Mean annual cycles of open water area within the Antarctic sea ice pack are presented for 1978-87 and 1987-97, compiled using mean daily data. A minimum of the open water area occurs in mid February, followed by a steady increase until a plateau is established in October, before a sudden surge in early November to a peak of 4.5¿106 km2. As this Antarctic Open water Surge (AOS) occurs on timescales of around one month, its temporal structure only becomes truly clear when using daily means. The AOS is shown to be the result of the relative timings of the sea ice area and extent annual cycles. It is suggested these reflect the late spring crossing of the Antarctic circumpolar trough over the sea ice edge, thus shifting the dominant winds (Ekman forcing) from westerly (northerly) to easterly (southerly). ¿ 1999 American Geophysical Union