The solar radiative output and its changes arrive at the top of the Earth’s atmosphere.
The radiative output determines the thermal structure of the Earth’s atmosphere and Earth’s radiation budget. It has an impact on the general circulation, ozone photo-chemistry, and weather-climate system. For illustration, Figure 1.8 shows the change in thermal structure of the Earth’s neutral atmosphere from solar minimum to maximum in the 11-year solar cycle.
Substantial effects due to variability over 11-year solar cycle and 27-day solar rotations are straightforward such as direct penetration of energetic photons that initiate photo-chemical processes. Solar signals impact ozone and temperature above approximately 25 km altitude. Below this altitude, the influence of the sun is less pronounced and oc-curs indirectly via complex dynamical processes. Focusing on the effects due to solar variability of electromagnetic radiation, these direct and indirect processes are briefly discussed in the following subsections [Brasseur et al., 2010, and references therein].
Not included in the ensuing discussion are the effects due to variability of particle pre-cipitation (protons and electrons). While most of these particles originate from various sources, some of them come from the sun, for example, as a result of solar flares di-rected towards the Earth [see, for example, Rohen et al., 2005; Randall et al., 2007;
Jackman et al., 2008].
1.3.1 Search for amplification mechanisms
Regular monitoring of the irradiance from space since early 1980s has shown that the solar constant varies about 0.08–0.1% between minimum and maximum of a solar
FIGURE 1.8: Illustration on the influence of changes of solar radiative output to the thermal structure of the Earth’s neutral atmosphere. Eleven-year cycle solar radia-tive ouput changes showed a negaradia-tive (posiradia-tive) response in the stratosphere (meso-sphere). From solar minimum to average solar maximum conditions, there is a 2–3%
decrease from its mean temperature value in the stratosphere, and 46% increase in 100 units of F10.7 cm solar radio flux in the upper mesosphere. Pressure increases by 5% in the stratosphere and 16–18% in the upper mesosphere compared to mean pressure values. The notationP andP′for pressure during solar maximum and solar minimum, respectively. The subscripts, m-, s-, and t- stand for meso-, strato-, and tropo-, respectively; and -p and -s for -pause and -sphere, respectively. Adapted from Figure 2.20 of Mohanakumar [2008]
cycle. With a mean value of 1366 W m−2 [de Toma et al., 2004a] from most TSI com-posites (or 1361 W m−2 [Kopp et al., 2005] from recent TIM aboard SORCE, PREMOS aboard PICARD, and corrected ACRIM III data)12, this is about 1.5 W m−2, of which the Earth intercepts 1.54 W m−2, the 11-year change of TSI times the ratio of Earth’s sunlit disk area over its surface area. Over longer time scales starting from the begin-ning of industrial era, a possible secular change in TSI is estimated to be 0.12 W m−2 with uncertainties of 0.06 to 0.30 W m−2. See Figure 1.1. Compared to the radiative forcing of 1.66 W m−2produced by enhanced concentrations of greenhouse gases, the secular change of TSI is considerably small. From a simple textbook model of radiative transfer processes (albedo of 0.31, shortwave transmission of close to 1.0, and long-wave transmission of 0.2), this secular change of TSI contributes an 0.07 K surface
12For more details, see Section 2.2.2.
temperature change, which is a factor of 10 smaller than the surface temperature trend observed since the start of industrial era. Therefore some amplification mechanisms have to occur in order for solar irradiance variability to play a stronger role in Earth’s climate change.
1.3.2 Absorption of solar radiation in the Earth’s atmosphere
The amount and spectral properties of the atmospheric constituents determine how the solar radiation is absorbed in the Earth’s atmosphere as a function of altitude, cf. middle panel of Figure 1.5. Above 100 km altitude in the thermosphere, X-ray and EUV radi-ation are absorbed. The Lyman-α line, 121.6 nm, penetrates down to 70 km altitude.
As shown in Figure 1.9 in terms of solar heating rate, the solar radiation in the wave-length 120–180 nm (Schumann-Runge continuum of molecular oxygen), 180–200 nm (Schumann-Runge bands), and 200–300 nm (Herzberg oxygen continuum and Hart-ley ozone bands) are absorbed above 80–120 km, 40–95 km (mesosphere and upper stratosphere), and below 50 km (stratosphere) altitude, respectively. Above 300 nm (Huggins and Chappuis ozone bands), solar radiation reaches the surface. Because 11-year solar cycle or 27-day solar rotation variability is larger at shorter wavelengths, which is mostly absorbed in upper layers of the atmosphere, the direct influence of solar variability decreases with altitude. In the mesosphere, temperature, water vapor and polar mesospheric clouds have been observed to be modulated by solar variability [Hervig and Siskind, 2006; Robert et al., 2010]. In the stratosphere, changes of temper-ature and ozone concentrations have been observed, too [Austin et al., 2008; Randel et al., 2009]. Down to the troposphere, the influence of solar variability has been been identified but is less well established in zonal mean temperature, surface pressure in the North Pacific, and global average surface temperature [Gleisner and Thejll, 2003;
Haigh and Blackburn, 2006; Matthes et al., 2006].
1.3.3 Stratospheric ozone photochemistry
The dominant solar UV and visible radiation absorbers are ozone (Hartley, Huggins, and Chappuis bands) and molecular oxygen (Schumann-Runge continuum and bands, Herzberg continuum). Stratospheric ozone photochemistry consists of the following chain reactions [Haigh, 2007].
FIGURE1.9: Diurnal average solar heating rate. The figure shows in log scale the di-urnal average solar heating rate in units of K d−1as a function of altitude for equinoctial conditions at the equator. Shown in the figure are contributions from the Schumann-Runge continuum and bands (SRC and SRB), the Herzberg continuum (Hz) and the Hartley (Ha), Huggins (Hu) and Chappuis (Ch) bands. Adapted with permission from Figure 25 of Haigh [2007, and references therein]. c⃝(2007) Max Planck Society and J. Haigh.
From top to bottom reactions, the following occurs.
(1) photodissociation of oxygen molecules, O2, at wavelengths less than 242 nm.
(2) oxygen atoms from (1) react with oxygen molecules to produce ozone molecules, O3. M is any other air molecule, whose presence is needed to conserve momen-tum and kinetic energy for a three-body collision.
(3) photodissociation of ozone, mainly by Hartley band absorption at wavelengths
(4) destruction of ozone by combination with an oxygen atom
(5)–(6) destruction of ozone by any catalyst X, which may include OH, NO, Cl, and Br.
The presence of the catalyst X lowers the energy of activation for the reaction and increases the efficiency of the decomposing an ozone atom and oxygen atom into two oxygen molecules.
The first two pathways, (1) and (2) for ozone formation, occur mostly in the lower strato-sphere (15 to 25 km), where little UVC radiation (100-290 nm wavelength) from the sun penetrates. The concentration of molecular oxygen is high in this region. Together with atomic oxygen the molecular oxygen rapidly forms into ozone. The rapid ozone forma-tion keeps the concentraforma-tion of atomic oxygen very low. The last two pathways, (5) and (6) for ozone destruction, occur mostly in the upper and middle stratosphere (25 to 50 km).
1.3.4 Atmospheric dynamics
In the absence of circulation or transport, most ozone would be produced at low lati-tudes in the upper stratosphere where photodissociation is most favorable. From ob-servations, however, ozone is higher at mid- and high latitude as a result of transport by the mean meridional circulation [Dobson, 1956]. The atmospheric circulation dis-tributes ozone in such a way that it is transported away from its source region towards the winter pole and downwards. In the lower stratosphere ozone has a long photo-chemical lifetime on the order of weeks to few months and is subject to advection and mixing processes. During winter ozone accumulates in the polar region due to en-hanced transport as result of planetary wave activity in the winter hemisphere driving the meridional circulation [see, for example, Weber et al., 2011]. After the sun returns in early spring, photochemical destruction of ozone begins and ozone at high latitudes decreases towards its seasonal minimum at the end of the summer. The amplitude of solar cycle variability at short wavelengths is larger than at long wavelengths, and ozone formation is more strongly modulated by solar activity than its destruction. The overall effect during high solar activity is a higher net production of stratospheric ozone particularly in the tropics. Solar cycle effects at higher latitudes are then mainly results of modulation in the thermal structure altering circulation.
The link establishing solar variability and climate (troposphere) requires a potential am-plification mechanisms that could be driven by meridional circulation and zonal winds [Brasseur et al., 2010, and references therein]. For a schematic diagram on the link, see Figure 1.10. Intriguing and still open for more research the following are three am-plification mechanisms that have been proposed to explain the effects of solar variability in the troposphere.
1. Top-down mechanism. Proposed by Haigh in 1996, Kodera and Kuroda in 2002, and Shindell and co-workers in 1999, this mechanism is the downward prop-agation of stratospheric perturbation that induce changes of weather patterns below the tropopause, or dynamical disturbances in the troposphere. Changes in UV directly impact the stratosphere, which indirectly impact the surface via stratosphere-troposphere coupling.
2. Bottom-up mechanism. Proposed by Meehl and co-workers in 2003, this mech-anism is forced by direct solar heating (mainly by changes in TSI) of the sea surface and dynamically coupled air-sea interaction, affecting, for example, evap-oration and low-level moisture.
3. Combined top-down and bottom-up mechanism. As proposed by Rind and co-workers in 2008 and Meehl and co-co-workers in 2009, the above two mechanisms play in the same direction and add together. That is, stratospheric disturbances propagate downward at the same time excess heat storage from ocean propa-gates upward during high solar activity to produce an amplified sea-surface tem-peratures, precipitation, and cloud response in the tropical Pacific.