Atmospheric Circulation Evolution and Recurrence During Boreal Summers Revealed by ISOMAP Trajectory Distances

Abstract

ABSTRACT Persistent climate anomalies are often associated with slowly evolving atmospheric circulation, but persistence can also occur when an evolving circulation repeatedly returns towards similar states. This study examines large‐scale circulation evolution associated with the 2020 Yangtze River flood and the 2022 East Asian heat and drought using local wave activity (LWA), isentropic potential vorticity (IPV) 2‐PVU contour length, and isometric feature mapping (ISOMAP). ISOMAP was applied to daily IPV anomalies on the 340‐K isentropic surface to represent atmospheric circulation as a trajectory through state space. Two complementary trajectory measures were used. Stepwise distance quantifies accumulated displacement between consecutive circulation states and thereby characterizes the magnitude of circulation evolution, whereas loop distance, calculated from pairwise distances among states within a selected period, characterizes the overall compactness or dispersion of the trajectory. Because loop distance does not retain temporal ordering, recurrence was additionally examined using delayed‐recurrence distances between temporally separated daily states. The physical and state‐space diagnostics reveal contrasting circulation behaviour during the two extreme summers. During much of summer 2020, Rossby‐wave activity continued to evolve and propagate across Eurasia while the circulation repeatedly approached previously occupied regions of state space. In contrast, circulation evolution became substantially weaker during the second half of August 2022, coinciding with persistent heat and drought conditions over East Asia. Thus, persistent regional climate anomalies can occur both while the large‐scale circulation continues to evolve and recur and when the circulation itself evolves relatively slowly. These results emphasize that slow circulation evolution and recurrence are related but distinct aspects of atmospheric persistence. ISOMAP trajectory distances provide a complementary state‐space framework for quantifying the evolution and geometry of the full circulation state, while LWA and IPV contour length provide physical measures of Rossby‐wave activity and upper‐tropospheric PV deformation. Together, these diagnostics provide a framework for distinguishing contrasting forms of circulation behaviour associated with persistent summer climate anomalies.