Attribution of the Hemispheric Asymmetries in Trends of Stratospheric Trace Gases Inferred From Microwave Limb Sounder (MLS) Measurements

Using Microwave Limb Sounder (MLS) satellite observations, ERA‐Interim reanalysis data, and a chemistry transport model simulation, we analyze and investigate the causes of the asymmetric hemispheric trends of N2O, CH4, and HCl in the stratosphere during the period 2004–2012. We find significant hemispheric asymmetries in the trends of these trace gases in the midlatitude middle and lower stratosphere. With regard to N2O and CH4, the enhanced downwelling branch of the residual circulation in the Northern Hemisphere (NH) middle and upper stratosphere transports more N2O/CH4‐poor air from the upper stratosphere to the lower stratosphere. The enhanced poleward meridional branch of the residual circulation in the Southern Hemisphere (SH) stratosphere brings more N2O/CH4‐rich air from lower to middle latitudes. These processes therefore contribute to the negative trends of N2O and CH4 in the NH lower stratosphere and the positive trends in the SH middle stratosphere. A corresponding positive trend is found for HCl in the NH, where the deep branch of the residual circulation located in the middle and upper stratosphere strengthens, bringing more HCl‐rich air downward to the lower stratosphere, while the shallow branch of the residual circulation in the lower stratosphere weakens and leads to enhanced conversion of chlorine‐containing source gases of different lifetimes to HCl. A reversed picture emerges in the SH, where the deep branch of the residual circulation in the middle and upper stratosphere weakens, while the shallow branch in the lower stratosphere strengthens, resulting in less HCl there. In addition, the southward shift of the upwelling branch of the residual circulation in recent decades can partly explain trace gas trends above 20 hPa, while the eddy mixing has a small effect on the trends. Understanding these contributions from different processes to the hemispheric asymmetries in trends of these trace gases can help us to evaluate more accurately future changes in stratospheric composition.