Mechanisms of Northward Propagation of Boreal Summer Intraseasonal Oscillation Revealed by Climate Model Experiments

Northward propagation of the boreal summer intraseasonal oscillation (ISO) has profound impacts on Northern Hemisphere extreme weather events. This study aims to test the hypotheses proposed in the previous studies to explain northward propagation of the ISO by drastically changing the climatological mean states through lowering the Tibetan Plateau (TP) with a fully coupled Earth system model, Nanjing University of Information Science and Technology Earth System Model version 3.0. The model reproduces realistic ISO over South Asia and the northern Indian Ocean. The results show that ISO northward propagations significantly weaken with decreased elevation of the TP. Lowering the TP reduces the vertical shear of the mean monsoon circulation over the northern Indian Ocean. The reduced vertical shear deteriorates the generation of positive vorticity anomalies and boundary layer moisture convergence to the north of ISO precipitation center, thereby weakening northward propagation of the ISO. On the other hand, the boundary layer moisture advection and air-sea interaction do not change appreciably when the TP elevation is reduced. These results suggest that the mean state vertical shear is most critical for the ISO northward propagation.