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A positive trend in western Antarctic Peninsula precipitation
over the last 50 years reflecting regional and Antarctic-wide
atmospheric circulation changes
John TURNER, Tom LACHLAN-COPE, Steve COLWELL, Gareth J. MARSHALL
British Antarctic Survey, Natural Environment Research Council, Madingley Road, Cambridge CB3 0ET, UK
E-mail: j.turner@bas.ac.uk
ABSTRACT. In situ observations of precipitation days (days when snow or rain was reported in routine
synoptic observations) from Faraday/Vernadsky station on the western side of the Antarctic Peninsula,
and fields from the 40 year European Centre for Medium-Range Weather Forecasts re-analysis (ERA-40)
project are used to investigate precipitation and atmospheric circulation changes around the Antarctic
Peninsula. It is shown that the number of precipitation days is a good proxy for mean sea-level pressure
(MSLP) over the Amundsen–Bellingshausen Sea. The annual total of precipitation days at the station has
been increasing at a statistically significant rate of +12.4days decade
–1
since the early 1950s, with the
greatest increase taking place during the summer and autumn. This is the time of year when the
Southern Annular Mode (SAM) has experienced its greatest shift to a positive phase, with MSLP values
decreasing in the Antarctic coastal zone. The lower pressures in the circumpolar trough have resulted in
greater ascent and increased precipitation at Faraday/Vernadsky.
INTRODUCTION
The Antarctic Peninsula is one of the most climatologically
interesting areas of the Antarctic. Over the last 50 years,
annual mean near-surface air temperatures on the western
side of the peninsula have risen by up to 0.68C decade
–1
(statistically significant at less than the 1% level), as large a
warming as anywhere on Earth (King and others, 2003). In
addition, a number of floating ice shelves on both the
eastern and western sides of the peninsula have disinte-
grated (Doake and Vaughan, 1991), and sea-ice extent over
the Bellingshausen Sea has decreased (Zwally and others,
2002). However, we have little knowledge of how pre-
cipitation has changed over the last 50 years.
Precipitation is extremely difficult to measure at the
research stations because of blowing-snow effects, which
can cause snow to be added to or removed from snow
gauges in the absence of precipitation, once the wind speed
exceeds about 10 m s
–1
. Some Antarctic Peninsula stations
have used snow gauges, but the results have not been
satisfactory. A more commonly used procedure has been to
measure net surface mass balance (for a definition of this
term and its relationship to precipitation and accumulation
see Turner and others (2002)) at the stations or remote sites
using a single stake or array of stakes. This method works
well at interior locations on the continent where there is
little blowing snow and the orography is fairly flat.
However, in the coastal region, where the winds are
frequently strong, stake measurements are of less value,
with the greatest increases observed often being associated
with strong wind events rather than heavy precipitation
(Turner and others, 1995).
Estimates of mass balance from ice cores and pits have
proved particularly valuable and have allowed the produc-
tion of maps of mean mass balance for the whole continent
(Bromwich, 1988). With careful analysis, ice cores can yield
annual mean accumulation, although the wind can mix
large amounts of snow on the surface, thus hindering the
identification of annual layers.
Atmospheric models are a powerful tool for examining
the precipitation and precipitation–evaporation (PE) over
the continent, providing data across regions where there are
no in situ observations. Model output has been used to
consider the temporal and spatial variability of these
quantities across the Antarctic (Turner and others, 1999)
and how precipitation may change in coming decades
(Budd and Simmonds, 1991). However, the relatively coarse
horizontal resolution of these models has meant that their
performance has been poor in areas of complex orography.
Recent experiments with high-resolution, limited-area mod-
els have had more success in the Antarctic Peninsula region
(Van Lipzig and others, 2004), but this is still a challenging
area for models.
Atmospheric models have been used as part of data
assimilation schemes incorporating in situ and satellite
observations to produce meteorological analyses. Recently,
some of the major meteorological agencies have re-
analyzed all the observational data using current analysis
schemes to produce the so-called re-analysis datasets
(Gibson and others, 1996; Kalnay and others, 1996). These
provide surface and upper-air meteorological analysis fields
at the four main synoptic hours each day, extending back to
the 1950s. As discussed below, these fields tend to be rather
poor at high latitudes in the pre-satellite era, but never-
theless are an extremely important tool for investigating
recent climate variability.
One final form of data that has been used to examine
precipitation variability and change is the observations of
precipitation occurrence from the stations, made as part of
the synoptic reporting programmes (Turner and Colwell,
1995). These observations cannot provide information on
the amount of precipitation that has fallen, but can yield
valuable data on precipitation frequency and the relative
occurrence of snow and rain in the Antarctic.
In this paper, we investigate the changes in precipitation
that have taken place around the Antarctic Peninsula over
the last 50 years, using the European Centre for Medium-
Range Weather Forecasts (ECMWF) re-analysis (ERA) data
Annals of Glaciology 41 2005 85
and observations from Faraday/Vernadsky station (65.48S,
64.48W). The data on precipitation days provide one of the
few reliable meteorological datasets on precipitation ex-
tending back to the 1950s. We have used this time series in
conjunction with the re-analysis fields to infer atmospheric
circulation variability and change in the pre-satellite era.
DATA AVAILABILITY AND CHARACTERISTICS
The re-analysis fields
The re-analysis fields produced by the ECMWF provide one
of the most reliable and consistent series of surface and
upper-air meteorological analyses available. Fields are
available every 6hours and have been produced with a
current, state-of-the-art data assimilation scheme that has
been run using the historical record of in situ meteorological
observations and satellite data. The fields cover the 44year
period 1957–2001, although the project is still known as
ERA-40.
In the pre-satellite era the only upper-air meteorological
data over the Southern Ocean were from widely spaced
island stations, while surface conditions had to be analyzed
from ship reports, which tended to come mainly from along
the major shipping lanes. It is therefore not surprising that
the quality of the ERA-40 fields around the Antarctic is poor
in the early years. Marshall (2003) compared the ERA-40
fields against the available in situ observations and
concluded that the mean sea-level pressure (MSLP) and
upper-level height fields can be used with confidence as far
back as 1973. However, he also noted that the fields from
over the Southern Ocean contain considerable errors in the
late 1950s and early 1960s. This makes them unsuitable for
investigation of atmospheric conditions around the Antarctic
Peninsula during this period. Figure 1 shows the differences
between the monthly mean MSLP data from Faraday and
ERA-40 at the location of the station, and confirms that from
1979 the Faraday MSLP is well represented in ERA-40,
although there is a small negative trend. From 1974 the
satellite temperature soundings from the Vertical Tempera-
ture Profile Radiometer were included in the analysis
system, so differences were usually limited to several hPa;
but before this, as would be expected given the lack of data,
there are very large differences between the Faraday data
and ERA-40. The Faraday observations themselves were
assimilated, but since the model fields to the west are
essentially climatological there is a mismatch between
conditions at Faraday and further west, hence the difficulty
of fitting the station data to the analysis scheme. It should
also be noted that the ERA MSLP data have a positive bias of
about 3 hPa at the location of Faraday in the period before
about 1970.
In this study, we have used the ERA-40 precipitation fields
and determined the number of precipitation days at the
location of Faraday. We have taken a precipitation day to be
any day with >0.1 mm of precipitation. This amount was
chosen to exclude the extremely small amounts of pre-
cipitation generated within the model by numerical in-
stabilities.
Although a number of studies of the quality of the ERA-40
MSLP and height fields have been carried out, it is unclear
how accurate the precipitation data, and especially the time
series of precipitation days, are for the Antarctic. Figure 2
shows the annual totals of the number of precipitation days
at Faraday and in ERA-40 at the location of Faraday. It shows
that in the period up to 1983 there was little correlation
between the two time series, as a result of the lack of
humidity data being assimilated into the analysis system.
However, from 1984 onwards there is good agreement
between the number of ERA and Faraday precipitation days
(correlation coefficient 0.8), indicating that the ERA analysis
system accurately simulates the hydrological cycle around
the Antarctic Peninsula. In the following we have there-
fore only used the ERA precipitation data over the period
1984–99.
Precipitation-day information
For this study we have used the time series of the number of
precipitation days occurring each month at Faraday/
Vernadsky station. We have defined a precipitation day as
any day when there was at least one report of snow, rain,
drizzle, hail or a shower in the past or present synoptic
weather reports. Reports of blowing snow and clear-sky
precipitation (‘diamond dust’) were not regarded as pre-
cipitation reports, and were therefore ignored.
The meteorological observing programme at the station
began in 1947, but for the first 3 years the weather reports
were made at different frequencies, so the number of
precipitation days is lower than in later years. A similar
marked drop in the number of precipitation days occurred
after 2000, indicating another change in observing practice.
Fig. 1. Differences between the monthly mean MSLP data from
Faraday and ERA-40 at the location of the station.
Fig. 2. Annual totals of the number of precipitation days at Faraday
and in ERA-40 at the location of Faraday.
Turner and others: Positive trend in western Antarctic Peninsula precipitation86
In the following we have therefore only used Faraday/
Vernadsky precipitation-day data for the period 1950–99,
when regular 3 hourly synoptic reports were made.
Although the synoptic reports provide information on
precipitation intensity, examination of the time series of
these observations shows marked jumps in the frequency
with which particular intensities, slight, moderate or heavy,
are reported, especially when there was a change of
observer. Although the World Meteorological Organization
guidelines on making observations provide information on
how to estimate snowfall intensity from the reduction in
visibility, this is still a very subjective process and dependent
on the observer. In this study we have therefore not used the
observations of precipitation intensity made by the ob-
servers. However, the question arises as to how the statistics
on precipitation days relate to the actual amount of
precipitation that fell. Figure 3 shows a scatter diagram of
the number of precipitation days at Faraday vs the monthly
totals of precipitation from ERA-40. The data are for all
months over the period 1984–99. It can be seen in Figure 3
that for any particular number of Faraday precipitation days
during a month there is a broad spread of ERA-40
precipitation amounts, but this is to be expected, as a
precipitation day can range from a few snowflakes to many
hours of heavy snowfall. However, Figure 3 does illustrate
the general increase in precipitation amount that is found
with a greater number of precipitation days in a month.
A further illustration of the value of precipitation-day data
from Faraday can be seen in Figure 4, which shows a
comparison of monthly precipitation-day totals from the
station and ERA-40. There is some scatter since the
precipitation-day values are produced by such different
means, but the correlation between these two datasets
indicates that the model reasonably represents the variability
of this quantity at the location of the station. The smaller
number of precipitation days in the Faraday record is to be
expected, as the observer does not watch the weather
continuously and observing conditions become difficult
during the long, dark winter nights.
TRENDS IN THE FARADAY/VERNADSKY RECORD
OF PRECIPITATION DAYS
Figure 5 shows the time series of the total number of
precipitation days over the year from Faraday/Vernadsky,
along with the annual mean temperature. It can be seen that
both quantities have increased over the 50year period:
temperatures at a rate of +0.568C decade
–1
and precipitation
days at a rate of +12.4decade
–1
, with both increases being
statistically significant at less than the 1% level. However,
there is only a relatively low correlation of 0.25 between the
two datasets, due to the different factors determining these
two quantities in a particular year and the timescales on
which they operate.
Temperatures at the station, especially during the winter,
are highly correlated with the sea-ice extent to the west of the
Antarctic Peninsula (King, 1994), years of extensive (little) sea
ice being characterized by low (high) temperatures. The sea-
ice extent in a particular year is heavily influenced by the
frequency of winds from the north or south, with northerlies
(southerlies) inhibiting (aiding) the advance of the ice. The
Fig. 3. Faraday precipitation days vs total monthly precipitation
from ERA-40. The data are for all months during the period
1984–99.
Fig. 4. Comparison of monthly totals of precipitation days from
Faraday and ERA-40. The data are for all months during the period
1984–99.
Fig. 5. Faraday/Vernadsky annual mean temperature and total
number of precipitation days, 1951–99.
Turner and others: Positive trend in western Antarctic Peninsula precipitation 87
winds affecting the area are in turn determined by the
synoptic conditions over the Amundsen–Bellingshausen Sea
(ABS) with low- (high-)pressure systems resulting in generally
northerly (southerly) flow. However, the sea ice does not
respond instantaneously to changes in the wind direction and
it has been shown that sea-ice anomalies, once created, can
last for long periods (King, 1994).
The number of precipitation days at Faraday is dictated to
a large extent by the air masses that arrive at the station:
moist, north-to-northwesterly (dry, southerly) air masses
result in a large (small) number of precipitation days (Turner
and others, 1995). So, as with sea-ice extent, the winds over
a year will influence the number of precipitation days
recorded at Faraday. But this quantity will respond almost
immediately to changes in wind direction and be more of a
direct measure of air-mass origin, and hence depression
activity over the ABS, than sea-ice extent or temperature.
This can be seen in Figure 6, which shows the monthly mean
number of precipitation days recorded throughout the year
at Faraday for the period 1951–99. This figure shows that the
greatest number of precipitation days occurs in the spring
and autumn, in association with the peaks in depression
activity as the circumpolar trough moves south and
intensifies as a result of the semi-annual oscillation.
Examination of the standard deviation of the number of
precipitation days throughout the year (not shown) does not
suggest any seasonal signal, which would be expected if the
quantity were being heavily influenced by the sea-ice extent
(as occurs with the temperature record) which has its largest
variability during the winter.
The close association between the number of precipita-
tion days at Faraday and depression activity over the ABS
can be further appreciated by noting that one of the largest
totals of precipitation days in the record occurs in 1979
(Fig. 2). This was the year of the First GARP (Global
Atmospheric Research Programme) Global Experiment,
when Physick (1981) noted that there were many deep
depressions and low MSLP values in the circumpolar trough.
The seasonal trends in the Faraday mean temperatures
and number of precipitation days are shown in Table 1. Here
it can be seen that the temperature trend has a clear
maximum in the winter, when it has almost double the
magnitude of any other season, then rapidly decreases
towards the spring. On the other hand, the trend in the
number of precipitation days is largest during the autumn,
although as can be seen in Figure 6, it varies considerably
over the year and does not have the smooth form of the
temperature trend as shown in King and others (2003).
RELATIONSHIP BETWEEN ATMOSPHERIC
CIRCULATION AND NUMBER OF PRECIPITATION
DAYS
In order to gain insight into the relationship between the
number of precipitation days at Faraday and the broad-scale
atmospheric circulation, the annual totals of precipitation
days from the station and the ERA-40 data at the location of
Faraday were correlated with the annual mean values of
MSLP across the whole of the Southern Hemisphere (Fig. 7).
The two fields are very similar considering the means by
which the number of precipitation days was computed using
the in situ data and ERA-40 fields. The greatest negative
correlations (a large number of precipitation days associated
with low MSLP) are over the ABS where values of –0.6 to
–0.8 are found with both the ERA and Faraday data,
although the area of correlation in this range is larger in
ERA. This picture of low MSLP over the ABS during periods
of greater number of precipitation events at Faraday is
consistent with our understanding of the means by which
warm air masses can arrive at the station. Case studies of the
arrival of different types of air mass on the western side of
the Antarctic Peninsula have shown that the location of the
low-pressure centres over the ABS is critical in determining
whether the area is affected by air of warm, mid-latitude or
cold Antarctic origin (Marshall and others, 1998). The
location of the highest values of negative correlation in the
southeastern corner of the Bellingshausen Sea is just about
optimal in ensuring that air arriving at Faraday has a long
fetch over the relatively warm Southern Ocean.
Around the rest of the Antarctic coastal zone the
correlations are quite low, indicating that over the year as
a whole it is synoptic activity over the ABS that mainly
determines the number of precipitation days at Faraday and
that conditions across the rest of the continent play a
relatively small role. However, it should be noted that
determining MSLP under the high-Antarctic orography has
little physical meaning, and correlations across the higher
parts of the Antarctic should be ignored.
Some areas of large positive correlation are found north
of the Antarctic in the Western Hemisphere, with values
from the ERA data being in excess of 0.8 over the South
Fig. 6. Faraday/Vernadsky mean monthly number of precipitation
days and the trend of 1951–99.
Table 1. Mean data and trends for Faraday/Vernadsky station. Data are for 1951–99
Annual Summer
(Dec–Feb)
Autumn
(Mar–May)
Winter
(Jun–Aug)
Spring
(Sep–Nov)
Mean number of precipitation days 267.1 58.7 67.3 65.6 70.6
Trend in precipitation days (days decade
–1
) +12.4 +2.8 +3.9 +3.2 +1.9
Trend in surface temperature (8Cdecade
–1
) +0.56 +0.25 +0.60 +1.08 +0.28
Turner and others: Positive trend in western Antarctic Peninsula precipitation88
Atlantic. Again this is to be expected since there is often an
anticorrelation between mean MSLP values in the Antarctic
coastal region and the 50–608S zone (Marshall and King,
1998), depending on the storm tracks over the South Pacific.
Similar maps of correlation between precipitation days
and MSLP have been produced for each month of the year.
In several of the months the area of high correlation has a
circular or wavenumber-3 form, with correlation values in
excess of 0.6 being found around much of the Antarctic
coastal region. This is particularly the case in summer and
autumn, as can be seen from the January map shown in
Figure 8. In January there is still a regional maximum of
correlation in the southeastern Bellingshausen Sea, but
similar high correlation values are found around much of the
coast of East Antarctica. This pattern suggests that changes in
the number of precipitation days at Faraday during certain
months are linked to Antarctic-wide, as well as local
synoptic, forcing factors in the immediate ABS region.
Figure 8 also shows that in January there were areas of high,
positive correlation in the 40–608S zone, suggesting Ant-
arctic–mid-latitude linkages play a role in modulating the
Faraday precipitation.
During the second half of the year, the maximum
correlation is generally located in the southeastern Bellings-
hausen Sea, with low correlations around the rest of the
continent, suggesting that at this time of the year local
synoptic conditions have most influence on Faraday
precipitation.
DISCUSSION
The number of precipitation days as determined from the
Faraday synoptic observations is clearly a valuable measure
of meteorological conditions over the ABS. The close
agreement between the time series of this quantity from
the station and similar measures derived from ERA-40 in
recent years suggests that it is a reliable measure of the
precipitation falling throughout the day. The high correlation
between the Faraday total of precipitation days and ERA-40
MSLP over the ABS indicates that it can be used as a proxy of
surface pressure in this area.
The time series of the annual total of Faraday precipitation
days (Fig. 5) shows that this quantity increased at a
statistically significant rate from the early 1950s up to the
Fig. 8. The correlation of the January totals of precipitation days
from Faraday with the January mean values of MSLP from ERA-40.
Fig. 7. Maps showing the correlation of the annual totals of
precipitation days from Faraday (bottom) and the ERA-40 data at the
location of Faraday (top) with the annual mean values of MSLP from
ERA-40.
Turner and others: Positive trend in western Antarctic Peninsula precipitation 89
late 1970s and has been essentially unchanged since that
time. The trend varies considerably throughout the year, but
generally peaks during the first half of the year. The
essentially circular, or ‘annular’, nature of the precipita-
tion-day/MSLP correlation maps, along with the low-/high-
latitude out-of-phase relationship, points to the Southern
Annular Mode (SAM) playing a role. This is the principal
mode of variability in the atmospheric circulation of the
Southern Hemisphere extratropics and high latitudes, and
involves synchronous anomalies of opposite sign in Ant-
arctica and the mid-latitudes. Following Marshall (2003), the
SAM can be defined as:
SAM ¼P
40SP
65S,
where P
40Sand P
65Sare the normalized monthly zonal
MSLP at 408and 658S respectively.
In recent decades the SAM has entered a positive phase,
with decreasing pressures over the Antarctic and increasing
values in the mid-latitudes (Marshall, 2003). As shown in
Figure 9, the SAM has changed most during the first half of
the year. Clearly the trends in the SAM and the number of
precipitation days are not directly comparable, but the
annular form of the correlation maps and the large trends in
the SAM and precipitation data in the summer and autumn
both point to the SAM playing a role in the changing nature
of precipitation at Faraday.
It is interesting to consider how the changes in the SAM
could have resulted in more precipitation days on the
western side of the Antarctic Peninsula. The ABS is at the
latitude of the circumpolar trough, which is the belt of low
pressure around the continent between about 608and 708S.
It is present because of the large number of depressions in
this zone that have either developed just north of the
Antarctic or moved south from mid-latitudes. The fact that
pressures have decreased in the trough does not tell us
whether the number of depressions has increased or the
mean depth of the lows has decreased, with the number of
systems remaining constant. Either way, the ready supply of
moisture over the Southern Ocean coupled with lower
atmospheric pressures will result in greater dynamical lifting
and increased precipitation.
As discussed earlier, the temperature at Faraday has
increased most during the winter. Examination of the July
and August number of precipitation days for Faraday shows
that this quantity has increased since the 1950s, pointing to
lower pressures over the ABS. However, the SAM has
changed less in winter than in summer, suggesting that other
factors are responsible for the decrease in sea ice over the
ABS. During winter, the sea-ice edge is close to the latitude
of Faraday so that any changes in the depth or frequency of
synoptic-scale weather systems over the ABS will change the
northerly component of the wind, thus amplifying variations
in the air masses arriving at the station via the ice–
atmosphere feedback mechanism. However, further work,
especially using coupled atmosphere–ocean models, is
needed to fully explain the marked winter temperature
increase over the western Antarctic Peninsula.
It is difficult to compare the Faraday record of precipita-
tion days directly with glaciological measurements of
accumulation from the Antarctic Peninsula. Ice cores are a
powerful tool for investigating accumulation variability, but
must be collected from areas with no summer melt, and
there is considerable melt along the coastal strip of the
western Antarctic Peninsula where the stations are located.
However, ice cores collected in the southeastern part of the
peninsula and on James Ross Island near the tip of the
peninsula show accumulation increases over the second half
of the 20th century, reflecting the precipitation increase
suggested by the Faraday data.
CONCLUSIONS AND FURTHER WORK NEEDED
The time series of the number of precipitation days at
Faraday is a good proxy of the MSLP over the ABS and
suggests that pressures were higher over the ocean to the
west of the Antarctic Peninsula in the 1950s and 1960s. The
greatest increase in precipitation days has occurred during
the summer and autumn, and appears to be associated with
a change in the nature of the SAM. But the reasons for the
marked increase in winter near-surface temperature at
Faraday over the last 50 years are still not fully understood.
Further work is needed to understand why the SAM has
changed, and the results of this shift into a positive phase.
Automatic depression-tracking software needs to be run on
the ERA-40 fields to understand how the changes in the SAM
have altered the synoptic-scale weather systems in the area.
Has there been a change in the number of lows and/or in
their depth? And have there been changes in the ratio of
systems developing in the circumpolar trough and moving
south from mid-latitudes?
Model experiments also need to be carried out to
determine whether the small changes in the SAM during
winter could have influenced the depressions over the ABS
sufficiently to change the meridional component of the wind
to the point where the ice–atmosphere feedback mechanism
could have been responsible for the marked change in
surface temperature.
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