Role of Vortex Tilting and Advection in Northward-Propagating Intraseasonal Oscillations During Western North Pacific Summer Monsoon

Abstract

The western North Pacific summer monsoon (WNPSM) is characterized by prominent northward propagation of intraseasonal oscillations (ISOs). However, the specific vorticity dynamics as an atmospheric precursor responsible for the northward propagation of ISOs remains unclear over the WNPSM. In this study, we utilized a multi-timescale vorticity diagnosis to identify the key dynamics of ISOs in WNPSM. Our findings indicate that both vortex tilting and vortex advection play important roles in vorticity generation and the subsequent development of new convection, ultimately leading to ISO propagation. Vortex tilting is primarily driven by the interaction between intraseasonal vertical velocity shear and background easterly wind shear, which is consistent with the ISOs during Indian summer monsoon (ISM). However, the impact of vortex tilting is confined near the equator in the eastern region of the WNPSM because of the decreased easterly wind shear from the Indian Ocean to the WNP. Meanwhile, vortex advection is the result of the transport of intraseasonal vorticity by the background southerly wind, which is significant over the region of tropical WNPSM. In summary, this differs from the tilting alone that controls northward-propagating ISOs during the ISM and is critical for advancing the unique framework for ISOs during the WNPSM.