The Antarctic Circumpolar Wave (ACW) is a coupled ocean/atmosphere wave that circles the Southern Ocean in approximately eight years at 6–8 cm/s (2.4–3.1 in/s).[1] Since it is a wave-2 phenomenon (there are two ridges and two troughs in a latitude circle) at each fixed point in space a signal with a period of four years is seen.[2] The wave moves eastward with the prevailing currents.