Summary

The first recorded voyage south of the Antarctic Circle, by Captain James Cook, provided the first broad description of high southern latitude weather sequences in the region. Some of the general conclusions reached by Cook are still valid: for example, ice in the Southern Ocean is further north in the Atlantic and Indian Ocean Sectors than in the Pacific Ocean Sector. Other mariners provided further initial information, and those who named the "roaring forties" also identified, with more alliteration, the "furious fifties" and "screaming sixties".

The middle and high latitude atmospheric circulation is indeed dominated by the westerly winds, that start south of 30°S and increase in intensity as latitude increases until the Antarctic Circumpolar Trough is reached near 65°S. A narrow zone of easterlies exists between coastal Antarctica and the Circumpolar Trough. The westerlies are marked by a half-yearly oscillation so that they are strongest in spring and autumn.

The oscillation in intensity of the westerlies is due in part to the large thermal inertia of the oceans that causes atmospheric temperature cycles to lag the annual cycle of solar insolation. This means that air temperatures over the oceans do not drop as rapidly in early autumn as temperatures over the continents. Latent heat released as sea ice forms in autumn also slows the high latitude temperature decrease as autumn and winter advance.

Again, increased storm activity in autumn drives warmer air from lower latitudes towards and over Antarctica. This is held responsible for a slight rise in temperature sometimes seen over the continent about June, before further cooling to a minimum in August. The absence of a single clearly defined minimum in winter temperatures, known as a "coreless" winter pattern, is a characteristic found in many parts of Antarctica. The pattern is strongest and occurs most frequently in the Ross Sea Sector.

The rotating storms, or cyclones, that develop in the middle latitude westerlies move gradually across higher latitudes towards Antarctica as they travel around the globe. The maximum frequency of cyclones is found in a concentric band centred about 50° S. A second region of high cyclone occurrence is found along a north-west to south-east band in the South Pacific.

Looking at the longitudinal variation of atmospheric pressures on a hemispheric scale, a long-wave trough (or region of relatively lower pressures) is dominant in the eastern Indian Ocean, whereas the Tasman Sea and the region southeast of New Zealand show persistent ridging (or relatively higher pressures). This pattern influences the movement of cyclones leaving the Indian Ocean, as they travel south of Australia and New Zealand and into the Pacific.

At higher latitudes, there is no clear relationship between seasonal cycles of mean sea-ice extent and cyclone activity. However, some relationships between fluctuations in sea ice and cyclone activity on very short (a few days) and very long (interannual) timescales have been found. For example, 3-day averages of sea ice position have shown that rapid growth of ice occurs to the west of cyclone centres. Annual latitudinal differences in sea-ice extent in the region south of eastern Australia have been shown to correspond with similar latitudinal shifts in the distribution of cyclone tracks.

There is a constant draining of cold air from the interior of Antarctica, outwards to the coast. This flow turns towards the west near the coast, producing the narrow ring of easterlies winds about the continent. Sometimes, persistent severe gales occur at coastal sites as a result of the forced cold air convergence and drainage off the plateau. These blizzards seldom extend more than a few kilometres offshore. The constancy in direction of winds at the coast was remarked on by early explorers such as Sir Douglas Mawson. The often violent coastal phenomena contrast with winds on the inland plateau, which are also remarkably constant in direction, but are usually relatively gentle.

Antarctica has a marked effect on the large-scale atmospheric circulation over the Southern Ocean. Heat sources and sinks, and mountain barriers, have a strong influence on the "standing wave patterns" in the atmosphere that set up the background environment under which smaller scale weather systems develop and move. Antarctica not only affects the atmosphere directly in this way, but its asymmetry about the Pole produces a corresponding asymmetry in ocean temperatures at high latitudes. At 55°S, surface water temperatures in the South Atlantic and South Indian Oceans are about 5°C colder than in South Pacific longitudes.

Numerical simulations of atmospheric flow still have a number of shortcomings, and the errors tend to be largest in polar latitudes. The relative contributions of the heat sink and land mass effect of Antarctica remain to be more precisely defined, but models are sufficiently meaningful to demonstrate that the presence of Antarctica has widespread effects on the Southern Hemisphere circulation.

Present research efforts, directed towards a better understanding of the role of Antarctica in the global climate system, are both observational and theoretical in nature, supported by numerical modelling. Antarctica represents a major global heat sink which sets up the temperature gradient that drives the atmospheric circulation of the Southern Hemisphere. Any high latitude changes can therefore have far-reaching consequences on, for example, cyclone tracks over the Southern

Ocean and thus on the regional climates of middle latitude land masses. Climate studies have also shown the sensitivity of high latitudes to global climate changes. The possible break-up of the West Antarctic Ice Sheet under the predicted "greenhouse" warming, and the appearance each spring since about 1980 of a "hole" in the ozone layer, are two examples of how global climatic variations can be amplified by the unique conditions of the Antarctic atmosphere and oceans.

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