5. MARCO TEÓRICO CONCEPTUAL
5.3 DESARROLLO ORGANIZACIONAL
Location
Named after Thaddeus von Bellingshausen, who led the Russian expedition to the south polar region in 1819–1821, the Bellingshausen Sea lies west of the Antarctic Peninsula at the southeast extremity of the Pacific Ocean. Situated between Alexander and Thur- ston islands, it lies north of Ellsworth Land and adja- cent to the Amundsen Sea in the west. Due to its remote location and extensive sea-ice coverage, the
region is relatively undersampled. The Russian expe- dition was followed by Belgica, which over-wintered in the ice (1897–1899), but only in the last two decades has interest developed with projects under the auspic- es of international programmes such as IGBP-JGOFS (Joint Global Ocean Flux Study), GLOBEC (Global Ocean Ecosystem Dynamics), and WCRP-WOCE (World Ocean Circulation Experiment). Much infor- mation has been obtained remotely by satellites, such as passive microwave measurements of sea-ice extent, sea-surface temperature from advanced very-high- resolution radiometers (AVHRR) and along-track scanning radiometers (ATSR), and surface chloro- phyll from Coastal Zone Colour Scanner (CZCS) and sea-wide field-of-view sensor (SeaWiFS) satellites. Numerical modelling of the area has been included in regional and global ocean-circulation models.
Setting
The shelf width ranges from 100 to 450 km, with aver- age water depths varying between 350 and 650 m. Ocean depths extend to 3500 m. Locally deep troughs are cut into the shelf. The upper continental shelf is
steeper in the eastern Bellingshausen Sea (13–17)
than in the west (1–4). The lower slope is cut by
erosional channels extending across the continental rise down to the abyssal plains. Elongated sediment mounds, associated with the channels, rise 1000 m above the adjacent sea floor. Features such as Peter I Island and Marie Byrd Seamount are volcanic peaks. The region is volcanic in origin, formed by the subduction of Pacific oceanic lithosphere beneath the continental margin of Gondwana, with the break- up of Gondwana leading to movements of crustal blocks along the region. The Antarctic Peninsula and eastern Ellsworth Land form a deeply eroded magmatic arc with much of the shelf and islands formed of clastic and volcanogenic sequences. The ice cap of the Antarctic Peninsula is drained by valley and outlet glaciers into the eastern Bellingshausen Sea. Locally these coalesce to form ice shelves, whose flow is confined by the local topography, islands, and peninsulas.
Climate
The climate is determined by the Antarctic circumpo- lar atmospheric pressure trough lying north of the coastline, which is characterised by many deep depres- sions and generally brief anticyclonic episodes. The leading modes of climate variability are the Antarctic BELLINGSHAUSEN SEA, OCEANOGRAPHY OF
Oscillation (AAO) and changes in the semiannual oscillation (SAO).
The AAO is the leading principal component of
sea-level pressure south of 20S. Most variability
associated with the AAO is zonal, but a nonannular component can be found, with a low-pressure anom- aly west of the Antarctic Peninsula for high values of the AAO index. The AAO has a clear and strong impact on the structure of the ocean currents over large parts of the water column. At the surface, trans- port anomalies associated with a positive phase of the AAO are directed toward the northwest at high
latitudes (south of 45S), inducing an upwelling
that features a maximum at around 65S and a down-
welling at about 45S, which is balanced by a south-
ward return flow below 1500 m. The nonannular response of the AAO gives rise to a low-pressure anom- aly in the Amundsen-Bellingshausen Sea sector during positive AAO years, when the Weddell and Belling- shausen seas are subject to more northerly winds, which induces a warming at the surface and a decrease in their ice-covered areas. The reverse is true of the Ross and Amundsen seas; hence, integrated over the Southern Ocean such regional differences cancel out.
The SAO is a twice-yearly contraction of the circumpolar pressure trough due to differences in energy uptake between Antarctica and its surround- ings. It results in a half-yearly wave in baroclinicity and depression activity. Latitudinal sea-ice extent in the Amundsen and Bellingshausen seas fluctuates with the movement of the circumpolar pressure trough, so a weakly developed SAO suppresses sea- ice growth.
Sea Ice
The climate is strongly influenced by sea-ice extent. Synoptic scale weather patterns, air temperatures, and the regional atmospheric circulation are all af- fected by the extent of sea ice.
The annual cycle of sea ice cover in the Belling- shausen Sea is unusual in its symmetry between ice growth and retreat. The more typical Southern Ocean cycle is one of slow growth and rapid retreat. Since rapid retreat in spring exceeds the available air–sea heat flux, the heat deficit must be made up from the underlying deep water. A more gradual retreat in the Bellingshausen Sea implies less deep-water influ- ence, consistent with a relatively low-salinity surface layer. The Amundsen and Bellingshausen seas also exhibit a low seasonal range in sea-ice extent com- pared to other sectors of the Southern Ocean. Sea ice is sensitive to changes in atmospheric circulation and
the frequency, depth, and track of depressions in and to the north of the circumpolar pressure trough. When depressions are slow moving and there is persistent meridional wind, large sea-ice anomalies may become established and can persist for several seasons.
Exceptional sea-ice retreats and advances occur in the Bellingshausen Sea, including sea-ice retreat dur- ing winter. It is usually associated with a combination of enhanced poleward flow and warm-air convection. The winter of 1993 was remarkable for the amplitude of excursions in the sea-ice edge. The changes were so pronounced because of the strong meridional winds induced by strong high–low pressure couplets on either side of the Antarctic Peninsula and the marked changes of near-surface temperature they brought about, which fluctuated rapidly. The rapid melting and refreezing of ice led to extensive movements in the ice edge.
Strong feedback between temperature over the Antarctic Peninsula and sea-ice cover in the Amund- sen and Bellingshausen seas is at least partly respon- sible for anomalous warming of this region in the last 50 years. A major decrease in sea-ice extent in the Bellingshausen Sea began in the late 1980s. The re- treat was correlated with increasing surface air tem- peratures on the west coast of the Antarctic Peninsula and coincided with more northerly surface winds and greater cyclonic activity. The last 2 decades have been the warmest in the last 5 centuries. During that time, the occurrence of the minimum extent of sea ice moved from March to February. The ice retreat, which was first observed in summer, now extends through all seasons, and the mean latitude of the Amundsen and Bellingshausen ice edge has shifted
southward by about 1.5latitude perC temperature.
Oceanography
The Bellingshausen Sea lies in the Antarctic Zone of the Southern Ocean, where the ocean circulation is dominated by the wind-driven, eastward-flowing Ant- arctic Circumpolar Current (ACC). While the main part of the current passes to the north, the broadening of the flow as it crosses the Southeast Pacific Ridge brings the southern ACC boundary into the Amund- sen and Bellingshausen seas. The Antarctic Zone com- prises the flow between the core of the ACC at the Polar Front and the southern ACC boundary. The zone south of the ACC lies between the southern ACC boundary and the coast. Prevailing easterly
winds south of 65S drive a westerly to southwesterly
flow of water, but the current regime is complex, with BELLINGSHAUSEN SEA, OCEANOGRAPHY OF
meanders and cyclonic gyres and no dominant pat- tern of flow.
Water masses upwell along isopycnals within the ACC, bringing warm, saline Circumpolar Deep Water (CDW) toward the sea surface. There is no marked front along the shelf break, allowing CDW to upwell onto the continental shelf where it causes intense melting at the base of the ice shelves. Limited observations confirm the lack of shelf water in the Bellingshausen Sea, one of the two prerequisites for the formation of bottom water. Thus, production of cold, dense water that sinks beneath CDW is unlikely. Rather, bottom water formed in the Ross Sea is cir- culated cyclonically around the deep southeast Pacific to reach the Bellingshausen Sea in modified form. Modified deep water from the Weddell Sea spreading westwards around the tip of the Antarctic peninsula may spread further westwards in a narrow, weak bottom current observed on the continental slope.
The surface layer consists of cold (<1.5C), low-
salinity Antarctic Surface Water.
Biogeochemistry
The upwelling of CDW across the southern ACC boundary provides a nutrient-rich input to the upper layer of the water column. Nutrient concentrations below 100 m are consistently high. In the surface layer, nutrient concentrations reflect recent biological activity; for example, in late spring, nitrate, phosphate, and silicate decrease from south to north while nitrite, ammonium, biogenic silica, and chlorophyll increase.
Micronutrient measurements are few. Observa- tions of dissolved iron show low levels, while the range of values includes the lowest measurements made in the Southern Ocean. Some enhancement of iron concentrations occurs to the north of the Polar Front and towards the Antarctic continent, where deeper waters may be affected by input from the continental margin sediments.
The Bellingshausen Sea acts as a sink for atmo- spheric carbon dioxide during summer, probably due to biological activity. Periods of increased uptake of carbon dioxide may be caused by enhanced produc- tivity or changes in circulation, and demonstrate that the area has the capacity for rapid and large changes in carbon uptake.