Atmosphere–sea‐ice low‐frequency variability with a simple model of the southern hemisphere

Abstract

AbstractA simple primitive‐equation general circulation model (SGCM) is used as a representation of an ‘aqua‐planet’ (with no orography) to model constant southern‐hemisphere‐winter conditions. The statistical significance of the decadal and quasi‐biennial variability is first assessed in a long‐time SGCM run by means of a frequency‐domain statistical decomposition. The modelled long‐wave atmospheric pattern is then associated with the observed southern‐hemisphere ‘annular mode’, as suggested by previous studies using the same SGCM.A simple linear relationship between the low‐level wind and the ice velocity is then introduced into the SGCM to investigate the feedbacks between the sea‐ice cover and the baroclinic waves. The zonal mean state reached by the sea‐ice cover in a long coupled run (a few hundred years) exhibits characteristics similar to the observations. Investigation of the low‐frequency variability in the coupled simulation leads to the following results: (1) statistically significant variability is observed on interdecadal, decadal, interannual and quasi‐biennial time‐scales; (2) interdecadal, decadal and quasi‐biennial signals are associated with the atmospheric annular mode that forces changes in the northward extent of the sea‐ice cover at all Southern Ocean longitudes; (3) the interannual signal (3–6 year period) exhibits spatio‐temporal patterns and propagation speed similar to the well‐known ‘Antarctic circumpolar wave’. Copyright © 2003 Royal Meteorological Society.