The Strength of the Meridional Overturning Circulation of the Stratosphere

The strength of the stratospheric circulation is important for the distribution of traces gases, such as water vapor and ozone, which are important for climate. We calculate the mean strength of the global meridional overturning circulation of the stratosphere between 2007-2011 from satellite data, using age of air derived from sulfur hexafluoride (SF₆) and nitrous oxide (N₂O). These two estimates agree in the lower stratosphere, and at 460 K (about 20 km or 60 hPa in the tropics) we find the global circulation strength is 7.3 ± 0.3x10⁹ kg/s. This is a data-based benchmark for models' circulation strength. We compare these data estimates to a state of the art climate model and to three different reanalysis products, from which we calculate the diabatic circulation strength directly. In the lower stratosphere the reanalyses broadly agree. Above 500 K, the N₂O is no longer valid as an age tracer globally. The SF₆-based estimate and the reanalyses diverge higher up in the stratosphere, and the reanalyses differ substantially from each other. The interpretation of SF₆ as an age tracer is complicated by its mesospheric sink, which we qualitatively explore using the climate model. At upper levels, there is around 100% uncertainty on the mean strength of the circulation, emphasizing the need for more data.