Characteristics of lower stratospheric transport as inferred from the age of air spectrum

Abstract. Trace gas transport in the lower stratosphere is investigated by analyzing seasonal and inter-annual variations of the age of air spectrum – the probability distribution of stratospheric transit times. Age spectra are obtained using the Lagrangian transport model CLaMS driven by ERA-Interim winds, and using a boundary-impulse-response method based on multiple tracer pulses. Seasonal age spectra show large deviations from an idealized stationary unimodal shape. Multiple modes emerge in the spectrum throughout the stratosphere, strongest at high latitudes, caused by the interplay of seasonally varying tropical 5 upward mass flux, stratospheric transport barriers and recirculation. The annual mean spectrum, on the other hand, is found to be well described by an idealized stationary spectrum. Inter-annual variations in transport (e.g., QBO, ENSO) cause significant modulations of the age spectrum shape. In fact, one particular QBO phase may determine the spectrum’s mode during the following 2-3 years. Interpretation of the age spectrum in terms of transport contributions due to the residual circulation and mixing is generally not straightforward. Advection by the residual circulation turns out to represent the dominant pathway in 10 the deep tropics and in the winter hemisphere extratropics above 500K, controlling the modal age in these regions. In contrast, in the summer hemisphere, particularly in the lowermost stratosphere, mixing represents the most probable pathway controlling the modal age. Analysis of the full age spectrum compared to mean age is highly beneficial for separating the effects of different transport processes, and is strongly recommended as a diagnostic for model inter-comparisons.