Role of SST meridional structure in coupling the Kelvin and Rossby waves of the intraseasonal oscillation

Abstract

The intraseasonal oscillation (ISO) is one of the most important modes of the tropical atmosphere, which influences global livelihood of hundreds of millions of people. The meridional structure of sea surface temperature (SST) has been found to be important for the ISO simulation in general circulation models (GCMs). Using a theoretical frictional skeleton model for the ISO, we investigate the effects of different SST structures on the ISO in this study. The model results show that the observed Madden-Julian oscillation (MJO), boreal summer ISO (BSISO) and quasi-biweekly oscillation can be simulated in this model with different SST structures. The Ekman pumping of the boundary layer associated with equatorially trapped SST favors the growth of eastward propagating Kelvin waves and prefers the fast eastward propagating signal. A broad SST provides a strong instability source for the Rossby waves, which will slow down the MJO. In the boreal summer, the high SST center in the off-equatorial region can trigger strong off-equatorial moisture pumping from the boundary layer, which enhances the Rossby waves and can simulate the northwest-southeast tilted rain band associated with the BSISO. When the Rossby component overwhelms the Kelvin component, the low-frequency westward component of the BSISO and the higher-frequency quasi-biweekly oscillation can be simulated.