Low-frequency variability in an idealized troposphere-stratosphere coupled system is investigated from a viewpoint of the annular variability. Our previous numerical experiment (Taguchi et al. 2001) with a simple global circulation model under a perpetual-winter condition is redone for a much longer period of 10000 model days. Amplitude of a sinusoidal surface topography of zonal wavenumber one, h(0), is changed from 0 m to 1600 m in 15 runs as an experimental parameter, to examine the role of forced planetary waves in the annular variability.
The parameter sweep experiment shows the ubiquity of the annular variability in the sense that the leading empirical orthogonal function exhibits a meridional seesaw pattern in the troposphere which has strong zonally symmetric component for all the runs. However, the numerical experiment also reveals qualitative change of the annular variability with the topographic amplitude in detailed spatial structure and temporal variability; the nature of the annular variability is classified into three regimes depending on h(0): In the regime (i) of 0 m less than or equal to h(0) less than or equal to 300 m, the annular variability consists almost only of zonally symmetric component and has long time scales. In the regimes (h) of 400 m less than or equal to h(0) less than or equal to 600 m and (iii) of 700 m less than or equal to h(0) less than or equal to 1600 m, the annular variability exhibits zonally asymmetric component of zonal wavenumber one as well as zonally symmetric component. The temporal variability is characterized by small standard deviation and negative skewness of time series of the leading mode in the regime (ii), while the standard deviation is large and the skewness is close to zero in the regime (iii).
The connection of the annular variability to the stratosphere is also different among the three regimes. The annular-variability signature substantially penetrates from the surface to the stratosphere with a time lag of about 20 days in the regime (i), while it is confined in the troposphere in the regime (ii). In the regime (iii), the annular variability leads to stratospheric sudden warmings with a time lag of about 30 days.
The present results suggest that the Northern and Southern annular modes in the real atmosphere can be different from each other in these aspects in contrast to the strong similarity noted by Thompson and Wallace (2000).