National Institute of Polar Research

Polar Meteorol. Glaciol., +2, 30ῌ++-, ,**.
῍ ,**. National Institute of Polar Research

Seasonal variation of air transport in the Antarctic
and Atmospheric Circulation in +331
Kazue Suzuki+ῌ, Takashi Yamanouchi+,,, Naohiko Hirasawa+,,,and Tetsuzo Yasunari+

Department of Polar Science, The Graduate University for Advanced Studies,
Kaga +-chome, Itabashi-ku, Tokyo +1--2/+/,
National Institute of Polar Research, Kaga +-chome, Itabashi-ku, Tokyo +1--2/+/
Hydrospheric Atmospheric Research Center, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya .0.-20*+
ῌCorresponding author. E-mail: suzuki@pmg.nipr.ac.jp(Received May 0, ,**.; Accepted August -*, ,**.)

Abstract: To better understand how present and past climates at Syowa Station,
Antarctica relate to climate elsewhere, we analyzed the tropospheric air transport to
Syowa Station for the year +331 using a dataset from the European Centre for Medium
Range Weather Forecasts (ECMWF). The ﬁve-day trajectories of the air parcels
were estimated and analyzed. In the middle troposphere in winter, air parcels wereusually from the lower troposphere over the Atlantic. However, in January, most of
the air parcels came from latitudes higher than 0*ῌS . The trajectories had little
vertical motion and were associated with a low pressure system that forms along thecoastal region of Antarctica only in summer. In the lower troposphere, trajectories
could be classiﬁed as originating in one of three regions: the Southern Ocean, thecontinental interior, and the east coast. In contrast to the middle troposphere, airparcels from the Southern Ocean had the lowest frequency, irrespective of the time ofyear. This is partially due to a low pressure system that blocks air parcels fromoutside the continent. Most trajectories are a#ected by the drainage ﬂow. An
ampliﬁed quasi-stationary planetary wave for September to November and a blocking
circulation in June make trajectories pass over Antarctica.
key words:

trajectory, Syowa Station, polar vortex, planetary wave+.

Introduction

The Antarctic ice sheet consists of water transported through the atmosphere andthen deposited at the surface (e.g., Giovinetto et al., +331). Concentrations of minor
atmospheric constituents, including greenhouse gases and aerosols that are recognized asstrong climate forcing factors, are highly inﬂuenced by the long-range transport to
Syowa Station (Morimoto et al., ,**-; Hara et al., ,**.). However, little is known
about this air transport. Moreover, to completely interpret the analyses of ice coresfrom the region, we need more knowledge about the transport of water vapor and otherminor constituents to the region.

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The transport route of air parcels to the Antarctic region has been studied using
trajectory analysis. Harris (+33,) showed some mid-tropospheric ﬂow patterns for theSouth Pole during +32/ῌ+323. Using snowfall signals detected in an ice core, Reijimer
et al. (,**,) estimated the origins of the water vapor coming to the Antarctic continent
using trajectory analysis for ﬁve locations in Antarctica (Vostok, Byrd, Dome F, DomeC, and DML*/). The results showed the typical ﬂow pattern and the origins of thewater vapor. Murayama et al. (+332) examined the transport of air with high Omixing ratios at Syowa Station using --D trajectory analysis. They suggested that themain transport path depended on the season, such that descending motion within thepolar vortex occurred in spring and autumn, whereas transport from the circumpolar
region was important in summer.
Air parcel trajectories are controlled by atmospheric ﬂow patterns that vary daily,
seasonally, and yearly. The average westerly wind between /*ῌ and 0/ῌS peaks in
March and September (Van Loon, +301). In summer and winter, cyclonic activity
generally decreases over the ocean at higher latitudes, the major pressure gradientappears further north, and westerly winds between /*ῌ and 0/ῌS weaken.
A stationary planetary wave of wave number +, hereafter WN+, exists all year
round. The amplitude at /** hPa peaks in the subpolar regions with a ridge in the
Paciﬁc and a trough in the Atlantic (Van Loon et al., +31,). The major features of theobserved stationary eddy ﬁelds remain the same between summer and winter in middle
and high latitudes, although the amplitude is larger in winter than in summer (Kitoh etal., +33*). These stationary planetary waves induce meridional transport of air overlarge scales such as that between the polar region and midlatitudes. Moreover, thesewaves inﬂuence the storm tracks that can mix air over synoptic scales.
Air parcels coming to Syowa Station are likely inﬂuenced by cyclones that passthrough the middle latitudes over the South Atlantic Ocean and the Southern Ocean.
At Syowa Station, synoptic-scale disturbances typically cause more than ,* blizzards per
year (e.g., Higashijima et al., ,**-). Also, at an inland station, Hirasawa et al. (,***)
showed that a blocking event could bring a large amount of air into Antarctica that
normally would not come.
The purpose of the present study is to describe the seasonal changes of air transport
to Syowa Station in the lower and middle troposphere. We also examine how theatmospheric circulation causes seasonal variations in the air transport.
,.

Data and analysis

We used the --D backward trajectory analysis developed by Sato and Tomikawa(Yamanouchi et al., ,**.) for air parcels arriving at Syowa Station (03ῌ**ῌS, -3ῌ-/ῌE)
in +331. Trajectories in +0 layers were calculated / days back because going further
back in time would have large uncertainties. These +0 layers each covered /* hPa
between +** and 2/* hPa. Each trajectory calculation started at +, UTC and the time
step was + hour. The objective analysis atmospheric data for this study were theTropical Ocean and Global Atmosphere (TOGA) Level III atmospheric data provided
by the European Centre for Medium Range Weather Forecasts (ECMWF). The data
are .-times daily on a ,./ῌ grid at +, mandatory pressure surfaces. Bromwich et al.

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K. Suzuki, T. Yamanouchi, N. Hirasawa and T. Yasunari(+33/) compared ECMWF analyses to East Antarctic rawinsondes and numericalanalyses. They found that the ECMWF analysis best reproduced sounding values forthe moisture ﬂux at each level. As one purpose of our study was to determine the vapor
circulation in the Antarctic, we chose to use the ECMWF analysis.
-.

Seasonal variation in the middle troposphere

Trajectory analysis
We ﬁrst discuss the monthly changes in the /-day trajectories that reach a height of/** hPa over Syowa Station (Fig. +). In January, most of the air parcel sources are
distributed in latitudes higher than 0*ῌS of the Atlantic Ocean sector. Figure , shows
the monthly averaged transport distance of air parcels in the middle and lower
troposphere. Among all months, the transport distance in January is the shortest.
From February to June, a high frequency of trajectories spread into the area between.*ῌ and 0*ῌS over the Atlantic Ocean. The lengths of the trajectories gradually
become longer through this duration. After June, the high frequency over the Atlantic
Ocean is maintained until November.
In June, several trajectories originate north of .*ῌS over the Indian Ocean, and passthrough inland Antarctica. These trajectories are associated with a blocking event
reported by Hirasawa et al. (,***). Also in July, there are trajectories passing over the
Indian Ocean that are associated with a developed synoptic-scale disturbance in the
Antarctic region.
In September, October, and November, many trajectories meander and pass over
the Antarctic continent. The trajectory lengths gradually decrease from September toDecember. In December, the air parcels come from the east, either from the continentor through the sector of the Indian Ocean in the Southern Ocean.
In January, most trajectories came to Syowa Station with little vertical motion,although a few trajectories came from the lower troposphere in middle latitudes (Fig.
+b). The latter trajectories came from the Indian Ocean in Fig. +a. In July, on theother hand, a large number of trajectories come from the lower troposphere in middle
latitudes (Fig. +b). This feature is generally seen from February to November withlarge variation in altitude.
-.+.

Geopotential height ﬁeld at /** hPa
In order to extract signiﬁcant circulation patterns from the distribution of
trajectories, monthly averaged geopotential height ﬁeld is examined here. This ﬁeld
represents stationary circulation patterns and also indicates the activity of synoptic
disturbances along the averaged westerly jet. Figure -a shows the monthly averaged
geopotential height at the /** hPa level, hereafter Z/**, in January and July. In
January and July, Z/** decreases rapidly with latitude in middle latitudes, corresponding to strong westerly winds. The Z/** over Antarctica is di#erent between the twomonths. In January, Z/** is low in the coastal region of the continent and relatively
high over the interior. Syowa Station is exposed to moderate easterly winds that appear
poleward of the low Z/** region. A large amount of air moves eastward on thenorthern side of the low Z/** region, turns southward, and then reaches Syowa Station-.,.

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Fig. +a.

Horizontal distribution of backward trajectories that reached a height of /** hPa over
Syowa Station in +331. Calculations ended either at / days or when a trajectory crossed amountain or sea surface.

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K. Suzuki, T. Yamanouchi, N. Hirasawa and T. YasunariFig. +a.

(Continued).

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Air transport in the Antarctic and Atmospheric Circulation

Fig. +b. Vertical distribution of trajectories in (a) for January and July, +331.

Fig. ,.

Monthly average of the transport distance of air parcels for / days. The solid line is for
trajectories that reached a height of /** hPa over Syowa Station; the dotted line is for
trajectories that reached 2/* hPa.

from the east. Some trajectories are trapped by a low Z/** cell centered near -*ῌE and0/ῌS. These air parcels have little vertical motion. On the other hand, a few airparcels from the region south of Africa have originated in the lower troposphere in thestrong westerly region.
In July, a center of low Z/** appears over Antarctica. Syowa Station is coveredby the southernmost region of the circumpolar westerlies. Trajectories in July areconcentrated around the center of the circumpolar westerlies in the Atlantic Ocean.

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K. Suzuki, T. Yamanouchi, N. Hirasawa and T. YasunariFig. -.

(a) Monthly averaged geopotential height (contoured) and zonal wind (shaded) at the /**hPa level (referred to as Z/**) in January and July. The contour interval for the solidlines is +** m. The lines for /+-* and /+0* m are dotted. Dark shading is easterly windand light shading is westerly wind.
(b) Time-latitude section of the geopotential height (contoured) and zonal wind (shaded)of the /** hPa level averaged between * and 3*ῌE in +331. The contour interval between
adjacent solid lines in +** m and the dotted lines are /*-m from the nearest solid line.
Shading is the same as that in (a).

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The daily Z/** weather maps (not shown here) showed that the air parcels came fromthe westerly wind zone to Syowa Station in association with synoptic-scale disturbancesthat develop in the circumpolar westerlies. Thus, the relevant air parcels at /** hPalevel over Syowa Station originated in the lower troposphere over the Atlantic Ocean.
Figure -b shows a time-latitude section of Z/** averaged between *ῌ and 3*ῌE.
The Z/** minimum in the coastal region of the continent (0*ῌῌ 1*ῌS) is relatively deep,
especially in January and December. However, in December, a stronger gradient in the
Z/** in the coastal region induces a stronger easterly wind than that in January.
Hence, the trajectories from the east are more elongated in December than in January.
In February and March, Z/** decreases in the polar region, and the low Z/** band
disappears from around the continent. Under these conditions, the circumpolar westerlies inﬂuence the area around Syowa Station and there is an increasing number oftrajectories from over the Atlantic. Later, from April to August, the southernmost
part of the westerly wind zone (indicated by a gradient in Z/**) just reaches thelatitudes near 1*ῌS, the latitude of Syowa Station (Fig. -b). Consequently, in thisduration, many trajectories originate in the westerly wind zone over the Atlantic.
The September to November period, on the other hand, is characterized byincreasing Z/** in the polar region and a retreat of westerly wind from the coastal
region. The temporal variation of the distribution of trajectories in this period is not
the opposite of the situation in February and March, but instead shows an increase in thenumber of trajectories passing over Antarctica. This increase means that the meridional ﬂow between the middle latitudes and Antarctica is stronger during this period. A
similar situation occurs in June. The relationship between the atmospheric circulationand the trajectories in these periods is described in Section /.
..

Seasonal variation in the lower troposphere

Trajectory analysis
In this section, we discuss the trajectories that reach a pressure level of 2/* hPa,
which correspond approximately to the boundary between the free atmosphere and theplanetary boundary layer. Figure . is similar to Fig. + but is instead for 2/* hPa.
Throughout the year, trajectories at 2/* hPa can be classiﬁed into one of three patterns:
those coming from inland of Syowa Station in Antarctica (continental interior), thosecoming along the east coastal region (east coast), and those coming from the Southern
Ocean, south of the Atlantic (south ocean). These are shown in Fig. /a. We classiﬁed the trajectories into one of these areas according to the parcel’s position two days
before reaching Syowa Station. This classiﬁcation indicates the direction, not theorigin. Figure /b shows the total number of air parcels for each area per month.
The continental interior generally has the highest number of parcels throughout theyear, except for January and December, when the number for the east coast is highest.
In January and July, almost all the trajectories from the east coast and the continental
interior go downward along the continental surface (Fig. .b). This indicates thatdrainage ﬂow, that is, the Katabatic wind, has a large e#ect in the lower troposphere
over Syowa Station. The relative amount of air coming over the south ocean is low formost of the year, except for March and June when it exceeds that from the east coast.

..+.

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K. Suzuki, T. Yamanouchi, N. Hirasawa and T. YasunariFig. .a.

Horizontal distribution of backward trajectories that reached a height of 2/* hPa over
Syowa Station in +331. Calculations ended either at / days or when a trajectory crosseda mountain or sea surface.

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Fig. .a.

105(Continued).

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K. Suzuki, T. Yamanouchi, N. Hirasawa and T. YasunariFig. .b. Vertical distribution of trajectories in (a) for January and July, +331.

Fig. /a. Map showing the three areas where air parcels had been two days before reaching Syowa
Station (03ῌ**ῌS, -3ῌ-/ῌE). The areas are the continental interior (1*ῌῌ 3*ῌS, *ῌῌ +-/ῌE),east coast (0*ῌῌ 1*ῌS, .*ῌῌ +-/ῌE), and south ocean (/*ῌῌ 1*ῌS, ./ῌWῌ.*ῌE).

In this case, most air parcels originate in the planetary boundary layer below the 2/* hPalevel.
Geopotential height ﬁeld at 2/* hPa
Figure 0a is similar to Fig. -a, but is for Z2/*, the geopotential height of 2/* hPa.
The surface air pressure over most of the Antarctic continent is lower than 2/* hPa
because of its high elevation, and thus we do not show contours over Antarctica. In..,.

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Fig. /b. Monthly total number of air parcel positions for trajectories that reached a height of 2/*
hPa over Syowa Station. The solid line connects adjacent values for the continental
interior area, the dotted line for the east coast, and the dashed line for the south ocean
area.

January and July, a large gradient of Z2/* exists at latitudes between .*ῌ and 0*ῌS.
As this also occurs for Z/**, this indicates that the westerly wind zone is deep in the
troposphere at those latitudes. Both months have low Z2/* values in the coastal
region. Syowa Station is located to the south of the low Z2/* region and thus is in aneasterly wind zone. In July, there is also an intensiﬁed westerly wind at middle
latitudes and an intensiﬁed easterly wind in the coastal region.
Figure 0b is similar to Fig. -b but is for Z2/*. The band in the latitude range of0*ῌῌ 1*ῌS has low Z2/* values throughout the year and is lowest in winter. The
geopotential height gradient in Z2/* is small around this band in January, whichindicates that the easterly wind near and over Syowa Station is weak.
From March to June, the easterly wind on average near Syowa Station gradually
becomes strong in association with a decrease in height of the low Z2/* band. The
deepening of the low Z2/* band is also consistent with the southernmost part of themean westerly wind shifting to the south of 0/ῌS (i.e., near the latitude of Syowa
Station) in this period. This should be a situation in which air parcels reach Syowa
Station from the westerly wind region. Although the height of the low Z2/* bandchanges little in July and August, the area with a small gradient in the low Z2/* band
becomes wide in latitude. This situation keeps westerly winds away from Syowa
Station; consequently, air parcels from the Atlantic Ocean do not arrive very often.

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K. Suzuki, T. Yamanouchi, N. Hirasawa and T. YasunariFig. 0.

(a) Monthly averaged geopotential height (contoured) and zonal wind (shaded) at the 2/*hPa level (referred to as Z2/*) in January and July. For the solid lines, the contour
interval is +** m. Lines under +,** m in January and under ++** m in July are shown
dotted and have a contour interval of ,* m. Dark shading is easterly wind and lightshading is westerly wind.
(b) Time-latitude section of the geopotential height (contoured) and zonal wind (shaded)at the 2/* hPa level averaged between * and 3*ῌE in +331. The contour interval between
adjacent solid lines in +** m and the dotted lines are /*-m from the nearest solid line.
Shading is the same as that in (a).

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Air transport in the Antarctic and Atmospheric Circulation/.

Trajectories that pass over the Antarctic continent

Quasi-stationary planetary wave in spring
In this section, we focus on the atmospheric circulation patterns in spring (between
September and November), which is when trajectories reach Syowa Station after passing
over the inland Antarctic continent. To estimate the frequency of such trajectories, we
calculate the residence times for the air parcels to be in the area from 1*ῌ to 3*ῌS, which
are indicated in percentage for the total time (+,* hours). Figure 1 shows the monthly
average residence times of trajectories starting in the middle (-** to 0** hPa) and thelower (0/* to 2/* hPa) troposphere. The residence time in the middle troposphere isroughly constant from February to August. It increases signiﬁcantly in September andremains large through November. In the lower troposphere, the monthly variation in
the residence time is similar to that for the middle troposphere, but the absolute values
are about twice those of the middle troposphere. In December, the residence time inthe lower troposphere decreases from that in November.
The monthly averaged Z/** for September shows a planetary wave of wave number-, hereafter WN -, with low pressure regions near -*ῌW, 3*ῌE, and +/*ῌW (Fig. 2).
The planetary wave has an almost barotropic structure through the troposphere (notshown). Such a planetary wave of WN - does not dominate in January and July.
Figure 3 shows a time-longitude section of -+-day moving-average anomalies fromthe zonal mean of the geopotential height of Z/** between //ῌ and 0/ῌS. The
planetary wave of WN + dominates in these latitudes throughout the year, and theincrement in the amplitude of the quasi-stationary planetary wave of WN - is foundespecially in September to November.

/.+.

Fig. 1.

Monthly average residence times for trajectories that began in the -**ῌ0** hPa range(middle troposphere) and 0/*ῌ2/* hPa(lower troposphere) over Syowa Station.

Fig. 2.

Monthly averaged geopotential height at the/** hPa level for September. The planetary wave shown has three low-pressure
regions: near -*ῌW, 3*ῌE , and +/*ῌW.

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K. Suzuki, T. Yamanouchi, N. Hirasawa and T. YasunariFig. 3.

Time-longitude section of -+-day moving-average anomalies from the zonal mean of thegeopotential height at the /** hPa level between //ῌ and 0/ῌS. The solid lines are positive
values; dotted lines are for negative values.

The ﬂow associated with disturbances can cause mixing of air parcels between
latitudes. To better understand the relationship between the trajectories and the ﬂow
generated by the planetary wave of WN -, we analyzed the trajectories coming throughthe continent on +3 September when there is a clear planetary wave. Figure +*a shows
three low-pressure regions near -*ῌW, 0*ῌ to 3*ῌE, and +/*ῌ to +1*ῌW. This feature
occurs on all ﬁve days from the +/th to the +3th of September. Following the trajectoryin Fig. +*a, the air parcel enters the continental interior through the eastern peripheryof the trough at 0*ῌ to 3*ῌE. The parcel comes to Syowa Station through the western
periphery of the same trough after traveling over the continent. Many air parcels came
to Syowa Station through the western periphery of the trough at 0*ῌ to 3*ῌE, which
comprises a quasi-stationary planetary wave of WN -, whereas some air parcels enter
into the continental interior through the eastern periphery of the trough at -*ῌW.

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Fig. +*.

(a) The 1-day backward trajectory of a +3 September air parcel that reached a height of/** hPa over Syowa Station. The trajectory began at +, UTC. The dots were drawn forevery +, UTC (i.e., once per day). The geopotential height at the /** hPa level on +3
September is shown by contours.
(b) Same as that for (a) but for ,* June.

Blocking in June
A blocking event occurred on ,* June that altered the air parcel trajectory over
Syowa Station. One trajectory is shown in Fig. +*b. The trajectory originates in thewesterly wind zone, passes near Syowa Station on the western side of the blocking
anticyclone, then enters into the continental interior, and ﬁnally circles back to thestation on the eastern side of the anticyclone.
The position of Syowa Station relative to the circulation pattern is similar for boththe planetary wave of WN - and the blocking, that is, there is a ridge to the west of
Syowa Station and a trough to the east.
/.,.

0.

Summary

Using trajectory analysis, we examined the seasonal changes of air transport
trajectories coming to Syowa Station in the middle and lower troposphere in +331. In
the middle troposphere, most air parcels came from the lower troposphere of thewesterly wind region in middle latitudes throughout the year, except for January. In
January, most air parcels came from latitudes higher than 0*ῌS, where a low
geopotential height band was located, and had little vertical motion, and consequently
few air parcels came directly from the westerly region to Syowa Station. In the lower
troposphere, trajectories were classiﬁed into three patterns: those coming from inland of
Syowa Station (continental interior), those coming along the east coast region (eastcoast), and those coming from the Southern Ocean, south of the Atlantic (south ocean).

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Air parcels from the Southern Ocean were the least common throughout the year,
partially due to a low geopotential height band surrounding the continent similar to thatat /** hPa in January. Instead, trajectories were mainly from the east coast andcontinental interior. These trajectories are supposed to be associated with a drainage
ﬂow system (Katabatic wind) in the planetary boundary layer over Antarctica. These
results are relevant to the origin of the stratiﬁed air over Syowa Station in winter.
Namely, air in the middle troposphere comes from the lower troposphere in middle
latitudes, whereas that in the lower troposphere comes from the middle troposphere overthe continent. This result is relevant to the advection of concentrations of minor
atmospheric constituents.
We also found that trajectories can pass through the continental interior when ablocking event develops over Antarctica or when the amplitude of a quasi-stationary
planetary wave of WN - becomes relatively large. In this case, for the air parcels tocome to Syowa Station from the continental interior, a ridge to the west of Syowa
Station and a trough to the east must form. Then, the air parcels take a path between
the ridge and trough.
We studied the seasonal variation of air transport to Syowa Station during +331.
It is not yet clear how these results apply to other years. To ﬁnd out, we should study
the climatology of the air transport for Syowa Station for at least +* years. In
particular, we need to better understand how the various planetary waves inﬂuence thetrajectories on daily and seasonal timescales. In addition, we aim to ﬁnd out more
about the relation between these variable trajectories and the transport of water vapor
and atmospheric minor constituents to Antarctic stations.
Acknowledgments
The trajectory calculation program used in this paper was developed by Y.
Tomikawa and K. Sato at the National Institute of Polar Research, Japan.
References
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