02 Dec 2022
02 Dec 2022

The role of tropical upwelling in explaining discrepancies between recent modeled and observed lower stratospheric ozone trends

Sean Davis1, Nicholas Davis2, Robert Portmann1, Eric Ray1,3, and Karen Rosenlof1 Sean Davis et al.
  • 1NOAA Chemical Sciences Laboratory, Boulder, CO, USA
  • 2Atmospheric Chemistry Observations and Modeling Laboratory, National Center for Atmospheric Research, Boulder, CO, USA
  • 3Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Boulder, CO, USA

Abstract. Several analyses of satellite-based ozone measurements have reported that lower stratospheric ozone has declined since the late 1990s. In contrast to this, lower stratospheric ozone was found to be increasing in specified dynamics (SD) simulations from version 4 of the Whole Atmosphere Community Climate Model (WACCM-SD) where the model was nudged using reanalysis wind/temperature fields. This paper demonstrates that the standard configuration of WACCM-SD fails to reproduce the underlying tropical upwelling changes present in the reanalysis fields used to drive the model. Over the period since the late 1990s, WACCM-SD has a spurious negative upwelling trend that induces a positive quasi-global lower stratospheric column ozone trend and accounts for much of the apparent discrepancy between modeled and observed ozone trends. Using a suite of SD simulations with alternative nudging configurations, it is shown that short-term (~2 decade) ozone trends scale linearly with short-term trends in tropical upwelling. However, none of the simulations capture the recent ozone decline, and the ozone/upwelling scaling in the WACCM simulations suggests that a large short-term upwelling trend (~6 % decade-1) would be needed to explain the observed satellite trends. The strong relationship between ozone and upwelling, coupled with both the large range of reanalysis upwelling trend estimates and the inability of WACCM-SD simulations to reproduce upwelling from their input reanalyses, severely limits the use of these simulations for accurately reproducing recent ozone variability. Contrary to expectations, a free-running version of WACCM using only surface boundary conditions and a nudged QBO more closely captures both interannual variability and decadal-scale ozone “trends” than the SD simulations.

Sean Davis et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Review on egusphere-2022-1267', Roland Eichinger, 03 Jan 2023
  • RC2: 'Comment on egusphere-2022-1267', Anonymous Referee #2, 06 Jan 2023

Sean Davis et al.


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Short summary
Several studies have noted that ozone in the lower part of the stratosphere ozone has not increased and perhaps even continued to decline in recent decades. This study demonstrates that the amount of ozone in this region is highly sensitive to the amount of air upwelling into the stratosphere in the tropics, and that some climate models fail to accurately capture the variations in upwelling that control these short term trends in ozone.