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«This review examines the operation of the Antarctic Circumpolar ocean current, its role in the so-called ‘great ocean conveyor belt’ of worldwide ...»

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The Antarctic Circumpolar Ocean Current

A review of its influence on global ocean currents and climate within

Antarctica and Europe

James S. B. Mason

GCAS Class 2006-7

Department of Antarctic Studies and Research

University of Canterbury

Abstract

This review examines the operation of the Antarctic Circumpolar ocean current, its role

in the so-called ‘great ocean conveyor belt’ of worldwide ocean currents and its influence

on climate, particularly in northern Europe.

The development in the understanding of ocean currents and their driving forces is described using historical sources, starting from the observations of early explorers to modern scientific analysis.

The interaction of the Antarctic Circumpolar current within the ocean conveyor belt and its influence on worldwide oceanic flow is reviewed with reference to its effect on the Gulf Stream The associated implications for climate change within Antarctica and Europe are discussed in the context of recently proposed scenarios.

Introduction The Antarctic Circumpolar Current ( ACC ) encircles Antarctica, flowing from west to east, and stretches over twenty thousand kilometers forming the world’s largest ocean current. The average flow rate is estimated (1) at 135 million cubic metres of water per second ( 135x106 m3 s-1 ) or 135 Sverdrup ( Sv ) with 1 Sv being the estimated flow of all the world’s rivers combined. Although the flow rate of the current is low, less than 20 cm s-1, the current can reach a width of 2000km and depths of 2000-4000m which accounts for the huge flow rate. The absence of any continental landmass allows the ACC to circulate around the globe allowing water transfer between oceans. The ACC is driven by the strong winds of the Polar Front which extend between the latitudes of 40°S and 60°S, where the average wind speed is between 15 and 20 knots.

The Gulf Stream begins in the Caribbean and ends in the northern North Atlantic. The flow of this current varies markedly along its length from 30 Sv in the Florida Current to a peak of 150 Sv at 55°N (2). The Gulf Stream is a boundary current, these flow close to and parallel with continental margins and they dominate surface current patterns in the world’s oceans. This flow is also seasonally dependant, reaching a maximum in the autumn and minimum in spring. Although the Gulf Stream undoubtedly provides a warming influence to Europe, there is considerable debate to its extent and importance for the European climate.

The ACC encircles Antarctica and thereby connects the ocean basins of the Atlantic, Indian and Pacific Oceans. The ACC smoothes out variations in water properties, for example salinity and temperature between these basins, and enables a global ocean circulation pattern. Ocean water at the high Antarctic latitudes is made cold and saline, through the formation of sea ice, causing it to sink. This cold, deep water flows to lower latitudes, equalising the temperature and salinity difference of deep ocean water, and warm, shallower water flows in the reverse direction to maintain water balance. The ACC is a key component in the so-called ‘ocean conveyor belt’ by allowing the free exchange of water between the ocean basins. Consequently the ACC is an important factor in worldwide ocean currents forming a linkage to the Gulf Stream.

The exchange of cold water from the Antarctic with warm water from lower latitudes regulates worldwide temperatures and is an important factor in worldwide climate. The European climate is influenced by the Gulf Stream which, in turn, is affected by the ACC. In the period between 1300 and 1800, Europe was so cold that the period has been dubbed the ‘Little Ice Age’. Many researchers believe that this was due to a slowdown in the ocean current system, including the Gulf Stream, carrying heat towards Europe (3).

At the same time, it has also been proposed that the southern ocean currents were stronger and that in a mirror image of the Little Ice Age, the Antarctic region warmed up by perhaps as much as 3°C. This review covers the development in understanding of the ACC and ocean conveyor belt, it then examines the implications for climate, specifically the linkage with the Gulf Stream and warming of Europe.

Early Observations

The early explorers of the southern latitudes discovered that both the air and sea became cooler as they sailed across the Southern Ocean. It was also observed that strong currents within this ocean set their ships east where the temperature change occurred. In 1775, this strong current was noted by James Cook on his second voyage with the ships Resolution and Adventure. Subsequent recordings of the current were made by Thaddeus Bellingshausen ( 1819-1821 ) and James Clark Ross ( 1839-43 ) in their journals. The southern latitudes have been known since early times for some of the strongest westerly winds on earth. This prompted sailors to term these winds the ‘Roaring Forties’ and ‘Furious Fifties’ and it is these winds which drive the ACC.

In the sixteenth century, it was observed that surface water in the Pacific Ocean off the coast of Peru and Northern Chile was unusually cold for that latitude. In 1811, Humboldt, whose name is given to the associated current, proposed that advection of cold water from southern latitudes was responsible for this lower than expected temperature.





In 1787, the scientist Richard Kirwan theorised that water in the high latitudes “is, by cold, rendered specifically heavier that that in the lower warm latitudes, hence there arises a perpetual current from the poles to the equator”. In 1798, Benjamin Thomson noted that the maximum densities of pure water and salt water occurred at different temperatures. He proposed that descending cold water, since it has higher density than water at the same depth in warmer latitudes, would spread on the sea floor and move towards the equator and that this would produce a surface current in the opposite direction. A similar theory was also proposed by Humboldt in 1814 (5). This was the start of understanding thermohaline circulation which is caused by differences in sea density caused by differences in temperature and salinity The action of wind above a water surface results in friction at the interface causing the water to move in the direction of the wind thereby setting up a surface ocean current.

During the latter part of the nineteenth century, there was scientific debate on the relative importance of wind, temperature and density differences in defining ocean circulation. In 1878, Karl Zöppritz suggested that the current was primarily set by the wind and had the same average direction. However this theory did not account for the Coriolis effect of the earth’s rotation. A second theory was proposed by Vihelm Bjerknes in 1904 which gave density and temperature differences as the driver for ocean circulation.

During the Fram expedition in the Arctic ( 1893-6 ), Fridtjof Nansen found that the drift of the vessel in the sea ice did not, according to the then general opinion, follow the wind’s direction but deviated between 20° and 40° to the right. This was later explained by V. Walfird Ekman to be as a consequence of the Earth’s rotation (6). He predicted that the current would spiral clockwise ( for the northern hemisphere ) with increasing depth and reduce in strength, the so-called Ekman spiral (7). The two apparently divergent theories where unified as Ekman showed that viscous effects were not able to transfer horizontal momentum vertically down the water column and that it was also necessary to consider downwards turbulence as well.

The Ekman spiral causes a net movement of water ( to the left of the wind in the southern hemisphere ) and this is known as Ekman transport. An example of this transport, called divergence, occurs when surface winds move water away from a region.

In this case, subsurface waters move upwards to replace this lost water and a prominent divergence occurs, forming a gyre centre. A gyre is circular surface rotation of ocean water due to this effect which rotates to the left in the southern hemisphere and the process of moving water in or out of a gyre centre is termed Ekman pumping.

A Norwegian oceanographer Harald Sverdrup joined Roald Amundsen’s North Polar expedition in 1917 and also worked under Bjerknes. During the 1930s, he developed fundamental theories on how wind surface stress transfers mechanical energy to the ocean and recognized this as the dominant factor (8). He also developed the theory of ocean circulation known as Sverdrup balance which was the first accurate description of the process. He was honoured by having the unit of volume flow for currents named after him. In 1957, Stommel suggested that the ACC is controlled by Sverdrup balance and although this is not universally accepted (9), it is still supported by some (Warren, 1996 ).

The Development of Oceanography and Linkages to Climate

During the 1950s, oceanography developed significantly and this was driven by two institutions, the U.S. Navy’s Office of Naval Research, which actively funded many aspects of ocean research and the International Geophysical Year of 1957-8 which set up an international committee on this subject. The circulation of ocean currents became better charted using the radioactive isotope carbon 14 contained within the carbon dioxide dissolved in sea water. Nuclear tests during the late 1950s had also placed tritium and other radioactive material into the atmosphere which were also used as tracers to map the main features of ocean circulation. These methods were later supplemented by remote sensing techniques using satellites. During the 1970s, the U.S. Government funded the Geochemical Ocean Sections Study for studying ocean circulation.

In the late 1960s, the first use of computers to model ocean circulation began with an early researcher being Kirk Bryan who started modeling the Gulf Stream. In 1969, he coupled his ocean basin model to Syukoro Manabe’s model of atmospheric circulation which allowed a significant simulation of climate and thermohaline ocean circulation.

Links between ocean currents and climate were suspected from early times and in 1904, Petterson suggested a relationship between outbursts of ice from Antarctica and fluctuations in the monsoon rainfall of India (10). In 1963, Jerome Namias proposed how a change in prevailing winds could change the ocean surface temperature which in turn affected the winds. It was later recognized that ocean-atmosphere feedback oscillations could occurs on timescales of a few years to decades. In the late 1950s, researchers such as Wallace Broecker and Henry Stommel began to make connections between disturbances in the ocean circulation system and changes in climate. These ideas were developed worldwide during the 1960s and 1970s, further assisted by the collected of ice cores collected from the Greenland ice cap and deep sea drilling projects such as those from the Glomar Challenger in 1968. This was also supplemented by research into deep sea geochemistry and investigation of the deep sea fossil records. In the 1980s, Broecker and others realized how much heat was actually carried by what he described as the “great conveyor belt” of sea water flowing northwards. They found that a huge amount of water was flowing slowly northward to the surface of the Atlantic and that this was as important in terms of heat flow as the well studied Gulf Stream. Although the importance of the Gulf Stream to Europe’s climate had long been recognized, it was now realized that if something shut down this conveyor belt then climate would change significantly over much of the northern hemisphere.

The Antarctic Circumpolar Current

The ACC is the world’s only global current, flowing eastwards around Antarctica in a closed current with its flow unimpeded by continents. The ACC is usually considered to be the northern border of the Southern Ocean which, because of the formation of sea ice, contains the world’s densest sea water. Since the ACC links the ocean basins of the Atlantic, Indian and Pacific Oceans, the waters carried in the ACC contain a mix of waters originating in different parts of the world. As water flows away from the ACC, it flows both to the north and to the south where it becomes a primary source for the Antarctic Bottom Water.

A difference between the ACC and other major currents is that the ACC is not a single flow but consists of two or more jets that run largely parallel to an ocean ridge system which surrounds Antarctica. Between the ACC and Antarctica there are two significant gyre systems, the Ross and Weddell Sea gyres.

An ocean front is defined as an abrupt change in the water properties, principally temperature and salinity, with lateral position. In 1939, George Deacon (13) recognised that there was more than one front in the Southern Ocean. The Polar Front (or Antarctic Convergence ) had been discovered earlier through noting changes in sea temperature and Deacon showed that it was circumpolar. Subsequently the Sub Antarctic Front was also been found to be circumpolar. These fronts separate distinctive zones of sea water, interestingly the largest water mass in the ACC is the Circumpolar Deep Water which is not Antarctic in origin but can be traced back to the outflow from the Mediterranean Sea.

In the Antarctic zone, this water mass lies beneath the winter water so that between 200m to 500m, water temperature increases with depth and this was first noted on Captain Cook’s second voyage.

The winds that drive the ACC are stronger than required even for a current as large as the ACC. Models of wind driven circulation which work well for other currents produce flows that are up to ten times to strong in the case of the ACC. Counter-acting forces which slow the ACC are believed to be the effect of the bottom ridges in the Southern Ocean and boundary currents associated with the ACC. Around New Zealand and South America, the ACC departs from its eastwards flow and turns to the north. At these locations, the ACC can be considered as a boundary current, like the Gulf Stream, and these are known to dissipate large amounts of energy (14) in the eddies and turbulence associated with such currents.

A phenomenon related to the ACC and its connection of the ocean basins is the Antarctic Circumpolar Wave ( ACW ) identified by Warren White and Ray Peterson (15) in 1996.



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