Southern Ocean hotspots Isabelle J. Ansorge explores the influence of a mountain ridge under the ocean at the Prince Edward Islands and their ecosystems.
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Figure 1
Top: This iceberg was spotted in 2004 south of Africa at 54°S. Icebergs drift at a rate of about 0.7 km/h. Their speed is affected by many things, including their size and shape, ocean currents, waves, and wind. Figure 1: Schematic map of the major currents in the southern hemisphere south of 20°S. Depths shallower than 3500 m are shaded. The two main cores of the Antarctic Circumpolar Current are shown – the Subantarctic Front (SAF) and the Antarctic Polar Front (APF) – as is the interbasin exchange between the Southern, Pacific, Atlantic and Indian Oceans. Source: S.R. Rintoul et al., “The Antarctic Circumpolar Current System”, in G. Siedler et al. (eds), Ocean Circulation and Climate – Observing and Modelling the Global Ocean (International Geophysics Series, vol. 77, Academic Press, 2001), pp.271–302.
he Prince Edward Islands are located in the Southern Ocean at approximately 38°E, 47°S. They form a very special habitat for seals, albatrosses and whales and have, for this reason, been designated a declared nature reserve. The ecology of the islands depends directly on their ocean environment. The advent of satellite data has revealed that these islands are in fact located on the northern edge of a region of remarkably high oceanic turbulence, which, in turn, influences the kind of food available to the islands’ top predators and could help to explain their feeding behaviour. The latest discoveries and explorations – which form a key part in South Africa’s involvement in the work of the International Polar Year – could also help us to understand better than ever before the high physical and biological variability found at the islands. Southern Ocean currents and the world’s climate The Southern Ocean is the name given to the oceanic region that surrounds the entire Antarctic continent1. It is made up of the southern extents of the Atlantic, Indian, and Pacific oceans (see Figure 1). The Antarctic continent forms its southern boundary, while the northern border normally coincides with the location of the Subtropical Convergence, which marks the boundary between warm, salty subtropical surface water to the north and cooler, fresher Antarctic waters to the south. The particular geography of the Southern Ocean makes it the only place where ocean currents can
Definitions Eddy: A current of water moving contrary to the main current, especially in a circular motion. Bathymetry: The measurement of the depth of the ocean floor from the water surface. Ecosystem: A community of organisms, interacting with each other, as well as the environment in which they live. GOOS and SOOS: Did you know that observing systems are in place all over the world’s oceans under the Global Ocean Observing System (GOOS) programme (http://www.iocgoos.org/). They are there to ensure that data collection continues and that gaps in our knowledge and understanding of ocean processes are slowly filled. A new system is now in place for the Southern Ocean (SOOS).
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run completely around the globe, unobstructed by land masses. Dominating the circulation of the region’s currents is the strong, deep, eastward-flowing Antarctic Circumpolar Current (ACC) – the only current that flows completely around the globe. The ACC is a complex system, comprising narrow regions of sharp temperature gradients known as frontal bands, which are separated by broad zones with less intense gradients. Variability in the pathway of this current occurs in the form of meanders or eddy-like features that result from the current’s interaction with shallow bathymetry such as underwater mountain chains and plateaux – making it behave a bit like a ‘speed wobble’. The ACC is often referred to as the West Wind Drift, named after the strong westerly winds that prevail between 40°S and 60°S2. The Southern Ocean plays a critical role in global ocean circulation and, thereby, on present-day climate. Extending unbroken around Antarctica, the ACC is by far the largest conduit by which water, heat, and salt are transferred between the Atlantic, Indian, and Pacific oceans (see Figure 1). These exchanges help to regulate global climate, so it is vital to monitor changes in the ACC’s flow, strength, and physical-chemical characteristics if we want to understand the importance of this ocean in the context of climate change. It is especially important to increase oceanographic surveys to the Southern Ocean, to improve our forecasting and modelling capabilities, and to continue to integrate interrelated sciences such as marine and terrestrial biology, physical oceanography, and marine meteorology. 1. The Southern Ocean is defined as the region between 40°S and the Antarctic continent. 2. South of the ACC, in a narrow zone around most of the Antarctic continent, there is a westward flowing coastal current termed the East Wind Drift, named after the prevailing easterly winds near the coast. Flowing westwards, this current is a counter-current to the ACC and can be found south of 66°S. Although the East Wind Drift is circumpolar, the shape of the Antarctic Peninsula partially impedes its flow.