Decadal Influence of the Solar Cycle (DISC)

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Although global climate is expected to warm over the next century in response to increasing levels of greenhouse gases, regional changes over the next decade or so are likely to be dominated by unforced natural variability of the climate system. Some of this natural variability is potentially predictable months or even years in advance because it is related to relatively slow processes, especially those in the ocean (El Niño, fluctuations in the thermohaline circulation, and large-scale anomalies of ocean heat content). Another potential source of long-term variability comes from the well-known 11-year solar variations in the Sun's output but the mechanisms for how the signal reaches the surface is not well understood and, because of this, it is not well represented in climate models used for decadal predictions.The percentage variation in total solar irradiance over an 11-yr cycle is very small, but the variation in the UV is much larger and this can impact stratospheric temperatures and ozone production. Various mechanisms have been proposed to explain how this stratospheric solar signal may extend its influence to the surface, including amplifying mechanisms through atmospheric circulation changes. Analysis of observations show that the surface response to solar variability is regional. There has been controversy surrounding the observed signal over Europe, but recent analysis of long-term observational records over Europe have confirmed a strong statistically significant signal at lags of 3-4 years. Climate models, including the Hadley Centre HadGEM model, are able to capture an upper stratospheric response to changing UV, but do not reproduce the observed signal in the lower stratosphere nor the observed lagged signal over Europe.Sensitivity tests with the HadGEM model with an exceptionally large UV change was able to reproduce the lagged nature of the signal, thus showing some promise for its ability to reproduce the signal, but the surface response amplitude is still much too weak, suggesting that there is room for significant improvements which should lead to improved decadal forecasts. This prime aim of the proposal is to improve the representation of mechanisms of solar influence on the Earth's surface in the HadGEM climate model so that forecasts using the Met Office DePreSys operational decadal forecast model can be improved. The project will employ both the HadGEM model and a simpler model for extensive testing of mechanisms, with a particular focus on improving the representation of those mechanisms that transfer the solar signal to the surface via stratospheric heating anomalies and a surface amplifying mechanism that involves atmosphere-ocean coupling in the North Atlantic. The resulting improvements to the HadGEM model will be tested by comparing results from re-forecasts (hindcasts) of selected years, with particular attention to improvements in the Atlantic / European sector. The project will be performed by researchers at Oxford University who will carry out the HadGEM investigations and Imperial College who will perform the mechanistic model investigations. Extensive support will be provided by Project Partners at the Met Office, who will be closely involved in the interpretation of the HadGEM experiments and implementation / testing of improvements in the DePreSys forecast system and a Project Partner at Kiel University who will advise on solar spectral forcing and contribute to interpretation of results in the context of other major climate models.