While there is strong evidence that global climate change is related to man-made increasing greenhouse gases, there remains uncertainties regarding the actual contribution of natural climate variability. The latter includes the 11-year solar cycle, which can have an influence on regional rather than global mean temperatures. Recent studies suggest that the winter circulation patterns, such as the North Atlantic Oscillation (NAO) could be modulated by the 11-year solar cycle. The NAO is the dominating weather pattern over the Atlantic-European region and a better understanding of mechanisms governing its predictability would be hugely important, not only for science but also for societal needs.
SOLENA used a new generation of coupled ocean-atmosphere climate models with a fully chemically interactive middle atmosphere. A special focus is the atmosphere-ocean interactions. SOLENA hence gained novel insights about the role of the Sun in influencing climate variability that may ultimately lead to improved decadal climate predictions, with special relevance for Northern Europe, the North Atlantic and the Arctic but also for other teleconnections with the Pacific region. The Intergovernmental Panel on Climate Change (IPCC) Coupled Model Intercomparison Project Phase 6 (CMIP6) now recommends a solar forcing that is composed not only of radiative but also particle fluxes. A set of decadal, coupled ocean-atmosphere ensemble experiments has been carried out with the Norwegian Climate Prediction Model (NorCPM) to investigate the relative roles of the solar radiative forcing and of the particle precipitation during the solar cycle 23. The latter includes both the low-energy electrons that precipitate to form the aurora and the more energetic particles that occasionally penetrate into the mesosphere principally during the declining phase of the solar cycle. This is the first time that such a comprehensive coupled ensemble experiment has been conducted. The solar radiative forcing induces a clear NAO signature in winter, originating from the stratosphere, with a tendency for a more positive phase during solar maximum. The NAO response is synchronised with the solar cycle, hence peaks during solar maximum, and the ocean-atmosphere coupling does not appear to be strong enough to induce a multi-year lag. On the other hand, enhanced particle precipitation fails to induce a stratospheric signal migrating down to influence the NAO toward a more positive phase, but only in specific years of strong precipitation and during late winter-spring. The potential predictability, an index that is measuring the ability to detect the solar signal amidst the large internal variability of the NAO remains quite small. A new finding resulting from the analysis of NorCPM simulations is that the amplitude of solar signal depends on the Pacific Decadal Oscillation (PDO), a major climate pattern with global impacts on sub-decadal to multi-decadal timescales. By itself, the PDO modulates the polar stratospheric variability and the PDO negative phase tends to be associated to a strengthened polar vortex. Solar maximum conditions are also associated with a stronger polar vortex. Yet, we demonstrated that the two effects are not purely additive, making that the solar signal reinforced during PDO negative phase. This signal is transferred into the troposphere, leading to a stronger polar jet and a weaker Aleutian Low.
A new examination for signatures of geomagnetic activity in the high-quality Japanese atmospheric re-analyses spanning the last 5 decades has been carried out, using more stringent statistical testing than before. One conclusion is that there is no statistically significant signature in stratospheric temperatures using geomagnetic activity indices. Long-term simulations with fixed geomagnetic forcing have also been carried out to distinguish its effect under different phases of the solar cycle. These simulations indicate a primary role of solar radiance change over geomagnetic activity. Simulations with more energetic electron forcing show decadal-mean deficits of mesospheric ozone and of stratospheric polar ozone. The latter are caused by chemical depletion at subpolar latitudes due to excess of nitrogen oxides and by changes in the mean meridional circulation. The changes in temperature are consistent with an early winter strengthening of the polar vortex in both hemispheres, with a weak but consistent tropospheric and surface signal.
Simulations for the conditions of a hypothetical Grand Solar minimum have been completed in a coupled ocean-atmosphere framework. Such a period with little or no sunspot activity on the sun occurred in the late 17th century (Maunder Minimum) and it has been speculated it could happen in the near-future. These simulations made it possible to better distinguish the effects of a reduction in UV radiation from a reduction in the visible part of the spectrum.
The project led to further understanding of the 11-year solar cycle influence on climate, with important implications for decadal predictability. The climatic influence of solar irradiance seems largely dominant over geomagnetic activity and particle precipitation. The weak signal-to-noise ratio associated to the solar forcing, e.g., over the North Atlantic, reflects the predominance of internal variability and is symptomatic of the signal-to-noise paradox common to prediction models. The simulations were among the first in the world to include both medium-to-high energy particle precipitation and ultraviolet irradiance forcings in coupled, high-top ocean-atmosphere experiments with interactive chemistry. As such they lead to new developments and accrued international visibility for the Norwegian Climate Prediction Model and the Norwegian Earth system Model. The project further cemented the cooperation between NILU, the Bjerknes Centre and the University of Oslo on climate modelling.
Although the solar forcing cannot account for the observed recent global warming, there is evidence that solar variability influences climate both globally and regionally.
SOLENA will address the solar forcing on climate arising from both radiance and particle flux variations. The effects of solar radiance and particle forcings will be examined first separately and then jointly, using a common state-of-the-art modelling framework. SOLENA will rely on performing simulations with a chemistry-climate model with a high-top middle atmosphere component and interactive chemistry, coupled to the ocean. The model will be the Norwegian Earth System Model (NorESM) in a version coupled to the Whole-Atmosphere Community Climate Model (WACCM). Through running time-slice natural variability or else transient historical simulations, the project will examine how the 11-year solar cycle and its attendant radiance and particle variations could affect the interannual-to-decadal variability of the North Atlantic and Arctic circulation patterns, as well as of other teleconnections with the tropical and Pacific regions. A special focus is the atmosphere-ocean interactions.
Analysis of existing centennial-to-millennial simulations, as well as dedicated decadal-scale sensitivity experiments, will examine if variations in solar radiative forcing, especially those associated with solar grand minima, could affect the variability of these patterns through similar processes. The implications for the interannual-to-decadal predictability of these North Atlantic and Arctic circulation patterns will be examined.
The multi-disciplinary project will bring together expertise in atmospheric dynamics, oceanography, climate modelling as well as middle atmosphere and solar-terrestrial physics.