Relative Effects of the Greenhouse Gases and Stratospheric Ozone Increases on Temperature and Circulation in the Stratosphere over the Arctic

Using a stratosphere-resolving general circulation model, the relative effects of stratospheric ozone and greenhouse gases (GHGs) increase on the temperature and circulation in the Arctic stratosphere are examined. Results show that stratospheric ozone or GHGs increase alone could result in a cooling and strengthening extratropical stratosphere during February, March and April. However, the contribution of stratospheric ozone increases alone on the cooling and strengthening Arctic stratosphere is approximately 2 fold that of the GHGs increase alone. Model simulations suggested that the larger responses of the Arctic stratosphere to the ozone increase alone are closely related to the wave fluxes in the stratosphere, rather than the wave activity in the stratosphere. In response to the ozone increase, the vertical propagation of planetary waves from the troposphere into the mid-latitude stratosphere weakens, mainly contributed by its wavenumber-1 component. The impeded planetary waves tend to result from the larger zonal wind shear and vertical gradient of the buoyancy frequency. The magnitudes of anomalies in the zonal wind shear and buoyancy frequency in response to GHGs increase alone are smaller than in response to the ozone increase, which is in accordance with the larger contribution of stratospheric ozone to the temperature and circulation in the Arctic stratosphere.

Large impacts, past and future, of ozone-depleting substances on Brewer-Dobson circulation trends: A multi-model assessment

Substantial increases in the atmospheric concentration of well-mixed greenhouse gases (notably CO2), such as those projected to occur by the end of the 21st century under large radiative forcing scenarios, have long been known to cause an acceleration of the Brewer-Dobson circulation (BDC) in climate models. More recently, however, several single-model studies have proposed that ozone-depleting substances might also be important drivers of BDC trends. As these studies were conducted with different forcings over different periods, it is difficult to combine them to obtain a robust quantitative picture of the relative importance of ozone-depleting substances as drivers of BDC trends. To this end we here analyze - over identical past and future periods - the output from 20 similarly-forced models, gathered from two recent chemistry-climate modeling intercomparison projects. Our multi-model analysis reveals that ozone-depleting substances are responsible for more than half of the modeled BDC trends in the two decades 1980-2000. We also find that, as a consequence of the Montreal Protocol, decreasing concentrations of ozone-depleting substances in coming decades will strongly decelerate the BDC until the year 2080, reducing the age-of-air trends by more than half, and will thus substantially mitigate the impact of increasing CO2. As ozone-depleting substances impact BDC trends, primarily, via the depletion/recovery of stratospheric ozone over the South Pole, they impart seasonal and hemispheric asymmetries to the trends which may offer opportunities for detection in coming decades.

The influence of mixing on stratospheric age of air changes in the 21st century

Climate models consistently predict an acceleration of the Brewer-Dobson circulation (BDC) due to climate change in the 21st century. However, the strength of this acceleration varies considerably among individual models, which constitutes a notable source of uncertainty for future climate projections. To shed more light upon the magnitude of this uncertainty and on its causes, we analyze the stratospheric mean age of air (AoA) of 10 climate projection simulations from the Chemistry Climate Model Initiative phase 1 (CCMI-I), covering the period between 1960 and 2100. In agreement with previous multi-model studies, we find a large model spread in the magnitude of the AoA trend over the simulation period. Differences between future and past AoA are found to be predominantly due to differences in mixing (reduced aging by mixing and recirculation) rather than differences in residual mean transport. We furthermore analyze the mixing efficiency, a measure of the relative strength of mixing for given residual mean transport, which was previously hypothesized to be a model constant. Here, the mixing efficiency is found to vary not only across models, but also over time in all models. Changes in mixing efficiency are shown to be closely related to changes in AoA and quantified to roughly contribute 10% to the long-term AoA decrease over the 21st century. Additionally, mixing efficiency variations are shown to considerably enhance model spread in AoA changes. To understand these mixing efficiency variations, we also present a consistent dynamical framework based on diffusive closure, which highlights the role of basic state potential vorticity gradients in controlling mixing efficiency and therefore aging by mixing.