Antarctic Peninsula

Antarctic Peninsula glaciated mountain region of W Antarctica , extending c.1,200 mi (1,930 km) N toward South America; in the south, volcanic peaks rise to c.11,000 ft (3,350 m). Most of its NE coast is fringed by the Larsen ice shelf. The peninsula is surrounded by numerous islands, including the South Shetlands and the Palmer Archipelago. The tip of the peninsula, 670 mi (1,078 km) from Cape Horn, is Antarctica's farthest point from the South Pole. The continent's only flowering plants are found on the peninsula.

The northwest coast of the peninsula is believed to have been mapped by the British navigator James Bransfield in Jan., 1820, and was explored by sealers in 1820-21. First considered to be part of the continent, the peninsula was later (1928) thought to be a group of islands; the John Rymill expedition (1934-37) proved its peninsularity. It was originally named Palmer Peninsula by Americans for Nathaniel Palmer, a U.S. captain who explored the area in Nov., 1820. In 1832, Britain claimed it and called it Graham Land and Trinity Peninsula. Argentina claimed it in 1940 as San Martin Land and Chile in 1942 as O'Higgins Land. In 1964, by international agreement, the entire feature was called the Antarctic Peninsula; Graham Land, Trinity Peninsula, and Palmer Land are used as local names. The peninsula is now the site of numerous research stations. The disintegration of a Rhode Island-sized section of the Larsen ice shelf over a few weeks time in 2002, although directly due to locally warmer temperatures, was also regarded by some scientists as a result of the more general global warming .

Antarctic climate The climate of Antarctica is determined by a number of factors, including the continent's geographical location, its topography, and its interaction with the high-latitude circulation of the Southern Hemisphere atmosphere and ocean. In common with the Arctic regions, the surface of the Antarctic continent receives less energy in solar radiation than it loses by infrared cooling. This is because the solar elevation remains low all year round and, for regions south of the Antarctic Circle, the sun does not rise above the horizon for part of the winter. Furthermore, the permanent snow and ice that cover over 97 per cent of the continent and the sea ice that covers up to 20 million square kilometres of the surrounding ocean in late winter both have a high albedo, reflecting up to 85 per cent of the incident solar radiation. The net cooling of the polar regions has to be balanced by the atmospheric transport of heat from lower latitudes. In the Arctic, a significant fraction of this transport is accomplished by the stationary planetary waves that are forced by the major land masses of the Northern Hemisphere. In the Southern Hemisphere, the planetary waves are much weaker and most of the heat transported to Antarctica is carried by the mean north–south circulation of the atmosphere and by transient weather systems.

The East Antarctic ice sheet is a high plateau that reaches a maximum elevation of over 4000 m (Fig. 1). Over the interior of the plateau, the surface slope is generally of the order of 1 in 500 or less, rising to 1 in 100 or greater around the coastal fringes. However, even the small interior slope has a profound effect on the continent's climate as a result of the persistent surface cooling, which generates a nearly permanent surface temperature inversion. The cold, dense air adjacent to the surface accelerates down the surface slope and is turned to the left by the action of the Coriolis force. Eventually, the downslope acceleration is balanced by frictional and Coriolis forces and the resultant flow is known as a katabatic wind. Such winds are a characteristic feature of the Antarctic interior. Over the gentle slopes of the plateau, katabatic wind speeds are rarely greater than 5 metres per second (m s⁻¹) and the wind is directed some 30–40° to the left of the local fall line. Because these winds are topographically driven, they exhibit great directional constancy and often appear to be effectively decoupled from disturbances in the free atmosphere. Over the steeper coastal slopes, the katabatic winds accelerate and may converge into coastal valleys, giving rise to extreme wind speeds. For example, at Cape Denison, on the Adélie Land coast, Mawson's 1912–13 expedition recorded a world record annual mean wind speed of 19.4 m s⁻¹ (approx. 43.4mph) and experienced gale-force winds on all but one of 203 consecutive winter days. Such extremes occur only in a few regions where the local topography favours convergence of the katabatic flow; around much of the Antarctic coast annual mean wind speeds are typically between 5 and 10 m s⁻¹.

The outflow of cold air associated with the katabatic winds is restricted to the lowest few hundred metres of the atmosphere and forms one half of a thermally direct circulation that transports heat to Antarctica to balance the surface radiative cooling. Above the cold outflow, there is a compensating inflow of warmer air that gradually subsides into the katabatic layers. Winds in this upper layer have a weak westerly component in contrast to the easterly component of the near-surface winds. The katabatic winds generated over the Antarctic ice sheets thus exert considerable control over the regional atmospheric circulation, and it is now believed that this influence extends well beyond the Antarctic continent itself.

The annual cycle of air temperature at Antarctic interior stations is markedly different from that at coastal stations (Fig. 2). At the coast, summer mean temperatures are around freezing and the coldest month is usually July or August. On the high plateau, temperatures are much lower as a result of both the high surface elevation and decreased solar heating at these latitudes. A record low temperature of –89.5 °C has been recorded at Vostok station. The most remarkable feature of the temperature records from these interior stations is the lack of any well-defined temperature minimum during the winter months. It is believed that this coreless winter results from the character of the annual cycles of both the surface radiation balance and the atmospheric transport of heat from lower latitudes.

Precipitation over Antarctica falls mostly as snow, although rain can occur during the summer in the Antarctic Peninsula and surrounding islands. The highest precipitation occurs around the coasts of the continent, where weather systems moving in from lower latitudes impinge on the steep coastal slopes, giving annual precipitation totals of 300–400 mm (water equivalent) per year around the East Antarctic coast. Few weather systems penetrate inland on to the high plateau. The air here is very cold and dry, with correspondingly low precipitation. Much of the area of the East Antarctic plateau is classed as a desert, with an annual precipitation of less than 50 mm (water equivalent). A large proportion of the precipitation over the plateau occurs as a near-continuous fall of ice crystals from a clear sky, a phenomenon known as diamond dust. Although precipitation rates over the interior of Antarctica are very small, the low temperatures mean that melting and evaporation are negligible and the low precipitation is able to maintain the continental ice sheets, which are over 3 km thick in many places.

The weather systems responsible for transporting heat and moisture towards Antarctica have their origins in the midlatitudes of the Southern Hemisphere or in the circumpolar trough of low pressure. This climatological feature marks a storm track that extends around Antarctica at a mean latitude of about 66° S. The latitude of the trough undergoes a semi-annual excursion. It is closest to Antarctica in March and October, and the surface pressure in the trough is also lowest during these months. A semi-annual oscillation in surface pressure, reflecting the movement of the circumpolar trough, can be seen in surface pressure records from all high-latitude Southern Hemisphere stations. To the north of the trough, the pressure gradient drives the strong westerly winds experienced over the Southern Ocean; surface winds to the south of the trough are easterly and surface pressure increases towards the continent, indicating the presence of a permanent low-level anticyclone over Antarctica.

In 1995 there were about thirty permanent stations making meteorological and climatological observations in Antarctica, most of them situated on the coast of East Antarctica and in the Antarctic Peninsula. Few of these stations have records extending back beyond the International Geophysical Year of 1957, and little direct information is therefore available on the variability of the Antarctic climate on decadal or longer timescales. Some information can be gleaned from the records kept by the expeditions that visited the continent before permanent bases were established while analysis of Antarctic ice cores provides ‘proxy’ records of temperature and precipitation extending back over many thousands of years. The longest instrumental records from continental Antarctica come from the Antarctic Peninsula, where there have been permanent stations since 1945. Temperature records from this region show a very high degree of interannual variability superimposed on a warming trend of about 0.6 °C per decade—much larger than trends measured elsewhere in Antarctica or in the middle latitudes of the Southern Hemisphere. This recent warming trend has been confirmed by observations of significant deglaciation in parts of the Antarctic Peninsula, including a reduction of the area of the Wordie Ice Shelf from 2000 km² in 1966 to 700 km² in 1989. However, the region affected by the warming trend appears to be limited, and at present there is little evidence to connect the trend with hemispheric or global warming. Records from East Antarctica are shorter but the climate of this region appears to be much less variable than that of the Antarctic Peninsula, and no significant temperature trends are seen. The high variability of the Antarctic Peninsula climate appears to result from complex interactions between atmosphere, ocean, and sea ice in this region.

The response of the Antarctic climate to global warming caused by an enhanced greenhouse effect is of great interest because increased melting of the ice sheets could contribute to global sea-level rise. Experiments with coupled ocean–atmosphere general circulation models suggest that, while the Arctic regions might be experiencing a warming that is significantly greater than the global average, any warming in the Antarctic will be relatively modest, at least in the short term. This is because the wind-driven overturning circulation of the Southern Ocean, known as the Deacon Cell, is able to sequester large amounts of heat from the atmosphere to the deep ocean, and thus balance any atmospheric warming tendency. Paradoxically, in a slightly warmer climate the Antarctic ice sheets would at first expand, because the warmer atmosphere could hold more moisture, and snowfall over the continent would increase. However, if global warming continued, most of the sea ice around Antarctica would eventually disappear and the ice shelves around the coast would start to melt as warmer ocean waters flowed on to the continental shelf. Once this stage was reached, the West Antarctic Ice sheet could decay quite rapidly, contributing about a metre to global sea-level rise over 500 years.

J. C. King

Bibliography

King, J. C. and and Turner, J. (1997) Antarctic meteorology and climatology. Cambridge University Press.
King, J. C. (1991) Global warming and Antarctica. Weather, 46, 115–20.

South-east Pacific Plate A lithospheric plate which is now coupled with the Antarctic Plate, but whose oceanic lithosphere is interpreted to have been subducted under southern Chile and the Antarctic Peninsula.