The Role of Tropical Upwelling in Explaining Discrepancies Between Modeled and Observed Recent Lower Stratospheric Ozone Trends

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 is 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, and that the WACCM-SD negative upwelling trend accounts for most of the apparent discrepancy between modeled and observed ozone trends. Using a suite of specified dynamics simulations with alternative nudging configurations, it is shown that short-term ozone trends scale linearly with short-term trends in tropical upwelling. However, none of the models capture the observed ozone decline, and the ozone/upwelling scaling in the models suggests a large short-term upwelling trend (~6% decade-1) would be needed to explain the satellite trends. The strong relationship between ozone and upwelling variability, coupled with the large range of reanalysis upwelling estimates and inability of the SD simulations to reproduce the upwelling from their input reanalyses, severely limits the use of SD simulations for attributing the dynamical drivers of recent ozone variability. A free-running version of WACCM using only surface boundary conditions and a nudged quasi-biennial oscillation (QBO) more closely captures both interannual variability and decadal-scale "trends" than the nudged model simulations.