropical latitudes in 2009, 2010 and 2012 during the five
days after the central day, but in 2006 only an equatorward
movement from mid- to subtropical latitudes. Therefore, it is
possible that the reversed westward wind from polar latitudes
is carried down from the stationary wave flux to 20◦ N in the
three displaced SSWs.
This equatorward movement of the stationary wave flux is
also considered as responsible for the southward spread of
the warming at 20 km in 2009, 2010 and 2012 in Figs. 5a and
5b. Additionally, we observed another unusual latitudinal effect during the SSW in 2012. The warming occurs in 2012
at 20 km between 45◦ N and 75◦ N but not at the pole like in
the other events considered here (see Figs. 5a and 5b). Therefore Fig. 12 shows the projection of the temperature from
MERRA at 20 km in the Northern Hemisphere five days after
the central day of the respective SSWs in 2006, 2009, 2010
and 2012. The cold part of the global temperature pattern
Ann. Geophys., 31, 1397–1415, 2013
1410
V. Matthias et al.: Latitudinal displacement of SSWs
Fig. 10. Amplitude of the respective dominating stationary wave of SSW events of 2006, 2009, 2010 and 2012 as a function of latitude and
height 5 days before, at the central day and 5 days afterwards. Geopotential height data are obtained from MLS. The vertical dashed line at
60◦ N is used for the help of orientation.
five days after the central day of the events in 2006, 2009
and 2010 is located between 45◦ W and 90◦ E but not on the
pole. In contrast to this, the cold part of the temperature pattern in 2012 also lies between 45◦ W and 90◦ E but is rotated
by 90◦ about the longitudinal axis and is located partly on the
pole. This rotation occurs only in the lower stratosphere. At
upper heights such rotations are not observable. The reason
for this unusual rotation of the cold temperature pattern after the SSW in the lower stratosphere in 2012 is unclear and
should be further investigated.
A distinctive cooler upper stratosphere occurs around
40 km between 40◦ and 60◦ N before the SSWs of 2009,
2010 and 2012 compared to that in 2006. Similar observations have been made by Orsolini et al. (2010). The aim of
their paper was to show that mesospheric H2 O and temperature measurements from the Odin satellite allow to distinguish between the formation of an elevated stratopause and
the descent of dry mesospheric air into the polar stratosphere.
Nevertheless, Orsolini et al. (2010) show among other things
the temperature from Odin between July 2001 and July 2009
as a function of latitude at 1 hPa. During the winter months
the cold polar and mid-latitudinal stratospheric temperatures
vary from year to year with respect to their latitudinal ex-
Ann. Geophys., 31, 1397–1415, 2013
tension. During some years, the cold temperatures reach as
far south as 30◦ N as in 2009, but during other years, for
example in 2006, cold temperatures are present only up to
50◦ N. Note that there is an altitudinal difference between
our study and Orsolini et al. (2010) which explains the latitudinal differences in the cold temperatures. Comparisons
of the stationary wave fluxes in Fig. 11b) between 2006 and
the other three events show no significant differences which
could explain the cold temperatures between 40◦ and 60◦ N
before central days of the SSWs of 2009, 2010 and 2012.
Since PWs draw their energy from the temperature difference between the cold polar and the warm lower latitudes,
we ask the question: Do the cold stratospheric temperatures
at mid-low latitudes occur due to the southward extended PW
activity or is the PW activity increased at lower latitudes due
to the cooler temperatures at mid-low latitudes? The answer
is much more complicated than the question suggests. PWs
are influenced by tropical phenomena like the QBO as for example discussed in Chen and Huang (1999). Labitzke (2004)
even shows a statistical relation of SSW on the QBO and on
the solar cycle, but the results for the SSW of 2009 did not
fit with this statistical relation (Labitzke and Kunze, 2009).
In our case, the normal SSW in 2006 lies on the westerly
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V. Matthias et al.: Latitudinal displacement of SSWs
1411
Fig. 11. Stationary wave activity flux vectors following Plumb (1985) of each SSW considered in this study (a) of a five day mean after the
central day averaged over the height range between 25 and 50 km, where the coloured background represents the flux divergence (red: source,
blue: sink); top with flux divergence, bottom without (better view on the arrows) and (b) of a 30 day mean before each corresponding central
day zonally averaged and scaled by (Fϕ , Fz ) → (p/p0 )(−1/2) (Fϕ , 100 · Fz ). Fluxes had been calculated using MERRA data provided by
NASA GMAO.
phase of the QBO while the other displaced SSWs lie on
the easterly phase in the hight region between 25 and 50 km
where the continuous westward wind band occurs (see http:
//www.geo.fu-berlin.de/en/met/ag/strat/produkte/qbo). Naito
and Yoden (2006) studied the PW activity before and after
SSWs depending on the phase of the QBO by a numerical
model. They found that the dominant zone of the upward and
equatorward Eliassen–Palm flux in the lower stratosphere is
shifted southward in the westerly phase and poleward in the
easterly phase of the QBO during a SSW. This result agrees
to our observations of the Plumb flux averaged between 25
and 50 km (see Fig. 11), where we found an equatorward
flux from pole to lower latitudes during the displaced SSW
events. So there might be a connection between the QBO
phase and the latitudinal displacement of SSWs but needs
further investigation to approve this result. However, the reason for the cold stratospheric temperatures before the exceptional warmings at mid- and lower latitudes down to 30◦ N,
www.ann-geophys.net/31/1397/2013/
which are connected with an increased PW activity between
30◦ and 50◦ N, is still unclear.
The previously closer investigated mesospheric warming
between 40◦ and 60◦ N around the central day (see Figs. 5a
and 5b) occurs during all vortex displacement events. From
local meteor radar temperatures and global temperature maps
we found distinctive longitudinal variations in the temperature at mid-latitudes depending on their location relative to
the disturbed polar vortex and therefore on the phasing of
PWs. Comparisons of the mesospheric temperature structure
at mid-latitudes (Figs. 5a and 5b) with the stationary wave
occurrence in Fig. 10 show an increased stationary wave 1
activity in the mid-latitudinal mesosphere during 2006 and
2012 around the warming and 2010 afterwards. This indicates that the increased mesospheric stationary wave activity
at mid-latitudes is responsible for the mid-latitudinal mesospheric warming.
Ann. Geophys., 31, 1397–1415, 2013
1412
Fig. 12. Projection of the MERRA temperature data at 20 km over
the Northern Hemisphere 5 days after the central day of the respective SSW.
Besides the latitudinal differences between the 3 displaced
events and 2006, we found an equatorward movement of
the warming in the stratosphere at 40 km during all considered events. Besides mesospheric variability in the tropics which are correlative in time with the SSW observed
at higher latitudes, Shepherd et al. (2007) found a warming
at stratospheric altitudes too. They explain the mesospheric
variability with an increased PW activity in the mesosphere.
Thus, we speculate that the stratospheric tropical warming
observed here after the SSW occurs due to the enhanced PW
activity not only in the mesosphere but also in the stratosphere at lower latitudes.
We summarise that the differences in the zonal wind and
temperature behaviour between the normal polar dominated
SSW in 2006 and the southward displaced SSWs in 2009,
2010 and 2012 are connected to the increased PW activity
(stationary and transient) between 30◦ N and 50◦ N and the
changed stratospheric dynamics during the three displaced
SSWs.
During this study, we could not find a reason for the southward extended PW activity during the displaced SSWs of
2009, 2010 and 2012. Our hypothesis is that during the generation of PWs in the troposphere a large scale disturbance is
responsible for the southward extension of the PW activity.
5
Conclusions
MF- and meteor radar winds at selected locations, global
satellite measurements and assimilated model data have been
used to investigate the impact of PWs on the latitudinal displacement of SSWs. A comparison was shown of the latitudinal structure of the zonal wind, temperature, PW activity
Ann. Geophys., 31, 1397–1415, 2013
V. Matthias et al.: Latitudinal displacement of SSWs
and stationary wave flux between the normal polar dominated SSW in 2006 and the southward displaced SSWs in
2009, 2010 and 2012. The continuous westward wind band
between the pole and 20◦ N as well as the southward spread
warming in the stratosphere during the three exceptional
warmings occur due to the equatorward stationary wave flux
from polar latitudes to 30◦ N.
The cold stratospheric temperatures at mid-latitudes before the displaced warmings are not connected with a
changed wave flux before the warming. In general, during
the displaced events in 2009, 2010 and 2012 an increased
PW wave activity (transient and stationary) between 30◦ N
and 50◦ N compared to that in 2006 is observed.
We also found a hint for a connection of the latitudinal
displacement of SSWs and the QBO phase.
An effect that occurs beside these differences is a midlatitudinal warming in the mesosphere around the SSWs during all displacement events considered in this study. This is
caused by an increased stationary wave 1 activity between
30◦ N and 50◦ N in the mesosphere around the warmings.
In addition, during all events considered in this study an
equatorward movement of the stratospheric warming and a
downward progression of the zonal wind and temperature
changes is observed.
This study does not only reveal latitudinal differences but
also longitudinal variability in both, wind and temperature
observations. These longitudinal differences seem to arise
from the phasing of stationary and transient waves. At this
point further investigations are needed to fit local measurements and zonal mean observations better together with circulation models into the global context. This issue will be
considered in a future work.
Acknowledgements. We thank the Jet Propulsion Laboratory/NASA for providing access to the Aura/MLS level 2.2
retrieval products. We acknowledge the Global Modelling and
Assimilation Office (GMAO) and the GES DISC for the dissemination of MERRA. We wish to thank Ralph Latteck, Werner Singer
and Dieter Keuer for their permanent support using the Meteor and
MF-radars at Andenes and Juliusruh. We thank also Gerd Baumgarten for providing MERRA data and Christoph Zülicke and the
ISSI team 217 for their helpful discussions. Last but not least, we
thank Timo Viehl for his grammatical corrections.
Topical Editor C. Jacobi thanks two anonymous referees for
their help in evaluating this paper.
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