Abstract
Abstract. We investigate the relative role of volcanic eruptions, El Niño–Southern Oscillation (ENSO), and the quasi-biennial oscillation (QBO) in the quasi-decadal signal in the tropical stratosphere with regard to 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 linear 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. The novel regression approach shows 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). This is caused by the alignment of these eruptions with periods of high solar activity. While it is feasible to detect a robust solar signal in the middle and upper tropical stratosphere, this is not the case in the tropical lower stratosphere, at least in a 45 yr simulation. The present results suggest that in the tropical lower stratosphere, the portion of decadal variability that can be unambiguously linked to the solar cycle may be smaller than previously thought.
Highlights
The Sun–climate connection is a topic of high relevance since solar variability is one source of natural variability in the climate system
Between 30 and 70 hPa a significant warming of 0.5– 0.7 ± 0.3 K is diagnosed in WACCM, which agrees with values reported from ERA-40/ERA-Interim reanalysis in Frame and Gray (2010)
We have investigated the attribution of quasi-decadal variations in tropical stratospheric temperature and ozone to the 11 yr solar cycle
Summary
The Sun–climate connection is a topic of high relevance since solar variability is one source of natural variability in the climate system. The 11 yr solar cycle is a welldocumented mode of variation of solar activity. Observations have shown decadal variations in the climate system that are commonly attributed to the 11 yr solar cycle (see review by Gray et al, 2010). A well-established decadal variability can be found in reanalysis data of stratospheric temperature (Crooks and Gray, 2005). An extended reanalysis data set from the European Centre for Medium-Range Weather Forecasts seems to confirm this pattern (Frame and Gray, 2010). A similar variability has been found in stratospheric ozone in three independent satellite data sets (Soukharev and Hood, 2006)
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