Dynamics of the Asian Summer Monsoon Anticyclone: Insights from Potential Vorticity Tendency Diagnostics

Abstract. The Asian Summer Monsoon Anticyclone (ASMA) is a dominant circulation in the upper troposphere and lower stratosphere (UTLS) during boreal summer and exhibits pronounced temporal and spatial variability. While its time-mean structure and climatological characteristics are well documented, the mechanisms governing its temporal evolution, particularly its eastward and westward propagation, episodic eddy-shedding events, and the emergence of multimodal behavior are remaining incompletely understood. Using multiple reanalysis datasets (JRA-3Q, ERA5, MERRA-2) from 2000–2020, this study analyzes the dynamic structures of the ASMA, the analysis shows that the ASMA exhibits a trimodal structure with centers over the Iranian Plateau, Tibetan Plateau, and western Pacific, identified via a multi-center vortex tracking algorithm applied to the 370 K isentropic surface to distinguish persistent and transient anticyclones. Further results show that mean zonal and meridional advection are the primary drivers of potential vorticity (PV) tendency, while total diabatic heating that plays a crucial modulating role in ASMA evolution. A key contribution of this work lies in the formulation and application of an ASMA-specific PV tendency diagnostic, which builds upon established PV budget and enables a more rigorous quantification of the respective roles of dynamical and thermodynamical processes in the anticyclone's intensification and zonal propagation across its three modes. This diagnostic decomposes total PV tendency into dynamic components (e.g., horizontal and vertical advection) and thermodynamic components (e.g., latent heating and background radiation), enabling the separation of the combined effects of anticyclone propagation and intensification by capturing a characteristic tripole pattern of PV tendency. Compared to conventional methods, this new diagnostic provides a more rigorous and precise quantification of the physical processes governing the ASMA's zonal movements and overall development, thereby advancing our scientific understanding of the ASMA's variability and underlying mechanisms in the UTLS region.