Diagnosing the dynamical and chemical stratospheric tropical width using model and observational data

Transport in the stratosphere is characterized by upwelling in the tropics and downwelling in the extratropics, and these regions are separated by the so-called turnaround latitudes. In the winter hemisphere, a region of intense wave breaking (the ‘surf zone’) separates the tropics from the polar vortex. In the summer hemisphere, easterly winds do not allow for penetration of planetary waves into the middle and upper stratosphere.

Observational evidence of this global transport circulation comes from the shape of long-lived tracer contours. While the overturning circulation tends to steepen latitudinal gradients of tracers, large-scale stirring by wave breaking leads to quasi-horizontal mixing and thus flattens the gradients. The observed shape of tracer contours results from the combined effects of the two transport processes, and is therefore not trivially related to the turnaround latitudes or mixing diagnostics.

In this study we compare several tracer-based and dynamical-based diagnostics of the tropical width computed both from a CESM1-WACCM model simulation and from satellite observations  and reanalysis data. We find notable differences between the dynamical and tracer metrics particularly in the seasonality, with good correspondence only in the equinox seasons. We also examine the interannual variability and long-term trends.