Simulations of the trend and annual cycle in stratospheric CO2

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Abstract
The distribution and evolution of stratospheric CO2 in response to the observed annual cycle, interannual variations, and long‐term trends in tropospheric CO2 is simulated with the GISS 23 layer stratospheric general circulation model. Carbon dioxide is a tracer of stratospheric transport which has essentially no local sources or sinks but still displays gradients due to the forcing at the surface. Consequently, observations of stratospheric CO2, until recently limited to a few flask samples, but now included as high frequency in situ sampling in aircraft campaigns, provide a test of tracer transport in stratospheric simulations independent of model chemistry. In our model, CO2 enters the stratosphere primarily through the tropical tropopause, where air parcels are effectively labeled in time by their CO2 values (although not uniquely because of the cycles in tropospheric concentration). Parcels of differing ages are subsequently mixed in the stratosphere. Only when the growth is purely linear can the CO2 offset in a parcel relative to the troposphere be interpreted as the average time since stratospheric air was last in contact with the troposphere, i.e., the “age” of the stratosphere. Our model is in qualitative agreement with multiyear averages of balloon soundings at northern mid‐ and high latitudes; the stratosphere at 30 km at mid‐latitudes is about 4 years (6 ppm of CO2) behind the troposphere. We predict significant propagation of the CO2 annual cycle into the lower stratosphere, an effect which must be accounted for when interpreting observations. While the annual cycle is negligible above the lower stratosphere, interannual oscillations, such as those associated with El Ninos, can propagate well into the middle stratosphere as positive offsets from the linear trend lasting significantly longer than their duration in the troposphere.