GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L14102, doi:10.1029/2004GL020282, 2004
The large-scale dynamics of the mesosphere--lower thermosphere
during the Southern Hemisphere stratospheric warming of 2002
Andrew J. Dowdy,1 Robert A. Vincent,1 Damian J. Murphy,2 Masaki Tsutsumi,3
Dennis M. Riggin,4 and Martin J. Jarvis5
Received 17 April 2004; revised 3 June 2004; accepted 10 June 2004; published 17 July 2004.
[ 1 ] An unprecedented major stratospheric warming
occurred in the Antarctic winter of 2002. We present
measurements of winds in the mesosphere-lower
thermosphere (MLT) made with MF radars located at Davis
(69S, 78E), Syowa (69S, 40E) and Rothera (68S, 68W).
The mesospheric wind field in 2002 was found to be
considerably different to other years due to increased
planetary wave activity throughout the winter. Zonal winds
were weaker than usual during the 2002 winter and also
during the transition to the summer circulation. The MLT
zonal winds showed a reversal about one week earlier than the
stratospheric reversal associated with the warming.
Meridional winds showed oscillations consistent with the
presence of traveling wave-1 planetary waves with periods
14 days. The results are compared with similar mesospheric
observations made during northern hemisphere stratospheric
warmings. Some similarities between hemispheres were
found, notably that the reversal in the mesospheric winds
I NDEX T ERMS : 3332
precedes the warming events.
Meteorology and Atmospheric Dynamics: Mesospheric dynamics;
3334 Meteorology and Atmospheric Dynamics: Middle atmosphere
dynamics (0341, 0342); 3384 Meteorology and Atmospheric
Dynamics: Waves and tides. Citation: Dowdy, A. J., R. A.
Vincent, D. J. Murphy, M. Tsutsumi, D. M. Riggin, and M. J. Jarvis
(2004), The large-scale dynamics of the mesosphere – lower
thermosphere during the Southern Hemisphere stratospheric
warming of 2002, Geophys. Res. Lett., 31, L14102, doi:10.1029/
2004GL020282.
1. Introduction
[2 ] Sudden stratospheric warmings result from the
propagation of planetary waves from the troposphere into
the stratosphere where they interact with the mean flow
[Matsuno, 1971; Andrews et al., 1987]. Following the
classification system of Labitzke and Naujokat [2000],
a major mid-winter warming must have a zonal-mean
temperature increase poleward from 60 degrees latitude at
10 hPa or below, with an associated circulation reversal and
hence a breakdown or splitting of the polar vortex. The
warming is classed as minor if there is a temperature
increase of at least 25K per week at any stratospheric level
in any area of the wintertime hemisphere, but the mean
zonal winds at the 10 hPa level do not reverse.
1
Department of Physics, University of Adelaide, Adelaide, Australia.
Australian Antarctic Division, Kingston, Tasmania, Australia.
National Institute of Polar Research, Tokyo, Japan.
4
Colorado Research Associates, Boulder, Colorado, USA.
5
British Antarctic Survey, Cambridge, UK.
2
3
Copyright 2004 by the American Geophysical Union.
0094-8276/04/2004GL020282
[3] Many major and minor warmings have been observed
in the northern hemisphere [Schoeberl, 1978; Hoffmann
et al., 2002]. In the southern hemisphere many minor
warmings have been observed [Shiotani et al., 1993], but
the first major warming ever observed in the southern
hemisphere occurred during the 2002 Antarctic winter
[Baldwin et al., 2003]. At 60S the winds at 10 hPa reversed
on 26 September, before returning to a weak winter-like
flow on about 15 October [Baldwin et al., 2003].
[4] Planetary waves in the southern hemisphere generally
have smaller amplitudes than in the northern hemisphere
where orographic and thermal forcing is stronger [Andrews
et al., 1987]. This causes the Antarctic winter vortex to be
stronger than in the Arctic and this is widely accepted as the
reason why major stratospheric warmings had never been
observed in the southern hemisphere prior to 2002.
[5] The state of the Antarctic stratosphere during winter
2002 was unusual in that there was a series of large
planetary wave events that weakened and warmed the polar
vortex. The largest of these planetary wave events occurred
during late September, producing a major stratospheric
warming and the splitting of the Antarctic ozone hole
[Baldwin et al., 2003].
[6] The mesosphere is known to be sensitive to changes
in the lower atmosphere [Labitzke, 1972]. Planetary wave
signatures have been observed previously in the Antarctic
MLT [Phillips and Vincent, 1989; Lawrence and Jarvis,
2001; Kamalabadi et al., 1997], and are commonly thought
to originate at lower altitudes. Given the unusual state of the
Antarctic stratosphere during the winter of 2002, it may be
expected that the Antarctic mesospheric wind field would
also show unusual behavior. MF radar observations made at
Davis (69S, 78E), Syowa (69S, 40E) and Rothera
(68S, 68W) allow us to explore the dynamical response
of the MLT to large-amplitude planetary waves and the
associated major warming event in the stratosphere.
2. Observations
2.1. Data Collection and Analysis
[7] The spaced-antenna MF radars located at Davis,
Syowa, and Rothera are very similar in construction and
operate at frequencies of 1.94, 2.4 and 1.98 MHz, respectively. Transmission uses pulse widths of 30 ms, which
correspond to range resolutions of about 4 km. Data are
taken every 2 minutes and oversampled at 2 km height
intervals.
[8] The raw data were analyzed using the full correlation
analysis method to derive the horizontal winds [Briggs,
1984]. The wind data were then filtered, with points lying
more than 2 standard deviations from the mean value
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excluded in order to remove outliers. This may have caused
small amounts of genuine wind data to be rejected, but
contamination by external RF interference or poor signal-tonoise was considerably reduced. Daily averaged zonal (EW)
and meridional (NS) wind fields were then calculated from
the filtered data at heights from 66 km to 100 km. Data were
available for the years 1994 to 2003 inclusive at Davis,
1999 to 2003 at Syowa, and from 1 July 2002 at Rothera.
2.2. Mean Wind Structure During 2002
[9] Figures 1 and 2 show, respectively, daily average
zonal and meridional winds from the start of July until the
end of November 2002 at Davis, Syowa and Rothera. It is
evident that the horizontal wind fields at these three
locations show strong similarities. Given the large difference in longitude between Rothera and Davis and Syowa
this means that many features are hemispheric in scale.
[10] Eastward zonal winds normally dominate the winter
MLT at these stations. However, many brief periods of
westward zonal winds occurred during 2002 (Figure 1).
The largest of these zonal wind reversals occurred during
the major stratospheric warming event in late September.
Figure 3 shows plots of the zonal winds at 10 hPa (32 km)
averaged around a latitude of 68S and the zonal winds
observed by the three radars averaged for a height of 80 km.
The stratospheric winds were derived from the UK Meteorological Office (UKMO) stratospheric assimilation. At all
three radar stations the zonal wind reversal occurred almost
simultaneously at all heights starting around day 259
(16 September), which is about 4 days earlier than the
winds started to reverse at 10 hPa (Figure 3). The winds in
the mesosphere became westward (easterly) on day 261
(18 September), about 6 days earlier than the 10 hPa winds
turned westward. Following the warming event the MLT
zonal winds returned to the eastward direction typical of
Figure 2. As for Figure 1, but for daily averaged
meridional winds. Positive (northward) and negative
(southward) winds are indicated by light and dark shading,
respectively.
winter for about a week, before the final mesospheric zonal
wind reversal occurred in early October, with westward
winds then persisting into summer (Figure 1).
[11] Planetary wave activity is apparent in the mesospheric
EW winds, but is especially apparent in the NS winds
(Figure 2). Oscillations with periods of about 14 days can
be seen from about August to October at the three stations.
Qualitatively the oscillations observed at Davis and Syowa
appear to be similar in phase, while an out-of-phase behavior
occurs at Rothera, suggesting a zonal wave number 1 type
structure and an associated flow over the pole. A more
quantitative analysis is presented below.
2.3. Comparison With Other Years
[12] The long data sets available for the Davis and Syowa
MF radars allow the 2002 Antarctic mesospheric winds to
be compared with long-term averages. Time series of daily
averaged zonal winds observed at 80 km at Davis and
Figure 1. Daily averaged zonal winds for Davis, Syowa
and Rothera shown from 1 July to 30 November 2002.
Positive (eastward) and negative (westward) winds are
indicated by light and dark shading, respectively. White
areas indicate where no data were available.
Figure 3. Zonal-mean zonal winds at 10 hPa for a latitude
of 68S (dashed) and zonal winds at 80 km averaged for
Davis, Syowa and Rothera (solid).
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Syowa are shown in Figure 4. The data were smoothed with
5-day running means. The dashed lines indicate the average
values for all years, excluding 2002.
[13] During 2002 the zonal winds at 80 km for both
stations were considerably different to the values observed
in other years. Prior to about mid-April the winds for 2002
followed the summer to winter trend evident for other years.
During the period from mid April to the end of September,
eastward winds were on average 10 ms 1 weaker than the
long term values. This behavior was observed at other
heights between 70 and 100 km.
[14] The winter to summer zonal wind transition normally
proceeds with little interannual variability, (Figure 4). But in
2002 the EW component was remarkably different from the
long-term average, being weaker (more eastward) by 10–
20 ms 1 during November. It was not until the start of
December that the zonal winds were comparable in strength
to other years.
[15] The seasonal behavior of the NS wind component
observed in 2002 (not shown) at Davis is not markedly
different to that in other years. Large wavelike variations
start at the end of March and diminish in amplitude going
from spring to summer. In general, the NS winds at Davis
fluctuated around 0 ms 1, presumably due to the passage
of traveling planetary waves. Stationary planetary wave
behavior may be evident at Syowa. In 2002 the NS winds
tended to be equatorward from May through July, whereas in
other years the mean flow was poleward during this period.
2.4. Planetary Waves
[16] Large amplitude oscillations with periods of about
14 days were evident at all three stations from about late
August until early October. A least-squares harmonic fit to
the daily average NS winds at the three stations was made to
estimate wave periods, amplitudes and phases. The optimum fit occurred for a period of 14 ± 1 day at each station.
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Figure 5. Daily average NS winds (solid) averaged over
the 76– 84 km height range plotted as a function of day of
year at Davis (top panel), Syowa (second panel) and
Rothera (third panel). The dotted lines represent leastsquares fitted 14-day oscillations. The bottom panel shows
the phases as a function of longitude, with the error bars
representing the standard deviation of the phase estimates.
The dotted line has a slope of a zonal wave number 1.
[17] Figure 5 shows daily values of the NS winds averaged
over the 76– 84 km height range during a 45-day period from
late August to early October, with a fitted 14-day oscillation
overplotted. Wave amplitudes were 10 ms 1 at Davis and
Syowa, but only 5 – 7 ms 1 at Rothera. Also shown in
Figure 5 are the phases of the three fitted waves plotted with
respect to longitude. The phase slope is consistent with
the 14-day wave being a westward propagating planetary
wave 1. No other wave structure, either eastward or westward
propagating is consistent with the data.
[18] Harmonic analysis for each height show that wave
energies decayed with increasing height. However, there is
some evidence of a phase tilt with height in a sense that
indicates upward propagation of energy, but the implied
vertical wavelength is very large, with values of 200 to
400 km. Comparisons with UKMO assimilation data show
that there is also evidence for linkage with planetary waves
in the stratosphere.
3. Discussion
Figure 4. Daily averaged EW winds at 80 km for Davis and
Syowa. The dark lines represent data for 2002, and the light
lines represent data for other years. The long-term average
values (excluding 2002) are indicated by the dashed lines.
[19] The many unusual features of the southern hemisphere stratosphere in the winter of 2002 were summarized
by Baldwin et al. [2003]. The circulation in the stratosphere
was characterized by a series of planetary-wave events
that weakened the vortex and preconditioned the atmosphere for the major warming in late September. At 20 hPa
(27 km altitude) wavenumber 1 was especially prominent
in August, while wavenumber 2 was larger in July and early
September. Our observations using three MF radars located
at a latitude of 68S show that many of these features were
also evident in the MLT during 2002.
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[20] The occurrence of the zonal wind reversal in the
mesosphere associated with the stratwarm was relatively
consistent between the three stations, although Davis and
Syowa were more similar to each other than they were to
Rothera. This may be due to zonal symmetry being better
over shorter longitudinal distances, or that the polar vortex
was not centered over the pole. A comparison of mesospheric
zonal winds averaged for the three stations with the zonalmean zonal wind derived from the UKMO stratospheric data
at the 10 hPa showed that the reversal event in the mesosphere led that in the stratosphere by several days (Figure 3).
[21] The August/September period was dominated by a
large amplitude wave with a period of about 14 days and a
zonal wavenumber 1 structure. It is apparent from Figure 5
that the wave amplitude was larger at Davis and Syowa than
at Rothera. Davis and Syowa are relatively close in longitude to each other compared with Rothera, so the smaller
amplitude at this latter station may be because the mean
zonal circulation was displaced off the pole. Inspection of
the UKMO data shows that the stratospheric vortex was
indeed displaced toward the South American sector, in the
vicinity of Rothera.
[22] In the northern hemisphere the mesospheric wind
field has been observed to be strongly influenced by
stratospheric circulation disturbances [Hoffmann et al.,
2002]. The southern hemisphere mesospheric response to
the major warming shows similarities to these northern
hemisphere observations. Similarities include mesospheric
zonal wind reversals associated with major stratospheric
warmings, the mesospheric reversals preceding the events in
the stratosphere by one or more days, and observations of
long-period oscillations at mesospheric heights with periods
of about 14 days at times of enhanced stratospheric planetary
wave activity. The mesospheric temperature at the South Pole
during the 2002 Antarctic winter was reported by Hernandez
[2003] to be cooler than usual, in a similar manner to the
mesospheric cooling often observed during northern hemisphere stratospheric warmings [e.g., Walterscheid et al.,
2000]. However, long-term OH temperature measurements
made at 87 km at Davis show that the winter of 2002
was warmer than the long-term average values in winter
(G. Burns, personal communication, 2004).
[23] The mesospheric response to stratospheric warmings
depends on a number of factors that are not yet fully
understood. Changes in momentum and energy budgets will
be caused by changes in both planetary wave and gravity
wave interactions with the mean flow. Changes in the
stratospheric circulation produce changes in gravity wave
filtering and hence in the gravity wave fluxes reaching the
mesosphere. Gravity wave momentum deposition drives the
pole-to-pole meridional circulation that in turn drives mesospheric temperatures away from radiative equilibrium at the
solstices, keeping the winter MLT warmer and the summer
MLT colder than would otherwise be the case [Holton, 1983].
Any reduction in the strength of eastward flow in the
stratosphere allows an increased flux of eastward propagating
gravity waves, which will produce an increased eastward
forcing in the MLT. Overall, there are complex changes in the
residual circulation in the high latitude MLT that will
influence the composition of important minor constituents,
such as atomic oxygen [Liu and Roble, 2002] and the effects
are likely to be longitudinally dependent due to planetary
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wave modulation of the gravity waves fluxes [Dunkerton and
Buchart, 1984]. We are now looking at gravity wave fluxes
measured with the three MF radars used in this study to
examine how the gravity wave fluxes changed between 2002
and other years and the possible impact on the polar MLT.
[24] Acknowledgments. The research at Davis was supported by the
Australian Antarctic Science Advisory Committee grants scheme project
674 and by Australian Research Council grant DP0346394. The research at
Syowa is supported by a Grant-in Aid for Scientific Research (14740287)
from Japan Society for the Promotion of Science (JSPS). MF radar
operations at Syowa were carried out by the 40th – 43rd Japanese Antarctic
Research Expeditions. Support for the Rothera radar comes through NSF
grant OPP-9813629. We thank the British Atmospheric Data Centre for
providing access to the UKMO stratospheric assimilation data.
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A. J. Dowdy and R. A. Vincent, Department of Physics, University of
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M. J. Jarvis, British Antarctic Survey, Madingley Road, Cambridge CB3
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