**2. CGR effects on atmosphere dynamics and cloud fields**

It is well known that the main reason for cloud formation is a vertical transport of water vapor which results in its cooling and condensation (for example, see [44]). So, the formation of cloud fields in the troposphere is determined by upward air movements which, in turn, are closely related to atmospheric circulation.

At extratropical latitudes most large-scale upward movements, with the horizontal extent being from several hundred to several thousand kilometers, are associated with low-pressure systems, cyclones and troughs. They result from a convergence of air flows near the Earth's surface to the cyclone center or to the trough axis. Upward movements in the atmosphere are also associated with atmospheric fronts which are narrow transition zones between cold and warm air masses. A front is called 'warm', if a warm air mass moves toward a cold one shifting it. Warm fronts are characterized by regular ascending movements of air sliding slowly along a frontal surface. These movements produce strong systems of frontal stratiform clouds Ns-As-Cs (nimbostratus Ns, altostratus As and cirrostratus Cs) with continuous precipitation. Cold fronts arise when a cold air mass moves toward a warm one. Cloud systems of slowly moving cold fronts are similar to those of warm ones. If a cold front is fast moving, vertical velocity of air movements before this front is higher than before a warm one; this contributes to the development of convective clouds, such as cumulonimbus (Cb) with storm precipitation and lightening (for example, [44]). A merging of the cold and warm fronts (so-called 'occlusion') in the process of cyclone evolution results in the formation of an occluded front with the most complex cloud systems. A cloud field of an atmospheric front is seen from satellites as a long band, with the width being usually less than 1000 km and the length reaching several thousand kilometers.

Let us consider variations of cloud cover at middle latitudes where the intensive cyclonic activity takes place and compare them with pressure variations. As experimental base for this study the cloud data from ISCCP (International Satellite Cloud Climatology Project) [46] available for the period from July 1983 to December 2009 were used. At present it is the most comprehensive and the longest archive of different cloud characteristics. According to ISCCP classification clouds are divided into three types depending on pressure at cloud top (CP): low (CP > 680 hPa), middle (440 hPa < CP < 680 hPa) and high (CP < 440 hPa) clouds. Cloud amount is defined as a fraction of the area covered by clouds of a definite type and is expressed as a percentage of the total area. Anomalies of cloud amount are determined as the difference between monthly values of cloud amount of the studied type and the climatic mean, i.e. cloud amount for a given month averaged over the whole period of observation. In this study we consider monthly values of low cloud anomalies (LCA) from the ISCCP-D2 archive based on infrared (11 μm) radiance measurements [47]. Low-level cloudiness involves stratus (St), nimbostratus (Ns) and stratocumulus (Sc), it may also involve convective cumulus (Cu). The data were taken for the mid-latitudinal belts 30–60° of both hemispheres which are regions of intensive extratropical cyclogenesis. At these latitudes the ISCCP data are in a rather good agreement with other satellite data (MODIS, UW HIRS), unlike polar ones [48]. In **Figure 2a** and **b**, temporal variations of LCA for the Northern and Southern hemispheres are presented. One can see a gradual decrease of low cloudiness from the early 1980s to 2009,

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**Figure 2.** *Left*: Temporal variations of LCA (monthly values) at the latitudes 30–60° in the Northern (a) and Southern (b) hemispheres. *Right*: Temporal variations of detrended values of LCA in the Northern and Southern hemispheres (c) and detrended values of LCA in the Northern hemisphere versus those in the Southern one. Thick lines show

linear (a, b) and polynomial (c) trends in LCA variations.

An extratropical cyclone is usually a frontal one, all its evolution being closely related to fronts. First, a cyclone arises as a wave at cold front; then, it passes to the stage of a young cyclone characterized by an existence of a warm sector, i.e. the area of warm air between its cold and warm fronts. At the stage of the maximum development of a cyclone the occlusion starts and an occluded front is formed. At the final stage of cyclone evolution the occlusion continues, a cyclone gets cold and slow and starts filling. A well-developed cyclone can be seen from a satellite as a cloud vortex with a spiral structure, the cloud field dimensions being comparable with those of a cyclone (see **Figure 1**). Thus, frontal cloudiness develops at all the stages of cyclone evolution. This results in a close connection between cloud fields and baric fields of the atmosphere, with baric field changes being accompanied by the evolution of cloud systems.

**Figure 1.** Cloud system of an extratropical cyclone over Alaska gulf (NASA Earth Observatory, photo by Jessy Allen and Robert Simon [45]). The center of the vortex is marked by A.

Let us consider variations of cloud cover at middle latitudes where the intensive cyclonic activity takes place and compare them with pressure variations. As experimental base for this study the cloud data from ISCCP (International Satellite Cloud Climatology Project) [46] available for the period from July 1983 to December 2009 were used. At present it is the most comprehensive and the longest archive of different cloud characteristics. According to ISCCP classification clouds are divided into three types depending on pressure at cloud top (CP): low (CP > 680 hPa), middle (440 hPa < CP < 680 hPa) and high (CP < 440 hPa) clouds. Cloud amount is defined as a fraction of the area covered by clouds of a definite type and is expressed as a percentage of the total area. Anomalies of cloud amount are determined as the difference between monthly values of cloud amount of the studied type and the climatic mean, i.e. cloud amount for a given month averaged over the whole period of observation.

cyclones and troughs. They result from a convergence of air flows near the Earth's surface to the cyclone center or to the trough axis. Upward movements in the atmosphere are also associated with atmospheric fronts which are narrow transition zones between cold and warm air masses. A front is called 'warm', if a warm air mass moves toward a cold one shifting it. Warm fronts are characterized by regular ascending movements of air sliding slowly along a frontal surface. These movements produce strong systems of frontal stratiform clouds Ns-As-Cs (nimbostratus Ns, altostratus As and cirrostratus Cs) with continuous precipitation. Cold fronts arise when a cold air mass moves toward a warm one. Cloud systems of slowly moving cold fronts are similar to those of warm ones. If a cold front is fast moving, vertical velocity of air movements before this front is higher than before a warm one; this contributes to the development of convective clouds, such as cumulonimbus (Cb) with storm precipitation and lightening (for example, [44]). A merging of the cold and warm fronts (so-called 'occlusion') in the process of cyclone evolution results in the formation of an occluded front with the most complex cloud systems. A cloud field of an atmospheric front is seen from satellites as a long band, with the width being usually

An extratropical cyclone is usually a frontal one, all its evolution being closely related to fronts. First, a cyclone arises as a wave at cold front; then, it passes to the stage of a young cyclone characterized by an existence of a warm sector, i.e. the area of warm air between its cold and warm fronts. At the stage of the maximum development of a cyclone the occlusion starts and an occluded front is formed. At the final stage of cyclone evolution the occlusion continues, a cyclone gets cold and slow and starts filling. A well-developed cyclone can be seen from a satellite as a cloud vortex with a spiral structure, the cloud field dimensions being comparable with those of a cyclone (see **Figure 1**). Thus, frontal cloudiness develops at all the stages of cyclone evolution. This results in a close connection between cloud fields and baric fields of the atmosphere, with baric field changes being accompanied by the evolution of cloud systems.

**Figure 1.** Cloud system of an extratropical cyclone over Alaska gulf (NASA Earth Observatory, photo by Jessy Allen and

Robert Simon [45]). The center of the vortex is marked by A.

less than 1000 km and the length reaching several thousand kilometers.

84 Cosmic Rays

In this study we consider monthly values of low cloud anomalies (LCA) from the ISCCP-D2 archive based on infrared (11 μm) radiance measurements [47]. Low-level cloudiness involves stratus (St), nimbostratus (Ns) and stratocumulus (Sc), it may also involve convective cumulus (Cu). The data were taken for the mid-latitudinal belts 30–60° of both hemispheres which are regions of intensive extratropical cyclogenesis. At these latitudes the ISCCP data are in a rather good agreement with other satellite data (MODIS, UW HIRS), unlike polar ones [48].

In **Figure 2a** and **b**, temporal variations of LCA for the Northern and Southern hemispheres are presented. One can see a gradual decrease of low cloudiness from the early 1980s to 2009,

**Figure 2.** *Left*: Temporal variations of LCA (monthly values) at the latitudes 30–60° in the Northern (a) and Southern (b) hemispheres. *Right*: Temporal variations of detrended values of LCA in the Northern and Southern hemispheres (c) and detrended values of LCA in the Northern hemisphere versus those in the Southern one. Thick lines show linear (a, b) and polynomial (c) trends in LCA variations.

a reason for this decrease being not quite clear. According to [49], the trends may be due to some changes in the satellite view angles. However, as it will be shown later, this decrease of cloudiness may be also associated with long-term weakening of cyclonic activity in the belts under study. In any case, for our analysis of cloud-GCR links, we detrend values of LCA and GCR intensity.

Removal of linear trend in LCA reveals (**Figure 2c**) that low cloud anomalies in the Northern and Southern hemispheres have a rather high similarity. The correlation coefficient between these values amounts to 0.62 (**Figure 2d**), the statistical significance being 0.95 according to the random-phase test [50]. LCA variations in both hemispheres seem to be also characterized by a roughly 20-year periodicity. This periodicity, which is close to the magnetic Hale cycle on the Sun, was detected in many climatic parameters (for example, see [51, 52]), including the intensity of extratropical cyclogenesis in the North Atlantic [53]. Thus, the ~20-year periodicity indicates a link between cloudiness and the evolution of dynamic processes in the lower atmosphere.

Let us now compare temporal variations of LCA and GCR intensity. To characterize GCR intensity we used monthly values of charged particle fluxes FCR measured in the stratosphere at ~15–20 km (in the maximum of the transition curve) at the mid-latitudinal station Dolgoprudny (geomagnetic cutoff rigidity Rc = 2.35 GV) near Moscow [54]. Variations of LCA and FCR monthly values, the linear trends being subtracted, are presented in **Figure 3**. We can see that till ~2000 LCA and GCR intensity varied in a similar way, but then this similarity was violated. Indeed, the correlation coefficients between yearly values of LCA and GCR fluxes for sliding 11-year intervals (**Figure 4**) show that cloud-GCR links were the closest in both hemispheres from the middle 1980s to the middle 1990s, the correlation coefficients amounting to ~0.6–0.8. The statistical significance levels (dotted lines in **Figure 4**) for the correlation coefficients were estimated on the base of Monte-Carlo simulations of sliding coefficients for surrogate time series obtained by a randomization of initial ones. In the indicated period the cloud-GCR correlations were most significant (the significance level 0.95–0.99), but since ~2000 they started to decrease sharply and in the early 2000s correlation became negative in both hemispheres.

Taking into account a close link between cloudiness and dynamic processes in the atmosphere, let us consider pressure variations at middle latitudes. As a characteristic of pressure we used geopotential heights of the pressure level 700 hPa (GPH700), taken from NCEP/NCAR reanalysis archive [55]. The indicated level is related to the free atmosphere where effects of Earth's surface friction on air motion are negligible, and its heights correlate well with surface pressure. Temporal variations of 12-month running averages of GPH700 values area-averaged over the belts 30–60° in both hemispheres are shown in **Figure 5** for the period 1948–2013. One can see that long-term variations of pressure differ noticeably in these belts, which implies that cyclonic processes at middle latitudes of the Northern and Southern hemispheres develop to a great extent independently. However, during the period of ISCCP observations (1983–2009) pressure in the studied belts was gradually increasing, i.e. cyclonic processes were weakening. As cloud fields are produced by upward air movements closely associated with largescale low-pressure areas, a weakening of cyclonic processes had to result in a decrease of cloud cover. Thus, LCA decrease during the period 1983–2009 is consistent with observed

**Figure 4.** Correlation coefficients between yearly values of LCA and GCR fluxes for sliding 11-year intervals in the Northern (solid red line) and Southern (dashed blue line) hemispheres. Dotted lines show the significance levels of the

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In **Figure 6a** and **b**, pressure (GPH700) anomalies in the belts 30–60°N (S), calculated similarly to low cloud anomalies, are compared with GCR variations, with the data being averaged over a year and the linear trends being removed. From the early 1980s to ~2000 pressure at middle latitudes of the Northern hemisphere and GCR variations developed in the opposite phases, i.e. GCR increases were accompanied by cyclone intensification and pressure decrease, which agrees well with the effects detected in [42]. However, this link was destroyed near 2000. A similar situation took place in the Southern hemisphere. However, unlike the Northern one, where GCR effects are pronounced in almost all the belt 30–60°, GCR effects on cyclone evolution in the Southern hemisphere are restricted by the areas of climatic lows near

pressure changes.

correlation coefficients.

**Figure 3.** Temporal variations of detrended monthly values of LCA and GCR fluxes in the Northern (a) and Southern (b) hemispheres. Thick lines show 12-month running averages of LCA.

a reason for this decrease being not quite clear. According to [49], the trends may be due to some changes in the satellite view angles. However, as it will be shown later, this decrease of cloudiness may be also associated with long-term weakening of cyclonic activity in the belts under study. In any case, for our analysis of cloud-GCR links, we detrend values of LCA and

Removal of linear trend in LCA reveals (**Figure 2c**) that low cloud anomalies in the Northern and Southern hemispheres have a rather high similarity. The correlation coefficient between these values amounts to 0.62 (**Figure 2d**), the statistical significance being 0.95 according to the random-phase test [50]. LCA variations in both hemispheres seem to be also characterized by a roughly 20-year periodicity. This periodicity, which is close to the magnetic Hale cycle on the Sun, was detected in many climatic parameters (for example, see [51, 52]), including the intensity of extratropical cyclogenesis in the North Atlantic [53]. Thus, the ~20-year periodicity indicates a link between cloudiness and the evolution of dynamic processes in the lower

Let us now compare temporal variations of LCA and GCR intensity. To characterize GCR intensity we used monthly values of charged particle fluxes FCR measured in the stratosphere at ~15–20 km (in the maximum of the transition curve) at the mid-latitudinal station Dolgoprudny (geomagnetic cutoff rigidity Rc = 2.35 GV) near Moscow [54]. Variations of LCA and FCR monthly values, the linear trends being subtracted, are presented in **Figure 3**. We can see that till ~2000 LCA and GCR intensity varied in a similar way, but then this similarity was violated. Indeed, the correlation coefficients between yearly values of LCA and GCR fluxes for sliding 11-year intervals (**Figure 4**) show that cloud-GCR links were the closest in both hemispheres from the middle 1980s to the middle 1990s, the correlation coefficients amounting to ~0.6–0.8. The statistical significance levels (dotted lines in **Figure 4**) for the correlation coefficients were estimated on the base of Monte-Carlo simulations of sliding coefficients for surrogate time series obtained by a randomization of initial ones. In the indicated period the cloud-GCR correlations were most significant (the significance level 0.95–0.99), but since ~2000 they started to decrease sharply and in the early 2000s correlation became negative in

**Figure 3.** Temporal variations of detrended monthly values of LCA and GCR fluxes in the Northern (a) and Southern

(b) hemispheres. Thick lines show 12-month running averages of LCA.

GCR intensity.

86 Cosmic Rays

atmosphere.

both hemispheres.

**Figure 4.** Correlation coefficients between yearly values of LCA and GCR fluxes for sliding 11-year intervals in the Northern (solid red line) and Southern (dashed blue line) hemispheres. Dotted lines show the significance levels of the correlation coefficients.

Taking into account a close link between cloudiness and dynamic processes in the atmosphere, let us consider pressure variations at middle latitudes. As a characteristic of pressure we used geopotential heights of the pressure level 700 hPa (GPH700), taken from NCEP/NCAR reanalysis archive [55]. The indicated level is related to the free atmosphere where effects of Earth's surface friction on air motion are negligible, and its heights correlate well with surface pressure. Temporal variations of 12-month running averages of GPH700 values area-averaged over the belts 30–60° in both hemispheres are shown in **Figure 5** for the period 1948–2013. One can see that long-term variations of pressure differ noticeably in these belts, which implies that cyclonic processes at middle latitudes of the Northern and Southern hemispheres develop to a great extent independently. However, during the period of ISCCP observations (1983–2009) pressure in the studied belts was gradually increasing, i.e. cyclonic processes were weakening. As cloud fields are produced by upward air movements closely associated with largescale low-pressure areas, a weakening of cyclonic processes had to result in a decrease of cloud cover. Thus, LCA decrease during the period 1983–2009 is consistent with observed pressure changes.

In **Figure 6a** and **b**, pressure (GPH700) anomalies in the belts 30–60°N (S), calculated similarly to low cloud anomalies, are compared with GCR variations, with the data being averaged over a year and the linear trends being removed. From the early 1980s to ~2000 pressure at middle latitudes of the Northern hemisphere and GCR variations developed in the opposite phases, i.e. GCR increases were accompanied by cyclone intensification and pressure decrease, which agrees well with the effects detected in [42]. However, this link was destroyed near 2000. A similar situation took place in the Southern hemisphere. However, unlike the Northern one, where GCR effects are pronounced in almost all the belt 30–60°, GCR effects on cyclone evolution in the Southern hemisphere are restricted by the areas of climatic lows near

Thus, the data presented in **Figure 6** (left) show that before ~2000 GCR increases in the solar minima contributed to extratropical cyclone intensification at middle latitudes of both hemispheres, but near 2000 the character of the pressure-GCR link was abruptly changed. This is confirmed by the temporal behavior of correlation coefficients for sliding 11-year intervals between detrended yearly values of GPH700 anomalies and GCR fluxes shown in **Figure 6** (right). From the middle 1980s to the middle 1990s, the strongest negative correlation, with R(GPH, FCR) reaching approximately −0.8 and statistically significant at the level 0.98 according to Monte-Carlo estimates, was observed throughout the mid-latitudinal belt of the Northern hemisphere and in the cyclonic areas of the Southern one. In this period we can see the most pronounced positive correlation between low clouds and GCR fluxes which is consistent with GCR effects on cyclone development. Then, a negative correlation between pressure and GCR fluxes started weakening and its sign reversal took place in the early 2000s. Simultaneously with the weakening of the pressure-GCR correlation, we observe the corresponding weakening of a positive correlation between low clouds and GCR variations, as well as this correlation turning negative in the early 2000s. Thus, the obtained results suggest that cloud-GCR correlation links at middle latitudes observed on the decadal time scale are closely related to GCR effects on the development of cyclonic

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

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

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

processes.

**on the lower atmosphere**

activity and, then, to cloud field formation.

**Figure 5.** Long-term variations of tropospheric pressure (12-month running averages of GPH700) in the belts 30–60° of the Northern (a) and Southern (b) hemispheres. Red lines show polynomial trends.

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**) were calculated for these cyclonic areas.

**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 between GPH700 and GCR fluxes.

Thus, the data presented in **Figure 6** (left) show that before ~2000 GCR increases in the solar minima contributed to extratropical cyclone intensification at middle latitudes of both hemispheres, but near 2000 the character of the pressure-GCR link was abruptly changed. This is confirmed by the temporal behavior of correlation coefficients for sliding 11-year intervals between detrended yearly values of GPH700 anomalies and GCR fluxes shown in **Figure 6** (right). From the middle 1980s to the middle 1990s, the strongest negative correlation, with R(GPH, FCR) reaching approximately −0.8 and statistically significant at the level 0.98 according to Monte-Carlo estimates, was observed throughout the mid-latitudinal belt of the Northern hemisphere and in the cyclonic areas of the Southern one. In this period we can see the most pronounced positive correlation between low clouds and GCR fluxes which is consistent with GCR effects on cyclone development. Then, a negative correlation between pressure and GCR fluxes started weakening and its sign reversal took place in the early 2000s. Simultaneously with the weakening of the pressure-GCR correlation, we observe the corresponding weakening of a positive correlation between low clouds and GCR variations, as well as this correlation turning negative in the early 2000s. Thus, the obtained results suggest that cloud-GCR correlation links at middle latitudes observed on the decadal time scale are closely related to GCR effects on the development of cyclonic processes.
