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Long-term trends in geomagnetic and climatic variability
Physics and Chemistry of the Earth 27 (2002) 427–431 www.elsevier.com/locate/pce
Long-term trends in geomagnetic and climatic variability V aclav Bucha
*
Geophysical Institute, Academy of Sciences of the Czech Republic, Bo cnı II, 141 31 Prague 4, Czech Republic Received 30 October 2001; received in revised form 7 January 2002; accepted 11 January 2002
Abstract Causes leading to global mean sea surface temperature (GT) variability and to variations of the global circulation including the North Atlantic Oscillation (NAO) and El Ni~ no (EN) events are examined. Statistically significant correlation coefficients between these variables were found and their relations are discussed with the aim to show possible causes leading to general year-to-year variability and to the global warming. At the same time, the results contribute to the verification of the hypothesis as given in our previous papers. We have suggested a link of processes generated by geomagnetic forcing that is followed by dramatic shifts in the atmospheric circulation patterns. At times of low geomagnetic activity in winter the meridional flow prevails contributing to the strong heat exchange of air between low and high latitudes. The arctic air penetrates from polar areas and participates in the cooling of middle latitudes; the NAO winter index (WI) is negative. At times of high geomagnetic activity the Icelandic low intensifies and influences the strengthening of the zonal flow in the Northern Hemisphere; the NAO WI is positive. A continuous zone of high pressure originates along middle latitudes and a little north–south motion of air takes place. In mid-latitudes above-normal temperatures occur while in polar areas the values are below normal. The strong Australian high at times of low geomagnetic activity seems to initiate EN events while the zonal flow in the Southern Hemisphere intensifies monsoon rains in the Indian Ocean when geomagnetic activity is high. EN and NAO events are shown to take part in the variability of GT. Controversial problems are discussed as well. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Geomagnetic forcing; Climatic variability; Meridional and zonal flow; Global warming
1. Introduction The time series of global mean sea surface temperatures shows significant variability. According to the WMO Statements 1996–2000 the global warming was observed following ENSO events, e.g. in 1973, 1977, 1983 and 1987 and cooling around 1992 following the 1991 eruption of Mount Pinatubo. The year 1996 was the eighteenth consecutive year with global temperatures above the 1961–1990 normal. On the other hand, the overall stratospheric cooling occurred during the past 50 years according to the WMO Statements 1996. The relative cooling over much of Eurasia in 1996 can be attributed to a strong change in the phase of the North Atlantic Oscillation (NAO). The highest global surface temperature in 1998, 0.57 °C above the 1961–1990 normal, is attributed to the unprecedented warmth of the Indian Ocean and of parts of every continent, especially North America, Eurasia and Australia. In the
*
Tel.: +420-2-6710339; fax: +420-2-72761549. E-mail address: [email protected] (V. Bucha).
year 2000 the global mean temperature anomaly was only 0.29 °C above the 1961–1990 base period average. The eastern tropical Pacific was, according to the WMO Statements 2000, colder than usual and much of Asia and west-central part of North America were colder than normal during the September–November period. It was shown that geomagnetic forcing has a much stronger effect on meteorological processes than solar activity forcing and leads to strong changes in the configuration of main pressure formations (Bucha, 1976; Bucha and Bucha, 1998, 2002; Lastovicka, 1996; Bochnıcek et al., 2001). Let us try to study whether there are positive relations also between geomagnetic activity, NAO, El Ni~ no (EN) events and fluctuations of global mean sea surface temperature.
2. Analyses and results The relations between EN events (Wallace et al., 1998) and GT show that global warming really follows EN events as given in the WMO Statement 1996. The highest correlation coefficient r between EN and GT is
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V. Bucha / Physics and Chemistry of the Earth 27 (2002) 427–431
Fig. 1. Temperature anomalies representing El Ni~ no (EN) events as defined by values in the equatorial area of the Pacific Ocean (curve a) (Wallace et al., 1998; World Climate News, 1997–2000), global mean sea surface temperatures (curve b), yearly averages of geomagnetic aa indices (curve c).
positive and equals to 0.43 for the time interval 1950– 2000 with a time lag of one year (Fig. 1). The crosscorrelation showed that these correlation coefficients are statistically significant at the 99% confidence level for the time lag of one year. From it follows that the EN effect on GT exists in the second half of the 20th century. The correlation is, however, not statistically significant for the period 1900–1950 (r ¼ 0:08). By studying relations between the NAO and GT for the time interval 1951–2000 we found the correlation coefficient r equal to 0.39. However, the cross-correlation showed that all correlation coefficients for nine positive and nine negative lags are significantly non-zero. This is most probably caused mainly by the upward trend of the two series. From these results it follows that a stronger effect of EN events exists on fluctuations of the global temperature while the relation between NAO and GT seems to be connected rather with the long-term upward trend. When we summarized both effects, i.e. EN + NAO on GT we obtained the correlation coefficient that equals to 0.54 for 1951–2000 and 0.63 for 1951–1995. Both are statistically significant at the 99% confidence level. The fluctuation of GT could be interpreted as a result of the NAO effect in the Northern Hemisphere and of the EN effect in the Southern Hemisphere. The correlation between EN and NAO events is, however, not statistically significant. Let us try to look for the causes of these relations.
3. Relations between geomagnetic activity and global mean surface temperature As a next step we examined the relations between geomagnetic activity and fluctuations of GT. Following correlation coefficients were obtained (Fig. 2): rðaa; GTÞ ¼ 0:58 for the time interval 1900–2000 with a time
Fig. 2. Sum of geomagnetic aa indices for Jan, Feb, Mar, Nov, Dec of the year-1 and for Jan, Feb, Mar of the year 0 (curve a), global mean sea surface temperatures (curve b) (World Climate News, 1997–2000), differences between mean values of geomagnetic aa indices for aaðMayÞ aaðFeb þ Mar þ AprÞ=3 (curve c).
lag of one year where aa is the sum of geomagnetic aa indices for Jan, Feb, Mar, Nov, Dec of the year-1 and Jan, Feb, Mar of the year 0. The correlation for the time interval 1920–1973, rðaa; GTÞ ¼ 0:68, is statistically significant both for short-term fluctuations with a time lag of one year and for a long-term trend of curves a, b in Fig. 2. However, for the time interval 1900–1920 and for 1974–2000 the correlation coefficients are not statistically significant and are negative, rðaa; GTÞ ¼ 0:20. For yearly averages of geomagnetic activity the correlation coefficients were similar. We succeeded in finding positive correlation between the difference aa fMay ðFeb þ Mar þ AprÞ=3g and GT when r ¼ 0:60 for 1985–2000. This would mean that GT in this time interval grows when geomagnetic activity is high in May while it is low in February, March and April, and vice versa. These results enable us to judge that enhanced geomagnetic forcing in the time interval 1920–1973 influenced mainly processes in the Northern Hemisphere. The strengthening of the zonal flow and shifts in the atmospheric circulation patterns occurred at times of high geomagnetic activity during northern winters participating in the warming of middle latitudes (Bucha and Bucha, 1998) and even in enhancing GT. On the other hand, it seems that geomagnetic forcing in 1985–2000 influenced stronger the processes in the Southern Hemisphere, led to significant shifts of atmospheric pressure centers during the southern winter, most probably in connection with EN events, and participated in global warming. The role of NAO and EN events will be discussed in more detail in Sections 4 and 5.
4. Relations between geomagnetic activity and the North Atlantic Oscillation Strong changes in the phase of the NAO were observed relatively frequently. It remains, however, to answer the question what is the relation between geo-
V. Bucha / Physics and Chemistry of the Earth 27 (2002) 427–431
Fig. 3. Time series of geomagnetic activity (aa index) for winter periods December–March (curve a), the NAO winter index WI (curve b, www.cru.uea.ac.uk/timo/projpages/nao_update.htm and Jones et al., 1997). The correlation coefficient r ¼ 0:71 for 1970–1996.
magnetic activity and NAO and in which way it could act on fluctuations of the global temperature. We suggest (Bucha and Bucha, 1998) that enhanced geomagnetic forcing leads to the intensification of the westerly flow in the Northern Hemisphere and to above normal temperatures in Europe, northern Asia and North America. The NAO winter index (WI) is positive when the Azores high is strong and the Icelandic low is deep, and negative when reversed. We have found statistically significant correlation coefficients between geomagnetic activity (aa index) and the normalized WI as given by Jones et al. (1997) and in www.cru.uea.ac.uk/timo/ projpages/nao_update.htm. They equal to rðaa; WIÞ ¼ 0:71 for the period 1970–1996 and rðaa; WIÞ ¼ 0:61 for 1961–1999 (Fig. 3). The cross-correlation for six positive and six negative lags has shown that the highest correlation coefficient is statistically significant at the 99% confidence level for the zero time lags only. Already earlier we found similar results between geomagnetic activity and the zonal index rðaa; ZOIÞ ¼ 0:68 for 1970– 1996 (Bucha and Bucha, 1998). The correlation coeffi-
429
cient between geomagnetic activity and NAO index for 1900–1950 (r ¼ 0:33) is negative. This can be explained as due to the much lower geomagnetic activity occurring in this time interval than in 1950–2000 and leading to the significant shift of atmospheric circulation patterns (Bucha and Bucha, 1998). To show the real effect of geomagnetic forcing on pressure changes in the Northern Hemisphere we constructed composites of the s. l. pressure distribution in the Northern Hemisphere (Die Grosswetterlagen Europas, 1970–2000) for 16 averages of months February and March with low and for 16 averages with high geomagnetic activity. The composite for 16 months when geomagnetic activity was decreased and the NAO WI was negative shows (Fig. 4(a)) that the Icelandic low is shallow and is displaced westward towards Newfoundland; the Siberian high strengthens and extends toward Europe. Positive pressure anomalies occur in the region of the northern Atlantic and Eurasia while a negative anomaly is located over the northern Pacific. On the other hand, as follows from the composite for 16 months with high geomagnetic activity the Icelandic low deepens considerably (Fig. 4(b)) and extends to the Northeast influencing the whole Europe and polar areas of Siberia and Canada. The Azores high strengthens, extends southeastward to southern Europe and along the strong Icelandic low a substantial intensification of the zonal flow occurs; the NAO WI is positive. A highpressure zone in mid-latitudes encircles the very deep Icelandic low. We also found a short-term atmospheric response and suggested (Bucha and Bucha, 1998) the strengthening of the zonal flow to be influenced by geomagnetic forcing. For the case of several couples with low values aa and enhanced values of the NAO index and temperatures T in Europe which occurred for winters 1977, 1988, 1997 and 1998 we found that geomagnetic activity (aa index) was low already during three preceding months October, November and
Fig. 4. Composites of 16 monthly averages for the sea level pressure at times of low geomagnetic activity (l.h.s.) and at times of high geomagnetic activity (r.h.s. of figure).
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December. This disagreement which decreases the value of the correlation coefficient, occurred relatively rarely during the past 30 years and can be explained as follows: between the Icelandic low and the Siberian high southerly winds prevail in Europe and participate in normal to above-normal temperatures here.
5. Relations between geomagnetic activity and El Ni~no events As a next step we examined the relations between geomagnetic activity and EN events. Correlation coefficients were found to be positive for relations between the difference of geomagnetic indices aafðFeb þ MarÞ= 2 Aprg and EN. They were equal to r ¼ 0:61 for 1967–2000 and 0.45 for 1935–2000 (Fig. 1). After compiling the cross-correlation for nine positive and nine negative lags we found that the highest r is statistically significant at the 99% confidence level for the zero time lag and for the sixth negative lag. This result indicates a possible oscillation of both variables equal to 6 years. For the period 1900–1950 r ¼ 0:32 only, again most probably due to the very low geomagnetic activity in this time interval. Van Loon and Shea (1987) have constructed maps of the s. l. pressure anomalies for the mean of May, June and July of six EN years when the mean s. l. pressure tended to be higher over Australia. At that time geomagnetic activity in April was low and the intensity of Asian monsoons decreased as follows from the statistically significant correlation coefficient r ¼ 0:48 between both variables (Bucha and Bucha, 2002). Similarly, Van Loon and Shea (1987) constructed the map of anomalies for four La Ni~ na years when the s. l. pressure across Australia was below normal. In this case geomagnetic activity in April was enhanced and most probably intensified the zonal flow in the Southern Hemisphere. Then, the Australian high was weak and did not block monsoon rains that were above normal. Because of the statistically significant correlation between AIRI (Webster et al., 1998) and ENSO (negative r ¼ 0:51 for 1962–1994) we can presume that the strong Australian high plays an important role in initiating the EN event. The correlation coefficient rfaaðFeb þ MarÞ=2 aa ðAprÞ; ENg equals to 0.61 for 1968–2000 and indicates that positive values of differences of geomagnetic activity aa correspond to the stronger Australian high persisting during the southern winter. Then, the strengthened southerly wind east of Australia to 140° W blocks, together with the intense Aleutian low, the outflow of warm air from equatorial areas. Easterly surface winds weaken and help to create conditions for the origin of the EN (warm) event. Naturally, geomagnetic activity should be considered as one factor only that could influence climatic fluctuations.
6. Conclusions The aim of this paper is to find out whether relations exist in the longer time interval between climatic variables represented by changes of global temperature, EN events and NAO and to what degree they are connected with geomagnetic forcing. In this way, we tried to verify the hypothesis dealing with the geomagnetic forcing on climatic fluctuations and suggested in our previous papers (Bucha and Bucha, 1998, 2002). Downward winds are generated in the polar thermosphere at times of high geomagnetic activity (Crowley et al., 1989). According to our hypothesis the winds penetrate through the stratosphere to the troposphere and accelerate the subsidence of the air especially along the northern margin of the Siberian high and the west coast North American ridge (Bucha and Bucha, 1998). This process here is connected with an increase of pressure, of the jet stream and of westerlies. We also found a response, i.e. surface temperatures increased in central Europe with a differing time lag of 6–10 days in dependence on that whether the effect of geomagnetic forcing occurred prevailingly in the region of Siberia or Alaska or the Nordic Seas (Bucha and Bucha, 1998). Due to the prevailing zonal flow the cold air persists in the polar stratosphere where the cooling is observed. On the other hand, at times of strongly decreased geomagnetic activity in winter the Icelandic low is often split into two cells: one is displaced westward towards Newfoundland and the other is situated over Scandinavia enabling the arctic air to penetrate southward, directly to central Europe. This leads to abrupt temperature decline here. What concerns the long-term effect of geomagnetic forcing on the climatic variability, following results were obtained: (a) The relation between EN events and GT was found to exist as given by statistically significant correlation coefficients in the second half of the 20th century (see Fig. 1). (b) Geomagnetic activity and fluctuations of GT show connections mainly between 1920 and 1973 as given by statistically significant correlation coefficient 0.68 (Fig. 2) indicating partly short-term partly longterm relations between both variables with the one year time lag. For 1985–2000 when geomagnetic activity was in average much higher than in 1920–1973 73, the correlation coefficient was found to be 0.60 between the differences aafMay ðFeb þ Mar þ AprÞ=3g and GT. (c) The positive relation between geomagnetic activity and NAO is given by the correlation coefficient r ¼ 0:71 for 1970–1996 (Fig. 3). It can be explained on the basis of our hypothesis that geomagnetic forcing leads to the intensification of the zonal flow in the Northern Hemisphere and to above-normal sea surface temperatures here (Bucha and Bucha,
V. Bucha / Physics and Chemistry of the Earth 27 (2002) 427–431
1998, 2002). At times of decreased geomagnetic activity penetrations of arctic air prevail in winter leading to below-normal temperatures mainly in Europe and northern Asia. Typical atmospheric circulation patterns are given by composites of 16 monthly averages for the sea level pressure anomalies at times of low and 16 averages at times of high geomagnetic activity (Fig. 4). (d) The Australian high is shown to play an important role in weakening the intensity of monsoon rains as given by AIRI and in initiating EN events. We have found statistically significant correlation coefficients between geomagnetic activity and AIRI (0.48 for 1962–1994), between geomagnetic activity and EN events (0.61 for 1968–2000 and 0.45 for 1935– 2000) as well as between AIRI and EN events ()0.51 for 1962–1994). On this basis we can conclude that geomagnetic activity participates in forcing EN events which have an effect on GT with a time lag of one year. Geomagnetic activity in the first half of the year fdifferences aaðFeb þ MarÞ= 2 aaðAprÞg is suggested to initiate the occurrence of EN events in the second half of the year. EN events then participate in the increase of GT during the next year frðEN; GTÞ ¼ 0:43g for 1950–2000. The EN effect on GT was found to be negligible in the first half of the 20th century (r ¼ 0:08). This can be explained again as due to the fact that geomagnetic activity in this time interval was significantly lower than that in the second half of the 20th century. (e) The correlation between geomagnetic activity and fluctuations of GT was found to be statistically significant (0.68) for 1920–1973 with the time lag of one year while it was low for 1974–2000 ()0.20) and negative. This discordance can again be explained as due to the fact that the value of geomagnetic activity increased considerably in the second half of the 20th century, especially during the past 30 years. Its stronger forcing led to the shift of atmospheric circulation patterns and to changes in relations between geomagnetic activity and meteorological variables. The distribution of pressure formations is also affected by seasonal changes so that the effect of geomagnetic activity can change during individual years. The value of the correlation coefficient can be influenced not only by values of geomagnetic activity in winter months but also in October and November. Then, the anomalous distribution of pressure formations in autumn can partly influence the shift of the atmospheric circulation patterns in
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winter. Also, the fact that the suggested possible influence of EN and NAO events on GT is not simultaneous but lagged in time can decrease the significance of the correlation. The detailed study of these open questions that may even cause the change of the sign of correlation coefficients, need to be addressed in the future with the aim to contribute to the separation of natural and anthropogenic effects participating in the global warming.
Acknowledgements This work was supported by the grant A3012806/ 1998 of the Grant Agency of the Academy of Sciences of the Czech Republic.
References Bochnıcek, J., Hejda, P., Pycha, J., 2001. Comparison of solar and geomagnetic activity effects on the Northern Hemisphere weather changes. Studia Geophys. et Geod. 45, 133–154. Bucha, V., 1976. Variations of the geomagnetic field, the climate and weather. Studia Geoph. et Geod. 20, 149–167. Bucha, V., Bucha Jr., V., 1998. Geomagnetic forcing of changes in climate and in the atmospheric circulation. J. Atm. Solar Terr. Phys. 60 (2), 145–169. Bucha, V., Bucha Jr., V., 2002. Geomagnetic forcing and climatic variations in Europe, North America and in the Pacific Ocean. Quaternary International 91, 5–15. Crowley, G.T., Emery, B.A., Roble, R.G., 1989. Thermospheric dynamics during September 18–19, 1984. Model simulations. J. Geophys. Res. 92, 16925–16944. Die Grosswetterlagen Europas, 1970–2000. Jones, P.D., Jonsson, T., Wheeler, D., 1997. Extension to the North Atlantic Oscillation using early instrumental pressure observations from Gibraltar and South West Iceland. Int. J. Climatol. 17, 1433– 1450, Available from www.cru.uea.ac.uk/timo/projpages/nao_update.htm. Lastovicka, J., 1996. Effects of geomagnetic storms in the lower ionosphere, middle atmosphere and troposphere. J. Atm. Terr. Phys. 58 (7), 831–843. Van Loon, H., Shea, D.J., 1987. The Southern Oscillation. Part VI. Anomalies of sea level pressure on the Southern Hemisphere and of Pacific sea surface temperature during the development of a Warm Event. Mon. Weather Rev. 115, 370–379. Wallace, J.M., Rasmusson, E., Mitchel, T., Kousky, V.E., Sarachik, E.S., Storch, H., 1998. On the structure and evolution of ENSOrelated climate variability in the tropical Pacific: Lessons from TOGA. J. Geophys. Res. 103 (C7), 14241–14259. Webster, P.J., Magana, V.O., Palmer, T.N., Shukla, J., Tomas, R.A., Yanai, M., Yasunari, T., 1998. Monsoons; Processes, predictability and the prospects for prediction. J. Geophys. Res. 103 (C7), 14451– 14510. WMO Statements, 1996–2000. World Climate News, WMO, 1997–2000.
Long-term trends in geomagnetic and climatic variability V aclav Bucha
*
Geophysical Institute, Academy of Sciences of the Czech Republic, Bo cnı II, 141 31 Prague 4, Czech Republic Received 30 October 2001; received in revised form 7 January 2002; accepted 11 January 2002
Abstract Causes leading to global mean sea surface temperature (GT) variability and to variations of the global circulation including the North Atlantic Oscillation (NAO) and El Ni~ no (EN) events are examined. Statistically significant correlation coefficients between these variables were found and their relations are discussed with the aim to show possible causes leading to general year-to-year variability and to the global warming. At the same time, the results contribute to the verification of the hypothesis as given in our previous papers. We have suggested a link of processes generated by geomagnetic forcing that is followed by dramatic shifts in the atmospheric circulation patterns. At times of low geomagnetic activity in winter the meridional flow prevails contributing to the strong heat exchange of air between low and high latitudes. The arctic air penetrates from polar areas and participates in the cooling of middle latitudes; the NAO winter index (WI) is negative. At times of high geomagnetic activity the Icelandic low intensifies and influences the strengthening of the zonal flow in the Northern Hemisphere; the NAO WI is positive. A continuous zone of high pressure originates along middle latitudes and a little north–south motion of air takes place. In mid-latitudes above-normal temperatures occur while in polar areas the values are below normal. The strong Australian high at times of low geomagnetic activity seems to initiate EN events while the zonal flow in the Southern Hemisphere intensifies monsoon rains in the Indian Ocean when geomagnetic activity is high. EN and NAO events are shown to take part in the variability of GT. Controversial problems are discussed as well. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Geomagnetic forcing; Climatic variability; Meridional and zonal flow; Global warming
1. Introduction The time series of global mean sea surface temperatures shows significant variability. According to the WMO Statements 1996–2000 the global warming was observed following ENSO events, e.g. in 1973, 1977, 1983 and 1987 and cooling around 1992 following the 1991 eruption of Mount Pinatubo. The year 1996 was the eighteenth consecutive year with global temperatures above the 1961–1990 normal. On the other hand, the overall stratospheric cooling occurred during the past 50 years according to the WMO Statements 1996. The relative cooling over much of Eurasia in 1996 can be attributed to a strong change in the phase of the North Atlantic Oscillation (NAO). The highest global surface temperature in 1998, 0.57 °C above the 1961–1990 normal, is attributed to the unprecedented warmth of the Indian Ocean and of parts of every continent, especially North America, Eurasia and Australia. In the
*
Tel.: +420-2-6710339; fax: +420-2-72761549. E-mail address: [email protected] (V. Bucha).
year 2000 the global mean temperature anomaly was only 0.29 °C above the 1961–1990 base period average. The eastern tropical Pacific was, according to the WMO Statements 2000, colder than usual and much of Asia and west-central part of North America were colder than normal during the September–November period. It was shown that geomagnetic forcing has a much stronger effect on meteorological processes than solar activity forcing and leads to strong changes in the configuration of main pressure formations (Bucha, 1976; Bucha and Bucha, 1998, 2002; Lastovicka, 1996; Bochnıcek et al., 2001). Let us try to study whether there are positive relations also between geomagnetic activity, NAO, El Ni~ no (EN) events and fluctuations of global mean sea surface temperature.
2. Analyses and results The relations between EN events (Wallace et al., 1998) and GT show that global warming really follows EN events as given in the WMO Statement 1996. The highest correlation coefficient r between EN and GT is
1474-7065/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 4 7 4 - 7 0 6 5 ( 0 2 ) 0 0 0 2 2 - 0
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V. Bucha / Physics and Chemistry of the Earth 27 (2002) 427–431
Fig. 1. Temperature anomalies representing El Ni~ no (EN) events as defined by values in the equatorial area of the Pacific Ocean (curve a) (Wallace et al., 1998; World Climate News, 1997–2000), global mean sea surface temperatures (curve b), yearly averages of geomagnetic aa indices (curve c).
positive and equals to 0.43 for the time interval 1950– 2000 with a time lag of one year (Fig. 1). The crosscorrelation showed that these correlation coefficients are statistically significant at the 99% confidence level for the time lag of one year. From it follows that the EN effect on GT exists in the second half of the 20th century. The correlation is, however, not statistically significant for the period 1900–1950 (r ¼ 0:08). By studying relations between the NAO and GT for the time interval 1951–2000 we found the correlation coefficient r equal to 0.39. However, the cross-correlation showed that all correlation coefficients for nine positive and nine negative lags are significantly non-zero. This is most probably caused mainly by the upward trend of the two series. From these results it follows that a stronger effect of EN events exists on fluctuations of the global temperature while the relation between NAO and GT seems to be connected rather with the long-term upward trend. When we summarized both effects, i.e. EN + NAO on GT we obtained the correlation coefficient that equals to 0.54 for 1951–2000 and 0.63 for 1951–1995. Both are statistically significant at the 99% confidence level. The fluctuation of GT could be interpreted as a result of the NAO effect in the Northern Hemisphere and of the EN effect in the Southern Hemisphere. The correlation between EN and NAO events is, however, not statistically significant. Let us try to look for the causes of these relations.
3. Relations between geomagnetic activity and global mean surface temperature As a next step we examined the relations between geomagnetic activity and fluctuations of GT. Following correlation coefficients were obtained (Fig. 2): rðaa; GTÞ ¼ 0:58 for the time interval 1900–2000 with a time
Fig. 2. Sum of geomagnetic aa indices for Jan, Feb, Mar, Nov, Dec of the year-1 and for Jan, Feb, Mar of the year 0 (curve a), global mean sea surface temperatures (curve b) (World Climate News, 1997–2000), differences between mean values of geomagnetic aa indices for aaðMayÞ aaðFeb þ Mar þ AprÞ=3 (curve c).
lag of one year where aa is the sum of geomagnetic aa indices for Jan, Feb, Mar, Nov, Dec of the year-1 and Jan, Feb, Mar of the year 0. The correlation for the time interval 1920–1973, rðaa; GTÞ ¼ 0:68, is statistically significant both for short-term fluctuations with a time lag of one year and for a long-term trend of curves a, b in Fig. 2. However, for the time interval 1900–1920 and for 1974–2000 the correlation coefficients are not statistically significant and are negative, rðaa; GTÞ ¼ 0:20. For yearly averages of geomagnetic activity the correlation coefficients were similar. We succeeded in finding positive correlation between the difference aa fMay ðFeb þ Mar þ AprÞ=3g and GT when r ¼ 0:60 for 1985–2000. This would mean that GT in this time interval grows when geomagnetic activity is high in May while it is low in February, March and April, and vice versa. These results enable us to judge that enhanced geomagnetic forcing in the time interval 1920–1973 influenced mainly processes in the Northern Hemisphere. The strengthening of the zonal flow and shifts in the atmospheric circulation patterns occurred at times of high geomagnetic activity during northern winters participating in the warming of middle latitudes (Bucha and Bucha, 1998) and even in enhancing GT. On the other hand, it seems that geomagnetic forcing in 1985–2000 influenced stronger the processes in the Southern Hemisphere, led to significant shifts of atmospheric pressure centers during the southern winter, most probably in connection with EN events, and participated in global warming. The role of NAO and EN events will be discussed in more detail in Sections 4 and 5.
4. Relations between geomagnetic activity and the North Atlantic Oscillation Strong changes in the phase of the NAO were observed relatively frequently. It remains, however, to answer the question what is the relation between geo-
V. Bucha / Physics and Chemistry of the Earth 27 (2002) 427–431
Fig. 3. Time series of geomagnetic activity (aa index) for winter periods December–March (curve a), the NAO winter index WI (curve b, www.cru.uea.ac.uk/timo/projpages/nao_update.htm and Jones et al., 1997). The correlation coefficient r ¼ 0:71 for 1970–1996.
magnetic activity and NAO and in which way it could act on fluctuations of the global temperature. We suggest (Bucha and Bucha, 1998) that enhanced geomagnetic forcing leads to the intensification of the westerly flow in the Northern Hemisphere and to above normal temperatures in Europe, northern Asia and North America. The NAO winter index (WI) is positive when the Azores high is strong and the Icelandic low is deep, and negative when reversed. We have found statistically significant correlation coefficients between geomagnetic activity (aa index) and the normalized WI as given by Jones et al. (1997) and in www.cru.uea.ac.uk/timo/ projpages/nao_update.htm. They equal to rðaa; WIÞ ¼ 0:71 for the period 1970–1996 and rðaa; WIÞ ¼ 0:61 for 1961–1999 (Fig. 3). The cross-correlation for six positive and six negative lags has shown that the highest correlation coefficient is statistically significant at the 99% confidence level for the zero time lags only. Already earlier we found similar results between geomagnetic activity and the zonal index rðaa; ZOIÞ ¼ 0:68 for 1970– 1996 (Bucha and Bucha, 1998). The correlation coeffi-
429
cient between geomagnetic activity and NAO index for 1900–1950 (r ¼ 0:33) is negative. This can be explained as due to the much lower geomagnetic activity occurring in this time interval than in 1950–2000 and leading to the significant shift of atmospheric circulation patterns (Bucha and Bucha, 1998). To show the real effect of geomagnetic forcing on pressure changes in the Northern Hemisphere we constructed composites of the s. l. pressure distribution in the Northern Hemisphere (Die Grosswetterlagen Europas, 1970–2000) for 16 averages of months February and March with low and for 16 averages with high geomagnetic activity. The composite for 16 months when geomagnetic activity was decreased and the NAO WI was negative shows (Fig. 4(a)) that the Icelandic low is shallow and is displaced westward towards Newfoundland; the Siberian high strengthens and extends toward Europe. Positive pressure anomalies occur in the region of the northern Atlantic and Eurasia while a negative anomaly is located over the northern Pacific. On the other hand, as follows from the composite for 16 months with high geomagnetic activity the Icelandic low deepens considerably (Fig. 4(b)) and extends to the Northeast influencing the whole Europe and polar areas of Siberia and Canada. The Azores high strengthens, extends southeastward to southern Europe and along the strong Icelandic low a substantial intensification of the zonal flow occurs; the NAO WI is positive. A highpressure zone in mid-latitudes encircles the very deep Icelandic low. We also found a short-term atmospheric response and suggested (Bucha and Bucha, 1998) the strengthening of the zonal flow to be influenced by geomagnetic forcing. For the case of several couples with low values aa and enhanced values of the NAO index and temperatures T in Europe which occurred for winters 1977, 1988, 1997 and 1998 we found that geomagnetic activity (aa index) was low already during three preceding months October, November and
Fig. 4. Composites of 16 monthly averages for the sea level pressure at times of low geomagnetic activity (l.h.s.) and at times of high geomagnetic activity (r.h.s. of figure).
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December. This disagreement which decreases the value of the correlation coefficient, occurred relatively rarely during the past 30 years and can be explained as follows: between the Icelandic low and the Siberian high southerly winds prevail in Europe and participate in normal to above-normal temperatures here.
5. Relations between geomagnetic activity and El Ni~no events As a next step we examined the relations between geomagnetic activity and EN events. Correlation coefficients were found to be positive for relations between the difference of geomagnetic indices aafðFeb þ MarÞ= 2 Aprg and EN. They were equal to r ¼ 0:61 for 1967–2000 and 0.45 for 1935–2000 (Fig. 1). After compiling the cross-correlation for nine positive and nine negative lags we found that the highest r is statistically significant at the 99% confidence level for the zero time lag and for the sixth negative lag. This result indicates a possible oscillation of both variables equal to 6 years. For the period 1900–1950 r ¼ 0:32 only, again most probably due to the very low geomagnetic activity in this time interval. Van Loon and Shea (1987) have constructed maps of the s. l. pressure anomalies for the mean of May, June and July of six EN years when the mean s. l. pressure tended to be higher over Australia. At that time geomagnetic activity in April was low and the intensity of Asian monsoons decreased as follows from the statistically significant correlation coefficient r ¼ 0:48 between both variables (Bucha and Bucha, 2002). Similarly, Van Loon and Shea (1987) constructed the map of anomalies for four La Ni~ na years when the s. l. pressure across Australia was below normal. In this case geomagnetic activity in April was enhanced and most probably intensified the zonal flow in the Southern Hemisphere. Then, the Australian high was weak and did not block monsoon rains that were above normal. Because of the statistically significant correlation between AIRI (Webster et al., 1998) and ENSO (negative r ¼ 0:51 for 1962–1994) we can presume that the strong Australian high plays an important role in initiating the EN event. The correlation coefficient rfaaðFeb þ MarÞ=2 aa ðAprÞ; ENg equals to 0.61 for 1968–2000 and indicates that positive values of differences of geomagnetic activity aa correspond to the stronger Australian high persisting during the southern winter. Then, the strengthened southerly wind east of Australia to 140° W blocks, together with the intense Aleutian low, the outflow of warm air from equatorial areas. Easterly surface winds weaken and help to create conditions for the origin of the EN (warm) event. Naturally, geomagnetic activity should be considered as one factor only that could influence climatic fluctuations.
6. Conclusions The aim of this paper is to find out whether relations exist in the longer time interval between climatic variables represented by changes of global temperature, EN events and NAO and to what degree they are connected with geomagnetic forcing. In this way, we tried to verify the hypothesis dealing with the geomagnetic forcing on climatic fluctuations and suggested in our previous papers (Bucha and Bucha, 1998, 2002). Downward winds are generated in the polar thermosphere at times of high geomagnetic activity (Crowley et al., 1989). According to our hypothesis the winds penetrate through the stratosphere to the troposphere and accelerate the subsidence of the air especially along the northern margin of the Siberian high and the west coast North American ridge (Bucha and Bucha, 1998). This process here is connected with an increase of pressure, of the jet stream and of westerlies. We also found a response, i.e. surface temperatures increased in central Europe with a differing time lag of 6–10 days in dependence on that whether the effect of geomagnetic forcing occurred prevailingly in the region of Siberia or Alaska or the Nordic Seas (Bucha and Bucha, 1998). Due to the prevailing zonal flow the cold air persists in the polar stratosphere where the cooling is observed. On the other hand, at times of strongly decreased geomagnetic activity in winter the Icelandic low is often split into two cells: one is displaced westward towards Newfoundland and the other is situated over Scandinavia enabling the arctic air to penetrate southward, directly to central Europe. This leads to abrupt temperature decline here. What concerns the long-term effect of geomagnetic forcing on the climatic variability, following results were obtained: (a) The relation between EN events and GT was found to exist as given by statistically significant correlation coefficients in the second half of the 20th century (see Fig. 1). (b) Geomagnetic activity and fluctuations of GT show connections mainly between 1920 and 1973 as given by statistically significant correlation coefficient 0.68 (Fig. 2) indicating partly short-term partly longterm relations between both variables with the one year time lag. For 1985–2000 when geomagnetic activity was in average much higher than in 1920–1973 73, the correlation coefficient was found to be 0.60 between the differences aafMay ðFeb þ Mar þ AprÞ=3g and GT. (c) The positive relation between geomagnetic activity and NAO is given by the correlation coefficient r ¼ 0:71 for 1970–1996 (Fig. 3). It can be explained on the basis of our hypothesis that geomagnetic forcing leads to the intensification of the zonal flow in the Northern Hemisphere and to above-normal sea surface temperatures here (Bucha and Bucha,
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1998, 2002). At times of decreased geomagnetic activity penetrations of arctic air prevail in winter leading to below-normal temperatures mainly in Europe and northern Asia. Typical atmospheric circulation patterns are given by composites of 16 monthly averages for the sea level pressure anomalies at times of low and 16 averages at times of high geomagnetic activity (Fig. 4). (d) The Australian high is shown to play an important role in weakening the intensity of monsoon rains as given by AIRI and in initiating EN events. We have found statistically significant correlation coefficients between geomagnetic activity and AIRI (0.48 for 1962–1994), between geomagnetic activity and EN events (0.61 for 1968–2000 and 0.45 for 1935– 2000) as well as between AIRI and EN events ()0.51 for 1962–1994). On this basis we can conclude that geomagnetic activity participates in forcing EN events which have an effect on GT with a time lag of one year. Geomagnetic activity in the first half of the year fdifferences aaðFeb þ MarÞ= 2 aaðAprÞg is suggested to initiate the occurrence of EN events in the second half of the year. EN events then participate in the increase of GT during the next year frðEN; GTÞ ¼ 0:43g for 1950–2000. The EN effect on GT was found to be negligible in the first half of the 20th century (r ¼ 0:08). This can be explained again as due to the fact that geomagnetic activity in this time interval was significantly lower than that in the second half of the 20th century. (e) The correlation between geomagnetic activity and fluctuations of GT was found to be statistically significant (0.68) for 1920–1973 with the time lag of one year while it was low for 1974–2000 ()0.20) and negative. This discordance can again be explained as due to the fact that the value of geomagnetic activity increased considerably in the second half of the 20th century, especially during the past 30 years. Its stronger forcing led to the shift of atmospheric circulation patterns and to changes in relations between geomagnetic activity and meteorological variables. The distribution of pressure formations is also affected by seasonal changes so that the effect of geomagnetic activity can change during individual years. The value of the correlation coefficient can be influenced not only by values of geomagnetic activity in winter months but also in October and November. Then, the anomalous distribution of pressure formations in autumn can partly influence the shift of the atmospheric circulation patterns in
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winter. Also, the fact that the suggested possible influence of EN and NAO events on GT is not simultaneous but lagged in time can decrease the significance of the correlation. The detailed study of these open questions that may even cause the change of the sign of correlation coefficients, need to be addressed in the future with the aim to contribute to the separation of natural and anthropogenic effects participating in the global warming.
Acknowledgements This work was supported by the grant A3012806/ 1998 of the Grant Agency of the Academy of Sciences of the Czech Republic.
References Bochnıcek, J., Hejda, P., Pycha, J., 2001. Comparison of solar and geomagnetic activity effects on the Northern Hemisphere weather changes. Studia Geophys. et Geod. 45, 133–154. Bucha, V., 1976. Variations of the geomagnetic field, the climate and weather. Studia Geoph. et Geod. 20, 149–167. Bucha, V., Bucha Jr., V., 1998. Geomagnetic forcing of changes in climate and in the atmospheric circulation. J. Atm. Solar Terr. Phys. 60 (2), 145–169. Bucha, V., Bucha Jr., V., 2002. Geomagnetic forcing and climatic variations in Europe, North America and in the Pacific Ocean. Quaternary International 91, 5–15. Crowley, G.T., Emery, B.A., Roble, R.G., 1989. Thermospheric dynamics during September 18–19, 1984. Model simulations. J. Geophys. Res. 92, 16925–16944. Die Grosswetterlagen Europas, 1970–2000. Jones, P.D., Jonsson, T., Wheeler, D., 1997. Extension to the North Atlantic Oscillation using early instrumental pressure observations from Gibraltar and South West Iceland. Int. J. Climatol. 17, 1433– 1450, Available from www.cru.uea.ac.uk/timo/projpages/nao_update.htm. Lastovicka, J., 1996. Effects of geomagnetic storms in the lower ionosphere, middle atmosphere and troposphere. J. Atm. Terr. Phys. 58 (7), 831–843. Van Loon, H., Shea, D.J., 1987. The Southern Oscillation. Part VI. Anomalies of sea level pressure on the Southern Hemisphere and of Pacific sea surface temperature during the development of a Warm Event. Mon. Weather Rev. 115, 370–379. Wallace, J.M., Rasmusson, E., Mitchel, T., Kousky, V.E., Sarachik, E.S., Storch, H., 1998. On the structure and evolution of ENSOrelated climate variability in the tropical Pacific: Lessons from TOGA. J. Geophys. Res. 103 (C7), 14241–14259. Webster, P.J., Magana, V.O., Palmer, T.N., Shukla, J., Tomas, R.A., Yanai, M., Yasunari, T., 1998. Monsoons; Processes, predictability and the prospects for prediction. J. Geophys. Res. 103 (C7), 14451– 14510. WMO Statements, 1996–2000. World Climate News, WMO, 1997–2000.