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Abstract This study investigates changes in stratosphere‐troposphere exchange (STE) of air masses and ozone concentrations from 1960 to 2099 using multiple model simulations from Chemistry Climate Model Initiative (CCMI) under climate change scenario RCP6.0. We employ a lowermost stratosphere mass budget approach with dynamic isentropic surfaces fitted to the tropical tropopause as the upper boundary of lowermost stratosphere. The multi‐model mean (MMM) trends of air mass STEs are all small over all regions, which are within 0.3 (0.1) % decade−1for 1960–2000 (2000–2099). The MMM trends of ozone STE for 1960–2000 are 0.3%, −2.7%, 3.4%, −0.9%, and −2.7% decade−1over the Northern hemisphere (NH) extratropics, Southern hemisphere (SH) extratropics, tropics, extratropics, and globe, respectively. The corresponding ozone STE trends for 2000–2099 are 3.0%, 4.3%, 0.8%, 3.5%, and 4.7% decade−1. Changes in ozone STEs are dominated by ozone concentration changes, driven by climate‐induced changes and ozone‐depleting substance (ODS) changes. For 1960–2000, small changes in ozone STEs in the NH extratropics are due to a cancellation between effects of climate‐induced changes and ODS increases, while the ODS effect dominates in the SH extratropics, leading to a large ozone STE magnitude decrease. Increased ozone transport from tropical troposphere to stratosphere for 1960–2000 is due to increased tropospheric ozone. A decreased global ozone STE magnitude for 1960–2000 was largely caused by ODS‐induced ozone loss that is partly compensated by climate‐induced ozone changes. For 2000–2099, about two‐thirds of global ozone STE magnitude increases are caused by ozone increases in the extratropical lower stratosphere due to climate‐induced changes. The remaining one‐third is caused by ozone recovery due to the phaseout of ODS.
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Calibrating the Tropospheric Air and Ozone Mass
Abstract We divide the atmosphere into distinct spheres based on their physical, chemical, and dynamical traits. In deriving chemical budgets and climate trends, which differ across spheres, we need clearly defined boundaries. Our primary spheres are the troposphere and stratosphere (∼99.9% by mass), and the boundary between them is the tropopause. Every global climate‐weather model has one or more methods to calculate the lapse rate tropopause, but these involve subjective choices and are known to fail near the sub‐tropical jets and polar regions. Age‐of‐air tracers clock the effective time‐distance from the tropopause, allowing unambiguous separation of stratosphere from troposphere in the chaotic jet regions. We apply a global model with synthetic tracer e90 (90‐day e‐folding), focusing on ozone and temperature structures about the tropopause using ozone sonde and satellite observations. We calibrate an observation‐consistent tropopause for e90 using tropics‐plus‐midlatitudes and then apply it globally to calculate total tropospheric air‐mass and tropopause ozone values. The tropopause mixing barrier for the current UCI CTM is identified by a transition in the vertical transport gradient to stratospheric values of 15 days km−1, corresponding to an e90 tropopause at 81 ± 2 ppb with a global tropospheric air mass of 82.2 ± 0.3%. The best e90 tropopause based on sonde pressures is 70–80 ppb; but that for ozone is 80–90 ppb, implying that the CTM tropopause ozone values are too large. This approach of calibrating an age‐of‐air tropopause can be readily applied to other models and possibly used with observed age‐of‐air tracers like sulfur hexafluoride.
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- Award ID(s):
- 2135749
- PAR ID:
- 10609992
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- AGU Advances
- Volume:
- 6
- Issue:
- 3
- ISSN:
- 2576-604X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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