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

Abstract

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. Plain language summary In boreal summer, a heavy precipitation belt slowly moves from the equatorial Indian Ocean to south Asian monsoon regions on a 2- to 6-week time scale, which affects extreme weather events over Asia. Therefore, understanding the causes of the northward movement of tropical precipitation belt is societally important. For this purpose, we performed a set of climate model experiments in which we reduced the topography over the Tibetan Plateau (TP). We found that northward movement of tropical precipitation significantly weakened with decreasing elevation of the TP. Lowering the TP changes the mean circulation over the eastern Indian Ocean. The changed mean circulation weakens boundary layer moisture convergence to the north of the precipitation belt, which deteriorates the northward movement of the tropical precipitation. We highlight that the mean state vertical shear is important for the northward movement of tropical precipitation anomalies.