NOTES AND CORRESPONDENCE Human-Induced Change in the Antarctic Circumpolar Current

Global climate models indicate that the poleward shift of the Antarctic Circumpolar Current observed over recent decades may have been significantly human induced. The poleward shift, along with a significant increase in the transport of water around Antarctica, is predicted to continue into the future. To appreciate the magnitude of the poleward shift it is noted that by century’s end the concomitant shrinking of the Southern Ocean is predicted to displace a volume of water close to that in the entire Arctic Ocean. A simple theory, balancing surface Ekman drift and ocean eddy mixing, explains these changes as the oceanic response to changing wind stress. The Antarctic Circumpolar Current (ACC) encircles the Antarctic continent, flowing eastward through the southern portions of the Pacific, Atlantic, and Indian Oceans. It is the world’s largest, and arguably most influential, ocean current (Nowlin and Klink 1986; Rintoul et al. 2001). While the speed of the ACC is not extraordinary (about 0.5 m s 1 at the surface), it’s depth (about 4 km) and width (500–1000 km) results in a massive transport of about 140 10 m of water per second, equivalent to about 150 times the flow of all the world’s rivers combined. Because the Pacific, Atlantic, and Indian Ocean basins are almost entirely surrounded by land except at their southern boundaries, the ACC is the primary means by which water, heat, and other properties are exchanged between ocean basins. At the same time water found at abyssal depths in the ocean at low latitudes rises along the sloping density surfaces associated with the ACC and toward the surface in the Southern Ocean. Where these surfaces outcrop, intense air–sea ice interactions drive water mass transformations. In fact, the characteristics of more than 50% of the World Ocean volume reflects the air– sea ice interactions taking place in the Southern Ocean. In short, the ACC profoundly influences, and is influenced by, the regional and global climate. The Southern Ocean is remote and historical data are relatively scarce. Even so, an examination of autonomous float observations collected during the 1990s and historical shipboard measurements suggests that the middepth (700–1100 m) Southern Ocean temperatures have risen since the 1950s (Gille 2002). This warming is faster than that of the global ocean and is concentrated in the ACC, where temperature rates of change are comparable with Southern Ocean atmospheric temperature increases (Gille 2002). A comparison of the data with earlier hydrographic data indicates that the warming is associated with a southward migration of the ACC since the 1950s of about 50 km in the Pacific (Swift 1995), as well as the Atlantic and Indian Oceans (Gille 2002). A more recent analysis of 32 yr (1966–98) of subsurface layer (200–900 m) temperature observations in the Indian sector of the Southern Ocean similarly show a warming trend concentrated in the ACC, indicative of a southward shift of the ACC of about 50 km (Aoki et al. 2003). These changes may be associated with long-term changes in the overlying atmospheric circulation as seen in National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data (Kalnay et al. 1996; Thompson et al. 2000; Fyfe 2003), as well as in Corresponding author address: Dr. John Fyfe, Canadian Centre for Climate Modelling and Analysis, Meteorological Service of Canada, University of Victoria, P.O. Box 1700, Victoria, British Columbia V8W 2Y2, Canada. E-mail: [email protected] 3068 J O U R N A L O F C L I M A T E VOLUME 18