1Universidad Complutense de Madrid, Spain
2National Center for Atmospheric Research, Boulder (CO), USA
3Instituto de Geociencias (IGEO), Madrid, Spain
4Universidad de Extremadura, Badajoz, Spain
Abstract. We investigate the relative role of volcanic eruptions, El-Niño Southern-Oscillation (ENSO) and the Quasi-Biennal-Oscillation (QBO) in the quasi-decadal signal in the tropical stratosphere in temperature and ozone commonly attributed to the 11 yr solar cycle. For this purpose, we perform transient simulations with the Whole Atmosphere Community Climate Model forced from 1960 to 2004 with an 11 yr solar cycle in irradiance and different combinations of other forcings. An improved multiple regression technique is used to diagnose the 11 yr solar signal in the simulations. One set of simulations includes all observed forcings, and is thereby aimed at closely reproducing observations. Three idealized sets exclude ENSO variability, volcanic aerosol forcing, and QBO in tropical stratospheric winds, respectively. Differences in the derived solar response in the tropical stratosphere in the four sets quantify the impact of ENSO, volcanic events and the QBO in attributing quasi-decadal changes to the solar cycle in the model simulations. It is shown that most of the apparent solar-induced lower stratospheric temperature and ozone increase diagnosed in the simulations with all observed forcings is due to two major volcanic eruptions (i.e., El Chichón in 1982 and Mt. Pinatubo in 1991), that are concurrent with periods of high solar activity. While in the middle and upper tropical stratosphere, it is feasible to detect a robust solar signal, this is not the case in the tropical lower stratosphere, at least in a 45 yr record. The present results suggest that in the tropical lower stratosphere, the portion of decadal variability that can be unambigously linked to the solar cycle may be smaller than previously thought.