A simple model for the interhemispheric coupling of the middle atmosphere circulation HeinerK¨ornich a , Erich Becker b a Department of Meteorology, Stockholm University, 10691 Stockholm, Sweden b Leibniz-Institute of Atmospheric Physics e.V. an der Universit¨ at Rostock, Schlossstraße 6, 18225 K¨ uhlungsborn, Germany Abstract The interhemispheric coupling of the middle atmosphere general circulation is characterized by a global anomaly pattern of the zonal mean temperature. This pattern reflects an anomalous stratospheric and mesospheric residual circulation, in which a weaker (stronger) stratospheric winter circulation is linked to an upward (downward) shift of its upper mesospheric branch reach- ing from the summer to the winter pole. This phenomenon is robust in obser- vational data and several middle-atmosphere general circulation models. In the present study, the recently proposed mechanism of the interhemispheric coupling is unequivocally proven within the framework of a zonally sym- metric model that excludes any additional effects due to resolved waves and non-zonally propagating gravity waves. Two simulations are conducted that differ in the strength of the polar vortex. A weaker polar vortex results in a downward shift of the winter mesospheric gravity wave drag. This leads to changes also in the summer upper mesosphere via a feedback solely between gravity-wave breaking and the zonal-mean state. The accompanying tem- perature anomaly reproduces the pattern of the interhemispheric coupling. Key words: middle atmosphere transport circulation, noctilucent clouds Preprint submitted to Advances in Space Research September 29, 2009