Seasonality of the Tropical Intraseasonal Oscillations: Sensitivity to Mean Background State

The Madden Julian Oscillation (MJO) and the Monsoon Intra-seasonal Oscillations (MISO) are the two most dominant components of the variability in the tropics on intra-seasonal time scales (20-100 days). These phenomena are known collectively as tropical intra-seasonal oscillations (TISO). Both MJO and MISO exhibit strong seasonality: MJO peaks in boreal winter near the equator, while MISO peaks in boreal summer north of the equator in the Indian and western Pacific Oceans. Observational evidence suggests that intra-hemispheric migration of key features of the mean background states of the low-level atmospheric circulation (AC) and the underlying sea surface temperature (SST) play important roles in the seasonal characteristics of TISO. This study aims to understand the relative importance of AC and SST in the seasonality of TISO. Thirty summer season simulations with the super-parameterized version of the Community Atmosphere Model version 4 (SP-CAM) coupled to a slab ocean model with prescribed Q-flux fixed in one season are compared with a 30-year free run. TISO is found to be relatively weak when AC and SST are out of phase. this suggests that coherent inter-hemispheric migration of AC and SST play an important role in the initiation, maintenance, propagation and dissipation of TISO events.