OVERVIEW

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 in particular, which can have a stronger influence on regional rather than global mean temperatures. In particular, 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 its governing mechanisms and predictability would be hugely important, not only for science but also for societal needs.

SOLENA aims to use a new generation of coupled ocean-atmosphere climate models, with a fully chemically interactive middle atmosphere. It will bring together a multidisciplinary team to address the role of the Sun in climate variability on seasonal, decadal and centennial time scales, considering jointly radiance and particle fluxes variations within a common state-of-the-art modelling framework. A special focus is the atmosphere-ocean interactions. SOLENA hence aims at gaining 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 tropical and Pacific regions.

As an effort to include energetic particle effects in climate simulations, 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. To this end, an electron precipitation forcing dataset has been reconstructed, extending back in time using proxy data, for use in climate model simulations such as those carried out in this project.

New simulations are being carried out using this new CMIP6 forcing in a whole-atmosphere chemistry-climate model. Pair of decadal, ensemble simulations covering one entire solar cycle (cycle 23), with or without the energetic electron forcing, are compared and show decadal-mean deficits of mesospheric ozone by up to 20 percents, and of stratospheric polar ozone by up to a few percents. The latter are caused by chemical depletion at subpolar latitudes due to excess of nitrogen oxides and by changes in the mean meridional circulation itself. 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.  In the Southern Hemisphere, the dynamical signature reverses in late winter, likely due to the radiative feedback of the stronger ozone deficit.

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 (up to 30 km), using previously used 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 for the conditions of a solar Maunder-like minimum have been completed in a coupled ocean-atmosphere framework.

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 likewise induces 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.

The stratospheric, tropospheric and surface impacts from the 11-year ultraviolet solar spectral irradiance (SSI) variability have been extensively studied using climate models and observations. We demonstrate using idealized model simulations that the Pacific Decadal Oscillation (PDO), which has been shown to impact the tropospheric and stratospheric circulation from sub-decadal to multi-decadal timescales, strongly modulates the solar-induced atmospheric response. To this end, we use a high-top version of the coupled ocean-atmosphere Norwegian Climate Prediction model (NorCPM) forced by the SSI dataset recommended for CMIP6.  We perform a 24-member ensemble experiment over the solar cycle 23 (SC-23) in an idealized framework. To assess the PDO modulation of the solar signal, we divide the model data into the two PDO phases, PDO+ and PDO-, for each solar (maximum or minimum) phase. By compositing and combining the four categories, we hence determine the component of the solar signal that is independent of the PDO and the modulation of the solar signal by the PDO, along with the solar signal in each PDO phase. Reciprocally, we determine the PDO effect in each solar phase. Our results show that the intensification of the polar vortex under solar maximum is much stronger in the PDO- phase. This signal is transferred into the troposphere, where we find a correspondingly stronger polar jet and weaker Aleutian Low. We further show that the amplification of the solar signal by the PDO- phase is driven by anomalous meridional advection of solar-induced temperature anomalies over northern North America and the North Pacific, which contributes to a decreased meridional eddy heat flux and hence to a decreased vertical planetary wave flux into the stratosphere.