Antarctic Circumpolar Wave

E. Linacre


A newly discovered feature of the ocean circulation around Antarctica is the Antarctic Circumpolar Wave. This wave travels westward, against the massive circumpolar current, in which it is embedded (Fig 1). The result is that the ACW travels eastward around Antarctica, but more slowly than the current, circling the globe each 8 - 9 years. The ACW has a wave number of two, i.e. there are two large regions of relatively warm water, each 3 to 6 thousand kilometres across, separated by two equally huge patches of cold water (1). The amplitude of the ACW is highest between 50-60° S. It is not clear how these waves are triggered and maintained, the likely factors are the strong westerly winds in the region, the bottom topography, and the meridional temperature gradient in the upper ocean. There may be a connection with the El Niño Southern Oscillation, but it appears weak at best.

The ACW clearly affects the overlying atmosphere, in particular the temperature and winds over the southern seas, but also the weather of the three southern continents bordering these seas. A warm region implies higher surface pressures and a tendency for longwave ridging in the upper troposphere, resulting in drier-than-normal wetter, especially just east of the ridge. In 1998 a cold region passed south of Tasmania (2). A warm pool should be to the south of Tasmania in 2000. A cold region implies a stronger meridional temperature gradient, and therefore a stronger jet stream and more frontal activity. This implies more winter rain along the southern fringes of Australia.

The structure of the AWC suggests a 4-to 5-year cycle of rainfall. In coastal regions of the Australian Bight, and as far as the Australian Alps, there is some evidence of a ~4 year cycle in the annual rainfall, and ACW cold phases correspond with above-normal precipitation (3). In Southwest Australia droughts recur every 3-12 years (Section 10.7); the spectral variation shows a weak peak at a period of around 4 years. The ACW alternation may be more important than El Niño’s in governing rainfall on the southern fringes of Australia. In New Zealand also temperatures and precipitation amounts in autumn and winter have a weak 4-6 year periodicity in sync with the ACW (4).

Thus precipitation onto Australia is affected by a changing combination of the ACW to the south, El Niño events to the east, and also the ‘Indian Ocean Dipole’ (IOD) to the west. There are other variations in the north Atlantic and north Pacific, but they are too remote to have any effect.

Fig 1. Southern hemisphere SST anomalies mapped every 6 months for a period of 4 years. This sequence is derived from observations between 1982-1995 through empirical orthogonal function (EOF) analysis. The spatial pattern of the dominant EOF mode is shown. A warm anomaly can be seen travelling from New Zealand to the south Atlantic in 48 months (4).

The IOD is related to the northwest cloud bands, i.e. rain-producing disturbances that stretch across Australia from northwest to southeast. They are the chief cause of rain in the centre of the country. The phenomenon involves warm water around Indonesia and New Guinea (especially in La Niña years), with colder water in the middle of the Indian ocean, west of Australia.



  1. White, W.B. and R.G. Peterson, 1996. An Antarctic circumpolar wave in surface pressure, temperature and sea-ice extent. Nature 380, 699-702.
  2. Baines, P. 1998. A new oceanic influence on our climate: the Antarctic Circumpolar Wave. An item in the October issue of Atmosphere, the newsletter of the CSIRO Division of Atmospheric Research, Melbourne.
  3. Anon 1999. Antarctic Circumpolar Wave. Bull. Aust. Ocean. Meteor. Soc., 12, 69.
  4. White, W.B. and N.J. Cherry, 1998. Influence of the Antarctic Circumpolar Wave on New Zealand temperature and precipitation during autumn-winter. J. Climate, submitted.