Phase Unlocking and the Modulation of Tropopause‐Level Trace Gas Advection by the Quasibiennial Oscillation

Abstract

Open questions about the modulation of near‐surface trace gas variability by stratosphere‐troposphere tracer transport complicate efforts to identify anthropogenic sources of gases such as CFC‐11 and N2O and disentangle them from dynamical influences. In this study, we explore one model's modulation of lower stratospheric tracer advection by the quasi‐biennial oscillation (QBO) of stratospheric equatorial zonal‐mean zonal winds at 50 hPa. We assess instances of coherent modulation versus disruption through phase unlocking with the seasonal cycle in the model and in observations. We quantify modeled advective contributions to the temporal rate of change of stratospheric CFC‐11 and N2O at extratropical and high‐latitudes by calculating a transformed Eulerian mean (TEM) budget across isentropic surfaces from a 10‐member WACCM4 ensemble simulation. We find that positive interannual variability in seasonal tracer advection generally occurs in the easterly QBO phase, as in previous work, and briefly discuss physical mechanisms. Individual simulations of the 10‐member ensemble display phase‐unlocking disruptions from this general pattern due to seasonally varying synchronizations between the model's repeating 28‐month QBO cycle and the 12‐month seasonal cycle. We find that phase locking and unlocking patterns of tracer advection calculations inferred from observations fall within the envelope of the ensemble member results. Our study bolsters evidence for variability in the interannual stratospheric dynamical influence of CFC‐11 near‐surface concentrations by assessing the QBO modulation of lower stratospheric advection via synchronization with the annual cycle. It identifies a likely cause of variations in the QBO influence on tropospheric abundances.

A Method for Estimating Global Subgrid‐Scale Orographic Gravity‐Wave Temperature Perturbations in Chemistry‐Climate Models

“…These occur with irregular patterns as soon as something is changed in the applied parameterizations. To test the reason for this, a simulation randomly perturbing the initial temperature of REF on the order of 10 −14 K, as done in previous work with CESM (Kay et al., 2015; Shah et al., 2022; Stone et al., 2019), led to similar tropospheric changes of these species, see Figure S3 in Supporting Information for ozone. Thus, these changes are apparently a result of dynamical changes, which originate from intermediate solutions at the smaller chemistry time step when applying the chemistry sub‐stepping and which then change the trace gas concentrations in the model.…”

A method for estimating global subgrid-scale gravity-wave temperature perturbations in chemistry-climate models

“…These occur with irregular patterns as soon as something is changed in the applied parametrizations. To test the reason for this, a simulation randomly perturbing the initial temperature of REF on the order of 10 −14 K, as done in previous work with CESM (Kay et al, 2015;Stone et al, 2019;Shah et al, 2022), led to similar tropospheric changes of these species, see Fig. S3 in the supplement for ozone.…”