The Antarctic Circumpolar Wave: A Combination of Two Signals?

Abstract

A simplified view of the possible mechanisms behind the Antarctic Circumpolar Wave (ACW) interannual variability is provided by a frequency-domain decomposition of several observed atmospheric and oceanic variables. Two significant interannual signals with different temporal and spatial characteristics are identified in the Southern Ocean, and most of the variance of the ACW in the interannual band can be accounted for by a linear combination of them. The first signal has a period of oscillation of around 3.3 yr and a zonal wavenumber-3 structure across the Southern Ocean. It involves self-sustained fluctuations inherent in the Southern Ocean and driven by coupled air?sea interactions in which the atmosphere and the ocean mutually force one another. This signal is represented by an atmospheric standing oscillation with centers at fixed locations around Antarctica and a propagating oceanic pattern, in which the surface Antarctic Circumpolar Current plays an essential role. The second signal has a periodicity of around 5 yr and a zonal wavenumber-2 structure across the Southern Ocean. It seems to be remotely forced by the tropical ENSO phenomenon and has a signature mainly in the eastern Pacific sector. This signal likely represents the southern high-latitude manifestation of a larger-scale (hemispheric or global) pattern. The constructive or destructive interference of these two superimposed signals with different periodicities and spatial characteristics gives rise to the observed irregular fluctuations of the ACW on the interannual timescale.