Characterising the seasonal and geographical variability in tropospheric ozone, stratospheric influence and recent changes

Abstract

Abstract. The stratospheric contribution to tropospheric ozone ( O3 )\nhas been a subject of much debate in recent decades but is known to have an\nimportant influence. Recent improvements in diagnostic and modelling tools\nprovide new evidence that the stratosphere has a much larger influence than\npreviously thought. This study aims to characterise the seasonal and\ngeographical distribution of tropospheric ozone, its variability, and its\nchanges and provide quantification of the stratospheric influence on these\nmeasures. To this end, we evaluate hindcast specified-dynamics\nchemistry–climate model (CCM) simulations from the European Centre for\nMedium-Range Weather Forecasts – Hamburg (ECHAM)/Modular Earth Submodel\nSystem (MESSy) Atmospheric Chemistry (EMAC) model and the Canadian Middle\nAtmosphere Model (CMAM), as contributed to the International Global Atmospheric Chemistry – Stratosphere-troposphere Processes And their Role in\nClimate (IGAC-SPARC) (IGAC–SPARC) Chemistry Climate Model\nInitiative (CCMI) activity, together with satellite observations from the\nOzone Monitoring Instrument (OMI) and ozone-sonde profile measurements from\nthe World Ozone and Ultraviolet Radiation Data Centre (WOUDC) over a period\nof concurrent data availability (2005–2010). An overall positive, seasonally\ndependent bias in 1000–450\u2009hPa ( ∼0 –5.5\u2009km) sub-column ozone is\nfound for EMAC, ranging from 2 to 8\xa0Dobson units (DU), whereas CMAM is found\nto be in closer agreement with the observations, although with substantial\nseasonal and regional variation in the sign and magnitude of the bias ( ∼ ± 4 \u2009DU). Although the application of OMI averaging kernels (AKs)\nimproves agreement with model estimates from both EMAC and CMAM as expected,\ncomparisons with ozone-sondes indicate a positive ozone bias in the lower\nstratosphere in CMAM, together with a negative bias in the troposphere\nresulting from a likely underestimation of photochemical ozone production.\nThis has ramifications for diagnosing the level of model–measurement\nagreement. Model variability is found to be more similar in magnitude to that\nimplied from ozone-sondes in comparison with OMI, which has significantly\nlarger variability. Noting the overall consistency of the CCMs, the influence\nof the model chemistry schemes and internal dynamics is discussed in relation\nto the inter-model differences found. In particular, it is inferred that CMAM\nsimulates a faster and shallower Brewer–Dobson circulation (BDC) compared to\nboth EMAC and observational estimates, which has implications for the\ndistribution and magnitude of the downward flux of stratospheric ozone over\nthe most recent climatological period (1980–2010). Nonetheless, it is shown\nthat the stratospheric influence on tropospheric ozone is significant and is\nestimated to exceed 50\u2009% in the wintertime extratropics, even in the lower\ntroposphere. Finally, long-term changes in the CCM ozone tracers are\ncalculated for different seasons. An overall statistically significant\nincrease in tropospheric ozone is found across much of the world but\nparticularly in the Northern Hemisphere and in the middle to upper\ntroposphere, where the increase is on the order of 4–6\u2009ppbv (5\u2009%–10\u2009%)\nbetween 1980–1989 and 2001–2010. Our model study implies that attribution\nfrom stratosphere–troposphere exchange (STE) to such ozone changes ranges\nfrom 25\u2009% to 30\u2009% at the surface to as much as 50\u2009%–80\u2009% in the\nupper troposphere–lower stratosphere (UTLS) across some regions of the\nworld, including western Eurasia, eastern North America, the South Pacific\nand the southern Indian Ocean. These findings highlight the importance of a\nwell-resolved stratosphere in simulations of tropospheric ozone and its\nimplications for the radiative forcing, air quality and oxidation capacity of\nthe troposphere.