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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. 

Abstract Quantifying the width of the tropics has important implications for understanding climate variability and the atmospheric response to anthropogenic forcing. Considerable effort has been placed on quantifying the width of the tropics at tropospheric levels, but substantially less effort has been placed on quantifying the width at stratospheric levels. Here we probe tropical width in the stratosphere using chemical tracers, which are accessible by direct measurement. Two new tracer‐based width metrics are developed, denoted here as the “1σ method” and the gradient weighted latitude (GWL) method. We evaluate widths from three tracers, CH₄, N₂O, and SF₆. We demonstrate that unlike previously proposed stratospheric width methods using tracers, these metrics perform consistently throughout the depth of the stratosphere, at all times of year and on coarse temporal data. The GWL tracer‐based widths correlate well with the turnaround latitude and the critical level, where wave dissipation occurs, in the upper and midstratosphere during certain months of the year. In the lower stratosphere, the deseasonalized tracer‐based widths near the tropical tropopause correlate with the deseasonalized tropopause‐height based metrics. We also find that tracer‐tracer width correlations are strongest at pressure levels where their chemical lifetimes are similar. These metrics represent another useful way to estimate stratospheric tropical width and explore any changes under anthropogenic forcing.