J Turner, British Antarctic Survey, Cambridge, UK
Copyright 2003 Elsevier Science Ltd. All Rights Reserved.
Introduction
Antarctica is the coldest, windiest, and driest conti-nent on Earth, with a remote location far from the major centers of population. Yet as one of the two heat sinks in the global climate system it plays a crucial role in the general circulation of the atmosphere and has a profound effect on the atmospheric and oceanic
conditions across the Southern Hemisphere. This article presents a brief overview of the climate of Antarctica, which is taken to be the area south of 601S (a map of the Antarctic indicating topographic features and the locations of many of the research stations is shown inFigure 1). Statistics on mean and extreme atmospheric conditions are provided, along with selected mean meteorological fields. Particular attention is paid to the factors that maintain the climate of the Antarctic and the interactions with lower latitudes.
South Orkney Islands
Weddell Sea
Halley Coats Land Anta
rctic Peninsula South
Shetland Islands Adelaide Island
Alexander Island Bellingshausen
Sea
Thurston Island Amundsen
Sea Hobbs Coast
Ross Ice Shelf McMurdo Ross Sea
Adélie Land Wilkes
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Antarctica
Victor ia Land
Dumont D′Urville Casey Lambert
Glacier Enderby
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Syowa Dronning Maud
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Filchaner Ice Shelf Berkner Island Tran
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ntains Ronne
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Ellsworth Land 90° W
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Marie Byrd Land
Amery Ice Shelf
Bryan Coast
Figure 1 A map of the Antarctic showing topographic features and the locations of a selection of research stations.
ANTARCTIC CLIMATE 135
The Antarctic Continent
The climate of the Antarctic is influenced heavily by the orography of the continent, the highest part of which is located close to but slightly offset from the South Pole. The Antarctic represents about 10% of the land surface of the Earth and elevations increase very rapidly inland of the coast, with much of the continent being above 2 km in height and small areas above 4 km. The high interior is therefore isolated from the warm air masses of midlatitudes and is characterized by very cold, dry, relatively cloud-free conditions (Table 1).
The other landmasses of the Southern Hemisphere are well north of the Antarctic, so that the oceanic and atmospheric flow is much more zonal than in the Northern Hemisphere. This further isolates the Ant-arctic from the influences of lower latitudes and is one of the reasons the Antarctic is significantly colder than the Arctic.
Weather Systems in the Antarctic
The large difference in temperature between the Antarctic and the tropics is responsible for the many active depressions found over the Southern Ocean.
These carry warm (cold) air southwards (northwards) on their eastern (western) flanks and attempt to remove the radiatively induced pole to tropics tem-perature difference. Because of the distribution of the landmasses in the Southern Hemisphere the atmos-pheric long waves have a smaller amplitude than their counterparts in the north, so that the depressions play a greater role in the poleward transport of heat than in the Northern Hemisphere.
The depressions over the Southern Ocean generally move from west to east, with those that developed at more northerly latitudes spiraling in towards the Antarctic coastal region. The meeting of cold Antarc-tic and midlatitude air masses in the AntarcAntarc-tic coastal region results in a moderate to strong horizontal temperature gradient (baroclinicity), so that this area is also one of frequent cyclogenesis (development of depressions). In fact, recent studies using the analyses
from numerical weather prediction systems have shown that the Antarctic coastal region has the highest incidence of cyclogenesis anywhere in the Southern Hemisphere. The zone of 60–701S is therefore char-acterized by many depressions, both declining and active, which results in a belt of low surface pressure called the circumpolar trough. This can be seen in the maps of mean sea-level pressure (MSLP) for the four seasons shown inFigure 2. The circumpolar trough is present throughout the year, and in the mean fields has an approximate wavenumber 3 pattern with low-pressure centers close to 301E, 901E, and 1501W. This pattern affects a number of aspects of the Antarctic climate, such as the northward extension of sea ice close to the Greenwich meridian, as a result of the climatological southerly flow at this longitude and the relatively mild temperatures experienced on the west-ern side of the Antarctic Peninsula because of the predominance of north to north-westerly airstreams affecting the area. FromFigure 2it can be seen that MSLP values within the circumpolar trough are lowest during the spring and autumn, and higher during the summer and winter. This semiannual oscillation can be detected in the MSLP observations from the coastal stations and also in the number of reports of precip-itation. The oscillation is a result of changes in the position and intensity of the circumpolar trough over the year, with it being furthest south (north) and most pronounced (weak) in the intermediate (summer and winter) seasons. The cycle is present because of the phase difference between the seasonal cycle of surface pressure values over the continent and the sub-Antarctic latitudes.
One aspect of the climate of the Antarctic that was apparent only once satellite imagery became generally available is the degree to which mesoscale low-pressure systems (mesocyclones) are a feature of the Southern Ocean and Antarctic coastal regions. These systems have a horizontal length scale of less than about 1000 km and a lifetime of less than about 1 day and are therefore difficult to analyse and forecast.
However, they have a major impact on the weather experienced at coastal sites and so are important in the forecasting processes. Although mesocyclones are rare
Table 1 Mean temperature data for selected Antarctic stations
Station Latitude Longitude Elevation
(m)
Period Mean annual temperature
Mean January temperature
Mean July temperature
Vostok 78.51S 106.91E 3488 1958–2001 !55.41C !32.21C !67.11C
South Pole 90.01S 2800 1958–2001 !49.51C !28.21C !59.71C
Mawson 67.61S 62.91E 16 1954–2001 !11.21C 10.11C !18.01C
Faraday Vernadsky 65.41S 64.41W 11 1951–97 !3.91C 10.71C !9.21C
Bellingshausen 62.21S 58.91W 16 1968–2001 !2.41C 11.51C !6.61C
136 ANTARCTIC CLIMATE
over the high interior, they are a common feature on the ice shelves. Here there is low-level convergence of air that has descended from the plateau, which aids the spin-up of vortices, coupled with the presence of mild, oceanic air masses that provide moisture for the formation of cloud. However, during the winter season the automatic weather stations on the Ross Ice Shelf have indicated that low-level mesoscale vortices can be present that do not have any cloud associated with them, but which have a clear surface circulation.
Because of the rapid increase in elevation inland of the coast, few major weather systems penetrate far into the interior of the continent. However, satellite
imagery does show that some frontal bands associated with lows in the circumpolar trough can be seen on the plateau, although automatic weather station data suggest that the pressure signals across these features are small. The conditions that favor depressions having an impact in the interior are amplified long waves and strong steering flow aloft. Under such conditions mild air masses over the plateau can give relatively large falls of precipitation, resulting in a significant fraction of the year’s accumulation falling in a day or two. When the longwaves are strongly amplified, maritime air masses can affect the South Pole and even Vostok Station on the high plateau of East Antarctica, but such conditions are rather rare.
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Figure 2 Average mean sea-level pressure fields (hPa) for the four seasons.
ANTARCTIC CLIMATE 137
The Role of Sea Ice
Another major influence on the climate of the Antarc-tic is the seasonally varying belt of sea ice that rings the continent. Unlike in the Arctic, most of the sea ice melts during the summer so that by February there is only about 3.5#106km2 of ice, most of which is located over the western Weddell Sea. From February and throughout the winter and early spring the sea ice advances in a divergent fashion around the whole continent, reaching a maximum in September, when the ice covers around 19#106km2.
The Antarctic sea ice is generally about 1 m thick and provides an effective cap on the upper layers of the ocean, limiting the fluxes of heat and moisture into the lower layers of the atmosphere. However, the effects of the many weather systems over the Southern Ocean on the sea ice is to open up linear cracks (leads) or larger areas of open water (polynyas), which can provide local sources of heat or moisture, resulting in cloud.
This can be important for the climate at the coastal stations during the winter months when the opening up of coastal leads and polynyas can significantly increase the temperature and humidity, sometimes leading to fog formation.
The other major effect of sea ice is to increase the surface albedo. This is not a significant factor during the period of winter darkness, but can be important in the spring when the sun returns to the high latitude areas slowing the surface heating.
The Temperature Field
Because of the low solar elevations in summer and the long period of winter darkness, the bulk of the Antarctic receives little incoming solar radiation and is always very cold. In addition, the snow-covered surface reflects much of the incoming solar radiation back to space, so giving rise to a positive feedback that helps maintain the frigid conditions.
The high plateau of East Antarctica experiences the coldest temperatures on Earth and at Vostok Station (78.51S 106.91E, 3488 m elevation) they have an annual mean temperature of !55.41C and have recorded the lowest temperature on Earth of
!89.61C, measured on 21 July 1983. The very cold temperatures are experienced not just because of the lack of midlatitude air masses reaching the area, but because of the total lack of solar heating during the winter months, the high elevation of the Antarctic plateau and the very limited amounts of cloud and water vapor in the atmosphere, which allow much of the emitted long-wave radiation from the surface to be lost to space. Although the near-surface layers on the plateau are very cold, a characteristic of the climate of
the high plateau is that temperatures increase with height over the lowest few hundred meters of the atmosphere. This temperature inversion is a result of the intense radiative cooling of the surface and lower atmosphere and is therefore strongest during the winter season, although in other seasons it is still significant. The mean strength of the winter inversion, i.e., the temperature difference between the surface and the maximum temperature in the lower tropo-sphere, varies from about 51C in the coastal region to more than 251C over the highest parts of East Antarctica.
Across the Antarctic there is a very large range of annual surface air temperatures, although it is only in the northernmost part of the Antarctic Peninsula that mean summer temperatures rise above freezing. Over the Antarctic Peninsula and along the coast of East Antarctica the annual cycle of temperatures is similar to those found in midlatitudes with a broad summer maximum and a minimum in July or August. How-ever, at more southerly latitudes the cycle is different, with a sharp summer maximum and a ‘coreless’
winter, during which temperatures vary by only a small amount. This form of the annual cycle comes about for a number of reasons, including the abrupt change in solar radiation at the start and end of the period of Austral winter darkness, the effects of the semiannual oscillation of the annual cycle of advection of warm air into the Antarctic, and the heat reservoir effect of the Antarctic snow pack.
At higher levels in the troposphere the atmosphere is strongly stratified, much more so than in the midlat-itude areas of the Southern Hemisphere. This is the case in all seasons, with the stability being strongest below about 4 km during the winter. In radiosonde ascents a tropopause in usually evident in the summer, but it can become very indistinct during the winter when the stratosphere cools rapidly.
The Wind Field
The strong, persistent winds recorded at a number of sites around the Antarctic are one of the most remarkable features of the continent’s climate. It is now known that these winds are katabatic in origin and occur because of the drainage of cold, dense air at low levels from the interior plateau down towards the coast. The katabatic winds are most pronounced during winter, when there is no incoming solar heating, and a large pool of cold air over the interior is formed to feed the katabatic flow.
Surface winds over the interior show a high direc-tional constancy, indicating that they are dictated by the local orography through katabatic forcing rather 138 ANTARCTIC CLIMATE
than by the highly variable synoptic-scale weather systems. The wind speeds are closely related to the slope of the orography, with the strongest winds being measured at stations on the coastal escarpment and the weakest on the flattest areas of the plateau. Along the coast of Ade´lie Land the orography channels the katabatic flow onto a small stretch of coast, resulting in very strong and persistent winds with a very high directional constancy. It was in this area that Maw-son’s 1912–13 expedition recorded the world record annual mean wind speed of 19.4 m s!1 and experi-enced gale-force winds on all but one of 203 consec-utive winter days.
As the katabatic winds descend from the plateau they turn to the left because of the Coriolis force and merge with the coastal easterlies that are present because of the circumpolar trough north of the coast. The near-surface flow therefore appears as an anticyclonic vortex, with cold air outflow from the continent. In some parts of the coastal region, such as south of the Weddell Sea, the coastal easterly comes up against high orography and the cold, stably stratified air at low levels does not have the kinetic energy to cross the barrier. The air is then dammed up against the barrier until a pressure gradient develops that results in the air moving north as a ‘barrier wind’. With the strong static stability encountered at low levels in the Antarctic, barrier winds are relatively common in the coastal areas of the continent.
Clouds and Precipitation
Clouds are very important in the climate system as they can reflect a significant proportion of the incom-ing solar radiation back to space. However, since the surface of the Antarctic already has a high albedo by virtue of the year-round snow cover, clouds over the continent tend to have less of an effect on the incoming solar radiation because the surface and cloud have similar albedos. Nevertheless, clouds play a very important role in controlling surface temperatures through their effect on the long-wave radiation budget.
In cloud-free conditions, the dry atmosphere allows most of the emitted terrestrial radiation to escape to space, resulting in very low temperatures. However, when thick cloud cover is present surface temperatures are much higher because of the downward long-wave radiation emitted from the cloud.
Since most of the research stations are located in the coastal region, it is difficult to get an accurate picture of the distribution of cloud across the continent.
However, using in situ data and satellite imagery, climatologies of cloud cover have been prepared.
These suggest that the greatest cloud cover is found over the ocean area north of the edge of the continent, with about 85% cloud cover throughout the year near 601S. In the coastal region near 701S, the surface observations indicate that the total cloud cover is about 45–50%, with little seasonal variability and only a small decrease during the winter months. Inland of the coast, the amounts of thick cloud decrease rapidly, since few synoptic-scale weather systems are found here. However, the interior is characterized by extensive, very thin cirrus cloud which gives a semi-permanent veil of ice crystals. This type of cloud causes problems for observers, who have to decide whether to report no cloud or 10/10 cloud cover. The mean annual cloud cover at South Pole station is 45%, but anyone using such statistics has to be aware of the nature of the cloud that occurs there and the problem facing observers of how to report the thin cirrus.
The amount of precipitation across the Antarctic generally follows the distribution of thick cloud. In other words, the greatest precipitation totals are found in the coastal region, with a rapid decrease inland.
Figure 3shows the mean annual precipitation across the continent as estimated from ice cores. These glaciological measurements of accumulation are very similar to precipitation, since there is little evaporation in the interior. However, they are not identical, because of the effects of blowing snow and summer melt in some areas. But with so fewin situ measure-ments of precipitation they have been used extensively as a proxy for precipitation. InFigure 3it will be noted that no data are presented for the northern part of the Antarctic Peninsula, since precipitation varies so rapidly in this area. It can be seen that the area of greatest precipitation is along the coast of the southern Bellingshausen Sea, where there is over 1 m water equivalent per year. This peak is found because of the high frequency of northerly airstreams bringing mild, moist air onto the coast. Other areas of high precip-itation are found where there is frequent cyclonic activity, such as north of Enderby Land and along the coast of East Antarctica. The lowest precipitation in the coastal region is found on the low-lying Ross and Ronne Ice Shelves. Inland of the coast the amounts of precipitation drop very rapidly, so that over the vast majority of East Antarctic there is less that 50 mm of precipitation a year.
A number of estimates have been made of the mean and total snow accumulation across the whole of the Antarctic ice sheet using glaciological data gathered in situ. These estimates have improved as additional surveys have been carried out, and the latest studies suggest a figure of around 160 mm water equivalent per year. This equates to a total input of approximately 2205 Gt year!1.
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The nature of the mechanisms behind precipitation is different across the Antarctic, with most precipita-tion in the coastal area coming from synoptic-scale weather systems, while in the interior most falls in the form of clear-sky precipitation, or ‘diamond dust’ as it is sometimes known. This is an almost continuous fallout of ice crystals from a thin veil of cirrus covering the sky. Clear-sky precipitation has not been investi-gated to any great degree, but is thought to result from the cooling of air over the plateau and the formation of ice crystals as it descends into the cold near-surface layer. Just inland of the coast there is a zone where both synoptic-scale weather systems and clear-sky precip-itation both play a role. Over Dronning Maud Land studies have shown that clear sky precipitation falls on most days, but that a few major weather systems can give a significant fraction of the year’s accumulation in a few days.
Climate Variability and Change
Both the Arctic and Antarctic exhibit a greater degree of interannual and interdecadal climate variability than locations at lower latitudes. This is thought to be a result of the complex interactions between the atmospheric circulation and the cryosphere, including a number of positive-feedback mechanisms that am-plify the climate variability. However, our
understand-ing of climate variability and change is limited in the Antarctic because of the shortness of the records and the fact that most research stations are on the coast, with only Vostok and South Pole stations having long records from the interior.
The time series of annual mean surface air temper-atures at a number of stations are shown inFigure 4.
These stations are located in different climatic re-gimes, at the South Pole (Amundsen–Scott), on the high interior plateau (Vostok), on the coast (Mirny and Halley) and at Orcadas in the South Orkney Islands. It can be seen that all the stations show a high degree of interannual variability, but that this is largest on the western side of the Antarctic Peninsula at Faraday/
Vernadsky Station. This station is located close to the northern limit of the sea ice in the Bellingshausen Sea, and small variations in the ice extent are amplified into much larger surface temperature variations, depend-ing on whether the ocean west of the station is ice-covered or ice-free in a particular winter. Most of the stations show a small warming trend, the exception being Amundsen–Scott Station, where there has been a slight cooling since the late 1950s. The warming on the western side of the Antarctic Peninsula is larger than elsewhere in the Antarctic, and even though the record is not long by the standards of stations outside the Antarctic, the warming trend is statistically significant at the 99% level. Antarctic-wide temperature trends
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Figure 3 Precipitation over the Antarctic estimated from ice core data. Lines are accumulation isopleths in 100 kg m!2year!1(or, equivalently, 100 mm year!1). (Reproduced with permission from Bromwich DH (1988) Snowfall in high southern latitudes.Reviews of Geophysics26: 152.rAmerican Geophysical Union.)
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