A method proposal for spatially-resolved Age of Air from satellitedata

The stratospheric overturning meridional circulation is an important element in the global climate system and observationally-based estimates of its strength and changes are important for model validation and process understanding. But such observational constraints are prone to significant uncertainties related to the low circulation velocities and uncertainties in available trace gas measurements. Here, we propose a method to calculate mean age of air, as a measure for the stratospheric circulation, from mixing ratios of multiple measurable trace gas species, like trichlorofluoromethane (CFC-11), dichlorodifluoromethane (CFC-12), chlorodifluoromethane (HCFC-22), methane (CH₄), nitrous oxide (N₂O) and sulfur hexafluoride (SF₆ ). The method is based on the correlations of these trace gases with mean age. The involved methodological error includes uncertainties due to atmospheric variability and non-compactness of the correlation, and additional instrument uncertainties as would be inherent for e.g. satellite instruments. The age calculation method is evaluated, globally and seasonally, in a model environment and compared against the true model mean age. We show that the tracer-age correlations are, in general, sufficiently compact in the age range between about 1 and 4 to 5 years, depending on the given species. Combination of the six chosen species reduces the resulting uncertainty of the derived mean age to below 0.3 years throughout most regions in the lower stratosphere. Even smaller scale, seasonal features in the global age distribution can be reliably diagnosed from the multi tracer-based mean age. Hence, the proposed mean age calculation method shows promise to reduce the error in mean age estimates from satellite trace gas observations.