Evaluating Stratospheric Tropical Width Using Tracer Concentrations

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 1 sigma 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. Plain Language Summary Studies of the expansion of the Earth's tropical belt have focused on the surface and the troposphere, with limited consideration to changes in the stratosphere. Further, these studies largely use measures of tropical width that depend on dynamical quantities like vertical and longitudinal velocities, stream function, and tropopause height. Changes to the position of the subtropical jet and to tropopause height have implications for stratospheric transport, mixing and upwelling strength as well as for long-lived trace gas variability in the lower stratosphere, motivating study of the width of the stratospheric tropics and potential metrics here. In this study, we argue that chemical tracers form a useful basis for stratospheric tropical width measurements, particularly for species that are directly measured. We develop two new tracer-based metrics that we call the 1s method and the gradient weighted latitude method. These metrics advance opportunities to measure changes in stratospheric tropical width and to assess relationships between tropospheric and stratospheric width changes.