Solar Forcing for CMIP6 (v3.1)

Abstract

Abstract. This paper describes the solar forcing dataset for CMIP6 and highlights in particular changes with respect to the CMIP5 recommendation. The solar forcing is provided for radiative properties, i.e., total solar irradiance (TSI) and solar spectral irradiance (SSI), and F10.7 cm radio flux, as well as particle forcing, i.e., geomagnetic indices Ap and Kp, and ionisation rates to account for effects of solar protons, electrons and galactic cosmic rays. This is the first time that a recommendation for solar-driven particle forcing is provided for a CMIP exercise. The solar forcing dataset is provided at daily and monthly resolution separately for the CMIP6 Historical Simulation (1850–2014), for the future (2015–2300), including an additional extreme Maunder Minimum-like sensitivity scenario, as well as for a constant and a time-varying forcing for the preindustrial control simulation. The paper not only describes the forcing dataset, but also provides detailed recommendations for how to implement the different forcing components in climate models. The TSI and SSI time series are defined as averages of two (semi-) empirical solar irradiance models, namely the NRLTSI2/NRLSSI2 and SATIRE-TS. A new and lower TSI value is recommended: the contemporary solar cycle-average is now 1361.0 W/m2. The slight negative trend in TSI during the last three solar cycles in CMIP6 is statistically indistinguishable from available observations and only leads to a small global radiative forcing of −0.04 W/m2. In the 200–400 nm range, which is also important for ozone photochemistry, CMIP6 shows a larger solar cycle variability contribution to TSI than CMIP5 (50 % as compared to 35 %). The CMIP6 dataset is tested and compared to its CMIP5 predecessor using timeslice experiments of two chemistry-climate models and a reference radiative transfer model. The changes in the background SSI in the CMIP6 dataset, as compared to CMIP5, impact on climatological stratospheric conditions (lower shortwave heating rates (−0.35 K/day at the stratopause), cooler stratospheric temperatures (−1.5 K in the upper stratosphere), lower ozone abundances in the lower stratosphere (−3 %), and higher ozone abundances (+1.5 % in the upper stratosphere and lower mesosphere). Between the maximum and minimum phases of the 11-year solar cycle, there is an increase in shortwave heating rates (+0.2 K/day at the stratopause), temperatures (~1 K at the stratopause), and ozone (+2.5 % in the upper stratosphere) in the tropical upper stratosphere using the CMIP6 forcing dataset. This solar cycle response is slightly larger, but not statistically significantly different from that for the CMIP5 forcing dataset. CMIP6 models with a well-resolved shortwave radiation scheme are encouraged to use SSI, as well as solar-induced ozone signals, in order to better represent solar climate variability compared to models that only prescribe TSI and/or exclude the solar-ozone response. Monthly mean solar-induced ozone variations will also be incorporated into the CCMI CMIP6 Ozone Database for climate models that do not calculate ozone interactively. CMIP6 models with interactive chemistry are encouraged to use the particle forcing which will allow the potential long-term effect of particles to be addressed for the first time. The consideration of particle forcing has been shown to significantly improve the representation of reactive nitrogen and ozone variability in the polar middle atmosphere, eventually resulting in further improvements of the representation of solar climate variability.

A semi-empirical model for mesospheric and stratospheric NO<sub><i>y</i></sub> produced by energetic particle precipitation

“…It has been successfully tested in simulations carried out with the EMAC model (Matthes et al, 2016).…”

“…Such a combined, de-biased data set, expressed as a homogenized A p index, has been generated as part of the solar forcing recommendations for CMIP6, available at http://solarisheppa.geomar.de/ solarisheppa/cmip6. A detailed description of the methodology for homogenization of these three indices can be found in Matthes et al (2016). We use here the extended A p index for the EPP-NO y reconstruction in the period 1850-2014, corresponding to the historical simulation as part of the CMIP6 DECK experiments (Eyring et al, 2016).…”

“…Solar forcing recommendations for the recently launched Climate Model Intercomparison Project Phase 6 (CMIP6) (Eyring et al, 2016) include, for the first time, the consideration of energetic particle effects (Matthes et al, 2016). EPP is strongly linked to solar activity and hence to the solar cycle, either directly by coronal mass ejections producing solar energetic particles or indirectly by the impact of the solar wind on the Earth's magnetosphere.…”

HEPPA-II model–measurement intercomparison project: EPP indirect effects during the dynamically perturbed NH winter 2008–2009

“…Solar forcing recommendations for the recently launched Climate Model Intercomparison Project Phase 6 (Eyring et al, 2016) include, for the first time, the consideration of energetic particle effects (Matthes et al, 2016). EPP is strongly linked to solar activity and hence to the solar cycle, either directly by coronal mass ejections producing sporadically large fluxes of solar energetic particles or indirectly by the quasi-continuous impact of the solar wind on the Earth's magnetosphere.…”

What can the annual 10Be solar activity reconstructions tell us about historic space weather?

“…Three well established proxies are found in sunspot observations, geomagnetic activity records, and the concentrations of cosmogenic radionuclides stored in natural archives (Owens et al, 2016a,b). These estimates of historical solar variability are invaluable for placing the comprehensive space-age solar observations in a proper context (Barnard et al, 2017), and for investigating plausible scenarios of future space climate change (Abreu et al, 2008;Steinhilber et al, 2012;Matthes et al, 2016).…”