A new study published in the journal Weather and Climate Dynamics by Ninghui Li of Tsinghua University and colleagues now offers a sharper lens on this problem. The team analyzed twenty-one years of warm-season data, from 2000 to 2020, drawn from three of the most modern atmospheric reanalysis products available: the European Centre for Medium-Range Weather Forecasts’ ERA5, NASA’s MERRA-2, and the Japan Meteorological Agency’s JRA-3Q. By combining a sophisticated vortex-tracking algorithm with a purpose-built diagnostic based on potential vorticity tendencies, they have produced one of the most detailed mechanistic accounts to date of how the anticyclone is born, moves, strengthens, and decays.

The first headline finding concerns structure. For years, scientists debated whether the anticyclone was fundamentally bimodal, alternating between centers over the Iranian Plateau and the Tibetan Plateau, or whether it simply drifted between them. The new analysis, performed on the 370 Kelvin isentropic surface using the Montgomery stream function, a dynamically consistent measure of flow on surfaces of constant potential temperature, confirms that the system is robustly trimodal. Three preferred centers emerge: one near 50 degrees east over the Iranian Plateau, one near 90 degrees east over the Tibetan Plateau, and a third over the Western Pacific near 135 to 150 degrees east, corresponding to the well-known Bonin High. All three reanalysis products agree on this threefold structure, and the agreement is particularly strong between JRA-3Q and ERA5.

The tracking algorithm itself is elegant in its simplicity. The researchers first locate the zero-zonal-wind ridgeline, the line separating easterly tropical flow from westerly subtropical flow, and then search for local maxima in the Montgomery stream function along that line. Each candidate center must pass strict filters: the relative vorticity at the center and its neighbors must be anticyclonic, the winds poleward must be westerly while those equatorward must be easterly, and the ridgeline must be long enough to exclude small-scale noise. Vortices that persist for more than eighteen hours and move less than ten degrees between successive six-hour snapshots are classified as persistent; the rest are transient. Persistent vortices accounted for more than 65 percent of all identified centers in ERA5 and JRA-3Q, underscoring how stable these features can be.

The seasonal choreography of the three modes is striking. Tibetan Plateau centers dominate in May, Iranian Plateau centers take over from mid-June to mid-July, and the balance shifts back toward the Tibetan Plateau as the Bonin High emerges in August and September, coinciding with the maturation of the Western North Pacific monsoon. The latitude of the vortices also migrates, drifting northward to about 40 degrees north between mid-July and mid-August before retreating southward. Many features show evidence of westward propagation, though eastward motion appears early and late in the season, and time-longitude diagrams reveal tails of low potential vorticity stretching in both directions, the telltale signature of eddy shedding, in which subsidiary anticyclones break away from the main circulation.

The heart of the study, however, lies in its diagnostic innovation. Potential vorticity, a quantity that combines rotation and stratification, is conserved in adiabatic, frictionless flow, making it a powerful tracer of atmospheric motion. But instantaneous potential vorticity fields cannot distinguish between a vortex that is moving and one that is intensifying in place. To resolve this ambiguity, the team developed a decomposition of the potential vorticity tendency, separating dynamical contributions from horizontal advection of potential vorticity by the winds from thermodynamic contributions arising from diabatic heating, both latent heating from condensation in convective clouds and radiative heating from sunlight and infrared exchange.

The results reveal a characteristic tripole pattern in the potential vorticity tendency around each anticyclonic vortex. An advective dipole, with positive tendencies to the west of the center and negative tendencies to the east, governs the eastward translation of the vortex: positive tendencies erode the low-potential-vorticity anomaly on the western flank while negative tendencies reinforce it on the eastern flank, nudging the entire core eastward. Superimposed on this dipole is a central monopole, driven mainly by diabatic heating and its vertical gradient, which modulates the vortex’s intensity without displacing it. A negative central tendency strengthens the anticyclone; a positive one weakens it. This separation of propagation from intensification is precisely what earlier diagnostics could not achieve.

The heating story is nuanced. Deep convective latent heating, concentrated below and to the east of the vortex centers, generates negative potential vorticity tendencies that intensify the anticyclone, particularly for the Tibetan Plateau mode, where heating along the southern slopes of the Himalayas is strongest. Radiative heating, by contrast, generally acts to spin the anticyclone down: because radiative heating increases with height through the upper troposphere and lower stratosphere, its vertical divergence produces positive potential vorticity tendencies that erode the low-potential-vorticity core. Latent heating partially offsets this radiative damping near the vortex centers. Meanwhile, the composite analysis shows that diabatic heating exerts a southward pull on the vortices, tugging them back toward the convective core of the monsoon and inhibiting their escape from it.

When it comes to propagation, the winds rule. Mean zonal advection by the subtropical westerly jet pushes all three vortex modes eastward relative to the surface, but meridional advection, the dominant term in Rossby wave propagation, pushes them westward relative to the background flow, and the two effects largely offset each other in the net. The balance of terms differs across the reanalysis products, with MERRA-2 showing stronger radiative damping and weaker advective tendencies than ERA5 or JRA-3Q, differences the authors attribute in part to how each system represents anvil cloud radiative effects and prognostic cloud ice. These inter-product discrepancies are a sobering reminder that mechanistic attribution in the upper troposphere and lower stratosphere still carries real uncertainty.

The implications extend well beyond academic bookkeeping. Because the anticyclone controls the leakage of Asian monsoon pollution and moisture into the global stratosphere, a reliable framework for separating its propagation from its intensification could improve predictions of stratospheric composition and, ultimately, of monsoon-related climate variability itself. The authors suggest that their potential vorticity tendency diagnostic could help clarify the differences between westward- and eastward-shedding eddies, processes first linked to baroclinic instability more than two decades ago but never fully quantified. Future work, they note, will need objective criteria for identifying eddy shedding events and a fuller accounting of data assimilation effects and parameterized momentum sources. For now, the study establishes a unified theoretical framework for one of the atmosphere’s most important summer circulations, showing that winds set the anticyclone’s course while heating sets its strength, a division of labor now made visible for the first time.

Subject of Research: Dynamics and potential vorticity tendency diagnostics of the Asian Summer Monsoon Anticyclone in the upper troposphere and lower stratosphere

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

Article References: Li, N., Wright, J. S., Rupp, P., Ming, A., Zhang, S., & Gao, J. (2026). Dynamics of the Asian Summer Monsoon Anticyclone: insights from potential vorticity tendency diagnostics. Weather and Climate Dynamics, 7(3), 1853-1874. https://doi.org/10.5194/wcd-7-1853-2026

Image Credits: AI Generated

DOI: 10.5194/wcd-7-1853-2026

Keywords: Asian Summer Monsoon Anticyclone, potential vorticity, upper troposphere, lower stratosphere, diabatic heating, vortex tracking, reanalysis, Bonin High, Tibetan Plateau, eddy shedding, monsoon circulation, atmospheric dynamics