2.1                                   The physical environment of the Antarctic

2.1.1                                The polar cell in the three cell structure of the Earth’s atmosphere

The location of the Antarctic continent corresponds somewhat fortuitously to the region of the southern polar cell in the three–cell structure of the meridional circulation of the Earth’s atmosphere. The factors, which influence the meridional cell structure, include the scale of the planet, the rotation rate, and the depth and differential meridional heat balance of the atmosphere.

Numerical modelling has been used to show that even without the high surface orography of the Antarctic continent (i.e. if it were replaced by a snow surface at sea level) a high surface pressure region would develop, in the mean, over the south polar domain approximately corresponding to the region south of the Antarctic circle. The cold air at the surface flowing outwards from the polar cell turns towards the west, from the Coriolis acceleration, to form strong south easterly to easterly surface winds by the edge of the domain around the Antarctic circle. With strong westerly winds prevailing north of 60º S, from the general circulation in the three cell structure of the Earth’s atmosphere, a strong cyclonic vorticity at the surface naturally occurs between the polar cell and the prevailing westerlies around the Antarctic domain.

As the cold air from the south moves over the warmer, oceanic surface to the north strong baroclinic instabilities and convection occur, which together with the high cyclonic vorticity result in the formation of numerous mesoscale and synoptic scale cyclonic systems around the edge of the Antarctic domain. These synoptic low–pressure centres tend to move with the westerlies around the Antarctic region and also drift southwards into the edge of the polar cell region.

The general outflow of cold air at the surface from the polar cell is balanced by the spiralling southwards of the upper–air westerlies to give subsidence over the central south‑polar region. The advective heat transport of the atmospheric circulation largely balances the heat loss from the negative net radiative balance, with a strong seasonal cycle, over the south–polar region.

This simple picture of the atmospheric circulation around the Antarctic is strongly modified by the orography (see Figure 2.2.1.1 and Section 2.1.3) of the Antarctic continent and also by the way in which the sub–marine orography influences the ocean and sea ice patterns around Antarctica.

2.1.2                                Radiation and heat balances

At the top of the Earth’s atmosphere there is a net radiation loss of heat, in the annual mean, for the domain south of about the latitude of 40º S. There is a strong seasonal cycle in the zone of net radiation loss associated with the tilt of the Earth’s axis. This domain of net radiation loss varies from south of about 12º S in winter to south of about 70º S in summer.


This means that the region covering most of the Antarctic continent experiences a net loss of heat from the atmosphere all year round. Near the South Pole this net radiation loss has an annual mean of about 80 Wm–2, varying from about 50 Wm–2 in summer to about 130 Wm–2 in winter.

 Figure 2.1.1.1     A map of Antarctica showing orographic contours at 500 m intervals.

  (Adapted from a map provided courtesy of the Australian Antarctic Division.)

The net radiative heat loss through the year is balanced by the heat transport of the atmosphere, with net inflow at upper levels, subsidence over the continent and net flow outwards near the surface. In addition to this general circulation pattern, the synoptic scale eddies which move around the continent giving rise to horizontal advection through the troposphere with flow inwards to the Antarctic on the eastern side of the low–pressure centres and outflow on the western sides, with intense mixing of heat and moisture. The greater radiation loss in winter is associated with greater cooling over the continent, with larger horizontal temperature gradients around the continent and more intense circulation with stronger winds.

The snow surface that covers all but about 3% of the Antarctic continent has a number of important influences on the heat balance. The high albedo, averaging about 0.85, means that most of the solar radiation in summer is reflected with relatively little absorbed. On the other hand the high emissivity (~0.97) relative to that of the atmosphere means that the strong radiation loss, along with the low snow thermal conductivity, tends to keep the surface temperature low allowing strong surface inversions to develop in the boundary layer over the interior, particularly during winter and when it is calm. The mean inversion strength in the central zone of the high East Antarctic plateau averages about 25ºC in winter and is mostly concentrated in the lowest few tens of metres of the atmosphere. The cold surface air tends to flow downslope, as a density current, towards the coast increasing in mean speed with the surface slope and the channelling of the flow by the surface orography. The katabatic flow is also strongly influenced by the synoptic scale pressure systems. At the coast the combination of strong katabatic flow with favourable synoptic gradients gives rise to the blizzards that are characterized by gale–force winds and turbulent snow transport reaching several hundred metres above the surface.

Although some loss of moisture occurs with the drift snow transport most moisture loss occurs in summer from evaporation and outward transport in the boundary layer. As a whole the moisture budget for the continent is positive with a net latent heat transport to the continental domain associated with the net precipitation gain over the Antarctic ice sheet. The net snow accumulation varies from about 1000 mm per year water equivalent near the coast, to about 20 mm per year over the high interior of East Antarctica, with a mean over the continent of about 150 mm per year. The latent heat contribution from the moisture budget, while averaging about 12 Wm–2, only contributes about 1/6 of the input to compensate for the radiative heat loss. The remainder is contributed primarily from the atmospheric sensible heat advection that comes from a combination of the mean flow and most importantly the transient eddies.

2.1.3                                Orography

The Antarctic continent has a profound influence on the atmospheric circulation over the south–polar region. Although the area of the continent is contained largely within the 60º S latitude circle, its high orography has a marked asymmetric character (see Figure 2.1.1.1). The Antarctic Peninsula forms a narrow protrusion northwards to about 57º S near 57º W but with the rest of the West Antarctic region of the continent as a whole being much less extensive than that of the eastern region. The mean elevation of the whole continents is about 2,000 m (~6,500 ft) but West Antarctica has most of its area below 2,000 m, although high mountain regions above 3,000 m (~9,800 ft) are common. Outside the Antarctic Peninsula and Dronning Maud Land (also known as Queen Maud Land), the West Antarctic coastline is primarily south of 73º S with the coast of the large ice shelf embayments of the Weddell and Ross Seas reaching about 77º S. In these regions the low, flat ice shelves extend much further south with the 100 m (~330 ft) elevation contour inland of the Ronne Filchner Ice Shelf reaching to about 83º S and that inland of the Ross Ice Shelf reaching to about 86º S. Nevertheless, the high coastal regions of Marie Byrd Land (~140 – 100º W) and the Antarctic Peninsula act as strong barriers to the prevailing westerly winds of the lower troposphere and to the movement of the low–pressure cyclonic systems.

The East Antarctic Ice Sheet has its highest central dome of over 4,000 m (~13,100 ft) elevation (Dome A) located at about 81º S, 79º E. The coastline in Enderby Land (~55º E) reaches as far north as about 66º S, as does the coast in Queen Mary Land (~102º E) and Wilkes Land (~112º E). From 0º to 170º E the ice sheet surface tends to rise steeply from the coast reaching to near 2,000 m elevation within about 400 km from the coast in most regions. The one exception is in the Amery Ice Shelf–Lambert Glacier region where Prydz Bay represents an embayment reaching near to 69º S and the 100 m elevation contour extends inland to about 72º S.

This asymmetry of the Antarctic continent combines with the asymmetry of the locations of the three major continents further north, together with the climatological patterns of the ocean surface, such as the sea surface temperature and sea ice distribution, to give strong orographic and surface forcing to the atmospheric circulation around the south polar region.

The prevailing mean westerly winds in the lower troposphere of the mid latitudes extend southwards to the minimum in the mean surface pressure distribution, which occurs around the Antarctic as a feature referred to as the Antarctic circumpolar trough. The circumpolar trough is located at about 63º S around East Antarctica and closer to about 68‑70º S around West Antarctica but with a break associated with the Antarctic Peninsula. Several dominant low–pressure centres within the circumpolar trough tend to be associated with the Antarctic orography and sea surface temperature (SST) pattern. These separate centres in the mean surface pressure field (see Section 2.6.1) contribute to the mean 3 to 5 wave number character of the lower troposphere circulation around Antarctica. In winter the dominant centres tend to be located near 20º E, 90º E and in the Ross Sea (~170º W) with a weaker centre over the Bellingshausen Sea (~100º W). This results in a predominant 3–4 wave pattern. In summer, with the reduction in sea ice, the Bellingshausen Sea centre usually deepens and a further centre tends to form in the Weddell Sea near 20º W. This results in an approach more towards a 4 – 5 wave number pattern. However, there is a very high variability in the mean patterns on inter–annual as well as seasonal time scales.

The high orography has a strong influence on surface temperature with the mean temperatures decreasing inland with elevation as well as increasing latitude. This places the central coldest region around the Dome A vicinity in East Antarctica where the annual mean surface temperature reaches about –60ºC. Immediately inland of the coast where the ice cap rises steeply the surface temperature tends to decrease at a rate close to the dry adiabatic lapse rate around 1ºC/100 m. Further inland where the plateau slope is smaller this rate may become much higher. This is a feature associated with the reduction in katabatic wind speeds with the reduction in slope. The katabatic winds tend to break down the strong inversions due to mixing. As a result the flatter regions tend to have lower mean temperatures, which is also a common feature of the large, flat ice shelves. Further inland again the mean surface temperature lapse rate reduces more towards the moist adiabatic lapse rate associated with the continual subsidence over the high interior.

The elevation of the ice sheet surface also has a dominant influence on the distribution of precipitation. The annual mean total column moisture decreases from about 2 to 3 mm of water near the coast to less than 0.1 mm over the high interior of East Antarctica. The net accumulation rate tends to follow this smooth distribution of the column moisture but is distorted from it primarily by the pattern of the mean wind transport of the lower and mid troposphere.

The mean cloud climatology is similarly influenced by the elevation and moisture transport. The Antarctic coastal region is characterised as being one of the cloudiest regions of the world with mean total cloud cover typically around 80%. Inland over the continent the low and mid level cloud are greatly reduced by the high orography with much thinner high cloud prevailing (see, for example, Figure 2.6.5.1.1).

In a similar way the large concentration of cyclones moving around the coast of Antarctica are obstructed by the high and steep orography of the ice sheet. Occasionally some of the deeper cyclonic systems penetrate inland over the high plateau, but the most frequent penetration occurs over the lower elevation regions such as those of the large ice shelves. The low amounts of cloud and air moisture over the interior are important factors contributing to the radiation loss from the surface. On the other hand the high variability in the moisture and cloud cover plays an important role in the radiation balance and the generation of anomalous temperature and pressure episodes over the interior plateau, which may change relatively quickly with changing synoptic events.