Multi - scale interactions in an idealized Walker circulation : Mean 1 circulation and intra - seasonal variability

5 A high resolution cloud resolving model (CRM) simulation is developed here for a two6 dimensional Walker circulation over a planetary scale domain of 40000 km for an extended 7 period of several hundred days. The Walker cell emerges as the time averaged statistical 8 steady state with a prescribed sinusoidal sea surface temperature (SST) pattern with mean 9 temperature of 301.15 K and horizontal variation of 4 K. The circulation exhibits intra10 seasonal variability on a time-scale of about 20 days with quasi-periodic intensification of 11 the circulation and broadening of the convective regime. This variability is closely tied to syn12 optic scale systems associated with expansion and contraction of the Walker circulation. An 13 index for the low frequency variability is developed using an Empirical Orthogonal Function 14 (EOF) analysis and by regressing various dynamic fields on this index. The low frequency 15 oscillation has four main stages: a suppressed stage with strengthened mid-level circulation, 16 intensification phase, active phase with strong upper level circulation and a weakening phase. 17 Various physical processes occurring at these stages are discussed as well as the impact of 18 organized convective systems on the large scale flow. 19