Multi-scale analysis of idealized Walker Cell

Two-dimensional Walker circulation over a planetary scale domain of 40000 km for an extended period of several hundred days is simulated with cloud resolving model. The Walker cell emerges as the time averaged statistical steady state with a prescribed sinusoidal sea surface temperature (SST) pattern with mean temperature of 301.15 K and horizontal variation of 4 K. The circulation exhibits intra-seasonal variability on a time-scale of about 20 days with quasi-periodic intensification of the circulation and broadening of the convective regime. This variability is closely tied to synoptic scale systems associated with expansion and contraction of the Walker circulation. An index for the low frequency variability is developed using an Empirical Orthogonal Function (EOF) analysis and by regressing various dynamic fields on this index. The low frequency oscillation has four main stages: a suppressed stage with strengthened mid-level circulation intensification phase, active phase with strong upper level circulation and a weakening phase. Various physical processes occurring at these stages are discussed. In particular, the impact of various scales is analysed with isentropic analysis applied. Also, prevailing spatio-temporal patterns of convective organization are revealed based on clustering of isentropic streamfunction.