Stratospheric Intrusions Over Hong Kong: Key Dynamical Mechanisms in Summer

Abstract Summertime stratospheric intrusions (SI) impacting the Pearl River Delta (PRD) differ from those in winter and spring in both intensity and transport pathways. Using long‐term ozonesonde measurements and reanalysis data, we find that stratospheric O 3 is initially injected into the troposphere near 30°N, and subsequently advected far southward by anomalously strong northerlies. This unique two‐step transport mechanism is facilitated by a synoptic configuration comprising two subtropical highs, an intervening westerly trough, and, in most cases, typhoons. Composite analysis of all summertime SI episodes confirms the key dynamical ingredients identified in the case studies, and shows that these indirect SI events statistically contribute 37.2 ppbv (40.3%) to tropospheric O 3 maxima in the middle troposphere. The findings underscore the complex interplay between mid‐latitude and tropical systems over summer monsoon regions, and highlight the important role of stratospheric sources in short‐term O 3 variabilities in warm months.