Linking the world’s oceans: the Antarctic Circumpolar Current

In her year as a New Zealand Science, Mathematics and Technology Teacher Fellow, Jenny Pollock has focused her attention on the ACC.

New Zealanders may sometimes forget that we dwell in the middle of vast, restless oceans through which large currents that control the world’s climate flow. These currents are like huge rivers within the ocean, responsible for transporting ocean water and with it heat, salts, dissolvedgases, nutrients, and marine life. Surface currents are driven by wind, and deep currents are driven by gradients in density, a function of salinity and temperature. The Earth’s spin and the topography of the ocean floor strongly affect the direction in which currents flow.

Antarctic Circumpolar Current

The huge, cold Antarctic Circumpolar Current (ACC) flows south of New Zealand, completely encircling the globe. Formed by persistently strong westerly winds which transfer large amounts of momentum to the current, the ACC flows eastward around Antarctica, connecting the Atlantic, Pacific, and Indian oceans. It transports 110–150 million cubic metres of water per second; by comparison, all the water flowing out of all the world’s rivers is about a million cubic metres persecond.

Unlike other major currents, the ACC reaches from the surface to the bottom of the ocean. In places it is as deep as 4000 metres and as wide as 2000 kilometres. Mostly the current flows unimpeded, but underwater topography, such as the Drake Passage between South America and Antarctica and the ridges and plateaus south of New Zealand, act as barriers that constrict, deflect, and alter the flow.

Measuring the ACC

Because the ACC is linked to the three major oceans and is important in global ocean circulation and climate, it is essential that scientists understand and monitor its flow and detect any changes. Research on ocean circulation in the Southern Ocean is difficult because the ocean is stormy, the weather is generally bad, and the area to cover is vast. Oceanographers must choose their methods and sites for gathering data very carefully.

It happens that New Zealand is well placed for studying the ACC. The Macquarie Ridge, an underwater mountain range that rises 2000–3000 m from the seabed and stretches south of New Zealand for 1400 km, is one of the few places where the ACC deviates from its relentless circling of the globe.

Currents driven by density gradients, such as the ACC, are known as thermohaline circulation (thermo = temperature, haline = salinity), and the ACC has a profound influence on the world’s climate because it is part of the global thermohalinecirculation. Cold, dense water sinking around Antarctica and in the north Atlantic drives this circulation. These waters, already cooled by the atmosphere, become even denser as a result of sea ice formation. When sea ice forms the salt from the sea water is expelled as brine, increasing the density of the water below and allowing it to sink into the deep ocean. From there it flows northwards, joining with the ACC. Branching off the ACC, Deep Western Boundary Currents (DWBC) carry this deep water into the Indian, Atlantic, and Pacific oceans, travelling 2–5 km below the surface. The DWBC flows eastwards around the Campbell Plateau, past the Chatham Rise along the Kermadec trench and into the north Pacific.

Data from the Macquarie Ridge

In 2007, NIWA scientists on board RV Tangaroa dropped nine instrumented moorings in two gaps in the Macquarie Ridge through which the ACC squeezes. The moorings were over 3500 m long and were anchored to the bottom of the ocean by old railway wheels. Current meters on the mooring lines measured and recorded the speed, direction, and volume of the current at fixed positions under the surface to create a picture of how the ACC flows through the ridge. The instruments collected data continuously for a year before the moorings were retrieved in April 2008. The data showed that the speed of the current passing through the Macquarie Ridge is about 4 km per hour – the speed an adult would walk quickly, and very fast for an ocean current. Other instruments on the moorings measured temperature and salinity.

The data from the Macquarie Ridge will be compared with data from other moorings, such as those placed in the Drake Passage, to form an idea of how much water is flowing out into the Pacific and how much is staying to circulate around the Southern Ocean. Scientists are also looking for climate-related changes in ocean circulation by comparing the salinity, temperature, and density data with similar data collected since the 1960s. All these data will inform climate-change research.

Such changes could have an as yet unknown effect on global climate as the thermohaline circulation is finely balanced. The energy and extent of the deep and shallow flows depend on a balance between evaporation and freshwater supply, temperature distribution through the ocean, and wind patterns. Any or all of these factors may change as global warming continues.

The author thanks Dr Mike Williams, Dr Mireille Consalvey (both NIWA), and Dr Lionel Carter (Victoria University of Wellington) for their contribution to this article.