7 Antarctic environment | 2.2 The Southern Ocean
State of the Environment 2011 Committee. Australia state of the environment 2011.
Independent report to the Australian Government Minister for Sustainability, Environment, Water, Population and Communities.
Canberra: DSEWPaC, 2011.
The Southern Ocean is the only ocean that encircles the globe uninhibited by land masses. It flows around Antarctica, connecting the world's major southern ocean basins, and also links the surface of the ocean with the deepwater layer.74 The Southern Ocean covers an area of approximately 20.3 million square kilometres. Much of its area is 4000-5000 metres deep and its temperatures are below 0°C. Various currents form the Southern Ocean. The major current in the Southern Ocean is the Antarctic Circumpolar Current, which flows mainly eastwards, although important north-south movements also occur in various water masses. The structure of the water current system is still being investigated. Recently, measurements of a deep current system 3 kilometres below the surface and flowing along the Kerguelen Plateau in the southern Indian Ocean found that more than 12 million cubic metres of cold water are transported here each second.75 This system - the Kerguelen Deep Western Boundary Current - makes a significant contribution to global ocean circulation as these deep currents transport Antarctic waters into deep layers of the major ocean basins.
In winter, the surface of the ocean bordering the Antarctic continent freezes. The exceptions are a few areas, called polynyas, where persistent katabatic winds, formed by cold air draining from the interior of the continent, blow from the land to the sea and keep the sea surface clear of ice. When sea ice forms, salt is forced out of the forming ice (brine rejection) making the water below this ice denser and more saline.60 The densest water mass of the Southern Ocean is the Antarctic bottom water, which forms in only a few locations near the Antarctic continent. Antarctic bottom water is derived from shelf waters that are dense, cold and oxygen rich. The bottom water spills over the edge of the continental shelf and reaches deep oceanic waters that move northwards along the ocean bottom. The warmer waters in the north flow south and fill the gap, and as they reach higher latitudes they cool and sink.76 Through this cycle of water movement and the connection of all major ocean basins, heat and other components are redistributed and make the bottom water a key driver in the world's 'conveyer belt' of ocean currents.77 These processes influence weather, rainfall patterns and temperatures around the world.13
Although generally nutrient rich, the productivity of the Southern Ocean is not as high as may be expected due to low levels of iron (an important micronutrient) and low light levels (because of persistent cloud cover), particularly during the southern winter.
The Southern Ocean is the least well understood ocean due to its vast size and the difficulties in obtaining measurements in winter, because large areas are covered by ice. Nonetheless, some measurements have been possible: in the region of the Weddell Sea from 35°S to 65°S, the Southern Ocean has warmed by 0.17 °C in the upper 1000 metres since the 1950s.78 This rate of warming is faster than anywhere else in the world.74
The Southern Ocean is one of the world's largest sinks for atmospheric carbon dioxide. Approximately 25-30% of the anthropogenic (caused by human activity) carbon dioxide released to the atmosphere has been taken up by the world's oceans - some 40% of which has been taken up by cold Southern Ocean waters that lie south of 40°S.79-81
While reducing the accumulation of carbon dioxide in the atmosphere, ocean uptake is making sea water more acidic. Current atmospheric carbon dioxide levels of approximately 391 parts per million are higher than they have been for at least the past 25 million years82 and models predict it could rise to >1000 parts per million by 2100.83-84 Compared with pre-industrial times (pre-1700s) when carbon dioxide levels were around 280 parts per million, the pH (measure of acidity) of the ocean has dropped from pH 8.2 to pH 8.1, indicating increased acidity.85 Thus, although the ocean is still alkaline, its level of acidity is increasing. This drop in pH is linked to the dramatic rate of increase of carbon dioxide in the atmosphere - the rate is one hundred times greater than that during any other time in the past 650 000 years.85 In the period from 2000 to 2004, the rate of global carbon dioxide emissions grew by 3.3% per year compared with 1% per year in 1990-99 (Figure 7.6).84
Source: Conway & Tans86
Figure 7.6 Recent global monthly mean carbon dioxide concentration, 2007-11
In April 2011, the global average carbon dioxide concentration was 391.55 parts per million. The red line shows the mean monthly values centred around the middle of each month; the black line is the corrected version based on a seven-month running average.
Ocean acidification is likely to affect the efficiency of the Southern Ocean as a sink for atmospheric carbon dioxide and will also have profound impacts on species and ecosystems (see Sections 2.3.1 and 3.1.1).
|Component||Summary||Assessment grade||Confidence in grade||Confidence in trend|
|Very poor||Poor||Good||Very good|
|Sea surface temperature||
Since 1950, the upper kilometre of the water column and densest part of Antarctic bottom water in the Weddell Sea warmed by 0.2°C at 700-1000 metres between 35°S and 65°S.
|Ocean acidity||Polar pH levels are changing twice as fast as tropical ones; pre-industrial pH 8.2 dropped to pH 8.1, indicating increased acidity|
|Ocean salinity||The coastal waters between the Ross Sea and the southern Indian Ocean are fresher now than 50 years ago, making the Antarctic bottom water that forms here less saline|
|Southern Ocean circulation and structure||Increase in wind strength is expected to affect the ACC and upwelling of circumpolar deep water, formation of different water masses and gyre activity|
In December 2009, data were obtained from about 135 locations from 250 tide gauges, but large gaps still exist in datasets
Sea level changes are not expected to be uniform across Earth. Sea level rise in the Southern Ocean south of the ACC is predicted to be less than in the Arctic
|Recent trends||Improving||Stable||Confidence||Adequate high-quality evidence and high level of consensus|
|Deteriorating||Unclear||Limited evidence or limited consensus|
|Evidence and consensus too low to make an assessment|
|Grades||Very good: There are no significant changes in physical and/or chemical processes as a result of human activities|
|Good: There are some significant changes in physical and/or chemical processes as a result of human activities|
|Poor: There are substantial changes in physical and/or chemical processes as a result of human activities that significantly affect ecosystem functions in some areas|
|Very poor: There are substantial changes in physical and/or chemical processes as a result of human activities that significantly affect ecosystem functions in much of the region|