WAVE ZONAL-FLOW INTERACTION AND ULTRA-LOW-FREQUENCY VARIABILITY IN A SIMPLIFIED GLOBAL CIRCULATION MODEL

The interactions between baroclinic waves and the zonal-mean mid-latit ude flow are investigated using a multilevel primitive-equation model of moderate resolution with idealized surface friction and diabatic he ating terms. Integrations corresponding to 100 years were performed un der perpetual solstice conditions. Empirical Orthogonal Function (EOF) analyses were carried out on the zonal-mean zonal wind, [u]. Adjustme nts to the winter hemisphere zonal jet due to the growth and decay of baroclinic waves can be described in terms of the first two EOF patter ns, which exhibit substantial interannual variability. For a single li fecycle, such adjustments take the form of a circuit of a phase-space whose axes are defined by the amplitudes of the two EOFs. The cumulati ve effects of multiple lifecycles lead to a low-frequency circulation in the phase-space, with a timescale of about 150 days. We suggest tha t this effect may be intrinsically related to the internal generation of ultra-low-frequency variability in the model. Normal-mode lifecycle calculations, including dissipation, reveal similar phase-space prope rties. The growth rates of normal-mode disturbances are shown to be hi ghly sensitive to the phase of the first EOF.