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Cyclone regeneration in the North Atlantic intensified by energetic solar proton events
Advances in Space Research 35 (2005) 470–475 www.elsevier.com/locate/asr
Cyclone regeneration in the North Atlantic intensified by energetic solar proton events S. Veretenenko a
a,*
, P. Thejll
b
Ioffe Physico-Technical Institute, Russian Academy of Sciences, Politekhnicheskaya 26, 194021 St. Petersburg, Russia b Danish Meteorological Institute, Lyngbywej 100, Copenhagen DK-2100, Denmark Received 29 October 2004; received in revised form 25 January 2005; accepted 26 January 2005
Abstract Atmospheric effects of energetic solar proton events (SPE) were studied in the North Atlantic region, for particle energies above 90 MeV, using NCEP/NCER reanalysis data and weather charts. A significant lowering of the pressure levels in the troposphere accompanied by an increase of the cyclonic vorticity was found near the south-eastern coast of Greenland on days following the event onsets. According to the weather charts, the detected effects are caused by the re-deepening (the regeneration) of well developed cyclones that seems to be intensified during the SPE under study. A joint analysis of the pressure and temperature variations showed a noticeable decrease of the temperature in the rear of the deepening cyclones that may be due to the cold advection increase. The results obtained suggest the influence of energetic SPE on the cyclone development as well as the importance of the frontal zone situated near the Greenland coast for this influence. The physical mechanism may involve the increase of cold advection due to changes in the temperature gradients in this region, resulting from radiative forcing and/or latent heat release related to variations of cloudiness. 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Solar activity; Solar proton events; Weather; Climate
1. Introduction The North Atlantic region seems to be of special interest for investigations of solar–atmospheric links, as a region of the intensive formation and development of extratropical cyclones strongly affecting the weather at middle latitudes. It has been shown that cyclonic activity may be influenced by different helio/ geophysical phenomena (Macdonald and Roberts, 1960; Roberts and Olson, 1973; Wilcox et al., 1974; Tinsley and Deen, 1991, etc.,); however, the physical mechanism responsible for these effects remains un*
Corresponding author. Tel.: +7 812 2479 981; fax: +7 812 24 71017. E-mail addresses: [email protected] (S. Veretenenko), [email protected] (P. Thejll).
clear. It is suggested that solar activity influences on the circulation of the lower atmosphere are closely related to solar and galactic cosmic rays, the energy of particles being from 0.1 to several GeV (e.g., Tinsley and Deen, 1991). Indeed Veretenenko and Thejll (2004) found, by considering aerological sounding data, a noticeable decrease of pressure in the Arctic front region of the North Atlantic (i.e., in the most cyclogenic area) correlated with energetic (>90 MeV) solar proton events. In this work, we continue studying the effects of energetic SPE on cyclonic activity in the North Atlantic, using NCEP/NCAR reanalysis data (Kalnay et al., 1996). The south-eastern Greenland coast was of particular attention for us as a specific region where cyclones formed near the eastern coast of North America may re-deepen (regenerate).
0273-1177/$30 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2005.01.079
S. Veretenenko, P. Thejll / Advances in Space Research 35 (2005) 470–475
471
2. Analysis of experimental data
Table 1 List of the SPE selected for the analysis
2.1. Pressure and vorticity variations in the troposphere
N
Date of onset
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
6 23 7 25 8 12 31 8 7 22 26 8 15 25 5 3 16 14 22 6 14 25 12 8 14 14 27 11 23 19 29 15 30 19 23 23 25 23 30 16 7 15 30 4 12 20 19 20
To study pressure variations associated with energetic SPE, we considered the daily mean values of geopotential (gp) heights of the main pressure levels in the troposphere in a regular grid (2.5 · 2.5) (Kalnay et al., 1996). A set of solar proton events with energy above 90 MeV was selected, with days of the event onsets being used as the key dates (t = 0) for a superposed epoch analysis (SPEA). In order to eliminate the effects of the preceding events, we considered only isolated SPE (separated by at least 3 days from another similar event). The SPE were selected only for the cold half of the year (October–March) which is the period of the most intensive cyclogenesis at extratropical latitudes. Information about particle enhancements was taken from Logachev catalogues (1990, 1998) covering the period from 1980 to 1996 and providing data of particle flux observation carried out by Institute of Applied Geophysics (Moscow, Russia) using METEOR satellite. The SPE observed by METEOR instrument were verified by other measurements (IMP8, GOES etc.,). For some events the METEOR data were supplemented by the data of balloon measurements provided by Lebedev Physical Institute of Russian Academy of Sciences as well as by neutron monitor and riometric data. These catalogues contain rather comprehensive information about SPE including not only the data on proton fluxes in several energy ranges, time profiles and energetic spectra, but also the most probable solar sources of the observed fluxes and the main features of these sources. A list of the events selected for this study is given in Table 1. The results of the SPEA for the geopotential height variations (departures from the mean chart calculated over the period ±10 days relative to the key dates) are presented in Fig. 1 for the next day after the event onset (t = +1 day), when the most pronounced SPE effects were observed. We can see a significant lowering of geopotential heights of all the pressure levels (a decrease of pressure) in the troposphere, localized mainly in the region which includes the south-eastern coast of Greenland and Iceland. This region is of special importance for cyclonic evolution as a region of the Arctic front separating, in winter, the cold Arctic air over Greenland from the warmer air of middle latitudes. It is characterized by a high frequency of occurrence of winter cyclones, increasing from the coast of North America; this indicates the predominance of cyclogenesis in this area. The data presented in Fig. 1 show that at t = +1 day the mean changes of geopotential heights near Greenland amount to about 50 gp.m in the lower troposphere (1000–700 mb), from 70 to 100 gp.m in the middle and upper troposphere (500–200 mb), and exceed two and three standard deviations of the mean
February November March March October October January February March November November December December December January February February March January February February March October November November December December March March October October November November March October December January March October November March March October March March February October October
1980 1980 1981 1981 1981 1981 1982 1982 1982 1982 1982 1982 1982 1982 1983 1983 1984 1984 1985 1986 1986 1988 1988 1988 1988 1988 1988 1989 1989 1989 1989 1989 1989 1990 1990 1990 1991 1991 1991 1991 1992 1992 1992 1993 1993 1994 1994 1995
(the standard deviation of the pressure variations observed on this day divided by the square root of the number of events). The results obtained are in good agreement with the pressure changes observed after the SPE onsets at the high-latitudinal stations in the North Atlantic (Veretenenko and Thejll, 2004). It should also be noted that a decrease of the pressure near Greenland is accompanied by an increase of the pressure near the eastern coast of North America, though this effect is statistically significant only in the lower troposphere (below the 500 mb level).
472
S. Veretenenko, P. Thejll / Advances in Space Research 35 (2005) 470–475
Fig. 1. Mean charts of geopotential height variations (in gp.m) for different pressure levels on the day after the SPE onset. Number of events N = 48 (1980–1996), cold period (October–March). White lines indicate the regions where the SPE effects exceed two and three standard deviations of the mean.
In order to understand the reasons for the pressure variations correlated with the SPE under study, we examined the variations of the relative vorticity, which characterizes the air rotation in the horizontal plane (Matveev, 1991). In the northern hemisphere the relative vorticity is positive in cyclones and negative in anticyclones. In this study we calculated the sums of the relative vorticity in the regions of the most considerable changes of pressure associated with the SPE under study, NCEP/NCAR reanalysis data on the horizontal components of the wind velocity being used (Kalnay et al., 1996). In Fig. 2 the mean variations of the relative vorticity sums in the region of the south-eastern coast of Greenland and Iceland (50–60N, 0–55W) and near the eastern coast of North America (37–55N, 55–85W) are presented for the levels 500 and 300 mb where the pressure variations were found to be most pronounced (cf. Fig. 1). The dashed line is the mean level of vorticity
across all the days shown and the dotted lines are the number (1, 2 or 3) of standard deviations (S.D.) from that mean. We can see that a lowering of pressure near the Greenland coast correlated with energetic SPE is accompanied by a statistically significant (at the 3 S.D. level) increase of the relative vorticity that provides evidence for cyclone generation or intensification in this region. Similarly, a decrease of the relative vorticity to negative values near the eastern coast of North America (i.e., in the region of increased pressure) indicates an enhancement of the anticyclonic (high-pressure) area. 2.2. Weather chart analysis and temperature variations To confirm the above conclusions, an analysis of the weather (synoptic) charts near the EarthÕs surface was carried out. As the weather charts provide data on the atmospheric state (i.e., on the distribution and character of air masses, the atmospheric fronts, the different baric
S. Veretenenko, P. Thejll / Advances in Space Research 35 (2005) 470–475
0.04
0.03 North Atlantic
Summed relative vorticity
0.03
50–60°N, 0–55°W
0.01
0.01
0
0
–0.01
−0.01
North Atlantic 37–55°N, 55–85°W
0.02
0.02
–0.02
500 mb
500 mb −0.02 −30
−20
−10
0
10
20
30
0.05
–0.03 –30
–20
–10
0.04 North Atlantic 50–60°N, 0–55°W
0.04 Summed relative vorticity
473
0
10
20
30
North Atlantic 37–55°N, 55–85°W
0.03 0.02
0.03
0.01
0.02
0 0.01
–0.01
0
–0.02
−0.01 −0.02 –30
–0.03
300 mb –20
–10
0
10
20
Days
30
–0.04 –30
300 mb –20
–10
0
10
20
30
Days
Fig. 2. Mean changes of the relative vorticity sums (in s 1) in different regions of the North Atlantic. Number of events N = 48 (1980–1996), cold period (October–March). Day 0 corresponds to the day of the event onset.
systems such as cyclones, anticyclones, troughs, crests, etc.,), a synoptic analysis allows us to follow changes in the evolution of weather systems in the regions under study. According to the weather chart analysis, the cases of new cyclone formation in the region of the Greenland coast and Iceland on the days following the event onsets were rather rare, whereas the deepening of already existing cyclones near the Greenland coast was detected in most (80%) cases. A strong deepening (by 15–35 mb) was observed for 60% of the SPE under study. A typical example of cyclone deepening is given in Fig. 3 for the event starting on 7th March 1982. Thus, the pressure lowering near Greenland accompanied by an increase of cyclonic vorticity was found to be related mainly to the cyclone strengthening correlated with energetic SPE. The pressure increase accompanied by the intensification of anticyclonic vorticity observed near the eastern coast of North America was found to be due to the strengthening of the high-pressure area which, as a rule, adjoins the cold front of the deepening cyclone at its south-western edge. The weather charts also allowed us to obtain important information about the physical mechanisms relating to the detected effects. It is known that there are several
stages in cyclone evolution: a wave, a young cyclone with a warm sector between the cold and warm fronts and an occluded cyclone when the cold and warm fronts merge in the cyclone center, with the so called Õoccluded frontÕ being formed (Vorobjev, 1991). At the stage of the occlusion a cyclone has already reached its maximum development and must start filling (being destroyed). However, the weather chart analysis showed that predominantly such cyclones, i.e., well developed and starting to occlude, were found to deepen near the Greenland coast after the event onset. A secondary deepening of the occluded cyclone is called Õcyclone regenerationÕ and needs an advection of cold air in its rear, which results in an increase of the temperature contrasts in the cyclone (Vorobjev, 1991). The regeneration may occur when the occluded cyclone approaches the Arctic front where the high temperature contrasts create conditions for cold air advection. Indeed, a joint analysis of pressure and temperature variations in the lower troposphere does reveal a decrease of the temperature by 2 C in the rear of deepening cyclones (cf. Fig. 4) that seems to provide evidence of an increase of cold advection. The results obtained suggest that the cyclone regeneration near the south-eastern coast of Greenland may be intensified by energetic SPE, with the mechanism
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S. Veretenenko, P. Thejll / Advances in Space Research 35 (2005) 470–475
Fig. 3. Cyclone deepening correlated with the SPE on 7th March 1982. The weather charts show the synoptic situation observed a day before the SPE onset (left) and several hours after the SPE onset (right).
Fig. 4. Superposition of the mean charts of the geopotential height variations of the 700 mb level (in gp. m, gray scale) and of the temperature variations in the layer 1000–500 mb (in C, white lines) on the day after the SPE onset. Number of events N = 48 (1980–1996), cold period (October–March).
involving cold advection when the cyclone approaches the Arctic front.
3. Discussion of the results The results obtained show that the SPE, with energies above 90 MeV, may influence appreciably the evolution of extratropical cyclones in the North Atlantic, the effects being localized mainly in the region of the Arctic front near the Greenland coast. This seems to indicate an important role of the frontal zone for SPE influences on cyclone development. Indeed, most cyclones at middle latitudes arise, and undergo significant changes, in the frontal zones where the horizontal gradients of temperature are greater by about a factor of ten than outside these zones. The temperature contrasts create favourable conditions for the advection of cold that is of particular importance for cyclone evolution. Cold advection is known to contribute to the generation or intensification of a
cyclonic vortex, whereas warm advection contributes to the generation or intensification of an anticyclonic one. Deepening of the cyclone continues while there is cold advection. As soon as the cold air spreads over the whole cyclone and the temperature field becomes uniform, cold advection stops and the cyclone starts filling (Matveev, 1991). However, if the input of the colder air takes place in the rear of such a cold filling cyclone, it starts deepening again – this is known as Ôcyclone regenerationÕ (Vorobjev, 1991). An intensification of cyclone regeneration observed on the days following energetic SPE suggests that these events seem to create conditions contributing to the cold advection increase when the cyclone approaches the Arctic front. A possible reason may be an increase of the temperature contrasts in the frontal zone. A physical mechanism of the temperature changes may involve, in particular, the radiative forcing of the high-level (cirrus) cloudiness changes in this region associated with cosmic ray variations (Pudovkin and Veretenenko, 1995). Highlevel clouds do not noticeably affect the input of solar radiation, but they may significantly reduce the outgoing long-wave radiation from the Earth and the atmosphere, the warming effect ranging from 10 W m 2 for optically thin cirrus clouds to 90 W m 2 for optically thick ones (Gorchakova, 1991). As the outgoing longwave radiation in the cold half of the year is greater over warmer ocean than over land, we can suggest that the warming effect of the cloudiness increase may be more pronounced over the ocean and then enhance the temperature contrasts in the front region near the Greenland coast. The latent heat release accompanying the formation of clouds may also contribute to the frontal zone activation. Cosmic ray influences on cloud formation as well as weather and climate changes resulting from these influences are widely discussed nowadays. According to Tinsley and Yu (2004), the detected correlations of cosmic ray variations and clouds may be considered in terms of two main microphysical processes: ion-mediated nucleation
S. Veretenenko, P. Thejll / Advances in Space Research 35 (2005) 470–475
(IMN) and ÔelectroscavengingÕ. The first mechanism suggests a condensation on charged clusters of water and sulphuric acid resulting in the formation of new aerosol particles which may grow up to the sizes of cloud condensation nuclei (0.1–1 lm). This mechanism depends directly on atmospheric ionization by cosmic rays. Another group of mechanisms involves an influence of cosmic rays, as well as of the local ionosphere potential, on the vertical current density through the atmosphere which contributes to the space charge buildup in a cloud. The space charge affects electrostatic charge capture by aerosol particles and, thus, enhances the rate of the collection of such charged particles by falling droplets (ÔelectroscavengingÕ). A possible result of ÔelectroscavengingÕ may be an enhanced production of ice particles in supercooled clouds if the scavenged particles are ice-forming nuclei. The results obtained in this study for the SPE, with energies above 90 MeV, which lose their energy at stratospheric heights (30–40 km) seem to suggest the indirect influence of cosmic rays on the nucleation processes, variations of the atmospheric current being involved. Indeed, there is evidence of a sharp increase of current density associated with charged particles from solar flares (e.g., Markson and Muir, 1980). However, the mechanism of cosmic ray effects on cloud microphysics is not yet fully understood.
4. Conclusions The results presented above provide evidence of an intensification of cyclonic activity in the North Atlantic correlated with Solar Proton Events, the energy of particles being enough to reach the stratosphere. The SPE effects on cyclone development were found to be localized in the region of the Arctic front near the south-eastern coast of Greenland; they seem to be due to the intensification of the cyclone regeneration, with the mechanism involving an increase of cold advection. A possible reason for the effects detected may be an increase of the temperature contrasts in the frontal zone resulting from radiative forcing and/ or latent heat release related to the changes of cloudiness.
475
Acknowledgments The authors thank Prof. V. Vorobjev (Russian Hydrometeorological University, St. Petersburg) for helpful discussions, as well as the referees for constructive remarks and suggestions.
References Gorchakova, I.A. Variability of integral fluxes of thermal radiation in case of cirrus clouds. Izvestia Academii Nauk SSSR, Fizika atmosphery i okeana 27, 983–987, 1991 (in Russian). Kalnay, E., Kanamitsu, M., Kistler, R., et al. The NCEP/NCAR 40year reanalysis project. Bull. Am. Meteorol. Soc. 77, 437–472, 1996. Logachev, Yu.I. (Ed.). Solar Proton Events. Catalogue 1980–1986. Soviet Geophysical Committee of the Academy of Sciences of the USSR, Moscow, 1990 (in Russian). Logachev, Yu.I. (Ed.). Catalogue of Solar Proton Events 1987–1996. Moscow University Press, Moscow, 1998 (in Russian). Macdonald, N.J, Roberts, W.O. Further evidence of a solar corpuscular influence on a large-scale circulation at 300 mb. J. Geophys. Res. 65, 529–534, 1960. Markson, R., Muir, M. Solar wind control of the EarthÕs electric field. Science. 208, 979–990, 1980. Matveev, L.T. Theory of General Atmosphere Circulation and of the EarthÕs Climate. Hydrometeoizdat, Leningrad, 1991 (in Russian). Pudovkin, M.I., Veretenenko, S.V. Cloudiness decreases associated with Forbush-decreases of galactic cosmic rays. J. Atmos. Sol.Terr. Phys. 57, 1349–1355, 1995. Roberts, W.O., Olson, R.H. Geomagnetic storms and wintertime 300mb trough development in the North Pacific-North America area. J. Atmos. Sci. 30, 1135–1140, 1973. Tinsley, B.A., Deen, G.W. Apparent tropospheric response to MeV– GeV particle flux variations: a connection via electrofreezing of supercooled water in high-level clouds?. J. Geophys. Res. 96, 22283–22296, 1991. Tinsley, B.A., Yu, F. Atmospheric ionization and clouds as links between solar activity and climate. In: Pap, J. (Ed.), Solar Variability and Its Effects on Climate. AGU Press, Washington, pp. 321–340, 2004. Veretenenko, S., Thejll, P. Effects of energetic solar proton events on the cyclone development in the North Atlantic. J. Atmos. Sol.-Terr. Phys. 66, 393–405, 2004. Vorobjev, V.I. Synoptic Meteorology. Hydrometeoizdat, Leningrad, 1991 (in Russian). Wilcox, J.M., Scherrer, P.H., Svalgaard, L., et al. Influence of solar magnetic sector structure on terrestrial atmospheric vorticity. J. Atmos. Sci. 31, 581–588, 1974.
Cyclone regeneration in the North Atlantic intensified by energetic solar proton events S. Veretenenko a
a,*
, P. Thejll
b
Ioffe Physico-Technical Institute, Russian Academy of Sciences, Politekhnicheskaya 26, 194021 St. Petersburg, Russia b Danish Meteorological Institute, Lyngbywej 100, Copenhagen DK-2100, Denmark Received 29 October 2004; received in revised form 25 January 2005; accepted 26 January 2005
Abstract Atmospheric effects of energetic solar proton events (SPE) were studied in the North Atlantic region, for particle energies above 90 MeV, using NCEP/NCER reanalysis data and weather charts. A significant lowering of the pressure levels in the troposphere accompanied by an increase of the cyclonic vorticity was found near the south-eastern coast of Greenland on days following the event onsets. According to the weather charts, the detected effects are caused by the re-deepening (the regeneration) of well developed cyclones that seems to be intensified during the SPE under study. A joint analysis of the pressure and temperature variations showed a noticeable decrease of the temperature in the rear of the deepening cyclones that may be due to the cold advection increase. The results obtained suggest the influence of energetic SPE on the cyclone development as well as the importance of the frontal zone situated near the Greenland coast for this influence. The physical mechanism may involve the increase of cold advection due to changes in the temperature gradients in this region, resulting from radiative forcing and/or latent heat release related to variations of cloudiness. 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Solar activity; Solar proton events; Weather; Climate
1. Introduction The North Atlantic region seems to be of special interest for investigations of solar–atmospheric links, as a region of the intensive formation and development of extratropical cyclones strongly affecting the weather at middle latitudes. It has been shown that cyclonic activity may be influenced by different helio/ geophysical phenomena (Macdonald and Roberts, 1960; Roberts and Olson, 1973; Wilcox et al., 1974; Tinsley and Deen, 1991, etc.,); however, the physical mechanism responsible for these effects remains un*
Corresponding author. Tel.: +7 812 2479 981; fax: +7 812 24 71017. E-mail addresses: [email protected] (S. Veretenenko), [email protected] (P. Thejll).
clear. It is suggested that solar activity influences on the circulation of the lower atmosphere are closely related to solar and galactic cosmic rays, the energy of particles being from 0.1 to several GeV (e.g., Tinsley and Deen, 1991). Indeed Veretenenko and Thejll (2004) found, by considering aerological sounding data, a noticeable decrease of pressure in the Arctic front region of the North Atlantic (i.e., in the most cyclogenic area) correlated with energetic (>90 MeV) solar proton events. In this work, we continue studying the effects of energetic SPE on cyclonic activity in the North Atlantic, using NCEP/NCAR reanalysis data (Kalnay et al., 1996). The south-eastern Greenland coast was of particular attention for us as a specific region where cyclones formed near the eastern coast of North America may re-deepen (regenerate).
0273-1177/$30 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2005.01.079
S. Veretenenko, P. Thejll / Advances in Space Research 35 (2005) 470–475
471
2. Analysis of experimental data
Table 1 List of the SPE selected for the analysis
2.1. Pressure and vorticity variations in the troposphere
N
Date of onset
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
6 23 7 25 8 12 31 8 7 22 26 8 15 25 5 3 16 14 22 6 14 25 12 8 14 14 27 11 23 19 29 15 30 19 23 23 25 23 30 16 7 15 30 4 12 20 19 20
To study pressure variations associated with energetic SPE, we considered the daily mean values of geopotential (gp) heights of the main pressure levels in the troposphere in a regular grid (2.5 · 2.5) (Kalnay et al., 1996). A set of solar proton events with energy above 90 MeV was selected, with days of the event onsets being used as the key dates (t = 0) for a superposed epoch analysis (SPEA). In order to eliminate the effects of the preceding events, we considered only isolated SPE (separated by at least 3 days from another similar event). The SPE were selected only for the cold half of the year (October–March) which is the period of the most intensive cyclogenesis at extratropical latitudes. Information about particle enhancements was taken from Logachev catalogues (1990, 1998) covering the period from 1980 to 1996 and providing data of particle flux observation carried out by Institute of Applied Geophysics (Moscow, Russia) using METEOR satellite. The SPE observed by METEOR instrument were verified by other measurements (IMP8, GOES etc.,). For some events the METEOR data were supplemented by the data of balloon measurements provided by Lebedev Physical Institute of Russian Academy of Sciences as well as by neutron monitor and riometric data. These catalogues contain rather comprehensive information about SPE including not only the data on proton fluxes in several energy ranges, time profiles and energetic spectra, but also the most probable solar sources of the observed fluxes and the main features of these sources. A list of the events selected for this study is given in Table 1. The results of the SPEA for the geopotential height variations (departures from the mean chart calculated over the period ±10 days relative to the key dates) are presented in Fig. 1 for the next day after the event onset (t = +1 day), when the most pronounced SPE effects were observed. We can see a significant lowering of geopotential heights of all the pressure levels (a decrease of pressure) in the troposphere, localized mainly in the region which includes the south-eastern coast of Greenland and Iceland. This region is of special importance for cyclonic evolution as a region of the Arctic front separating, in winter, the cold Arctic air over Greenland from the warmer air of middle latitudes. It is characterized by a high frequency of occurrence of winter cyclones, increasing from the coast of North America; this indicates the predominance of cyclogenesis in this area. The data presented in Fig. 1 show that at t = +1 day the mean changes of geopotential heights near Greenland amount to about 50 gp.m in the lower troposphere (1000–700 mb), from 70 to 100 gp.m in the middle and upper troposphere (500–200 mb), and exceed two and three standard deviations of the mean
February November March March October October January February March November November December December December January February February March January February February March October November November December December March March October October November November March October December January March October November March March October March March February October October
1980 1980 1981 1981 1981 1981 1982 1982 1982 1982 1982 1982 1982 1982 1983 1983 1984 1984 1985 1986 1986 1988 1988 1988 1988 1988 1988 1989 1989 1989 1989 1989 1989 1990 1990 1990 1991 1991 1991 1991 1992 1992 1992 1993 1993 1994 1994 1995
(the standard deviation of the pressure variations observed on this day divided by the square root of the number of events). The results obtained are in good agreement with the pressure changes observed after the SPE onsets at the high-latitudinal stations in the North Atlantic (Veretenenko and Thejll, 2004). It should also be noted that a decrease of the pressure near Greenland is accompanied by an increase of the pressure near the eastern coast of North America, though this effect is statistically significant only in the lower troposphere (below the 500 mb level).
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S. Veretenenko, P. Thejll / Advances in Space Research 35 (2005) 470–475
Fig. 1. Mean charts of geopotential height variations (in gp.m) for different pressure levels on the day after the SPE onset. Number of events N = 48 (1980–1996), cold period (October–March). White lines indicate the regions where the SPE effects exceed two and three standard deviations of the mean.
In order to understand the reasons for the pressure variations correlated with the SPE under study, we examined the variations of the relative vorticity, which characterizes the air rotation in the horizontal plane (Matveev, 1991). In the northern hemisphere the relative vorticity is positive in cyclones and negative in anticyclones. In this study we calculated the sums of the relative vorticity in the regions of the most considerable changes of pressure associated with the SPE under study, NCEP/NCAR reanalysis data on the horizontal components of the wind velocity being used (Kalnay et al., 1996). In Fig. 2 the mean variations of the relative vorticity sums in the region of the south-eastern coast of Greenland and Iceland (50–60N, 0–55W) and near the eastern coast of North America (37–55N, 55–85W) are presented for the levels 500 and 300 mb where the pressure variations were found to be most pronounced (cf. Fig. 1). The dashed line is the mean level of vorticity
across all the days shown and the dotted lines are the number (1, 2 or 3) of standard deviations (S.D.) from that mean. We can see that a lowering of pressure near the Greenland coast correlated with energetic SPE is accompanied by a statistically significant (at the 3 S.D. level) increase of the relative vorticity that provides evidence for cyclone generation or intensification in this region. Similarly, a decrease of the relative vorticity to negative values near the eastern coast of North America (i.e., in the region of increased pressure) indicates an enhancement of the anticyclonic (high-pressure) area. 2.2. Weather chart analysis and temperature variations To confirm the above conclusions, an analysis of the weather (synoptic) charts near the EarthÕs surface was carried out. As the weather charts provide data on the atmospheric state (i.e., on the distribution and character of air masses, the atmospheric fronts, the different baric
S. Veretenenko, P. Thejll / Advances in Space Research 35 (2005) 470–475
0.04
0.03 North Atlantic
Summed relative vorticity
0.03
50–60°N, 0–55°W
0.01
0.01
0
0
–0.01
−0.01
North Atlantic 37–55°N, 55–85°W
0.02
0.02
–0.02
500 mb
500 mb −0.02 −30
−20
−10
0
10
20
30
0.05
–0.03 –30
–20
–10
0.04 North Atlantic 50–60°N, 0–55°W
0.04 Summed relative vorticity
473
0
10
20
30
North Atlantic 37–55°N, 55–85°W
0.03 0.02
0.03
0.01
0.02
0 0.01
–0.01
0
–0.02
−0.01 −0.02 –30
–0.03
300 mb –20
–10
0
10
20
Days
30
–0.04 –30
300 mb –20
–10
0
10
20
30
Days
Fig. 2. Mean changes of the relative vorticity sums (in s 1) in different regions of the North Atlantic. Number of events N = 48 (1980–1996), cold period (October–March). Day 0 corresponds to the day of the event onset.
systems such as cyclones, anticyclones, troughs, crests, etc.,), a synoptic analysis allows us to follow changes in the evolution of weather systems in the regions under study. According to the weather chart analysis, the cases of new cyclone formation in the region of the Greenland coast and Iceland on the days following the event onsets were rather rare, whereas the deepening of already existing cyclones near the Greenland coast was detected in most (80%) cases. A strong deepening (by 15–35 mb) was observed for 60% of the SPE under study. A typical example of cyclone deepening is given in Fig. 3 for the event starting on 7th March 1982. Thus, the pressure lowering near Greenland accompanied by an increase of cyclonic vorticity was found to be related mainly to the cyclone strengthening correlated with energetic SPE. The pressure increase accompanied by the intensification of anticyclonic vorticity observed near the eastern coast of North America was found to be due to the strengthening of the high-pressure area which, as a rule, adjoins the cold front of the deepening cyclone at its south-western edge. The weather charts also allowed us to obtain important information about the physical mechanisms relating to the detected effects. It is known that there are several
stages in cyclone evolution: a wave, a young cyclone with a warm sector between the cold and warm fronts and an occluded cyclone when the cold and warm fronts merge in the cyclone center, with the so called Õoccluded frontÕ being formed (Vorobjev, 1991). At the stage of the occlusion a cyclone has already reached its maximum development and must start filling (being destroyed). However, the weather chart analysis showed that predominantly such cyclones, i.e., well developed and starting to occlude, were found to deepen near the Greenland coast after the event onset. A secondary deepening of the occluded cyclone is called Õcyclone regenerationÕ and needs an advection of cold air in its rear, which results in an increase of the temperature contrasts in the cyclone (Vorobjev, 1991). The regeneration may occur when the occluded cyclone approaches the Arctic front where the high temperature contrasts create conditions for cold air advection. Indeed, a joint analysis of pressure and temperature variations in the lower troposphere does reveal a decrease of the temperature by 2 C in the rear of deepening cyclones (cf. Fig. 4) that seems to provide evidence of an increase of cold advection. The results obtained suggest that the cyclone regeneration near the south-eastern coast of Greenland may be intensified by energetic SPE, with the mechanism
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Fig. 3. Cyclone deepening correlated with the SPE on 7th March 1982. The weather charts show the synoptic situation observed a day before the SPE onset (left) and several hours after the SPE onset (right).
Fig. 4. Superposition of the mean charts of the geopotential height variations of the 700 mb level (in gp. m, gray scale) and of the temperature variations in the layer 1000–500 mb (in C, white lines) on the day after the SPE onset. Number of events N = 48 (1980–1996), cold period (October–March).
involving cold advection when the cyclone approaches the Arctic front.
3. Discussion of the results The results obtained show that the SPE, with energies above 90 MeV, may influence appreciably the evolution of extratropical cyclones in the North Atlantic, the effects being localized mainly in the region of the Arctic front near the Greenland coast. This seems to indicate an important role of the frontal zone for SPE influences on cyclone development. Indeed, most cyclones at middle latitudes arise, and undergo significant changes, in the frontal zones where the horizontal gradients of temperature are greater by about a factor of ten than outside these zones. The temperature contrasts create favourable conditions for the advection of cold that is of particular importance for cyclone evolution. Cold advection is known to contribute to the generation or intensification of a
cyclonic vortex, whereas warm advection contributes to the generation or intensification of an anticyclonic one. Deepening of the cyclone continues while there is cold advection. As soon as the cold air spreads over the whole cyclone and the temperature field becomes uniform, cold advection stops and the cyclone starts filling (Matveev, 1991). However, if the input of the colder air takes place in the rear of such a cold filling cyclone, it starts deepening again – this is known as Ôcyclone regenerationÕ (Vorobjev, 1991). An intensification of cyclone regeneration observed on the days following energetic SPE suggests that these events seem to create conditions contributing to the cold advection increase when the cyclone approaches the Arctic front. A possible reason may be an increase of the temperature contrasts in the frontal zone. A physical mechanism of the temperature changes may involve, in particular, the radiative forcing of the high-level (cirrus) cloudiness changes in this region associated with cosmic ray variations (Pudovkin and Veretenenko, 1995). Highlevel clouds do not noticeably affect the input of solar radiation, but they may significantly reduce the outgoing long-wave radiation from the Earth and the atmosphere, the warming effect ranging from 10 W m 2 for optically thin cirrus clouds to 90 W m 2 for optically thick ones (Gorchakova, 1991). As the outgoing longwave radiation in the cold half of the year is greater over warmer ocean than over land, we can suggest that the warming effect of the cloudiness increase may be more pronounced over the ocean and then enhance the temperature contrasts in the front region near the Greenland coast. The latent heat release accompanying the formation of clouds may also contribute to the frontal zone activation. Cosmic ray influences on cloud formation as well as weather and climate changes resulting from these influences are widely discussed nowadays. According to Tinsley and Yu (2004), the detected correlations of cosmic ray variations and clouds may be considered in terms of two main microphysical processes: ion-mediated nucleation
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(IMN) and ÔelectroscavengingÕ. The first mechanism suggests a condensation on charged clusters of water and sulphuric acid resulting in the formation of new aerosol particles which may grow up to the sizes of cloud condensation nuclei (0.1–1 lm). This mechanism depends directly on atmospheric ionization by cosmic rays. Another group of mechanisms involves an influence of cosmic rays, as well as of the local ionosphere potential, on the vertical current density through the atmosphere which contributes to the space charge buildup in a cloud. The space charge affects electrostatic charge capture by aerosol particles and, thus, enhances the rate of the collection of such charged particles by falling droplets (ÔelectroscavengingÕ). A possible result of ÔelectroscavengingÕ may be an enhanced production of ice particles in supercooled clouds if the scavenged particles are ice-forming nuclei. The results obtained in this study for the SPE, with energies above 90 MeV, which lose their energy at stratospheric heights (30–40 km) seem to suggest the indirect influence of cosmic rays on the nucleation processes, variations of the atmospheric current being involved. Indeed, there is evidence of a sharp increase of current density associated with charged particles from solar flares (e.g., Markson and Muir, 1980). However, the mechanism of cosmic ray effects on cloud microphysics is not yet fully understood.
4. Conclusions The results presented above provide evidence of an intensification of cyclonic activity in the North Atlantic correlated with Solar Proton Events, the energy of particles being enough to reach the stratosphere. The SPE effects on cyclone development were found to be localized in the region of the Arctic front near the south-eastern coast of Greenland; they seem to be due to the intensification of the cyclone regeneration, with the mechanism involving an increase of cold advection. A possible reason for the effects detected may be an increase of the temperature contrasts in the frontal zone resulting from radiative forcing and/ or latent heat release related to the changes of cloudiness.
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Acknowledgments The authors thank Prof. V. Vorobjev (Russian Hydrometeorological University, St. Petersburg) for helpful discussions, as well as the referees for constructive remarks and suggestions.
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