Impacts of northern summer monsoon on intraseasonal oscillation

Based on observations, we hypothesize that the Northern Hemisphere Summer Monsoon (NHSM) and underlying moist static energy distribution play a fundamental role in regulating boreal summer ISO. In this paper, we use an intermediate tropical atmospheric model to asses impacts of NHSM on low-frequency equatorial waves and to investigate the processes via which the monsoon regulates ISO. With climatological July mean circulation and moist static energy distribution, the model simulates a life cycle of low-frequency disturbances that consists of (1) an equatorial eastward propagation of a coupled Kelvin-Rossby wave packet, (2) an emanation of moist Rossby waves in the Western North Pacific (WNP), (3) a westward propagation and amplification of moist Rossby waves in South Asian monsoon regions, and (4) re-initiation of 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 controlled experiments reveal that reduction of the boundary layer moist static energy over the eastern-central Pacific suppress equatorial convection, which leads to disintegration of the Kelvin-Rossby wave packet and emanation of Rossby waves in the WNP. The strong easterly vertical shear and high boundary layer humidity in summer monsoon regions further amplify the Rossby waves (of the gravest meridional mode), making their structures highly asymmetric about the equator. When approaching Arabian Sea, the intensified Rossby waves start to stall and decay due to the "blocking" of the sinking dry airmass over Middle-East and North Africa. Meanwhile, convection and associated equatorial Kelvin-Rossby wave packet develops over the equatorial Indian Ocean. The mean Hadley circulation appears to play a critical role in re-initiation of the equatorial Indian Ocean disturbances.