Solar Influences on Climate

Lead Research Organisation: University of Reading
Department Name: Meteorology


One of the greatest science policy issues today is to determine what actions should be taken in response to human-induced changes in the Earth's climate. In order to attribute observed effects to human activity, however, it is essential that we have confidence in our ability to distinguish human-induced changes from those due to natural causes. The fundamental source of energy for the climate system is the Sun but the contribution of solar variability to recent climate change is not well known due to uncertainties in both the magnitude of the Sun's variations and the mechanisms of the climate response. Satellite measurements of total solar irradiance over the past 26 years show that it varies by ~0.1% over the 11-year solar cycle. However, with no reliable direct measurements having been made before the satellite era, studies of the role of solar variability in determining historical climate rely on reconstructions based on proxy activity indicators such as sunspot numbers. There are large uncertainties in these reconstructions and the spectral composition of the irradiance, which is important in determining the impact on atmospheric temperature and composition, is even less well known. Better specification of the temporal variation of total and spectral irradiance is necessary to provide the input required for climate studies. Signals of solar activity throughout the atmosphere have been detected in meteorological data but details of the links remain uncertain. For example, the direct effect on the temperature of the upper atmosphere is fairly well-understood but cannot explain the observed signal of solar variability at lower altitudes. One possible mechanism, based on the observation that variability in solar ultraviolet radiation is much greater than overall, suggests that the direct effects of the UV variations on the stratosphere may indirectly influence the atmosphere below by dynamical coupling, although details of how this takes place are unclear. Furthermore, the stratospheric impact may be associated with solar-induced changes in ozone but this response is not well established, with estimates from satellite data showing different structures from those predicted by theoretical models. In this project we will address all the key issues of uncertainty outlined above. The work will be carried out through a coordinated programme involving the participation of a number of overseas scientists who perceive the benefits of being involved in such an interdisciplinary collaborative project. We will use advanced theoretical models of the solar atmosphere to determine the relationship between solar irradiance and surface magnetic features (such as sunspots) and use this model, with the sunspot record, to determine the total and spectral irradiance over the past 300 years. Atmospheric measurements will be analysed to identify robust signals of solar influence on winds, temperature and chemical composition from the surface to the thermosphere. The irradiance data will be used in a number of different global circulation models of the Earth's atmosphere to investigate the impact of the solar variability and the results compared with the observational analyses. Discrepancies will be used to define further model experiments and to identify the key dynamical and chemical mechanism(s) through which solar variability influences tropospheric climate. The advances in understanding gained through these analyses will be used to improve the representation of the relevant processes in climate models. Beneficiaries will include all interested in climate change including researchers, policymakers and the general public. The irradiance reconstructions will be made available to climate modelling centres. The meteorological data analyses will be submitted to international assessments of trends in temperature and ozone while our advances in understanding of the processes involved will help to advance medium and long-range forecasting.


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Description This text summarises and updates the SOLCLI final report for the University of Reading dated November 2012.

Observational analyses
1. An earlier analysis of the solar signal in stratospheric temperature and winds was updated and extended over a third solar cycle to 2008, during which no major volcanic eruptions occurred. This allowed the solar and volcanic signals to be separated more completely, and confirmed earlier results, with a peak temperature response over the solar cycle of ~1.5K in the equatorial upper stratosphere and an additional local maximum response in the equatorial lower stratosphere (Frame and Gray 2010).
2. A regression analysis of the 11-year solar cycle signal in Hadley Centre sea level pressure and sea surface temperature (SST) datasets was undertaken (Gray et al. 2013). The analysis showed no concurrent statistically significant response over the Atlantic / European sector. However, a very clear lagged response was detected, in which a positive North Atlantic Oscillation (NAO) anomaly lagged the solar maximum periods by 3-4 years. The SST analysis was consistent with this response and provides strong corroboration of the lagged signal from an independent dataset. The detection of this signal has potential implications for improving seasonal to decadal scale forecasting in this region (Scaife et al. 2013).
3. A comparative study was performed employing different indices of solar variability, including the traditional total solar irradiance (TSI) and F10.7 cm solar radio flux compared with the open solar magnetic flux (Lockwood et al. 2010). The most notable difference between the indices is whether there is a long-term drift, particularly in the values at solar minimum. The study found larger stratospheric responses when open solar flux (OSF) is employed. Atmospheric blocking over the Atlantic / European region was found to occur more frequently during periods of solar minima, and greater, more consistent correlation of blocking frequency was found with OSF than with the traditional indices (Woollings et al. 2010, Lockwood et al. 2010).
4. A study assessing the ERA-40 global atmospheric analyses (Lu et al. 2008, 2009) confirmed that the solar signals in the northern hemisphere winter extratropics are indeed dependent on the phase of the stratospheric Quasi-Biennial Oscillation (QBO), moving poleward and downward as winter progresses with a faster descent rate during westerly QBO than during easterly QBO.

Model studies
1. A mechanism was identified in a simplified atmospheric model by which heating and cooling in regions of the lower stratosphere affects mid-latitude climate. The mechanism involves dynamical interactions between weather systems in the mid-latitude storm tracks and the mean atmospheric state (Simpson et al. 2009). It explains how stratospheric forcing can trigger a so-called "annular mode" response that involves a mutual displacement of the jet stream and storms poleward or equatorward. The response was found to depend on the model's background climate, being stronger if the jet stream and storm track are located further equatorward (Simpson et al. 2010, 2012). This is due to a change in the character of the natural variability of the model's annular modes, which have intrinsically longer timescales for lower latitude jet streams (Sparrow et al. 2009). Consistent with existing theoretical arguments, this leads to a stronger response to forcing.
These results are of wider importance due to their generic nature, but are particularly relevant to forcing by solar variability. A more active sun heats the tropical lower stratosphere, and the jet streams and storm tracks are observed to be displaced poleward. The model results identify a mechanism by which this displacement can occur, in all seasons in both hemispheres. The results imply that climate models whose jet streams are biased equatorward of observations, true of the majority of models analysed for the 2007 IPCC report, are likely to respond too strongly to a variety of climate forcings, including solar variability, stratospheric ozone depletion and increasing concentrations of greenhouse gases, all of which affect lower stratospheric temperatures.
2. A model in which dynamical heating is fixed was used to investigate the relative influence of 11-year solar irradiance and ozone variations on the temperature of the stratosphere (Gray et al. 2009). Upper stratospheric temperature variations were found to be primarily due to the irradiance changes while those in the lower stratosphere were due to the imposed ozone changes. The equatorial lower stratospheric structure was reproduced even though, by definition, the model calculations exclude dynamically driven temperature changes, suggesting an important role for an indirect dynamical effect through ozone redistribution. The results also suggest that differences between satellite (Stratospheric Sounding Unit/Microwave Sounding Unit) and ERA-40 estimates of the solar cycle signal can be explained by the poor vertical resolution of the satellite measurements.
3. An intermediate complexity model of the troposphere and stratosphere was used to study the atmospheric response to an idealized solar forcing in the subtropical upper stratosphere during Northern Hemisphere early winter (Cnossen et al. 2011). It was found that the tropospheric response was systematically different depending on the absence or presence of sudden stratospheric warmings (SSWs). When only years with SSWs are examined, the tropospheric signal appears to have descended from the stratosphere, while the tropospheric signal appears disconnected from the stratosphere when years with SSWs are excluded. Different troposphere-stratosphere coupling mechanisms therefore appear to be dominant for years with and without SSWs.
Exploitation Route Our research contributes evidence that changes in the stratosphere, including those forced by solar variability, affect the jet streams and storm-tracks and thereby influence surface climate. It is important that monthly, seasonal and longer-term climate forecasts are made using models that accurately represent stratospheric circulation and the external processes that cause it to vary.
Sectors Environment

Description Our research contributed evidence that led the UK Met Office to increase the resolution of its model used for seasonal forecasting, to better represent stratospheric processes (Dec2010 - GloSea4).
First Year Of Impact 2010
Sector Environment
Impact Types Policy & public services