1 Introduction
Many observational studies have found solar influences on climate on decadal and longer timescales (see part 3 for details). There is little evidence that solar vari-ations are a major factor in driving recent global climate change but consider-able evidence for a solar influence on the climate of particular regions as well as throughout the atmosphere. The Sun impacts the stratosphere (15–50 km), the troposphereand at the Earth’s surface as well as in the ocean (see part 3). During high solar activity, higher temperatures and larger ozone concentrations are ob-served in the tropicalstratosphere, and stronger pressure gradients are observed over the North Atlantic and Europe. During solar maxima, the stratospheric win-ter polar vortexis stronger with positive zonal wind anomalies extending down, through thetroposphere, to the ground. At the surface this can be expressed as a positive phase of theNorth Atlantic Oscillation (NAO), which is a feature of the natural variability of the climate characterised by the difference in surface pressure between the Icelandic Low and Azores High. A positive phase of the NAO is asso-ciated with strongerwesterlies, more northerly storm tracks and milder winters in Europe and North America (Figure1). During solar minima, the reverse picture is true: the stratospheric winter polar vortexis weaker and negative zonal wind anomalies extend to the surface. This represents a negative phase of the NAO and hence weaker westerlies and cold, snowy winters in Europe and North America.
Recent work suggests that the NAO response peaks a few years after thesolar cycle maximum, possibly due to atmosphere–ocean interactions over the North Atlantic (see Chapter 4.2 for details). Solar cycle signals have been also observed in the tropics where stronger trade winds, a stronger overturning (Walker) circulation
1GEOMAR Helmholtz Centre for Ocean Research Kiel, D¨usternbrooker Weg 20, 24105 Kiel, Germany; and Christian-Albrechts Universit¨at zu Kiel, Kiel, Germany
2Grantham Institute, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom
3Universit¨at Graz, Universit¨atsplatz 5, 8010 Graz, Austria
c
EDP Sciences 2015 DOI: 10.1051/978-2-7598-1733-7.c102
and shifts in precipitation, as well as changes in sea surface temperature patterns, occur during solar maxima (Figure 2). There is some observational evidence for a connection between geomagnetic activity and surface air temperatures. During active geomagnetic activity, which are characterised by energetic particle precipi-tation towards the Earth and which peak a few years after asolar cyclemaximum, typical regional surface temperatures pattern have been reported (see Chapter4.6 for more information).
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Figure 1.2.1: The positive phase of the North Atlantic Oscillation and its relation to the stratospheric as well as oceanic circulations. When the NAO index is high, the North Atlantic storm track is stronger, and northern Europe experiences mild winters. During solar maximum years, the winter stratosphere is colder, the polar vortex is stronger and this associates with a more positive NAO at the surface. !
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Fig. 1. The positive phase of the North Atlantic Oscillation and its relation to the stratospheric as well as oceanic circulations. When the NAO index is high, the North Atlantic storm track is stronger, and northern Europe experiences mild winters. During solar maximumyears, the winterstratosphereis colder, thepolar vortexis stronger and this associates with a more positive NAO at the surface. Copyright c2007 Woods Hole Oceanographic Institution, All Rights Reserved.
The coincidence of an unusual long, deep solar minimum around 2008 to-gether with the occurrence of a few cold and snowy winters in Europe since 2009 revived the discussion of solar influence on climate. On longer timescales, the Sun is currently declining from a grand maximum and moving towards a new grand minimum (see Chapter2.5and Infobox2.2) although predictions ofsolar activity are very difficult so that the date of the next grand minimum is very uncertain. If the Sun were to enter a new Maunder-like minimum, it would make only a small compensation for human-induced global warming but it might well affect regional climate patterns. Over thesolar cycleof about 11 years, solar variability offers a degree of predictability in particular for regional climate, especially in winter in
Kleareti Tourpaliet al.: The Earth’s atmosphere: an introduction 15
the Northern hemisphere. Understanding solar variability might therefore help reduce the uncertainty of future regional climate predictions on decadal time scales.
The identification of solar signals in climate observations is often difficult be-cause the available observational records are shorter than 100 years which would be at least needed to reliably detect robust solar signals (see part 3 for more de-tails). Sufficient observations covering the atmosphere from the surface to the stratosphere and mesosphere only exist for the last three solar cycles and often solar signals are “hidden”/overshadowed by other signals, such as volcanic erup-tions orENSOor man-made climate change. For these reasons, the extraction of solar signals from data using statistical techniques needs to be very thorough and interpretation of the results should be carried out with caution.
There are a number of mechanisms proposed for how solar variability could impact climate. Climate modelsin combination with findings from observations are useful to test the proposed mechanisms and understand the pathways. A brief review of existing mechanisms is now presented (see part 4 for an in-depth overview).
2 Mechanisms for solar influence on climate
Figure3gives a schematic overview of how solar variability can influence climate.
The influences can be categorised into two groups: the first is due to changes in the Sun’s radiant output (TSI: Total Solar Irradiance; UV: Ultra Violet) and the second due to the Sun’s influence on the energetic particles reaching Earth (SEPs:
Solar Energetic Particles, GCRs: Galactic Cosmic Rays).
Total Solar Irradiance (TSI), which is the sum of all irradiance coming from the Sun, varies by only about 0.1% with thesolar cycle(see Chapter2.2for more details). The irradiance is directly absorbed by the Earth and ocean surface and would lead to a small warming of about 0.1 K particularly in the tropics (see arrow with “TSI” in Figure 3) but is thought to be modulated through air–sea coupling; it changes precipitation and vertical motions which in turn influence trade winds and ocean upwelling. Duringsolar maximum, this so-called “bottom-up” mechanism is proposed to lead to stronger Hadley and Walker circulations and associated colder Sea Surface Temperatures (SSTs) in the tropical Pacific (Figure2). However, the details of the bottom-up mechanism and even the sign of SST response in the tropical Pacific are still under debate (see Chapter4.2 for more details).
Fractional variations in the UV part of the Sun’s radiative output (Solar Spectral Irradiance: SSI) are much larger than those in the Visible (VIS) and Near Infrared (NIR) reaching 5 to 8% in the region important forozonechemistry (see Chapter2.2for more details). Enhanced UV radiation duringsolar maximum leads to a warming in the tropics around the stratopause and to greater ozone formation down below. The ozone absorbs UV radiation at longer wavelengths and gives an additional warming (see arrow with “UV” in Figure3). The warming
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Figure 1.2.2: (a) Composite average sea surface temperature anomaly in the Pacific sector for December, January, and February (DJF) for 11 peak solar years (°C). (b) same as (a) but for composite average surface precipitation anomaly from three available peak solar years (mm/s). Adapted from Meehl et al. (2009) by Gray et al. (2010).
1.2.1 Mechanisms for Solar Influence on Climate
Fig. 1.2.3 gives a schematic overview of how solar variability can influence climate. The influences can be categorized into two groups: the first due to changes in the Sun’s radiant output (TSI: Total Solar Irradiance; UV: Ultra Violet) and the second due to the Sun’s influence on the energetic particles reaching Earth (SEPs: Solar Energetic Particles, GCRs:
Galactic Cosmic Rays).
Total Solar Irradiance (TSI), which is the sum of all irradiance coming from the Sun, varies by only about 0.1% with the solar cycle (see section 2.2 for more details). The irradiance is directly absorbed by the Earth and ocean surface and would lead to a small warming of about 0.1K in particular in the tropics (see arrow with „TSI“ in Fig. 1.2.3) but is thought to be modulated through air-sea coupling and changes precipitation and vertical motions which in turn influence trade winds and ocean upwelling. During solar maximum this so-called
„bottom-up“ mechanism is proposed to lead to stronger Hadley and Walker circulations and associated colder sea surface temperatures (SSTs) in the tropical Pacific (Fig. 1.2.2.;).
Fig. 2.(a) Composite average sea surface temperature anomaly in the Pacific sector for December, January, and February (DJF) for 11 peak solar years (C). (b) Same as (a) but for composite average surface precipitation anomaly from three available peak solar years (mm s−1). Adapted from Meehl et al. (2009) by Gray et al. (2010) (with permission).
in the tropics leads to stronger winds in the subtropical upperstratosphere(∆U in Figure3), which influence the background state forplanetary wavespropagating upward from the troposphere(thick black arrows in Figure3). Through complex interactions between the atmospheric flow andplanetary waves, the signal in the middle atmosphere is transmitted down to the troposphere, where it modifies e.g., the NAO and leads to measurable regional effects (Figure 1). In addition to the effects in the polar region, there is also a warming in the tropical lower stratosphere which affects the vertical propagation of synoptic scale waves (thin black arrows in Figure 3). These small scale waves deposit momentum in the tropopauseregion and thus influence the strength and position of the sub-tropical
Kleareti Tourpaliet al.: The Earth’s atmosphere: an introduction 17
Summer pole Equator Winter pole
VIS/NIR
Fig. 3.Schematic representation of possible mechanisms for solar influence on climate based on Kodera and Kuroda (2002) and redrawn from Gray et al. (2010). Shown are direct and indirect effects through solar irradiance changes (VIS/NIR and UV) during solar maxima as well as energetic particle effects (SEPs: Solar Energetic Particles and GCRs: Galactic Cosmic Rays). The two double arrows denote the coupling between different atmospheric layers (troposphere andstratosphere) or theatmosphere and the ocean. See text for details.
jets. The changes in the vertical structure may also influence convection in the tropicaltroposphere. Since a number of other factors, such as ENSO, the QBO, or volcanic eruptions, influence the temperature of the lower stratosphere, the detection of solar signals is complex here (see Chapter 4.3 for more details on this). The processes, involving stratospheric heating by solar UV and subsequent dynamical adjustment, are sometimes characterised as a “top-down” mechanism and have been confirmed in a number of climate modeling studies (for a review, see Gray et al., 2010; see Chapter4.2for more details).
Besides the two solar irradiance effects, which are likely to work in combi-nation, the effects of energetic particles need to be considered. Solar Energetic Particles (SEPs), such as solar protons or energetic electrons, which enter the at-mosphereat polar latitudes during high geomagnetic activity, ionise atmospheric particles and form radicals, such as NOx, which are transported down into the stratosphere during winter (arrow with “SEPs” in Figure3; for more details see Chapters 4.5 and 4.6). These radicals play an important role in ozone chem-istry and there is evidence that air depleted in ozonefollowing an SEP event is transported over a period of months from the polar upper stratosphereto lower altitudes and latitudes. Some climate models already include particle effects (in particular solar proton events and auroral particles), but the magnitude of the different effects, as well as their possible long-term climate impact, are still under investigation (see Chapter4.6).
Even less well established are the possible climate effects ofGalactic Cosmic Rays (GCRs). In response to the strength of the heliospheric magnetic field, the incidence of GCRs on Earth is inversely correlated withsolar activity(see Chapter 2.3). GCRs ionise the lower atmospherewith a peak inionisation rates near the tropopause(arrow with “GCRs” in Figure3). It is proposed that ionisedaerosols can act preferentially as cloud nuclei and hence a higher incidence of GCRs might increase cloud cover. The processes necessary for this to take place are under critical revision, specifically with cloud chamber experiments at the CERN particle accelerator in Geneva (see Chapter 4.7 for more details). GCRs also contribute to variations in the Global Electric Circuit (GEC) and another area of research concerns whereby cloud formation could be affected (see Chapter 4.8 for more details).
The impact of solar variability on climate has been studied for a long time and some of the mechanisms involved are now being clarified. Climate modelsare continuously improving but new and continuous measurements of solar activity, as well as of atmospheric parameters, are needed in order to improve our under-standing. The inclusion of all important aspects in climate models may contribute to the improvement of regional climate predictions.
Further reading
Gray, L.J. et al.: 2008,Solar Influences on Climate, Rev. Geophys., 48, RG4001.
Kodera, K. and Kuroda, Y.: 2002, Dynamical response to the solar cycle, J.
Geophys. Res., 107, 4749.
Meehl, G.A., J.M. Arblaster, K. Matthes, F. Sassi, and H. van Loon: 2009, Ampli-fying the PacificClimate SystemResponse to a Small 11-Year Solar Cycle Forcing, Science, 325, 1114.