Atmos. Chem. Phys., 10, 10273–10289, 2010
www.atmos-chem-phys.net/10/10273/2010/
doi:10.5194/acp-10-10273-2010
© Author(s) 2010. CC Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
Dynamics of the Antarctic and Arctic mesosphere and lower
thermosphere – Part 1: Mean winds
D. J. Sandford1 , C. L. Beldon1 , R. E. Hibbins2 , and N. J. Mitchell1
1 Department
2 British
of Electronic and Electrical Engineering, University of Bath, Bath, UK
Antarctic Survey, Cambridge, UK
Received: 15 June 2010 – Published in Atmos. Chem. Phys. Discuss.: 20 July 2010
Revised: 17 October 2010 – Accepted: 19 October 2010 – Published: 4 November 2010
Abstract. Zonal and meridional winds have been measured
in the upper mesosphere and lower thermosphere at polar latitudes using two ground-based meteor radars. One radar is
located at Rothera (68◦ S, 68◦ W) in the Antarctic and has
been operational since February 2005. The second radar is
located at Esrange (68◦ N, 21◦ E) in the Arctic and has been
operational since October 1999. Both radars have produced
relatively continuous measurements. Here we consider measurements made up to the end of 2009. Both radars are of
similar design and at conjugate geographical latitudes, making the results directly comparable and thus allowing investigation of the differences in the mean winds of the Antarctic
and Arctic regions. The data from each radar have been used
to construct climatologies of monthly-mean zonal and meridional winds at heights between 80 and 100 km. Both Antarctic and Arctic data sets reveal seasonally varying zonal and
meridional winds in which the broad pattern repeats from
year to year. In particular, the zonal winds display a strong
shear in summer associated with the upper part of the westward summertime zonal jet. The winds generally reverse
to eastward flow at heights of ∼90 km. The zonal winds
are eastward throughout the rest of the year. The meridional winds are generally equatorward over both sites, although brief episodes of poleward flow are often evident near
the equinoxes and during winter. The strongest equatorward
flows occur at heights of ∼90 km during summer.
There are significant differences between the mean winds
observed in the Antarctic and Arctic. In particular, the westward winds in summer are stronger and occur earlier in the
season in the Antarctic compared with the Arctic. The eastward winds evident above the summertime zonal wind reversal are significantly stronger in the Arctic. The summertime
equatorward flow in the Antarctic is slightly weaker, but occurs over a greater depth than is the case in the Arctic.
Correspondence to: D. J. Sandford
([email protected])
Comparisons of these observations with those of the
URAP and HWM-07 empirical models reveal a number of
significant differences. In particular, the zonal winds observed in the Antarctic during wintertime are significantly
weaker than those of URAP. However, the URAP zonal
winds are a good match to the observations of the Arctic.
Significant differences are evident between the observations
and HWM-07. In particular, the strong wintertime zonal
winds of the Arctic in HWM-07 are not evident in the observations and the summertime zonal winds in HWM-07 are
systematically stronger than observed. The agreement with
meridional winds is generally poor.
There is a significant amount of inter-annual variability in
the observed zonal and meridional winds. Particularly high
variability is observed in the Arctic zonal winds in spring and
is probably associated with stratospheric warmings.
1
Introduction
The dynamics of the mesosphere and lower thermosphere
(MLT) are dominated by atmospheric tides, planetary waves,
and gravity waves, all of which are superimposed on a background flow, which can itself reach large speeds. These
winds, waves and tides are all strongly coupled together.
Gravity waves launched from the lower atmosphere dissipate and deposit their energy and momentum into the middle atmosphere. The momentum deposited influences the
planetary-scale circulation of the stratosphere and largely
drives the planetary-scale circulation of the mesosphere. The
momentum deposition in the mesosphere acts to close the
middle atmosphere zonal jets and actually reverses the direction of the summertime zonal jet, resulting in strong eastward
flows at heights above ∼90 km. The momentum deposition
also acts with the pressure gradient and Coriolis forces to
produce a pole-to-pole meridional circulation. The vertical
winds required by continuity considerations to sustain this
Published by Copernicus Publications on behalf of the European Geosciences Union.
10274
meridional circulation result in an upwelling of air over the
summer polar region and a corresponding downwelling of
air in the winter polar region. The adiabatic cooing and heating resulting from this vertical motion acts to drive the atmosphere into states far removed from those of pure radiative equilibrium and leads to the well-known cold summer
mesopause and the reversed pole-to-pole mesospheric temperature gradient (e.g., McIntyre, 1989; Lieberman, 1999;
Holton et al., 2003). This means that there is a strong coupling between the wave dynamics of the mesosphere and the
planetary scale circulation and temperature structure. Attempts to understand these coupling processes therefore require accurate determination of the background winds of the
mesosphere.
The general circulation and seasonal variability of the
MLT at middle and low latitudes has been investigated by
ground based radars and satellites. These studies have determined the general characteristics of the zonal and meridional
winds of the MLT. In particular, strong zonal wind shears are
evident in summer, in which the westward winds of the top
of the middle atmosphere zonal jet reverse in the mesosphere
to give eastward flows at heights above ∼90 km. Associated
with this zonal wind reversal in summer is a region of strong
equatorward flow in which the meridional winds at heights
near 90 km can reach speeds in excess of 10 m s−1 . These
meridional winds are important in the transport of chemical
species (e.g., Plane et al., 1999; Smith, 2004).
In contrast to the situation at middle and low latitudes,
there have been relatively few studies of MLT region mean
winds at polar latitudes. This is because of the relatively
small number of ground based MLT radars deployed at polar
latitudes. Nevertheless, in the Arctic MF and Meteor radars
have been used to establish a limited number of simple climatologies of the zonal and meridional mean winds. These
include the MF radar studies of Portnyagin et al. (1992,
1993, 2004); Dowdy et al. (2001, 2007a); Hall et al. (2003);
Kishore et al. (2003) and the meteor radar studies of Hocking
(2001); Mitchell et al. (2002); Hall et al. (2003).
Similar to the Arctic there have been few studies made of
the mean winds of the Antarctic MLT region. This is largely
because of the difficulty in operating ground-based radars for
extended intervals in Antarctica. Despite these difficulties,
MF and meteor radars have been used to measure the seasonal variability of MLT region mean winds in the Antarctic
in a limited number of studies (e.g., Portnyagin et al., 1992,
1993; Vincent, 1994; Dowdy et al., 2001, 2007a; Kishore et
al., 2003; Hibbins et al., 2005; Baumgaertner et al., 2005;
Merzlyakov et al., 2009).
It is well known that there are significant differences in
the mean winds and dynamics of the Arctic and Antarctic
stratosphere. For example, the winds of the stratospheric
polar vortex are known to be significantly stronger in the
Antarctic through strong planetary wave activity (Holton et
al., 2003). Another significant difference lies in the gravity wave fluxes of the two polar regions. The mountainous
Atmos. Chem. Phys., 10, 10273–10289, 2010
D. J. Sandford et al.: Part 1: Mean winds
region of the Antarctic peninsula and the southern Andes experience strong prevailing tropospheric westerlies. This generates a localised “hot spot” of intense gravity wave activity
and gravity wave fluxes which has no parallel in the Arctic (Wu and Jiang, 2002; Alexander and Teitelbaum, 2007;
Baumgaertner and McDonald, 2007). Despite there being
significant stratospheric differences, the differences between
the polar MLT regions remain poorly understood.
Those studies that have attempted to quantify the differences between the mean winds of the two MLT regions include: Portnyagin et al. (1993), Dowdy et al. (2001, 2007a),
Kishore et al. (2003). These studies generally revealed significant differences between the Antarctic and Arctic.
The present study presents mean wind climatologies of
the polar MLT region obtained using meteor wind radars
located at Rothera (68◦ S, 68◦ W) in the Antarctic and Esrange (68◦ N, 21◦ E) in the Arctic. Datasets of 5 and 10
years are available from the two instruments, respectively,
allowing a study of the inter-annual variability in the winds.
These two radars are similar in design and are located at conjugate geographical latitudes in each polar region, allowing
a comparison of observations free from biases caused by different techniques or small differences in latitude. The use of
an identical technique is particularly important because there
are known and significant biases between some of the most
common techniques used for measuring MLT-region winds
(e.g., Meteor and MF radar, Manson et al., 2004). A second
companion study, part 2, will report the inter-hemispheric observations of the 12- and 24-h tides, in the MLT region, also
using the same two systems.
In Sect. 2 we present a description of the meteor radar data
analysis and comparisons between meteor distributions from
the two systems used in this study. Section 3 will consider
the monthly-mean winds measured over the height range 80–
100 km and compare these with the results of the URAP
and HWM-07 mean-wind models. In Sect. 4 we compare
the results with those of other studies and review the interhemispheric differences of the observations.
2
Data analysis
The first radar used in this study is located at Rothera Base
(68◦ S, 68◦ W) on the Antarctic Peninsula. This system has
been in continuous operation since its installation in February 2005. The second radar, similar in design to the Rothera
radar, is located at Esrange (68◦ N, 21◦ E), near Kiruna in
northern Sweden. This system has been in continuous operation since October 1999.
Both radars are commercially-produced SKYiMET
meteor-radar systems. Both radars use solid-state transmitters of 6 kW peak power and operate with a duty cycle
of 15%, pulse repetition frequency of 2144 Hz and a radio
frequency of 32.5 MHz. Crossed-element Yagi antennas
are used for both transmitting and receiving. A single,
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Younger, P. T., Astin, I., Sandford, D. J. and Mitchell, N. J.: The sporadic radiant and distribut
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D. J. Sandford et al.: Part 1: Mean winds 2831-2841, 2009.
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AVERAGE NUMBER OF METEORS
AVERAGE NUMBER OF METEORS
3-element antenna, acts as the transmitter and five separate
ROTHERA − MEAN DAILY METEOR COUNT RATE
8000
2-element antennas as receivers. The radars operate in
7000
an all-sky configuration with the radiated power being
6000
largely independent of azimuth and maximum gain at ∼30◦
5000
elevation. Only under-dense echoes are recorded. A detailed
4000
3000
description of the systems and data-processing technique
2000
used is given by Hocking et al. (2001).
1000
Typical meteor distributions will now be presented to com0
0
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pare the difference between the performances of the two sysTIME (Day of Year)
tems. Figure 1a and b presents the average number of me(a)
teor echoes received per day by the radars over the course
ESRANGE − MEAN DAILY METEOR COUNT RATE
of a year. The data from Rothera are from February 2005 to
8000
April 2009. The data from Esrange are from October 1999 to
7000
April 2009. Both plots show the existence of an annual cycle,
6000
superimposed upon which brief episodes of higher meteor
5000
4000
counts are found during meteor showers. Examples of me3000
teor showers can be seen over Esrange in January (the Quad2000
rantids shower) and December (the Geminids shower).
1000
The highest meteor count rates, discounting showers, are
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found during the summer solstice (days 355 over Rothera
TIME (Day of Year)
and days 172 over Esrange). Over both sites the count rates
(b)
minimise during spring. Daily meteor counts vary between
∼3000 and 7000 over Rothera and between ∼2000 and 5000
Fig.
1. Composite-year
dailya)meteor
counts
(a) Rothera
and wind radars
Fig.count
1a-b.rate
Composite
meteor
counts for the
Rothera
andfor
b)the
Esrange
meteor
over Esrange. The yearly-averaged meteor
per day year
(b)
Esrange
meteor
wind
radars
(i.e.,
average
number
of
meteors
over Rothera is 4738 meteors, compared with that seen over
results have been averaged
over 7per
years
from
to 2009.
resultsover
over about 4 ye
detected
day).
The1999
Rothera
resultsThe
haveRothera
been averaged
Esrange which is 3465 meteors. The differences between the
about 4 years from 2005 to 2009. The Esrange results over about
2009. arises from
count rates recorded over Rothera andtoEsrange
7 years from 1999 to 2009.
differences in the levels of radio interference at the two sites,
small differences in the performance of the radars and differences in the strengths of the visible sporadic meteor raEsrange). This behaviour is again a consequence of the visdiants. The annual cycle of count rates is a consequence of
ibility above the horizon of the astronomical sources of spothe visibility above the horizon of the astronomical sources
radic meteor radiants. A more detailed examination of these
of sporadic meteors (radiants) (e.g., Younger et al., 2009).
variations is presented in Younger et al. (2009).
Figure 2a and b presents the normalised diurnal variation
The normalised distribution of meteor echoes with height
of meteor counts in local time for Rothera and Esrange, reover
Rothera and Esrange is presented in Fig. 4a and b. All
spectively. A clear diurnal cycle is apparent. The diurnal cyrecorded
meteors are used. The meteor counts over both sites
cle over the two sites is almost identical. The highest meteor
peak
at
a
height of 90 km and have very similar distribucount rates occur in the early hours of the morning, at about
tions.
These
distributions of meteor echoes determine the
06:00 LT and the lowest count rates occur in the late afterheights
and
times
at which mesospheric winds can be calcunoon at about 18:00 LT. The ratio of maximum to minimum
lated, since sufficient meteors must be present in a particular
count rates over both sites is about 2:1.
height-time interval to allow20
a robust determination of the
An annual cycle in the diurnal distribution of meteor count
winds. A minimum of 5 meteors were used for this deterrates is also observed. Figure 3a and b present monthly normination. Generally, this meant that sufficient meteors were
malised hourly meteor counts between 00:00 and 24:00 LT
available to calculate winds even during the low-count-rate
for Rothera and Esrange, respectively. Data from all years
part of the day in the outermost height gates.
available are used and so the figures present composite years.
Note that the Rothera year is shifted by six months to ease
In routine calculation of mesospheric winds, the individual
comparison with Esrange. The figure thus shows how the dimeteor echoes are sorted into six independent height gates
urnal cycle in meteor count rates varies throughout the year.
covering the ranges 78–83, 83–86, 86–89, 89–92, 92–95,
In particular, the hour of day at which the greatest meteor
and 95–100 km. A least-squares fitting was then used to
count rates occurs changes from ∼07:00–08:00 LT in late
fit to the azimuth and horizontal component of the drift vespring, summer and early autumn (November–March over
locity to determine the zonal and meridional winds within
Rothera and April–September over Esrange) to earlier in the
a particular height gate. This fitting was performed for all
day in the remainder of the year e.g., 03:00–06:00 LT (from
meteors observed in each height gate using a two-hour winApril to October over Rothera and October to March over
dow incremented through the data set in steps of one hour.
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NORMALISED METEOR COUNT
NORMALISED METEOR COUNT
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TIME (LT Hours)
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. 2a-b. Normalised diurnal variation in the meteor counts with respect to local time for the a) Rothera met
0.2
0.1
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ar between 2005 and 2009 and b) Esrange meteor radar between 1999 and 2009.
0
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14
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0
24
2
4
6
(a)
8
10
12
14
TIME (LT Hours)
16
18
20
22
24
(b)
Fig. 2. Normalised diurnal variation in the meteor counts with respect to local time for the (a) Rothera meteor radar between 2005 and 2009
g. 2a-b. Normalised
diurnal
variation
in 2009.
the meteor counts with respect to local time for the a) Rothera met
and (b) Esrange meteor
radar between
1999 and
ar between 2005 and 2009 and b) Esrange meteor radar between 1999 and 2009.
24
1
22
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ESRANGE − DISTRIBUTION OF METEORS IN LOCAL TIME
1
NORMALISED METEOR COUNT
. 3a-b.
ROTHERA − DISTRIBUTION OF METEORS IN LOCAL TIME
24
8
Fig. 3. Normalised diurnal
variation in the meteor counts versus0.3month with8the peak hours of counts marked with the0.3white line, for the (a)
6
6
Rothera
meteor radardiurnal
between
2005
and 2009 andin
(b) the
Esrange
meteor
radar
between versus
1999 and 2009.
Normalised
variation
meteor
counts
month with 0.2the peak hours
0.2
4
4
2
0.1
2
0.1
0
0
0
of cou
rked with the black line, for the a) Rothera meteor radar between 2005 and 2009 and b) Esrange met
0
J
F
M
A
M
J
J
A
S
O
N
D
For the available data, this method lead to ∼7.2% datagaps
3 Results
TIME (Months)
for the
upperand
and 2009.
lower most height gates for Rothera and
ar between
1999
∼10.3% for Esrange. Errors in the resulting two-hour mean
3.1 Observations of mean winds over Rothera
(a)
(b)
winds are typically only a few ms−1 . Because of the uneven
and Esrange
distribution of meteors with height, the height gates are assigned representative centres of 80.8, 84.6, 87.5, 90.4, 93.4
To investigate the behaviour of the background winds over
g. 3a-b. and
Normalised
diurnaltovariation
in the
meteor
counts
versus
monthwinds
with
peakforhours
97.1 km, corresponding
the mean height
recorded
in
the
two sites,
monthly-mean
werethe
calculated
each of
each height gate, calculated using the Esrange data. The
height gate, using individual months of data. The monthlyrked with
for the
a) zonal
Rothera
meteor radar
between
and 2009
and b)between
Esrange
basethe
datablack
product line,
is thus hourly
spaced
and meridional
mean winds
for each2005
year measured
over Rothera
winds in six height gates over each site.
February 2005 and April 2009 are presented in Figs. 5a–
ar between 1999 and 2009.ROTHERA − HEIGHT DISTRIBUTION OF METEORS
ESRANGE
DISTRIBUTION
OF METEORS
e 110
and 6a–e
for− HEIGHT
zonal
and meridional
winds, respectively.
110
In these figures the zero-wind line is marked with a heavy
105
105
dashed contour. Figures 5f and 6f present the standard devi100
100
ations
of the monthly-mean winds measured each year, providing
a simple quantification of inter-annual variability. The
95
95
reader
should bear in mind that the use of monthly-mean
90
90
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A
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(b)
4. Normalised variation of meteor counts with height for the (a) Rothera meteor radar between 2005 and 2009 and (b) Esrange meteor
Fig. 4a-b.Fig.
Normalised
variation
of meteor counts with height for the a) Rothera meteor radar between 2005 a
radar
between 1999 and
2009.
2009 and b) Esrange meteor radar between 1999 and 2009.
winds means that shorter-period fluctuations may be overlooked. Note that in this study, for Arctic results winter will
refer to December–February, spring to March–May, summer
to June–August and autumn to September–November. For
the Antarctic, this is shifted by 6 months, so that winter refers
to June–August, spring to September–November, summer to
December–February and autumn to March–May.
In the observed zonal winds over Rothera, the seasonal
pattern is seen to repeat in general form from year to year.
The most conspicuous features apparent are the strong summertime wind shears in December, January and February.
This shear is the top of the westward summertime mesospheric jet. The strongest westward winds are observed in
early summer (November–December) where the winds become as strong as −50 m s−1 in 2007 at heights near 80 km.
At greater heights and as summer progresses the winds become increasingly eastward ultimately reaching flows as
strong as ∼30 m s−1 at heights near 97 km in late summer
(February–March). In contrast, the wintertime winds are
generally more variable from year to year but are largely eastward at all heights. Maximum values of ∼20 m s−1 are frequently seen in August over Rothera. The zonal wind standard deviations of Fig. 5f reveal that the greatest variability
of the monthly-mean zonal winds occurs at the end of spring
during the build up to the strongest westward winds.
In the meridional winds observed over Rothera, a seasonal
pattern that repeats from year-to-year, is also evident, at least
in summer. The most conspicuous feature is a period of equatorward flow during the summer, which generally occurs over
all heights observed and reaches velocities of up to 14 m s−1 .
However, the meridional winds in summer vary significantly
from year-to-year. There is a generally tendency for the flow
to be equatorward, particularly so in the upper heights observed. However, episodes of poleward flow do occur during the winter, especially at the lower heights observed. The
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greatest variability during winter occurs at the upper heights.
For example meridional winds reach more than 14 m s−1
equatorward above 94 km in July 2005, whereas at the same
height and time in 2006 the winds are actually poleward and
reach −6 m s−1 . In spring and autumn, the meridional flow
is often near zero with a tendency for poleward in autumn
and equatorward in spring. The variability is revealed in the
21meridional winds standard deviations of Fig. 6f, which suggest greatest variability occurs in the lower heights during
summer (December) and the upper heights during the winter
(July).
A similar analysis for Esrange is presented in Figs. 7a–k
and 8a–k, for zonal and meridional winds, respectively. The
data cover the interval of between October 1999 and April
2009. For both zonal and meridional winds, a generally similar pattern is observed, with an annually-repeating seasonal
cycle which looks superficially similar to that seen in the
zonal winds over Rothera. However, a number of significant differences are apparent, which will be discussed below. Figures 7l and 8l present the standard deviations of the
monthly-mean zonal and meridional winds. These latter figures indicate that the greatest level of inter-annual variability
occurs in late winter and spring over Esrange and particularly at heights below ∼88 km. This high level of variability
probably represents inter-annual differences arising from the
mesospheric manifestation of stratospheric warmings.
For the zonal winds, a generally similar pattern to Rothera
is observed, with an annually repeating seasonal cycle which
looks superficially similar to that seen in the zonal winds
over Rothera. Again, strong shears are observed in summer. The strongest westward winds are observed in summer
(June–August) where the winds reach speeds of ∼−30 m s−1
near 80 km. At greater heights the winds become eastward,
reaching flows as strong as ∼35 m s−1 at heights near 96 km
in late summer (August). The wintertime winds are generally
Atmos. Chem. Phys., 10, 10273–10289, 2010
−20
10
84
−20
0
−30
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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
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WINDSPEED
SPEED(ms
(ms ) )
HEIGHT (km)
HEIGHT (km)
98
98
1020
96
20
10
96 10
20
94
10
10
94
20
0
92
92
0
90
10
20
1010
90
10 10
−10
88−10
20
−10
−20
10
88 0 0
−20
−10
86
86
−10
10
84
0
10
84
−10−30−40
82
0
−40
82
10
−30
10
80 −30
−30
−20
20
−30
0
80 Jan −20
Feb Mar
Apr May Jun Jul
Nov Dec
−30
20 Aug Sep Oct−20
Jan Feb Mar Apr MayTIME
Jun (Months)
Jul Aug Sep Oct Nov Dec
TIME (Months)
(d)
(a)
98
−20−40
−10
−30
−40
10
10
−10
−10
−30
80−30
0
20
−50−40
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
−50
−50
50
50
40
40
30
30
20
20
10
10
0
0
−10
−10
−20
−20
−30
−30
−40
−40
−50
−50
STD OF ZONAL MEAN WINDS OVER ROTHERA
ZONAL MEAN WINDS OVER ROTHERA
FOR 2007
6
98
20 10
98
6
420
2
2
96
4
10
10
0
96
4
94
20
10
94
92
92 4
4
4
6
90
10 −10
20
90
10
10
4
10 2
8 6−20
88
2
−10
2 0
4
−30 −10
20 4
88 0
4
86
86
10
10
84
−40
4
−20
2
84
8
2
4
824
6
82
4
10
10
80
−10
4−10−30
−40
80−30Jan Feb
0 Apr May
20 Jun Jul Aug Sep Oct Nov −50
Mar
Dec
Jan Feb Mar Apr MayTIME
Jun(Months)
Jul Aug Sep Oct Nov Dec
TIME (Months)
14
50
12
40
30
10
20
810
60
−10
4
−20
2−30
−40
0
−50
(f)
(c)
STD OF ZONAL MEAN WINDS OVER ROTHERA
10
20 10
0
−20
84
ZONAL MEAN WINDS OVER ROTHERA FOR 2009
50
20
0
D. J. Sandford et al.: Part 1: Mean winds
(e)
(b)
ZONAL MEAN WINDS OVER ROTHERA FOR 2008
−20
−30 −10
(c)
ZONAL MEAN WINDS OVER ROTHERA FOR 2009
10
ZONAL
MEAN WINDS
OVER ROTHERA FOR 2006
20 10
50
50
40
40
30
30
20
20
10
10
0
0
−10
−10
−20
−20
−30
−30
−40
−40
−50
−50
−10
10
(b)
98
20
20
98
0
20
10
96
96
10 20 10
94
20
94
0
10
20
92
92
20
10 0−10
90
10
10
10
90
10
−20 −10
88 −10
10
0
0
88
86
−10
−30
86
10
84
20
−20
84
−20
0
20
82
−10
82
10 0−10
10
10
−30
−40
−40
−20
20
80
−30
80 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Jan Feb Mar Apr May TIME
Jun (Months)
Jul Aug Sep Oct Nov Dec
TIME (Months)
10
10
20
10
82
−40
−30
10
(a)
ZONAL MEAN WINDS OVER ROTHERA FOR 2008
ZONAL MEAN WINDS OVER ROTHERA
FOR 2005
20
20
−10
0
10
84
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
98
−10
HEIGHT
HEIGHT(km)
(km)
10278
10
88
0
−1
WIND
))
WINDSPEED
SPEED(ms
(ms−1
86
−10
−10
30
20
92
WIND SPEED (ms−1)
−10
10
94 10
STANDARD DEVIATION
(ms−1)
WIND SPEED (ms−1)
0
88
30
10
10
50
6
98
14
6
4
STANDARD DEVIATION (ms−1)
0
10
0
20
90
HEIGHT (km)
10
10
92
WIND SPEED (ms−1)
10
90
20
10
94
−1
20
HEIGHT (km)
HEIGHT (km)
30
0
10
92
WIND SPEED (ms )
20
20
94
Fig. 5a-f. (a)-(e) Monthly-mean zonal winds measured by the Rothera meteor radar for individual years between
20
20
92
20
10
20
10
94
HEIGHT (km)
HEIGHT (km)
94
30
WIND SPEED (ms−1)
10
10
96
40
10
30
20
20
92
10
0
96
2
4
2
92
10
4
2005 and 2009. (f) The standard deviation of all years of monthly-mean zonal winds.
90
10
88 −10
10
10 0−10
−20 −10
10
0
86
−30
84
20
−30
10 0−10
10
−10
−20
−20
20
82
0
−30
−40
−40
−20
20
80
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
90
20 10
10
10
88−10 0
86
−10
−20
20
0
−10
−20
84
10
−30
−10−30−40
0
−40
−40
82
−50
80 −30
−30
−20
20
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
10
(d)
12
4
94
HEIGHT (km)
40
WIND SPEED (ms−1)
0
20
10
96
90
4
4
88
4
4
86
8
6
2
2
2
4
8 6
6
4
4
10
84
−40
824
−50
80
4
2
8
2
4
2
6
4
0
4
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
(e)
(f)
Fig. 5. (a–e) Monthly-mean zonal winds measured by the Rothera meteor radar for individual years between 2005 and 2009. (f) The standard
Fig. 5a-f.
(a)-(e)
zonal
deviation
of allMonthly-mean
years of monthly-mean
zonal winds
winds. measured by the Rothera meteor radar for individual years between
2005 and 2009. (f) The standard deviation of all years of monthly-mean zonal winds.
0
8
8
4
0
−4
0
86
0
4
4
−8
−4
84
8 12
82
0
−8
−12
80
−12
4
90
88
12
0
8
0
86
0
8
0
0
−4
4 0
8 12
8
4
4
4
8
0
80 12
−4
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
20
20
16
16
12
12
8
84
−4
−8
−8
−12
04
−4
4
8 12
8280
−4
0
−8Jun Jul Aug 0Sep Oct Nov Dec
80 Jan Feb Mar Apr May
−12
Jan Feb Mar Apr May TIME
Jun (Months)
Jul Aug Sep Oct Nov Dec
TIME (Months)
4
0
98
98
96
96
94
−12
−12
−16
−16
−20
94
92
92
90
8
0
−20
80
−4
4
4
0
8
90
88
12 8
88
86
86
84
84
82
4
4
0
4
0
4 0 4 8
4
0
0
40
0−4
4
8
0
12
8
84
8
8
0
4
4
4
−4
0
8
0
0
4
8
4
82
80
−8
4 0
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
80 12
−4
Jan Feb Mar Apr MayTIME
Jun (Months)
Jul Aug Sep Oct Nov Dec
TIME (Months)
−20
−4
0
98
16
96
0
−4
−8
−4
−4
4
−4
−8
−12
−16
0
0
−20
−8 −4
−4
−8
−8
−12
−12
−16
−16
−20
STD OF MERIDIONAL MEAN WINDS OVER ROTHERA
8 4
MERIDIONAL MEAN WINDS OVER ROTHERA FOR 2007
98
98
96
96
94
−4
8
0
4
94
92
92
90
4 8
612
12
0
4
−4
48
0
2
90
88
88
86
8 4
4
0
0
4
8
86
84
84
82 2
2
4
4
−4
4
−4
8
8
4
60
12
8
0
2
4
2
4
4
2
4
−4
82
0
80
0
Sep Oct Nov Dec
−8 Jun Jul Aug −8
80 Jan Feb Mar Apr May
−4
Jan Feb Mar Apr MayTIME
Jun(Months)
Jul Aug Sep Oct Nov Dec
TIME (Months)
−20
14
20
12
16
10
12
2
2
4
2
4−4
−8
2
−12
0−16
−20
(f)
(c)
MERIDIONAL MEAN WINDS OVER ROTHERA FOR 2009
20
8
0
(e)
(b)
MERIDIONAL MEAN WINDS OVER ROTHERA FOR 2008
8
4
4
0
4
84
82
8 4
0
4
8
86
−16
20
20
16
16
12
4
12
0
12
12
4
90
20
16
48
92
88
12
0
−4
(c)
0
−8−4
0
4 8
12
0
4
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
MERIDIONAL MEAN WINDS OVER ROTHERA FOR 2009
4
MERIDIONAL MEAN0 WINDS OVER ROTHERA
FOR 2006
(d)
(a)
9812
−8
−20
40
0−4
0
0
8684
4
8482
−4
4
82
HEIGHT
(km)
HEIGHT
(km)
4
4
8
4
−16
−1
WIND
SPEED
(ms(ms
) −1)
WIND
SPEED
HEIGHT (km)
HEIGHT (km)
0
4
−4
0
9088
8886
120
0
−4
0
−4
8
94
(b)
MERIDIONAL MEAN WINDS OVER ROTHERA FOR 2008
0
9812 MERIDIONAL −4
MEAN WINDS OVER ROTHERA FOR
2005
4
12 4
8
9896
−8
4
9694
4
4
8
(a)
8
4
0
84
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
9492
9290
8
0
92
96
12
8
8
98
16
HEIGHT
(km)
HEIGHT
(km)
88
4
4
94
20
−1 −1
WIND
SPEED
(ms(ms
) )
WIND
SPEED
90
8
4
WIND SPEED (ms−1)
8
8
0
−4
MERIDIONAL MEAN WINDS OVER ROTHERA FOR 2007
0
−8−4
STANDARD DEVIATION
(ms−1)
WIND SPEED (ms−1)
12
4
92
96
HEIGHT (km)
12
98
16
HEIGHT (km)
−4
MERIDIONAL MEAN WINDS OVER ROTHERA FOR 2006
20
−1
0
WIND SPEED (ms )
4
94
HEIGHT (km)
12 4
8
96
WIND SPEED (ms−1)
MERIDIONAL MEAN WINDS OVER ROTHERA FOR 2005
98
4
STD OF MERIDIONAL MEAN WINDS OVER ROTHERA
20
98
16
96
8 4
14
8
12
94
4
92
8
0
6
12
4
94
92
2
2
between 2005 and 2009. (f) The standard deviation of all years of monthly-mean meridional winds.
8
90
0
4
−4
4
8
12
0
88
0
4
86
84
82
80
4
−4
4
−4
−8
0
−12
4 0
4
−4
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
90
88
4
4
82
−20
80
4
4 8
0
−4
4
84
−16
4
4
0
86
Atmos. Chem. Phys., 10, 10273–10289, 2010
(d)
8
0
0
−8
−12
−4
0
4
4
−8
0
8
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
(e)
2
90
2
4
4
2
88
86
2
84
4
10
8
6
4
2
−16
82 2
−20
80
4
2
4
2
STANDARD DEVIATION (ms−1)
0
4
12
HEIGHT (km)
0
92
0
0
WIND SPEED (ms−1)
HEIGHT (km)
94
12
HEIGHT (km)
4
−8
96
WIND SPEED (ms−1)
Fig. 6. (a–e) Monthly-mean meridional winds measured by the Rothera meteor radar for individual years between 2005 and 2009. (f) The
Fig. 6a-f.
(a)-(e) Monthly-mean meridional winds measured by the Rothera meteor radar for individual years
standard deviation of all years of monthly-mean meridional winds.
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
www.atmos-chem-phys.net/10/10273/2010/
(f)
D. J. Sandford et al.: Part 1: Mean winds
10279
10
90
0
10
88
−10
86
20
−20
20
90
10
10
86
82
−40
82
−50
80
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
−30
−10 −30
−20
10
−10
10
−10
−20
10
84
−30
10
20
10
80
0
−10 0
−20
10
0
10
86
−40
82
−50
80
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
−10
−20
−10
−30 −20
0
HEIGHT (km)
92
88
10
−10
−10
−10
20
98
40
96
10
10
0
10
0
−10
86 2020
−20
84
82
50
0
10
80
−20
0
30
88
20
10
10
0
−20
−30
−20−10
0
50
98
40
96
−30
0
2010 −10
10
0
−10
−10
86
−20
10
40
30
20
10
−10
−20
0
−30
−10
98
40
96
94
30
10 10
−1
−50
80
(j)
0
−10
−20
20
84
82
10
20
86
−40
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
20
88
30
−30
30
10
40
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
(k)
−50
0
(i)
50
10
−40
10
STD OF ZONAL MEAN WINDS OVER ESRANGE
96
9010
0
−10
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
98
0
10
0
10
84
40
10
10
20
86
50
0
50
0
88
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
10
10
90
80
10
−50
10
92
−50
10
−20
92
−40
10
−10 0
−20
94
80
−30
10
82
−20
ZONAL MEAN WINDS OVER ESRANGE FOR 2007
−50
−20
84
−10
10
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
82
−10
−20
86
80
10
10
−10
80
−40
HEIGHT (km)
20
0 −10
−50
82
20
92
88
84
−40
30
0
86
−30
WIND SPEED (ms )
HEIGHT (km)
10
40
90
10
10
0
10
−10
ZONAL MEAN WINDS OVER ESRANGE FOR 2009
30
10
94
20
10
10
(h)
30
20
−100
0 10
0
90
(g)
0 −10
30
20
0
0
88
82
20
ZONAL MEAN WINDS OVER ESRANGE FOR 2008
96
40
0
90
−40
30
92
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
98
20
20
50
92
84
−30
−20
20
10
10
30
(f)
94
HEIGHT (km)
10
−1
30
0
20
10 0
10 20
0
(e)
30
10
−30
ZONAL MEAN WINDS OVER ESRANGE FOR 2006
94
90
−20
0
94
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
WIND SPEED (ms )
−10 10
−10
−10
−30
(d)
96
0
−20
−10
ZONAL MEAN WINDS OVER ESRANGE FOR 2005
98
10 10
−10
0
96
10
10
84
98
40
20
20
88
50
30
10
0
90
−10
−20
ZONAL MEAN WINDS OVER ESRANGE FOR 2004
HEIGHT (km)
82
86
−50
6
14
6
4 6
2
4
94
HEIGHT (km)
−10
88
0
10
10
30
92
0
−10
−40
−1
10
10
20 10
10
0
0 −10
88
10
−20
−30
80 10
−20
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
WIND SPEED (ms )
0 10
0
10
20
90
−50
HEIGHT (km)
92
10
0
94
30
20
92
(c)
30
20
10
10
82
−1
96
20
90
40
10
84
−1
HEIGHT (km)
98
40
30
9410
86
50
50
30
−40
WIND SPEED (ms )
10
HEIGHT (km)
40
10
−100
(b)
30
30
−10
0
ZONAL MEAN WINDS OVER ESRANGE FOR 2003
−1
20
10
0
4030
10
94 10
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
WIND SPEED (ms )
10
−20
−20
(a)
96
−10
10
20
10
ZONAL MEAN WINDS OVER ESRANGE FOR 2002
98
30
0
10
0
0
0
84
96
10
20
−10
20
98
40
20
10
0
10
88
−30
10
30
92
84
80
10
0
50
WIND SPEED (ms−1)
92
94 20
0
WIND SPEED (ms−1)
20
10
92
4
2
12
2
2
10
4
4
8
90
2
8 6
88 6
4
86
82
−50
80
8
8
4
10
14 10
6
4
6
84
−40
6
2
6
6
2
STANDARD DEVIATION (ms−1)
30
ZONAL MEAN WINDS OVER ESRANGE FOR 2001
30
HEIGHT (km)
20
40
20
WIND SPEED (ms )
HEIGHT (km)
94
HEIGHT (km)
10
10
0
−1
96
WIND SPEED (ms−1)
98
40
WIND SPEED (ms−1)
ZONAL MEAN WINDS OVER ESRANGE FOR 2000
50
96
WIND SPEED (ms )
ZONAL MEAN WINDS OVER ESRANGE FOR 1999
98
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
(l)
Fig. 7. (a–k) Monthly-mean zonal winds measured by the Esrange meteor radar for individual years between 1999 and 2009. (l) The standard
Fig. 7a-l.
(a)-(k)
Monthly-mean
winds measured by the Esrange meteor radar for individual years bedeviation
of all years
of monthly-mean zonal
zonal winds.
tween 1999 and 2009. (l) The standard deviation of all years of monthly-mean zonal winds.
www.atmos-chem-phys.net/10/10273/2010/
Atmos. Chem. Phys., 10, 10273–10289, 2010
10280
more variable from year to year but are largely eastward at all
heights. Maximum values of ∼25 m s−1 are seen in January–
February over Esrange.
In the meridional winds observations over Esrange, again
a seasonal pattern is observed. The most striking feature is
the clearly defined region of equatorward flow found during the summer months (May to August). This flow varies
in shape and strength, but maximises at heights between
about 84 and 90 km in all years observed with speeds of
up to 16 m s−1 . In late summer (July–August) the meridional winds reverse to give poleward flow above heights of
about 94 km in all of the observed years. The standard deviations of Fig. 8l suggest that the equinox and wintertime flows
are more variable. In some years, the winds are predominantly poleward, reaching speeds of up to 10 m s−1 (e.g.,
February 2001) and in contrast, in other years the winds are
predominantly equatorward, reaching speeds of −10 m s−1
(e.g., February 2003).
The focus of the present work is to determine the differences between the mean winds of the Antarctic and Arctic
regions. We will therefore now turn our attention to a detailed comparisons of the monthly mean winds recorded over
Rothera and Esrange.
3.2
Similarities and differences in the mean winds over
Rothera and Esrange
To investigate systematic differences between the winds of
the Antarctic and Arctic, composite-year monthly-mean climatologies were constructed by averaging the data from all
the individual years. Figures 9 and 10 present these composite years. Figure 9 presents the zonal-mean winds. The data
from Rothera have been displaced by 6 months to help make
seasonal comparisons. These figures show the behaviour of
the mean winds with the inter-annual variability to some extent smoothed out. A Student’s t-test was employed to highlight the significance of the differences between the winds of
the Arctic and Antarctic. Using a confidence limit of 95%,
a number of significant differences are apparent between the
winds. These include:
1. The westward summertime wind over Rothera is significantly stronger than that over Esrange. Further,
the strongest westward winds occur approximately one
month earlier over Rothera than over Esrange. For example, the strongest westward flow of ∼−40 m s−1 occurs over Rothera in November–December at the lowest heights, whereas the strongest westward winds over
Esrange reach only ∼−25 m s−1 in June–July, which is
slightly later in the season. Because the reversal to westward winds occurs at approximately the same point in
the season over Rothera and Esrange, this means that
the rate of change of zonal winds with time is greater
over Rothera, since the strongest westward winds occur less than 2 months after the wind reversal, where
Atmos. Chem. Phys., 10, 10273–10289, 2010
D. J. Sandford et al.: Part 1: Mean winds
as over Esrange, the strongest winds occur more than
3 months after the reversal. Note that, in fact, the reversal to westward winds occurs slightly later in the season
over Rothera than over Esrange, the differences generally amounting to less than 1 month (see Point 3 below). To illustrate this general point, if we consider the
winds at a height of 80 km, the accelerations required
to produce the strongest westward winds from the point
at which the winds reverse into the summer circulation
are: −24.6 m s−1 per month for Rothera and −9.5 m s−1
per month for Esrange.
2. The eastward summertime flow at the upper heights
over Rothera is significantly weaker than that over Esrange. For example, the strongest eastward flow of
more than 25 m s−1 occurs at the upper heights in
a three-month interval over Esrange (June to August),
whereas the eastward winds over Rothera only reach
25 m s−1 briefly in February. Further, the strongest eastward flows over Rothera are significantly weaker than
over Esrange. Specifically, the strongest monthly-mean
winds over Rothera reach only 25 m s−1 where as over
Esrange it reaches more than 35 m s−1 .
3. The location of the zero-wind line shows that the summertime westward flow commences about one month
later over Rothera than over Esrange (October over
Rothera cf. late March over Esrange).
4. The zero wind line over Rothera does not reach higher
than ∼95 km (which occurs in November). Examination of the individual years (Fig. 5a–e) shows that the
winds never reverse to become westwards at the upper
heights observed. In contrast, over Esrange the winds
reverse to become westward at all heights in April/May.
Examination of the individual years (Fig. 7a–k) shows
that the winds reversed at all heights in eight of the nine
years observed (2007 being the exception).
5. During autumn and winter, of the two polar regions,
the composite-year winds generally agree to within
about 5–10 m s−1 . However, it is noticeable that in late
winter/early spring, differences are apparent at heights
above about 90 km. In particular, in the Antarctic data
in August–September the zonal winds exceed 20 m s−1 ,
whereas, in February–March in the Arctic, they are
generally only 5–10 m s−1 . However, these differences
were not found to be significant.
Figure 10a and b presents a similar analysis for the meridional winds. During spring and late winter, the meridional
winds are not significantly different. However, differences
are apparent throughout the rest of the year, even though amplitudes are not as large as in the zonal case. The features
include:
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MERIDIONAL MEAN WINDS OVER ESRANGE FOR 2004
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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
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MERIDIONAL MEAN WINDS OVER ESRANGE FOR 2003
0
WIND SPEED (ms−1)
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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
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(b)
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MERIDIONAL MEAN WINDS OVER ESRANGE FOR 2002
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0
0
(a)
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84
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0
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0
WIND SPEED (ms−1)
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MERIDIONAL MEAN WINDS OVER ESRANGE FOR 2001
WIND SPEED (ms−1)
4
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6
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86
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2
846
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2
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4
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STANDARD DEVIATION (ms−1)
8
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20
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MERIDIONAL MEAN WINDS OVER ESRANGE FOR 2000
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WIND SPEED (ms−1)
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20
WIND SPEED (ms−1)
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HEIGHT (km)
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WIND SPEED (ms−1)
MERIDIONAL MEAN WINDS OVER ESRANGE FOR 1999
98
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
(l)
Fig. 8. (a–k) Monthly-mean meridional winds measured by the Esrange meteor radar for individual years between 1999 and 2009. (l) The
Fig. 8a-l.
(a)-(k)
Monthly-mean
meridional
winds
measured by the Esrange meteor radar for individual years
standard
deviation
of all years of monthly-mean
meridional
winds.
between 1999 and 2009. (l) The standard deviation of all years of monthly-mean meridional winds.
www.atmos-chem-phys.net/10/10273/2010/
Atmos. Chem. Phys., 10, 10273–10289, 2010
10282
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10
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−50
0
Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
TIME (Months)
(a)
ZONAL MEAN WINDS OVER ESRANGE
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20
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WIND SPEED (ms−1)
2. Over Rothera this summertime flow is present over the
entire height range observed. The greatest flow speeds
exceed 10 m s−1 in December. In contrast, over Esrange the meridional flow in summer is more localised
with a slightly stronger equatorward flow maximising
at heights near 87 km (wind speeds exceed 12 m s−1 ).
However, above this height, the winds actually reverse
to become poleward at heights above 94 km during July
and August. Examination of Fig. 8a–k, shows this region of poleward flow occurs over Esrange in each of
the 9 summers observed, but no corresponding feature
was seen over Rothera in any of the 5 summers observed.
ZONAL MEAN WINDS OVER ROTHERA
98
HEIGHT (km)
1. Throughout most of the year the flow is equatorward
and the strongest equatorward flow occurs during summer. Episodes of poleward flow occur in autumn and
winter. However, there is a slight tendency for the
strongest poleward flows to persist for longer after the
summer solstice in the Arctic than the Antarctic.
D. J. Sandford et al.: Part 1: Mean winds
−20
−40
3. The standard deviation plot of Fig. 6f shows that
−10
80
−50
10
−20
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
the greatest variability of the meridional winds over
TIME (Months)
Rothera occurs during winter (June–August). Simi(b)
larly the standard deviation plot of Fig. 8l shows that
the greatest variability over Esrange also occurs during winter (December to February).Fig.
In 9a-b.
fact, examina9. winds
Monthly-mean
winds
by (a) the
Rothera
Monthly-meanFig.
zonal
measuredzonal
by (a)
the measured
Rothera meteor
radar
between 2005 and
meteor radar between 2005 and 2009 and (b) the Esrange meteor
tion of Fig. 8a–k reveals that the monthly mean merid(b) the Esrange meteor radar
between
that(a)
figure
(a) has
beeninshifted
radar
between1999
1999and
and2009.
2009. Note
Note that
has been
shifted
time in time by
ional winds can blow either poleward or equatorward in
by six months
(i.e.,(a)
summers
to aid
comparisons.
are to
inaid
thecomparisons
centre of both
and (b))are in the centre of
different years. For example, strong
wintertime
pole-(i.e., Summers
both
(a
and
b)).
ward flows are evident in 2001, 2004, 2006 and 2009.
This year-to-year difference probably represents the behaviour of stationary planetary waves (see Sect. 4).
We will now compare our observations and URAP at the
latitude
of Rothera (68◦ S) and Esrange (68◦ N). Figure 11a
The meridional summertime jets over both sites occur siand b presents the monthly-mean zonal winds from URAP at
multaneously with the strong shear in the zonal wind. The
68◦ S and 68◦ N, respectively. The figures are plotted such
maximum equatorward meridional winds occur on or just bethat the summers are in the centre.
low the zero-wind line in the zonal winds.
Firstly, comparing these winds with those recorded by the
3.3 Comparison of mean winds with the UARS
Rothera meteor radar (Fig. 9a), it can be seen that URAP repreference atmosphere project
resents the summertime winds remarkably well. One difference which is apparent, however, is that the time in spring at
The radar observations will now be compared with the URAP
which the zonal winds reverse from eastward to westward, is
(UARS Reference Atmosphere Project) empirical model.
systematically different between the radar winds and URAP.
URAP uses measurements from UARS (Upper Atmosphere
In particular the reversal in URAP generally occurs in late
Research Satellite). The data comprises zonal-mean winds
September or early October at heights up to 90 km, whereas
derived from HRDI (High Resolution Doppler Imager) meathe radar observations, show
25the reversal to occur approxisurements in the middle atmosphere made from January
mately 2 weeks later, i.e. in mid to late October. The differ1992 to December 1995, supplemented by data from the UK
ences noted above are evident in each of the 5 years of radar
Met Office Stratospheric data assimilation system and other
data.
sources (e.g., Swinbank and Ortland, 2003). Note that the
In contrast to the good agreement in summer, the eastHRDI instrument measures to latitudes of 72◦ , with the reward
wintertime winds in URAP are significantly stronger
gion between 40◦ S and 40◦ N being the highest sampled.
than those observed over Rothera, often exceeding 30 m s−1 ,
This model contains the zonal mean zonal winds only.
whereas over Rothera the winds are generally in the 10–
15 m s−1 range. Examination of the individual years of zonal
winds over Rothera suggests that there are regular short-
Atmos. Chem. Phys., 10, 10273–10289, 2010
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D. J. Sandford et al.: Part 1: Mean winds
10283
URAP ZONAL MEAN ZONAL WINDS AT 68S
0
4
92
8
8
96
12
94
8
4
4
4
0
88
4
0
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0
−8
84
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82
8
80
8
4
0
10
30
0
92
20
10
88
20
−10
20
−20
8430
−16
82
−20
80
0
30
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10
−4
16
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12
94
0
4
−12
0
−8
−4
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0
−12
−4
0
−8
−4
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80
98
8
88
82
20
−12
0
HEIGHT (km)
HEIGHT (km)
94
−8
−30
−40
−40 −30
−10
−20
0
20
−50
Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
TIME (Months)
WIND SPEED (ms−1)
0
−4
−10
−30
URAP ZONAL MEAN ZONAL WINDS AT 68N
−4
−4
0
0
30
(a)
0
92 −4
90
10
86
MERIDIONAL MEAN WINDS OVER ESRANGE
96
0
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−10
(a)
0
10
30
90
Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
TIME (Months)
98
40
20
30
20
40
30
20
92
10
90
88
50
10
10
0
−10
10
0
0
−10
86
−20
−20
84
−16
82
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80
10
−30
−10
10
0
−30−20−10
−1
90
16
WIND SPEED (ms )
HEIGHT (km)
94
50
20
−1
−4
98
HEIGHT (km)
96
0
WIND SPEED (ms−1)
4
WIND SPEED (ms )
MERIDIONAL MEAN WINDS OVER ROTHERA (Note: Colour Bar Reversed)
4
98
20
−40
−50
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
TIME (Months)
(b)
(b)
Fig. 2005
11.average
The
average
winds empirical m
Fig.
10. Monthly-mean
meridional
measured
byThe
(a)
the
Fig.
11a-b.
longitudinally
monthly-mean
zonalmonthly-mean
winds from zonal
the URAP
Monthly-mean
meridional
winds measured
by (a)winds
the Rothera
meteor
radar
between
and longitudinally
2009
Rothera meteor radar between 2005 and 2009 and (b) the Esrange
from the URAP empirical model for an average year (a) for the
◦
◦ S)S)and
average
year
(a)
for been
the southern
hemisphere
(68
and
northern
hemisphere (68◦ N
Esrange meteor
radar
between
2009.Note
Note
that
figure
has
shifted in
time by (68
Southern
Hemisphere
(b)(b)
forfor
the the
Northern
Hemisphere
meteor
radar
between1999
1999 and
and 2009.
that
(a) has
been(a)
shifted
◦
N). Note
that
(a) hasto
been
in time by six months to aid
timehas
bybeen
six months
andtothe
bar hasfigure
been
reversed
to aid
(a)
has blue
been
shifted
in time
byindicates
six
months
aidshifted
comparisons.
nd the colourinbar
reversed
aidcolour
comparisons.
(i.e.,
The
side
of (68
the
scale
comparisons (i.e., the blue side of the scale indicates equatorward
comparisons.
lived episodes where the wintertime zonal winds do exceed 20 m s−1 . However, the strong and persistent westward
winds of URAP are simply not observed over Rothera. The
differences between our observations and URAP during the
winter are generally in the order of 10–20 m s−1 .
Secondly, we will now compare the observations and
URAP at the latitude of Esrange (68◦ N). Figure 11b presents
the monthly mean zonal winds from URAP at 68◦ N. Comparing these with the observations over Esrange (Fig. 9b),
It can be seen that the agreement is generally quite good.
However, there are again a number of significant differences.
In contrast to the situation in the Antarctic, over Esrange
the wintertime winds agree quite well and it is the summers
which disagree. In particular,
26 in April/May the winds reverse to be westwards at all heights over Esrange, whereas
in URAP, the winds above ∼92 km are always eastwards and
never reverse. In fact, the individual years of radar observations presented in Fig. 7a–k, show this to be the case in 8
of the 9 early summers observed. Further, in the radar observations the zonal winds over Esrange reverse almost simultaneously at all heights in late March to the summertime
westward flow. The height of the zero-wind line associated
with the second summertime reversal from westward back
to eastward winds is systematically lower by about 2 km in
the observations over Esrange. A consequence of this is that,
since the magnitude of the summertime wind shear is about
the same in URAP and our radar observations, the eastward
winds observed in the upper heights are stronger over Esrange than in URAP by about 10 m s−1 . In contrast to the
Antarctic, the wintertime winds of URAP in the Arctic are of
generally similar magnitude to those observed over Esrange;
with differences being at most 5 m s−1 .
The strong vertical structure of the zonal winds in summer allows an examination of the systematic differences between URAP and the radar data. The best agreement over
Rothera (measured by minimising differences) occurs when
27
the URAP winds are assigned to a height 1.7 km lower than
in the URAP climatology (or equivalently the Rothera winds
are assigned to a height 1.7 km higher). Similarly, the best
agreement over Esrange occurs when the URAP winds are
assigned to a height 1.2 km lower than in the URAP climatology (or equivalently the Esrange winds are assigned to
a height 1.2 km higher). This might indicate a systematic
error in one or both of the data sets. However, it should be
borne in mind, that the years of observation are different for
flow and theflow
red and
sidethe
indicates
red sidepoleward.)
indicates poleward).
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Atmos. Chem. Phys., 10, 10273–10289, 2010
10284
HWM−07 ZONAL MEAN ZONAL WINDS AT 68S, 68W
40
30
20
10
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10
90
10
88
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0
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10
0
86
−10
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84
−30
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82
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0
80
20
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−50
Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
TIME (Months)
(a)
HWM−07 ZONAL MEAN ZONAL WINDS AT 68N, 21E
10
40
50
30
40
HEIGHT (km)
94
20
10
20
90
88
30
20
92
40
20
30
30
10
10
0
WIND SPEED (ms−1)
96
Comparison of mean winds with the
HWM-07 model
10
20
94
98
3.4
50
30
10
10
96
WIND SPEED (ms−1)
98
HEIGHT (km)
the satellite and radar data sets and that the URAP winds
are zonal averages rather than the local measurements of the
radars.
In summary, the summertime winds observed in the
Antarctic agree very well with those of URAP, however,
there are significant differences in winter with predicted
winds sometime being twice as large as those observed. The
springtime reversal is also ∼2 weeks later in the observations than the model. In the Arctic, it is the wintertime winds
which agree quite well with differences being at most 5 m s−1
and the summers which disagree. The springtime reversal is
observed at all heights over Esrange, but not above ∼92 km
in the model.
D. J. Sandford et al.: Part 1: Mean winds
0
−10
The second model against which we will compare our obser50
86
−20
20
50
vations is the provisional HWM-07 horizontal wind model
40
84
−30
−10
(Drob et al., 2008). This model succeeds the earlier HWM82
−40
0
30
20 10
93 model of Hedin et al. (1996). This latest version of HWM
80 40
−50
40
−10
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
is based on the previous version with the addition of an extenTIME (Months)
sive set of new measurements from ground-based and space(b)
based wind instruments. It contains both zonal and meridional components.
Fig.zonal
12. Monthly-mean
zonal
winds from
the HWM-07
Fig. 12a-b.
Monthly-mean
winds from the
HWM-07
empirical
modelempirical
for an average yea
Firstly we will consider the zonal winds
of HWM-07
model for an average year (a) for the Southern Hemisphere (68◦ S)
◦
which are presented in Fig. 12a and b,southern
for Antarctic
and (68◦and
hemisphere
S) (b)
andfor
(b)thefor
the northern
hemisphere
that
figure (a) has
Northern
Hemisphere
(68◦ N).(68
NoteN).
thatNote
(a) has
been
Arctic latitudes, respectively. The HWM-07 winds capture
shifted in time by six months to aid comparisons.
in time by six months to aid comparisons.
some of the character of the meteor radar observations in
summer in both the Antarctic and Arctic (Fig. 9a and b).
However, significant differences are apparent. In particuobserved winds are generally in the range 5–15 m s−1 at all
lar, in the Antarctic the spring reversal of the zonal winds
heights, whereas the HWM-07 winds reach values exceeding
from eastward to westward occurs about one month earlier
50 m s−1 below 90 km.
in HWM-07, than observed, at heights below about 90 km.
Secondly, we will consider the meridional winds of
The summertime westward winds in HWM-07 are slightly
HWM-07 which are presented in Fig. 13a and b for Antarcweaker than those observed by ∼10 m s−1 and the summertic and Arctic latitudes, respectively. In comparison to the
time eastward winds are slightly stronger than those observed
meteor-radar observations of Fig. 10a and b, it can be seen
by more than 10 m s−1 . In the other seasons, the observed
that there are significant differences. In the case of the
winds and those of HWM-07 are generally in the range 10–
Antarctic, the observations show poleward winds throughout
20 m s−1 .
the year except in autumn and the lowest heights in winter. In
contrast the HWM-07 winds are poleward except at the upIn contrast, the Arctic observations and HWM-07 winds
per heights in summer and at the lower heights in winter. The
(Figs. 9b and 12b) reveal striking differences. The summermagnitude of the poleward flows in HWM-07 is very much
time westward winds of HWM-07 are significantly weaker
stronger than the poleward flows observed. For example, the
than observed. Further, the observed reversal in spring to
observed winds reach only −4 m s−1 in the upper heights in
westward flow which occurs at all heights during March is
May, whereas the HWM-0728
winds reach −16 m s−1 at the
missing from HWM-07, where the winds remain eastwards
same height and time. The strongly equatorward flows of
above ∼85 km. The summertime eastward winds in HWMHWM-07 are not evident in the observations at all. Further,
07 are significantly stronger than those observed. The obserthe deep region of equatorward flow occurring in summer in
vations show the summertime eastward winds increasing in
the observations occurs only in upper heights of HWM-07
strength at all heights until the end of August when the winds
and the winds below ∼88 km are strongly poleward.
abruptly decrease to values near 10 m s−1 . At the same time,
the HWM-07 winds remain strong at the upper heights. In
In the Arctic, large differences are also apparent. In parautumn and winter there is generally very poor agreement
ticular, in spring, autumn and winter, HWM-07 includes
between the observation and HWM-07. In particular, the
regions of strong poleward flow, with speeds sometimes
Atmos. Chem. Phys., 10, 10273–10289, 2010
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D. J. Sandford et al.: Part 1: Mean winds
10285
4
Discussion
In this section we will consider our measurements of the
winds of the Antarctic and Arctic MLT and consider how
they relate to those reported in other studies and with the
URAP and HWM-07 empirical models. We will then specifically consider our results in the context of those studies attempting to identify inter-hemispheric differences. There are
a useful number of studies that report the winds of the Arctic
MLT, a somewhat smaller number that report the winds in
the Antarctic MLT and very few that compare the winds of
both polar regions using measurements made by essentiallyidentical instruments.
Studies of mean winds in the Antarctic MLT include those
of Portnyagin et al. (1992, 1993, 2006), Vincent (1994),
Dowdy et al. (2001, 2007a), Kishore et al. (2003), Hibbins
et al. (2005), Merzlyakov et al. (2009). Here, we will not
consider further the studies of Portnyagin et al. (1992), Merzlyakov et al. (2009) because they used meteor radars without height-finding capability and so reported winds that are
www.atmos-chem-phys.net/10/10273/2010/
WIND SPEED (ms−1)
WIND SPEED (ms−1)
HEIGHT (km)
HEIGHT (km)
exceeding 12 m s−1 . In contrast, the observations from EsHWM−07 MERIDIONAL MEAN ZONAL WINDS AT 68S, 68W
0
−12−8
98
20
range reveal predominantly weak equatorward flow in these
16
96
−1
0
−16
−12
seasons. There is a region of poleward flow of ∼2 m s
12
94
−4
−4
4
in late winter over Esrange (February–March), but the cor−8
8
92
4
responding flow in HWM-07 is significantly stronger with
0
900
0
4
speeds greater than 8 m s−1 at heights below 88 km. The
88 4
−4
12
86
distinctive region of equatorward flow observed to peak in
12
−8
8
4
−4
84
June–July at heights of ∼86 km is also evident in HWM−12
8
0
−8
82
−16
07; although in the model peak wind speed occur about one
0
−4
80
−20
−16
4
month earlier than observed. In both cases the equatorward
Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
−1
TIME
(Months)
wind speeds maximise at ∼12 m s .
In summary, zonal HWM-07 winds capture some of the
(a)
features observed in summer of both regions. However, sigHWM−07 MERIDIONAL MEAN ZONAL WINDS AT 68N, 21E
16 12
9812 8
20
nificant differences are apparent. In the Antarctic, model
4
4
8
−4 0
16
−8
96
summertime westward winds are slightly weaker than ob12
12
94
served and eastward winds are slightly stronger. Below
0
8
92
∼90 km the spring reversal occurs ∼1 month earlier in
4
−4
90
12
8
0
HWM-07. In the other seasons the amplitudes are in a similar
0
−4 4 8
88
4
−8
8
−12 −8
−4
range. In the Arctic, the differences are more striking. Model
−4
0
86
−8
4
−4
−8
summertime westward winds are much weaker than observed
0
84
−12
4
and eastward winds are stronger. The spring reversal is ob82
−16
8
−4
12
80
−20
served to occur at all heights, but is missing in HWM-07.
20 12
0
16
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Also the autumn and winter agreement is generally very poor.
TIME (Months)
The meridional winds of HWM-07 do not agree well with
(b)
the observations. Antarctic observations show equatorward
winds throughout most of the year, when the HWM-07 shows
Fig.poleward
13a-b. Monthly-mean
meridional
winds from
the HWM-07
model for
an average yea
Fig.
13. Monthly-mean
meridional
winds empirical
from the HWM-07
emgenerally poleward winds. At times when
flow is
pirical
model
for
an
average
◦ year (a)
◦ for the location of Rothera in
observed, it is much stronger in the model.
In the Arctic
fromin the southern hemisphere (68 S,
location
of Rothera
68 W) and (b) for the location of Esrange in
the Southern Hemisphere (68◦ S, 68◦ W) and (b) for the location of
autumn to spring the HWM-07 has regions of strong pole◦
◦
◦ E).
Esrange
in thefigure
Northern
Hemisphere
(68◦ N,
(a) and the co
hemisphere (68 N, 21 E).
Note that
(a) has
been shifted
in 21
time
byNote
six that
months
ward flow, where the observations show weak equatorward
has been shifted in time by six months and the colour bar has been
been reversed
to aid
flow. The equatorward jet seen during summer
is evident
in comparisons.
reversed to aid comparisons.
both observations and the model, with similar amplitudes,
however, the model peaks about one month earlier.
averages across the meteor region, rather than revealing the
vertical structure of the winds.
Portnyagin et al. (1993, 2006), Vincent (1994), Dowdy et
al. (2001, 2007a), Kishore et al. (2003), Hibbins et al. (2005)
used selections of MF and Meteor radars at: Syowa (69◦ S,
40◦ E - MF), Molodezhnaya (68◦ S, 45◦ E - MWR), Mawson
(67◦ S, 63◦ E - MF), Davis (69◦ S, 78◦ E - MF), Scott Base
(78◦ S, 167◦ E - MF), MaMurdo (78◦ S, 166◦ E - MF) and
Rothera (68◦ S, 292◦ E - MF) to measure zonal and meridional winds in the Antarctic MLT region. In general, the
seasonal pattern of winds described in Sect. 3.1 agrees well
with those reported in the above studies. However, there are
a number of significant differences. In particular, in the zonal
29
winds, the strong wind shear evident over Rothera in summer
that extends from 80–98 km is not observed above ∼92 km
in any of the above results. For example, over Rothera,
the summertime zonal winds reach speeds of up to 25 m s−1
in February at a height of about 97 km (Fig. 9a). In contrast, the zonal winds at the same height and time in the MF
radar studies are generally in the range 5–10 m s−1 . These
differences probably do not result from real differences in
Atmos. Chem. Phys., 10, 10273–10289, 2010
10286
the zonal winds over different sites, but rather appear to be
typical of differences reported when meteor- and MF-radar
measurements are compared.
These differences have been characterised by, e.g., Hocking and Thyaparan (1997), Manson et al. (2004). In general at heights above about 85–90 km MF radars record significantly weaker winds than meteor radars. Engler et al.
(2008) estimated the winds measured by MF radar in the
upper mesosphere to be too small by anywhere between 20–
40%. Similar results have been found by, e.g., Stubbs (1973),
Cervera and Reid (1995), Hocking and Thyaparan (1997),
Manson et al. (2004), Jacobi et al. (2009). The exact cause
of these differences remains uncertain. The possibility of discrepancies highlight the importance of the meteor radar technique in measuring MLT winds above 85–90 km.
Hibbins et al. (2005) used a combination of MF radar,
falling sphere, radiosonde and model data to build up a climatology of zonal mean winds between 0 and 100 km above
Rothera (67◦ S, 68◦ W). The MF radar is located at the same
site as the MWR used here. In fact, the antennas of the MWR
are actually located within those of the MF radar, making
these radars the most co-located MF/MWR pair in the world.
The Rothera MF-radar and falling-sphere data both showed
that the maximum in the summertime westwards jet occurred
much lower and earlier in the season than that observed at
Andoya (69◦ N, 16◦ E), a similar latitude Northern Hemisphere site (Müllemann and Lübken, 2005). This difference
was attributed to an atypically strong gravity-wave field over
the Antarctic Peninsula and an interpretation supported by
the height of the summertime zero-wind line which was observed to be around 87 km, somewhat lower than other similar latitude Southern Hemisphere sites (e.g., Portnyagin et al.,
1993). In our study, the summer time zero-wind line is found
to be around 95 km altitude, more in line with results reported
elsewhere, and significantly different from the MF-radar results reported in Hibbins et al. (2005). We note, however, that
the data used in Hibbins et al. (2005) were not contemporaneous with the observations reported here, and a subsequent
analysis of the Rothera MF-radar data from 2005 onwards
shows a much higher summer time zero-wind line more in
line with the results presented here (not shown). It is interesting to note that the MF radar data used in Hibbins et al.
(2005) were recorded almost exclusively during high solar
activity, whereas the data presented here is recorded during
the declining phase of solar cycle 23 and the deep solar minimum between cycles. However, whether the altitude of the
summertime zero-wind line is directly related to changes in
solar activity remains undetermined.
Studies of mean winds in the Arctic MLT include those of
Portnyagin et al. (1992, 1993, 2004, 2006), Hocking (2001),
Dowdy et al. (2001, 2007a), Hall et al. (2003), Kishore et
al. (2003). Again, we will not consider further the studies
of Portnyagin et al. (1992) because they used meteor radar
without height finding capability. Portnyagin et al. (1993,
2004, 2006), Hocking (2001), Dowdy et al. (2001, 2007a),
Atmos. Chem. Phys., 10, 10273–10289, 2010
D. J. Sandford et al.: Part 1: Mean winds
Hall et al. (2003), Kishore et al. (2003) used data from
some or all of the following sites: Svalbard (78◦ N, 16◦ E–
MR), Tromsø (70◦ N, 19◦ E–MF), Andenes (69◦ N, 19◦ E–
MF), Kiruna (68◦ N, 20◦ E–MWR), Esrange (68◦ N, 21◦ E–
MWR), Heiss Island (80◦ N, 58◦ E–MWR), Dickson Island
(72◦ N, 80◦ E–MWR), Poker Flat (65◦ N, 213◦ E–MF), Resolute Bay (75◦ N, 265◦ E–MWR) to measure the zonal and
meridional winds. The type of radar is indicated by MF
(Medium Frequency) or MWR (Meteor Wind Radar).
In general, the seasonal pattern of Arctic winds reported in
Sect. 3.1 agrees well with those reported in the above studies. Again however, the strong wind shear towards the upper
height gates >92 km evident in the MWR observations tends
to be much larger than those of MF observations. For example, the zonal winds over Resolute Bay (Hocking, 2001) from
the MWR reach values as large as 25 m s−1 and the MWR at
Esrange, presented here, reaches wind speeds of >35 m s−1 .
In contrast the zonal winds at the same height and time in
the MF radar studies are generally in the range 5–10 m s−1 .
Once more, these differences probably do not result from actual atmospheric differences, but are more likely to be a result
of MWR/MF radar measurement uncertainties.
A particular focus of this study has been to compare the
winds of the Antarctic and Arctic MLT region. In Sect. 3.2,
a number of similarities and significant differences have been
identified in the winds of the two polar regions. The earlier
inter-hemispheric studies of Portnyagin et al. (1993, 2006),
Dowdy et al. (2001, 2007a), Kishore et al. (2003) also suggested significant differences between the Antarctic and Arctic MLT. The similarities are that firstly, both the Antarctic
and Arctic stations measure the top of the summertime westward jet between 85 and 96 km and eastward winds throughout the rest of the year at these heights. Secondly, the meridional winds peak, with the strongest equatorward values at
heights near 88 km. This maximum in the meridional winds
is generally coincident in height with the reversal of the zonal
winds (i.e., the zero-wind line).
The main difference observed between the two hemispheres, is that in the Antarctic both the zonal and meridional
winds reach their maximum values near the summer solstice.
However, the strongest winds in the Arctic are delayed by
∼1 month compared to this. Dowdy et al. (2001) suggest
that radiative effects are therefore stronger in the Antarctic
and wave driving-effects are more important in the Arctic in
determining the state of the summer polar MLT. Dowdy et al.
(2007a) suggest that the equatorward flow near the summer
mesopause (∼88 km), persists for longer in the Arctic than
the Antarctic. This is also seen over Rothera and Esrange in
Fig. 10a and b.
Kishore et al. (2003) found that the wintertime zonal
winds over Davis (69◦ S, 78◦ E) were nearly twice as large
as those seen over Poker Flat (65◦ N, 147◦ W). However,
their observations covered only 2 years of data and in the
observations presented here there is a high degree of interannual variability in the zonal wintertime winds. Such
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D. J. Sandford et al.: Part 1: Mean winds
differences from year-to-year may thus not represent a climatological difference between the two polar regions. Alternatively, such differences may represent the contribution
of stationary planetary wave. Kishore et al. (2003) also
found that the Antarctic meridional winds were predominantly poleward throughout the year and the Arctic predominantly equatorward. In our observations, we find that both
Arctic and Antarctic winds are predominantly equatorward.
Again, the differences may arise because of stationary planetary waves and/or measurement biases between the MF-radar
technique used by Kishore et al. and the meteor radar technique used here.
The mean-wind standard deviation plots (Figs. 5f, 6f, 7l
and 8l) indicate that inter-annual variability is greatest in different seasons in the two polar regions. The significantly
greater level of variability evident in late winter/spring over
Esrange compared to that over Rothera almost certainly reflects the major sudden stratospheric warming events that
occur regularly in the Arctic, but are largely absent in the
Antarctic (e.g., Hoffmann et al., 2007; Dowdy et al., 2007b).
Planetary waves have been shown in modelling studies to exert a drag on the mean flow through Eliassen-Palm flux divergence (e.g., McLandress and McFarlane, 1993; Pogoreltsev, 1996). As stratospheric warmings are a disruption in
the planetary wave field, they could lead to the differences
observed. A possible explanation for the summertime variability over Rothera is because of the gravity wave activity
which causes the reversal the zonal winds. Rothera is located
on the Antarctic Peninsula and due to the relief of the land is
known to be an area of intense gravity wave excitation and
strong gravity wave momentum fluxes – at least in the troposphere and stratosphere (Alexander et al., 2008). Beldon and
Mitchell (2009) also found significant inter-hemispheric differences in the mesopause region using the same two meteor
radars. They found stronger Antarctic gravity-wave activity
particularly in the spring compared with the Arctic. Variations in these fluxes could in turn result in significant variations in the momentum deposited in to the MLT, resulting in
fluctuations in the zonal winds. Another possible explanation
for the observed summmertime variability in the mesosphere
over Rothera could come from planetary wave variability in
the winter stratosphere of the northern hemisphere. Becker
and Fritts (2006); Karlsson et al. (2009) both present modelling studies in which planetary-wave activity in the wintertime stratosphere causes variability in the mesopause of the
opposite hemisphere through the shifting of the gravity-wave
driven meridional circulation.
Our results reveal that, overall, there is a better agreement
between our observations and the zonal winds of URAP than
is the case for HWM-07. The zonal URAP predictions fit
the observations very well during the summertime. Wintertime winds, however, do not agree so well, in both URAP
and particularly HWM-07. In the case of URAP, the differences are greatest in the Antarctic winter. In the case of
HWM-07 it is the Arctic winds where the most significant
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10287
differences occur and the differences are significant throughout the year. There are two likely explanations of the wintertime differences. The first is the greater natural variability during this season, which probably includes significant
contributions from stationary planetary waves. Stationary
planetary waves will produce a longitudinally varying wind
field. The observed winds may thus be somewhat different
from the zonally-averaged winds of URAP. It should also be
noted that these zonally-averaged winds could also contain
a component of the migrating tides aliased onto them from
the HRDI measurements, this could account for the differences observed since the semi-diurnal tide is known to have
large amplitudes during winter. The Second is decadal scale
variability in the circulation of the MLT. For example our
Antarctic observations span the interval 2005–2009; whereas
URAP is based on observations made between 1992 and
1995 (Such decadal scale variability has been suggested by,
e.g., Portnyagin et al., 1993). In the case of the meridional
winds there is generally poor agreement with HWM-07 and
the strong poleward flows of the model are not observed.
Finally, we note that Portnyagin et al. (1993) suggested
that there are inter-annual trends in the polar winds. They
reported that the zonal winds at ∼95 km in the Antarctic exhibited a trend of decreasing speed from 1968 to 1977. This
was especially noticeable in July and January. This trend
was not apparent in the Arctic. They also reported maxima
in meridional winds occurring when the zonal winds are at
a minimum (e.g., 1971 and 1982/83 from Portnyagin et al.,
1993). Such long-term trends may, in part, account for some
of the differences between our observations and the other observations and models discussed above, where the various
studies used different years of observation. Merzlyakov et
al. (2009) also reported trends in long-term mean-wind measurements in the Antarctic. They found that there was a tendency for the winter zonal winds to decrease in amplitude
between 1970 and 2006. A similar trend was found in the
summertime zonal winds between 1970 and 1990, however,
they found an increasing trend after, between 1993 and 2005.
We regard our Antarctic observations as being of too short
in duration to make any meaningful estimate of trends in the
mean winds. For the longer Arctic dataset, we carried out
a linear regression analysis of the winds in each month and
in each height gate (not shown). In this simple analysis, no
significant tends in either the zonal or meridional winds were
found.
5
Conclusions
Meteor radars at Rothera (68◦ S, 68◦ W) and at Esrange
(68◦ N, 21◦ E) have been used to measure winds in the
Antarctic and Arctic mesosphere and lower thermosphere
region at heights of 80 to 98 km. The inter-hemispheric
comparisons suggest that there are some similarities as well
as significant differences in both the zonal and meridional
Atmos. Chem. Phys., 10, 10273–10289, 2010
10288
winds. The reversal of the zonal winds in summertime
occurs about one month later over Rothera than over Esrange. The summertime wind shear is shifted between the
two hemispheres giving stronger westward flow over Rothera
and stronger eastward flow over Esrange but with very similar wind shear. The meridional winds over both sites show
flow which is equatorward throughout most the year with significantly stronger equatorward flows during summer. Over
Rothera the summertime jet is present over the entire height
range, whereas over Esrange it is more localised in height.
These differences in wind speeds and variability between the
two polar regions may well originate from differences in the
strength of gravity-wave driving of the mean flow and the
behaviour of the Antarctic and Arctic stratospheric polar vortex, in particular sudden stratospheric warmings.
Comparisons with URAP reveal a generally good agreement in the zonal winds, although significant differences exist in the Antarctic winter where URAP winds are noticeably stronger than observed. Comparisons with HWM-07
reveals significant differences in Arctic winter where HWM07 winds are much stronger than observed. The meridional
winds of HWM-07 are significantly more poleward than observed. At least some of these differences may be due to
stationary planetary waves producing a longitudinally varying wind field or may reflect decadal scale changes in the
MLT circulation. Evaluating the magnitude of such effects
represents an interesting and important area for future study.
Acknowledgements. The authors would like to thank the teams
who have working on the URAP and HWM models. This study
was supported by the Science and Technology Facilities Council
(STFC) of the UK. The Rothera Meteor Radar was funded under a
Natural Environment Research Council (NERC) Antarctic Funding
Initiative (AFI) Grant: AF15/38.
Edited by: A. Baumgaertner
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