**4. Conclusion**

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 character-

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

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

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

istics and phenomena associated with solar activity.

on the decadal and longer time scales.

characteristics.

92 Cosmic Rays

near 2000 may be explained.

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.
