| 17 Dec 2025
Tropical tropopause ozone modulated by tropopause height
Abstract. Ozone is a key radiative species near the tropical tropopause, which acts as a gateway to the stratosphere for ascending air. Ozone concentrations at fixed heights in this region fluctuate seasonally and interannually as the strength of stratospheric upwelling varies, influencing local temperatures and stratospheric composition. Models ranging in complexity suggest that an accelerated stratospheric circulation, along with tropospheric expansion, could reduce tropical lower stratospheric ozone following surface warming. These modes of variability are often equated with variability at the tropical tropopause; however, tropopause height varies seasonally and interannually, and it is expected to rise as Earth’s surface warms. Here, we explore how tropical tropopause ozone varies when considering changes to tropopause pressure. We first examine 15 years of observations to distinguish variability at the tropical tropopause ozone from fixed pressure levels on annual-to-interannual timescales. We show that changes to tropopause pressure drive the annual cycle of ozone mixing ratios at the tropical tropopause to be substantially smaller and out of phase from those at 95 or 105 hPa. We then investigate how tropical tropopause ozone responds to surface warming under a range of forcing scenarios using output from the Chemistry-Climate Modeling Initiative (CCMI). We find that pressure-dependent ozone production coupled with tropospheric expansion leads tropical tropopause ozone variability to remain distinct from fixed pressure levels following surface warming, with divergent trends in the strongest forcing scenario. Finally, we show that increases to tropical tropopause ozone correspond with local warming in CCMI projections, while tropospheric expansion increases lower stratospheric ozone.
Review of “Tropical tropopause ozone modulated by tropopause height” by Stephen Bourguet
General comments
This new study examines how ozone variability at the tropical tropopause differs from changes at nearby fixed pressure levels and why this distinction matters for understanding present and future climate change. Using 2005–2019 MERRA2-GMI data and an ozone budget framework, the author shows that seasonal and interannual ozone variability at the tropopause is weaker and often out of phase with that at fixed pressure levels because changes in tropopause height counteract stratospheric upwelling. An additional budget term associated with tropopause pressure changes is required to explain observed tropopause ozone variability. Analysis of CCMI model projections further indicates that, under surface warming, tropical tropopause ozone trends remain distinct from those at fixed pressure levels and can increase under strong forcing due primarily to enhanced ozone production from tropospheric expansion, with implications for tropopause temperatures, stratospheric water vapor, and lower stratospheric ozone.
The study is carefully designed and presents sound, robust, and well-supported results that advance understanding of ozone variability at the tropical tropopause. The manuscript is clearly written, well structured, and concise, making the scientific arguments easy to follow. The topic is well within the scope of Atmospheric Chemistry and Physics and is relevant to ongoing discussions of tropopause processes and climate change. Overall, only a few minor clarifications may be required, the paper appears to be very close to being suitable for publication.
Specific comments
line 7: The abstract only refers to “observations” in a generic sense. For clarity, it would be helpful to be more specific about the data source used here (e.g., MERRA-2/MERRA2-GMI reanalysis). Explicitly naming the dataset would better inform readers about the observational basis of the analysis.
line 9: The choice of the fixed pressure levels (95 and 105 hPa) used for comparison with the tropopause is not explained. A brief justification for selecting these levels (e.g., their proximity to the mean tropical tropopause or data availability) would help clarify the rationale.
line 47: The manuscript states that the WMO lapse-rate tropopause “follows an objective definition.” While the WMO criterion is indeed a standardized and widely used operational definition, its historical development appears to be based on community consensus rather than on a uniquely derived physical or mechanistic argument. As such, the term “objective” may be somewhat misleading. The author may wish to clarify this wording (e.g., by referring to the WMO tropopause as a standardized or operational definition) or briefly explain what is meant by “objective” in this context.
line 55: The choice of the 2005–2019 period, coinciding with the availability of MLS ozone profile assimilation, is very reasonable and well motivated. However, since MERRA-2 also incorporates other ozone-related observations (e.g., column ozone products) and is evaluated against additional limb-sounding datasets (such as ACE-FTS or MIPAS), the author may wish to clarify whether MLS is the primary constraint motivating this period selection, or briefly acknowledge the role of other datasets in the reanalysis.
line 74: The ozone budget includes chemical and advective tendencies only. Since MERRA2-GMI also provides moist process and turbulence tendencies, it would be helpful to briefly state whether these terms were examined and found to be negligible near the tropical tropopause, or explain why they are omitted from the budget.
line 144: For the multiple linear regression analysis using ENSO and QBO indices, it would be helpful to clarify whether the predictors were detrended or lagged, and whether autocorrelation in the ozone time series was accounted for when assessing the regression fit and significance.
Technical corrections
line 145: The phrase “multiple linear regressions fit” is potentially misleading. If a single model with multiple predictors was used, “multiple linear regression” would be the more standard terminology.