**3. Polar vortex as a possible reason for the variability of GCR effects on the lower atmosphere**

Antarctic coasts in the South Atlantic and the Indian ocean, as well at climatic Polar fronts over the South Pacific [42]. So, GPH700 anomalies for the Southern hemisphere (**Figure 6b**)

**Figure 6.** *Left*: Temporal variations of GPH700 anomalies in the belts 30–60° and GCR fluxes (detrended yearly values) in the Northern (a) and Southern (b) hemispheres. *Right*: Correlation coefficients for sliding 11-year intervals between LCA and GCR fluxes (dashed lines), GPH700 anomalies and GCR fluxes (solid lines) in the Northern (c) and Southern (d) hemispheres. In the Southern hemisphere, the correlation coefficients are shown for the whole belt 30–60°S (light green line) and for the cyclonic areas (dark green line). Dotted lines show the significance levels of the correlations coefficients

**Figure 5.** Long-term variations of tropospheric pressure (12-month running averages of GPH700) in the belts 30–60° of

were calculated for these cyclonic areas.

88 Cosmic Rays

between GPH700 and GCR fluxes.

the Northern (a) and Southern (b) hemispheres. Red lines show polynomial trends.

It is well known that temporal variability is a characteristic feature of solar-atmospheric links (see, for example, [56]). Correlation links observed between lower atmosphere characteristics and phenomena related to solar activity may weaken, disappear and even change sign depending on time period. So, a violation of the cloud-GCR link in the 2000s is not an extraordinary event. Herman and Goldberg [56] suggested that a reason for temporal variability of solar-atmospheric links may be long-term processes of the Sun which do not influence sunspot numbers and/or some changes of atmospheric conditions. Veretenenko and Ogurtsov [42, 43] showed that temporal behavior of correlation links between surface pressure at extratropical latitudes and sunspot numbers is characterized by a roughly 60-year periodicity caused by changes in the epochs of the large-scale atmospheric circulation. The reversals of the correlation signs were found in the end of the nineteenth century, in the early 1920s, the 1950s and the early 1980s coinciding with climatic regime shifts at middle latitudes [57], as well as with the transitions between cold and warm epochs in the Arctic [58]. So, a violation of the cloud-GCR link in the 2000s seems not to be unexpected and may be associated with the next change of the circulation epochs resulting in the change of GCR contribution to extratropical cyclonic activity and, then, to cloud field formation.

According to the suggestions in [58], the changes of the circulation epochs are closely related to the state of the polar vortex. The polar vortex is a cyclonic circulation forming in a cold air mass in the polar region of the Northern and Southern hemispheres and spreading from the middle troposphere to the upper stratosphere. A circular air motion in the vortex results in a decrease of heat exchange between polar and middle latitudes and this contributes to a temperature drop inside the vortex and an increase of temperature gradients at its edges (see **Figure 7**). The vortex can also be seen as a region of enhanced velocity of zonal winds in the stratosphere during cold months for the given hemisphere, the highest values being observed at latitudes 50–80° at the pressure levels above 50 hPa.

The polar vortex is an important factor of the large-scale atmospheric circulation and climate variability. Gudkovich et al. [58] showed that the rotation of cold and warm epochs in the Arctic are caused by changes of the vortex intensity, warm and cold epochs being associated with a strong and weak vortex, respectively. Indeed, under strong vortex conditions cyclone tracks are shifted to the north [59] and more North-Atlantic cyclones arrive in the polar region bringing warm air. An important feature of the polar vortex is its influence on the troposphere-stratosphere coupling via planetary waves. Propagation of planetary waves upward depends on stratospheric circulation (for example, [60, 61]). If the vortex is strong and a velocity of western winds in the stratosphere exceeds some critical value, these waves are reflected back to the troposphere. If the vortex is weak, planetary waves propagate freely upward. So, the stratosphere may influence the troposphere under a strong vortex regime. Under a weak vortex, only the troposphere may influence the stratosphere. This point seems to be of importance to understand the observed temporal variability of solar activity/GCR effects on tropospheric circulation.

Let us consider variations of the polar vortex intensity and compare them with temporal behavior of GCR effects on cyclonic activity and clouds. To characterize the vortex strength, we used zonally averaged velocity of western winds (i.e. the U-component of wind velocity directed from west to east from [55]) at the level 50 hPa (~20 km). In **Figure 8** (top panel) there are presented variations (detrended values) of mean zonal wind velocity in the belts 60–80° in both hemispheres averaged for six cold months (October-March in the Northern hemisphere and April-September in the Southern one). The correlation coefficients R(GPH, FCR) and R(LCA,FCR) for sliding 11-year intervals are shown in **Figure 8** (bottom panel).

From the middle 1980s to the middle 1990s the polar vortices in both hemispheres were enhanced. The vortex enhancement was the most prominent in the Northern hemisphere, the wind velocity increasing up to 4–7 m.s−1 relative to the trend values. This agrees well with the data [62] showing no strong sudden stratospheric warming destroying the vortex in the indicated period. The vortex in the Southern hemisphere is more stable than that in the Northern one; however, we can also see a noticeable increase in wind velocity. The data in **Figure 8** show that in the period of the vortex enhancement most statistically significant correlations between pressure (cyclonic intensity) and GCR fluxes, as well as between low clouds and GCR fluxes take place. In the late 1990s both the vortices and the correlations GPH–GCR and LCA–GCR in both hemispheres started weakening. A sharp decrease of the wind velocities resulting in the vortex transition to its weak state occurred near 2000 in both hemispheres. The reversal of correlation coefficients under study coincided well with the transition of the vortices to a weak state. Thus, the presented data allow suggesting that the violation of the correlation between low cloudiness and GCR intensity detected on the decadal scale is closely

**Figure 8.** *Top*: Variations of mean zonal velocity of western winds (the U-component) at the stratospheric level 50 hPa in the belt 60–80° for cold months in the Northern (a) and Southern (c) hemispheres. Thick lines show 11-year running averages and polynomial fits. *Bottom*: Correlation coefficients for sliding 11-year intervals between LCA and GCR fluxes (dashed lines), GPH700 anomalies and GCR fluxes (solid lines) in the Northern (b) and Southern (d) hemispheres. Dotted lines show the significance levels of the correlation coefficients. The period of an enhanced vortex is marked by vertical dashed lines.

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The results of this study confirm that the changes of the atmosphere state, in particular, of the intensity of the stratospheric polar vortex, may be a real reason for the observed temporal variability of solar-atmospheric links. Indeed, sign reversals of correlation links between

related to the change of the polar vortex strength.

**Figure 7.** Distribution of mean monthly temperature at the pressure level 20 hPa (a) and of magnitude of horizontal temperature gradients (b) in the Northern hemisphere in January 2005. White asterisk indicates a minimum of temperature in the vortex; thick black line connects the points of maximal values of the temperature gradient at given latitude.

Galactic Cosmic Rays and Low Clouds: Possible Reasons for Correlation Reversal http://dx.doi.org/10.5772/intechopen.75428 91

in a decrease of heat exchange between polar and middle latitudes and this contributes to a temperature drop inside the vortex and an increase of temperature gradients at its edges (see **Figure 7**). The vortex can also be seen as a region of enhanced velocity of zonal winds in the stratosphere during cold months for the given hemisphere, the highest values being

The polar vortex is an important factor of the large-scale atmospheric circulation and climate variability. Gudkovich et al. [58] showed that the rotation of cold and warm epochs in the Arctic are caused by changes of the vortex intensity, warm and cold epochs being associated with a strong and weak vortex, respectively. Indeed, under strong vortex conditions cyclone tracks are shifted to the north [59] and more North-Atlantic cyclones arrive in the polar region bringing warm air. An important feature of the polar vortex is its influence on the troposphere-stratosphere coupling via planetary waves. Propagation of planetary waves upward depends on stratospheric circulation (for example, [60, 61]). If the vortex is strong and a velocity of western winds in the stratosphere exceeds some critical value, these waves are reflected back to the troposphere. If the vortex is weak, planetary waves propagate freely upward. So, the stratosphere may influence the troposphere under a strong vortex regime. Under a weak vortex, only the troposphere may influence the stratosphere. This point seems to be of importance to understand the observed temporal variability of solar activity/GCR

Let us consider variations of the polar vortex intensity and compare them with temporal behavior of GCR effects on cyclonic activity and clouds. To characterize the vortex strength, we used zonally averaged velocity of western winds (i.e. the U-component of wind velocity directed from west to east from [55]) at the level 50 hPa (~20 km). In **Figure 8** (top panel) there are presented variations (detrended values) of mean zonal wind velocity in the belts 60–80° in both hemispheres averaged for six cold months (October-March in the Northern hemisphere and April-September in the Southern one). The correlation coefficients R(GPH, FCR)

**Figure 7.** Distribution of mean monthly temperature at the pressure level 20 hPa (a) and of magnitude of horizontal temperature gradients (b) in the Northern hemisphere in January 2005. White asterisk indicates a minimum of temperature in the vortex; thick black line connects the points of maximal values of the temperature gradient at given

and R(LCA,FCR) for sliding 11-year intervals are shown in **Figure 8** (bottom panel).

observed at latitudes 50–80° at the pressure levels above 50 hPa.

effects on tropospheric circulation.

90 Cosmic Rays

latitude.

**Figure 8.** *Top*: Variations of mean zonal velocity of western winds (the U-component) at the stratospheric level 50 hPa in the belt 60–80° for cold months in the Northern (a) and Southern (c) hemispheres. Thick lines show 11-year running averages and polynomial fits. *Bottom*: Correlation coefficients for sliding 11-year intervals between LCA and GCR fluxes (dashed lines), GPH700 anomalies and GCR fluxes (solid lines) in the Northern (b) and Southern (d) hemispheres. Dotted lines show the significance levels of the correlation coefficients. The period of an enhanced vortex is marked by vertical dashed lines.

From the middle 1980s to the middle 1990s the polar vortices in both hemispheres were enhanced. The vortex enhancement was the most prominent in the Northern hemisphere, the wind velocity increasing up to 4–7 m.s−1 relative to the trend values. This agrees well with the data [62] showing no strong sudden stratospheric warming destroying the vortex in the indicated period. The vortex in the Southern hemisphere is more stable than that in the Northern one; however, we can also see a noticeable increase in wind velocity. The data in **Figure 8** show that in the period of the vortex enhancement most statistically significant correlations between pressure (cyclonic intensity) and GCR fluxes, as well as between low clouds and GCR fluxes take place. In the late 1990s both the vortices and the correlations GPH–GCR and LCA–GCR in both hemispheres started weakening. A sharp decrease of the wind velocities resulting in the vortex transition to its weak state occurred near 2000 in both hemispheres. The reversal of correlation coefficients under study coincided well with the transition of the vortices to a weak state. Thus, the presented data allow suggesting that the violation of the correlation between low cloudiness and GCR intensity detected on the decadal scale is closely related to the change of the polar vortex strength.

The results of this study confirm that the changes of the atmosphere state, in particular, of the intensity of the stratospheric polar vortex, may be a real reason for the observed temporal variability of solar-atmospheric links. Indeed, sign reversals of correlation links between troposphere pressure in the Northern hemisphere and sunspot numbers during the twentieth century coincided with the changes in the evolution of the large-scale circulation, which, in turn, were associated with the polar vortex transitions from one state to another [42, 43]. A roughly 60-year periodicity was revealed in the vortex strength, the phases of the Arctic Oscillation being used as a proxy of the vortex intensity [43], which is consistent with a similar periodicity found earlier in correlation links between pressure at extratropical latitudes and sunspot numbers [42]. Thus, the data presented in this study revealed the next change of the vortex state, which resulted in the reversal of correlation links between atmosphere characteristics and phenomena associated with solar activity.

**4. Conclusion**

**Author details**

**References**

LA>2.0.CO;2

Svetlana Veretenenko<sup>1</sup>

\*, Maxim Ogurtsov<sup>1</sup>

\*Address all correspondence to: s.veretenenko@mail.ioffe.ru

1991;**96**:18537-18549. DOI: 10.1029/91JD01992

1 Ioffe Institute, Sankt-Petersburg, Russia

2 Natural Resources Institute, Finland

The question of cloud-GCR links remains controversial and requires new studies, both experimental and theoretical, to evaluate a real contribution of galactic cosmic rays to solar activity influence on the Earth's climate. The data presented in this chapter show that possible links between clouds and GCR variations on the decadal and longer time scales could involve not only direct (microphysical) effects, but mostly indirect ones mediated by circulation changes. This should be taken into account when considering long-term GCR effects on the cloudiness state. An important part in the formation of long-term GCR effects on cloud cover at extratropical latitudes seems to be played by the stratospheric polar vortex. The state of the vortex controls the stratosphere-troposphere coupling creating more favorable conditions for GCR influence on extratropical cyclonic activity and, consequently, on cloud cover under a strong vortex regime. In this connection, a high positive correlation of low cloudiness and GCR variations in the 1980s–1990s, which was the period of a strong vortex, may be explained by a pronounced intensification of extratropical cyclones associated with GCR increases in the minima of the 11-year solar cycle. A sharp change of the vortex state near 2000 both in the Northern and Southern hemispheres altered the character of GCR effects on cyclone evolution and, thus, resulted in a violation of cloud-GCR correlation links observed earlier under strong vortex conditions.

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93

, Markus Lindholm2

[1] Harrison EF, Minnis P, Barkstrom BR, Ramanathan V, Cess RD, Gibson GG. Seasonal variation of cloud radiative forcing derived from the earth radiation budget experiment. Journal of Geophysical Research. 1990;**95**:18687-18703. DOI: 10.1029/JD095iD11p18687

[2] Ardanuy PE, Stowe LL, Gruber A, Weiss M. Shortwave, longwave, and net cloud-radiative forcing as determined from Nimbus 7 observations. Journal of Geophysical Research.

[3] Dickinson RE. Solar variability and the lower atmosphere. Bulletin of the American Meteorological Society. 1975;**56**:1240-1248. DOI: 10.1175/1520-0477(1975)056<1240:SVAT

and Risto Jalkanen2

The detected modulation of long-term solar activity/GCR effects on troposphere dynamics by the polar vortex state seems to be due to its role in troposphere-stratosphere coupling via planetary waves. As it was said above, the stratosphere may influence the troposphere only under a strong vortex regime when planetary waves are reflected back to the troposphere. Hence, a strong vortex regime seems to be more favorable to transfer a signal produced in the polar stratosphere by GCRs (or other solar activity phenomena) to the troposphere, as changes in the vortex formation region may influence its intensity and, then, conditions for propagation of planetary waves. Indeed, we can see that GCR effects on cyclonic activity are most pronounced under a strong vortex (see **Figures 6** and **8**) that agrees well with the previous data [42]. Thus, the results of this study provide new evidence for an important part of the polar vortex in the mechanism of solar activity/GCR influence on the troposphere dynamics on the decadal and longer time scales.

Let us note a favorable location of the vortex for GCR effects on the lower atmosphere. The vortex is formed in the region of low geomagnetic cutoff rigidities (Rc < 2–3 GV) that allows particles with a broad energy range to precipitate, including low-energy GCR component which is strongly modulated by solar activity. Wind velocities in the vortex reach maximal values at the heights ~20–30 km where the maximum of the transition curve is observed [63]. This height range also involves the layer of stratospheric aerosols consisting mainly of water solution of sulfuric acid (the Junge layer) (for example, [64]). This creates conditions for influence of ionization changes on aerosol formation which, in turn, may influence the radiative-thermal balance and temperature in the stratosphere and, as a result, the vortex characteristics.

We should also stress that the data presented above do not imply a lack of GCR influence on microphysical processes in clouds. However, they suggest that the formation of cloud-GCR correlation links differs depending on the time scale. GCR variations may influence nucleation rates and growth of particles in clouds according to IMN and/or electric mechanisms [3–11], but this influence may be detected only on rather short time scales (from hours to several days) until the response of atmosphere dynamics to radiative forcing of cloud changes enhances or weakens initial microphysical effects. On longer time scale direct effects of GCRs on cloud formation are masked by more powerful indirect effects through circulation changes associated with GCR variations, these indirect effects depending on the polar vortex state. Taking into account this suggestion, the violation of cloud-GCR correlation links detected near 2000 may be explained.
