Abstract
Abstract. Variations in the solar spectral irradiance (SSI) with the 11-year sunspot cycle have been shown to have a significant impact on temperatures and the mixing ratios of atmospheric constituents in the stratosphere and mesosphere. Uncertainties in modelling the effects of SSI variations arise from uncertainties in the empirical models reconstructing the prescribed SSI data set as well as from uncertainties in the chemistry–climate model (CCM) formulation. In this study CCM simulations with the ECHAM/MESSy Atmospheric Chemistry (EMAC) model and the Community Earth System Model 1 (CESM1)–Whole Atmosphere Chemistry Climate Model (WACCM) have been performed to quantify the uncertainties of the solar responses in chemistry and dynamics that are due to the usage of five different SSI data sets or the two CCMs. We apply a two-way analysis of variance (ANOVA) to separate the influence of the SSI data sets and the CCMs on the variability of the solar response in shortwave heating rates, temperature, and ozone. The solar response is derived from climatological differences of time slice simulations prescribing SSI for the solar maximum in 1989 and near the solar minimum in 1994. The SSI values for the solar maximum of each SSI data set are created by adding the SSI differences between November 1994 and November 1989 to a common SSI reference spectrum for near-solar-minimum conditions based on ATLAS-3 (Atmospheric Laboratory of Applications and Science-3). The ANOVA identifies the SSI data set with the strongest influence on the variability of the solar response in shortwave heating rates in the upper mesosphere and in the upper stratosphere–lower mesosphere. The strongest influence on the variability of the solar response in ozone and temperature is identified in the upper stratosphere–lower mesosphere. However, in the region of the largest ozone mixing ratio, in the stratosphere from 50 to 10 hPa, the SSI data sets do not contribute much to the variability of the solar response when the Spectral And Total Irradiance REconstructions-T (SATIRE-T) SSI data set is omitted. The largest influence of the CCMs on variability of the solar responses can be identified in the upper mesosphere. The solar response in the lower stratosphere also depends on the CCM used, especially in the tropics and northern hemispheric subtropics and mid-latitudes, where the model dynamics modulate the solar responses. Apart from the upper mesosphere, there are also regions where the largest fraction of the variability of the solar response is explained by randomness, especially for the solar response in temperature.
Highlights
Solar ultraviolet (UV) radiation is largely absorbed in the stratosphere and mesosphere, thereby heating these regions and forming the ozone layer, filtering the most harmful part out of the solar spectrum and protecting life on Earth
Modelling studies range from early investigations with two-dimensional atmospheric and chemistry models (Garcia et al, 1984; Haigh, 1994) and three-dimensional general circulation models (GCMs) (e.g. Matthes et al, 2004) to studies with advanced chemistry– climate models (CCMs) (SPARC CCMVal, 2010) and chemistry–climate model (CCM) coupled to an ocean model, as partly used within the Coupled Model Intercomparison Project Phase 5 (CMIP5) (Mitchell et al, 2015; Misios et al, 2015; Hood et al, 2015)
To analyse the solar response, time series of anomalies have been calculated for each simulation performed by ECHAM/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) and Whole Atmosphere Chemistry Climate Model (WACCM) using one of the five solar spectral irradiance (SSI) data sets constructed for solar maximum conditions with respect to the time series of the reference simulations of both models using the ATLAS3-based SSI near solar minimum
Summary
Solar ultraviolet (UV) radiation is largely absorbed in the stratosphere and mesosphere, thereby heating these regions and forming the ozone layer, filtering the most harmful part out of the solar spectrum and protecting life on Earth. SPARC CCMVal (2010) identified a large model spread in solar responses for ozone and temperature of 18 CCMVal CCMs, mainly caused by differences in the spectral resolution of the SW radiation parameterisations or the treatment of photolysis in the CCMs. The simulated solar response in annual mean tropical (25◦ S–25◦ N) temperature (1960–2004) near the stratopause ranges from 0.45 to 1.4 K, whereas the Stratospheric Sounding Unit (SSU) satellite data (1979–2005) show 0.85 K for a comparable height region, and ERA-40 reanalyses (1979–2001) show 1.4 K. The aim of this study is to estimate the uncertainty of the solar cycle signal resulting from the two above-described sources of uncertainty: the specification of the 11-year solar cycle SSI amplitude and the models’ SW radiation and photolysis schemes and their dynamical characteristics In this Part 1 of our study we concentrate on the annual mean solar response in heating rates, temperature, and ozone, while Part 2 (Kruschke et al, 2020) focuses on the dynamical solar and auroral responses in northern winter.
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