A Model for the Boreal Summer Intraseasonal Oscillation

The tropical intraseasonal oscillation (ISO) exhibits pronounced seasonality. The boreal summer ISO is more complex than its winter counterpart due to the coexistence of equatorial eastward, off-equatorial westward, and northward propagating, low-frequency modes and their interactions. Based on observational evidence and results obtained from numerical experiments, a mechanism is proposed for the boreal summer ISO in which the Northern Hemisphere summer monsoon (NHSM) circulation and moist static energy distribution play essential roles. With a climatological July mean basic state, the life cycle of model low-frequency waves consists of four processes: an equatorial eastward propagation of a coupled Kelvin–Rossby wave packet, an emanation of moist Rossby waves in the western Pacific, a westward propagation and amplification of the Rossby waves in South Asian monsoon regions, and a reinitiation of the equatorial disturbances over the central Indian Ocean. The life cycle spans about one month and provides a mechanism for self-sustained boreal summer ISO. Analyses of the model experiments reveal that the monsoon mean flows and spatial variation of moist static energy trap equatorial disturbances in the NHSM domain. The reduction of moist static energy over the eastern central Pacific suppresses equatorial convection, leading to disintegration of the equatorial Kelvin–Rossby wave packet and the emanation of Rossby waves in the western North Pacific. Strong easterly vertical shears and seasonally enhanced boundary layer humidity in the NHSM further amplify the Rossby waves (of the gravest meridional mode), making their structures highly asymmetric about the equator. The intensified Rossby waves start to stall and decay when approaching the Arabian Sea due to the ‘‘blocking’’ of the sinking dry air mass over North Africa, meanwhile triggering equatorial convection. The mean Hadley circulation plays a critical role in reinitiation of the equatorial Kelvin–Rossby wave packet over the equatorial Indian Ocean.