INTRASEASONAL VARIABILITY IN A BAROTROPIC MODEL WITH SEASONAL FORCING

It has recently been suggested that oscillatory topographic instabilit y could contribute to low-frequency variability over the Northern Hemi sphere midlatitudes. A barotropic potential vorticity model, with a hi erarchy of forcing and topography configurations on the sphere, is use d to investigate the nature of low-frequency oscillations induced by s uch instabilities. Steady-state solutions of the model include multipl e unstable equilibria that sustain oscillatory instabilities with peri ods of 10-15 days, 35-50 days, and 150-180 days, for a realistic forci ng pattern. Time-dependent solutions exhibit chaotic behavior with epi sodic oscillations, featuring both the intraseasonal (35-50 day) and b iweekly (10-15 day) modes. The former is dominated by standing spatial patterns, the latter by traveling wave patterns. The phases of the in traseasonal oscillation are robust for all cases, exhibiting a clear o scillatory exchange of atmospheric angular momentum with the solid ear th via mountain torque. It is demonstrated, through linear stability a nalysis on the sphere, that the intraseasonal oscillations are induced by topographic instabilities. The role of the seasonal cycle is studi ed by prescribing an annual cycle in the forcing. In this case, the wi nter forcing is more favorable than the summer for the occurrence of e pisodic intraseasonal oscillations. Recent observations are consistent with this model result.