Table 1: Default model parameter values; different values are used in some calculations as stated in the text Parameter Value Definition t 37 d Thermal relaxation time H 16 km Tropopause height d 4 km Depth of layers in which meridional flow occurs T0 300 K Reference temperature ?z 60 K Vertical potential temperature stratification ?y 50 K RCE equator-pole temperature gradient ?00 330 K Background tropospheric-mean potential temperature eu 10-8 s-1 Background Rayleigh drag vd 2.5 m s-1 Parameterized eddy momentum flux coefficient kv 7786 m2/s Diffusivity on v ß 2 × 10-11m-1 s-1 Meridional gradient of Coriolis parameter