The Footprint of Growing Stratospheric Transport Contribution in Tropospheric Ozone

The long-term ozone changes in the middle to upper troposphere, of importance to climate radiative forcing and setting the baseline for tropospheric ozone pollution, are not well quantified from observations in East Asia. In this study, three decades of observations from ozone soundings are examined with meteorological reanalysis data and climate model simulations to characterize regional features in tropospheric ozone and investigate their driving factors along the northwestern Pacific coastal region. A rapid increase in ozone is observed in the middle to upper troposphere over Naha (26 ^oN), Tsukuba (36 ^oN), and Sapporo (43 ^oN) in the warm season during 1990‒2020. Coincidentally, tongue-shaped hotspots with high tropospheric ozone extending from the stratosphere occur during the warm season. The timing for these ozone hotspots shows a time lag from late spring to summer with the increasing latitude of the observation sites. Intensified stratospheric intrusion (i.e., tropopause folding) events and enhanced buildup of ozone abundance in the lowermost stratosphere (likely driven by climate change) are the key factors causing the large increase over this region. With the aid of the chemistry climate model EMAC, the enhanced downward transport of stratospheric ozone into the troposphere is revealed by a surge in the stratospheric ozone tracer in the troposphere.