Dynamics of the Asian Summer Monsoon Anticyclone: insights from potential vorticity tendency diagnostics

The Asian Summer Monsoon Anticyclone (ASMA), the dominant circulation in the upper troposphere and lower stratosphere (UTLS) during the boreal summer, undergoes pronounced spatio-temporal variability. While the climatological mean state of the ASMA is well documented, the mechanisms governing its longitudinal propagation, episodic eddy shedding events, and multimodal transient behavior remain incompletely understood. In this study, we investigate the dynamic behavior of the ASMA using the reanalysis products JRA-3Q, ERA5, and MERRA-2 during 2000–2020. The ASMA is primarily trimodal, with core centers situated over the Iranian Plateau, the Tibetan Plateau, and the Western Pacific in all three reanalyses. To identify and analyze the evolution of individual anticyclonic features, we apply a multi-center vortex tracking algorithm to Montgomery stream function on the 370 K isentropic surface. Although potential vorticity (PV) tendencies are dominated by mean zonal and meridional advection, total diabatic heating plays a critical modulating role in the evolution of anticyclonic vortices within the ASMA. For further insight, we develop and apply an ASMA-specific PV tendency diagnostic that decomposes the total PV tendency into dynamical (isentropic horizontal advection) and diabatic (latent heating and background radiation) components. The diagnostic highlights a characteristic tripole pattern of PV tendency within vortices, separating propagation effects from intensification and thereby providing a clearer physical attribution of the evolution of vortices within the ASMA.