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On the signal of the 11-year sunspot cycle in the stratosphere and its modulation by the quasi-biennial oscillation
Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1151 – 1157
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On the signal of the 11-year sunspot cycle in the stratosphere and its modulation by the quasi-biennial oscillation Karin Labitzke∗ Meteorologisches Institut, Freie Universitat Berlin, Carl-Heinrich Becker Weg 6-10 12165 Berlin, Germany
Abstract In earlier studies we examined the global structure and the size of the signal of the 11-year sunspot cycle (SSC) in the stratosphere and troposphere. For the correlations between the solar cycle and heights and temperatures at di3erent pressure levels we used mainly the whole data set, and only during northern winters, the years were strati5ed according to the phase of the quasi-biennial oscillation (QBO). This work is expanded here and it is shown that the QBO must be introduced throughout the year, because the solar signal is very di3erent in the respective phases of the QBO, particularly over the tropics and subtropics. The structure of the solar signal appears to indicate that the mean meridional circulation systems (Hadley and Brewer–Dobson Circulation) are in:uenced by the SSC, and stronger so during the east phase of the QBO. c 2004 Elsevier Ltd. All rights reserved. Keywords: 11-year solar cycle; Stratosphere; Quasi-biennial oscillation; Mean meridional circulation systems
1. Introduction We have shown in several publications (Labitzke, 1987; Labitzke and van Loon, 1988; van Loon and Labitzke, 1994, 2000) that there exists a strong signal of the 11-year solar cycle in the atmosphere during the late northern winters— but only if the data are strati5ed according to the phase of the QBO. If one uses the unstrati5ed data, no clear signal emerges. In winter, the arctic stratosphere is a region with large, dynamically induced temperature variations, and very cold, stable periods as well as warm, disturbed periods are observed. Because of this high variability, the identi5cation and explanation of the solar signal is very complex (Kodera and Kuroda, 2002). Therefore, this study concentrates at 5rst on the northern summer which is a dynamically less disturbed
∗
Tel.: +0049-30-8387-1166; fax: +0049-30-8387-1167. E-mail address: [email protected] (K. Labitzke).
season. The emphasis of this study is placed on the modulation of the solar cycle by the QBO.
2. Data and methods The NCEP/NCAR re-analyses (Kalnay et al., 1996) are used for the period 1968–2002. The re-analyses were less reliable earlier, i.e. before 1968, mainly because of the lack of radiosonde stations over the southern hemisphere, the lack of high reaching balloons in the early years and the scarce satellite information before 1979. But it should be mentioned that the inclusion of the early data yields similar but somewhat weaker results. As a measure of the SSC, the monthly mean values of the 10:7 cm solar :ux are used. The :ux values are expressed in solar :ux units: 1 s.f.u. =10−22 W m−2 Hz−1 . This is an objectively measured radiowave, highly and positively correlated with the SSC and with the UV part of the solar spectrum, which varies about 6–8% between solar maxima and minima (Chandra and McPeters, 1994).
c 2004 Elsevier Ltd. All rights reserved. 1364-6826/$ - see front matter doi:10.1016/j.jastp.2004.05.011
1152
K. Labitzke / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1151 – 1157
(a)
(b)
Fig. 1. (a), (left-hand side): Vertical meridional sections of the correlations between the 10:7 cm solar :ux and the detrended zonal mean temperatures in July; shaded for emphasis where correlations are above 0.5; (b), (right-hand side): The respective temperature di3erences (K) between solar maxima and minima, shaded where the correlations are above 0.5. Upper panels: all years; middle panels: only years in the EAST PHASE of the QBO; lower panels: only years in the WEST PHASE of the QBO (NCEP/NCAR re-analyses, 1968–2002)(Fig. 4 in Labitzke (2003)).
For the range of the SSC the mean di3erence of the 10:7 cm solar :ux between solar minima (about 70 units) and solar maxima (about 200 units) is used, i.e. 130 units. Any linear correlation can be represented also by a regression line with y = a + bx, where x in this case is the 10:7 cm solar :ux and b is the slope. This slope is used here, multiplied by 130, in order to get the di3erences between solar minima and maxima, as presented in this paper. The statistical signi5cance of the correlations cannot be determined with certainty because we have less than four solar cycles and the degrees of freedom are therefore limited. The QBO is an oscillation in the atmosphere which is best observed in the stratospheric winds above the equator, where the zonal winds are changing between east and west. The period of the QBO varies in space and time, but it lies on an average near 28 months at all levels (Labitzke and van Loon, 1999).
The QBO modulates the solar signal and it is therefore necessary to stratify the data into years when the equatorial QBO in the lower stratosphere (about 45 hPa) was in its west phase and years when it was in its east phase. 3. The solar signal during the northern summer We have shown that the correlations between stratospheric temperatures and heights are large during the northern summer, but that the signal is much stronger during the east phase of the QBO (Labitzke, 2003). Fig. 1a (left-hand side) shows for July, vertical meridional sections of the correlations between the zonal mean temperatures and the 11-year solar cycle, while Fig. 1b (right-hand side) gives the temperature di3erences between solar maxima and minima. The three panels show the results for all data (upper panel), for the years in the east phase of the QBO (middle
K. Labitzke / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1151 – 1157
panel) and for the years in the west phase of the QBO (lower panel). The di3erences of the size of the solar signal in the two di3erent phases of the QBO is striking and the result for all data is misleading. The structure of the signal hints to a connection to the mean meridional circulation systems, namely the Hadley Circulation (HC) in the troposphere and the Brewer–Dobson Circulation (BDC) in the stratosphere, as also reported by Soukharev and Hood (2001) and by Salby and Callaghan (2000, 2004). The two maxima of response in Fig. 1b for the east phase, at 30 hPa/25N and S, are probably an indirect e3ect of the direct warming observed in the upper stratosphere during solar maxima in connection with increased UV radiation and increased ozone, e.g., Chandra and McPeters (1994). They must be explained by vertical motions, i.e. adiabatic warming over the tropics and subtropics due to increased downwelling, which in turn means weaker upwelling, i.e. a weakening of the BDC. At the same time a weaker BDC leads to a colder polar stratosphere over the winter hemisphere. A realisation of this signal is given in Fig. 2 for the zonal mean 30-hPa temperatures at 25N and S in the form of scatter diagrams. The di3erence between the two phases of the QBO is very striking, as is the similarity of the solar signal between the two latitude regions which are 5500 km apart, and at one it is summer while it is winter at the other. Note, that the warming of the stratosphere after the volcanic eruptions of El Chichon (April 1982) and Pinatubo (June 1991) does not in:uence the correlations (Labitzke and van Loon, 1996). The third maximum of the correlations for the east phase in Fig. 1a directly over the equator at 70 hPa is likely connected with the QBO itself. It was shown in Labitzke (2003) that the 30-hPa height di3erences between solar maxima and minima in July are largest directly over the equator in the east phase of the QBO, implying during solar maxima an anomalous west wind, i.e. weaker east winds during solar maxima, (see discussion for Fig. 5). Similarly, in the west phase most of the time the height di3erences have a minimum during solar maxima, implying anomalous east winds, i.e. weaker west winds in solar maxima (cf. Fig. 5). The respective data of the QBO (winds over the equator) are given in Fig. 3. Again, the size of the solar signal in the east phase is surprisingly large: The di3erence of −14 m/s between solar maxima and minima is more than half of the total range of the data. The east phase of the QBO is generally connected with rising motion over the equator which results in low temperatures due to adiabatic cooling. The solar signal indicates in the east phase warming during solar maxima in the lower stratosphere over the equator (Fig. 1b) and this implies anomalous downwelling—against the BDC in the stratosphere and against the HC in the tropical troposphere. In the west phase of the QBO the correlation and the size of the solar signal in the QBO data are weaker, Fig. 3, and accordingly also the temperature di3erences,
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Fig. 2. Scatter diagrams of the zonal mean detrended 30-hPa temperatures (C) in July at 25N and 25S against the 10:7 cm solar :ux. Left: years in the EAST PHASE of the QBO (n = 16); right: years in the WEST PHASE (n = 19). The numbers indicate the respective years; r= correlation coeMcient; NT = temperature di3erences between solar maxima and minima (NCEP/NCAR re-analyses, 1968–2002).
Fig. 3. Scatter diagrams of the zonal wind (m/s) over the equator at (40 + 50 hPa)/2 in July (absolute values) against the 10:7 cm solar :ux. Left: years in the EAST PHASE of the QBO (n = 16); right: years in the WEST PHASE (n = 19). The correlation coeMcients (r) and the di3erences in the speed between maxima and minima are also given. The numbers indicate the respective years. Period: 1968–2002 (Data: QBO data set, FU Berlin).
Fig. 1b. But the connection to the meridional circulation system is consistent: A weaker QBO west phase during solar maxima (Fig. 3) is connected with weaker downwelling over the equator, i.e., an intensi5cation of the rising branch of the HC which results in adiabatic cooling above the tropopause, see also discussions in Labitzke and van Loon (1995) and van Loon and Meehl (2003). Here, at about 70 hPa, the temperature di3erence between solar maxima and minima is only about 0:2 K, in contrast to the east phase when it is above 2:5 K, Fig. 1.
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K. Labitzke / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1151 – 1157
4. March of the solar signal through the year 4.1. 30-hPa temperatures As discussed above for July (Fig. 1), the correlations with the SSC and the connected temperature di3erences between solar maxima and minima are especially large at the 30-hPa level during the east phase of the QBO, maximizing over the subtropics, at about 25N and S. Therefore, this pressure level is chosen here for the discussion of the solar signal throughout the year. Fig. 4a shows again for the years in the east phase of the QBO (left-hand panels) that the correlations of the 30-hPa temperatures are largest during the northern summer, July/August, not only over the northern, but also over the southern hemisphere (top/left). The two regions of large correlations are connected with two regions of large, positive temperature di3erences (bottom/left) which imply anomalous downwelling, i.e. a weakened BDC (see discussions above). Over the northern hemisphere, the positive correlations extend towards the Arctic during the northern summer and autumn and remain con5ned to the subtropics (25–30S) over the southern (winter and spring) hemisphere, and over the southern middle latitudes weak negative correlations are observed. Over the Arctic, negative correlations are observed from autumn (October/November) through late winter (January/February), see e.g., Labitzke (1987) and Labitzke and van Loon (1988, 2000). The maxima of the positive correlations (NH and SH) move towards the tropics during the northern winter/southern summer. During the southern spring season positive correlations extend from the tropics to the Antarctic. For the years in the west phase of the QBO, Fig. 4b (right-hand panels), the structure of the solar signal is very di3erent and often opposite to the years in the east phase of the QBO, Fig. 4a. Over the northern hemisphere the maximum of the correlations is reached much earlier (April/May)(top/right). It is much weaker than during the east phase years and much more con5ned to the subtropics. During this time of the year a weak secondary maximum is observed over the southern subtropics. During the northern winter, positive correlations and a very large positive temperature response is observed over the Arctic (bottom/right). This re:ects the often described fact that during west phase winters a warmer (more disturbed) stratospheric polar vortex is observed in the maxima of the SSC (see, e.g., van Loon and Labitzke, 2000). This phenomenon maximizes during late winter. The stratospheric arctic warmings are connected with intense downwelling (adiabatic warming) over the Arctic and at the same time with upwelling (and connected adiabatic cooling) over the subtropics and tropics of both hemispheres (Labitzke, 2002; Salby and Callaghan, 2003). Therefore, compared to the east phase, the solar signal of the southern summer
is disturbed here during west phase winters in January/ February. Similar to the arctic polar region, there appears to exist a signal of a warmer and weaker polar vortex over the Antarctic during solar maxima in the west phase of the QBO, during the southern spring (September till November) which is the time of the Final Warmings over the Antarctic. And concurrently the correlations and temperature di3erences are negative from 30S towards the Arctic (September through December). This is the same hemispheric interaction as described before for the northern winters. But as the correlations are weak over the Antarctic, one has to be careful with an interpretation, although the correlations above 0.6 in October/November over 60S hint in the right direction. A more detailed analysis of the antarctic spring is under preparation (Labitzke, 2004). 4.2. 30-hPa heights For an easier comparison of the solar signal, i.e. di3erences between solar maxima and minima in the west and east phase of the QBO, respectively, Fig. 5 gives the zonal mean 30-hPa height di3erences separately for two-month means. The height of a pressure level is the result of the integrated temperatures below the respective pressure level, and in this presentation the integral of the temperature differences below 30-hPa. Again, during the east phase of the QBO, the size of the solar signal is impressively large over a wide range of latitudes throughout the year. And the di3erences are positive except for high latitudes during winter when they turn to negative values, while the west phase signal becomes strongly positive. During the east phase, the maximum of the height di3erences is situated directly over the equator most of the time, with lower values poleward, and therefore the anomalous winds connected with this structure are from the west. This implies for the QBO/east itself: weaker east winds during solar maxima, as discussed above for July with Fig. 3. This structure with the maxima of the height di3erences between solar maxima and minima directly over the equator exists from March through October, that is for 8 months. Only during the northern winter, a very weak anomalous height gradient exists. Therefore, one 5nds an in:uence of the solar cycle on the QBO for the whole year, as discussed by Soukharev and Hood (2001) and Labitzke (2003), (cf. discussion for Fig. 6). During the west phase of the QBO, most of the year the solar signal is much weaker than during the east phase. During the northern spring and summer (April through August) a clear signal exists from the northern subtropics to the Arctic, with a secondary maximum over the southern hemisphere. The positive height di3erences over the Arctic in late winter are again connected with the warmer arctic stratosphere in late winter during the west phase of the QBO in solar maxima. At the same time, the solar signal during the southern
K. Labitzke / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1151 – 1157
1155
(a)
(b) Fig. 4. (a), EAST PHASE (top/left): 2-month running (DJ = December/January, etc.) correlations between the 10:7 cm solar :ux and the detrended zonal mean 30-hPa temperatures (n = 16); (bottom/left): temperature di3erences (K) between solar maxima and minima; (b), same as Fig. 4(a), but for the WEST PHASE, right-hand side panels (n = 19) (NCEP/NCAR re-analyses, 1968–2002).
Fig. 5. Meridional pro5les (60N–60S) of the detrended zonal mean 30-hPa height di3erences (gpm) between solar maxima and minima (2-month means); dashed: for the EAST phase, solid for the WEST Phase (NCEP/NCAR re-analyses, 1968–2002).
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K. Labitzke / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1151 – 1157
of the QBO in the tropics with almost no signal directly over the equator in the west phase and a strong signal (almost 1 K) in the east phase (Fig. 6, top). 5. Conclusion
Fig. 6. (top), Meridional pro5les (80N–80S) of the Constructed Annual Mean 30-hPa temperature di3erences (K); 6 (bottom), same as 6 (top), but of the 30-hPa height di3erences (gpm); both arranged for the QBO WEST PHASE (solid lines) and QBO EAST PHASE (dashed lines), respectively.
summer is much reduced compared to the results obtained during northern summer. This is due to the dynamical interactions between high and low latitudes, as discussed above. Over the equatorial region the height di3erences show a structure completely opposite to the other phase of the QBO. A minimum of the height di3erences is found over the equator most of the year, implying anomalous east winds, that is a weaker QBO-west wind in solar maximum. Fig. 6 summarizes the di3erences discussed above in Fig. 5. As it is practically impossible to derive an annual mean for the QBO data, the data shown in Fig. 6 display a constructed annual mean of the di3erences between solar maxima and minima, where the data given in Fig. 5 are linearly averaged, and similarly so for the 30-hPa temperatures. The main features are clearly identi5ed: In the middle stratosphere the solar signal (di3erences between solar maxima and minima) is much stronger during the east phase of the QBO, from 60N till 50S. This is the fact for the 30-hPa temperatures and heights. The height di3erences (Fig. 6, bottom) have a clear maximum over the equator during the east phase, and a clear minimum during the west phase. These di3erences are connected with the described weaker QBO winds (in both phases) during solar maxima. The summarized solar signal in the 30-hPa temperatures shows nicely two maxima in the subtropics in both phases of the QBO, but large di3erences between the two phases
It is generally accepted that there exists a strong signal of the 11-year solar cycle in the atmosphere during the late northern winters—but only if the data are strati5ed according to the phase of the QBO. If one uses the unstrati5ed data, no clear signal emerges. The strati5cation of the data leads to a reduction of the number of years in each group. Therefore we avoided this before. But the results shown here indicate clearly that we cannot identify the size of the solar signal if we are using the undivided data, and therefore we cannot understand the mechanisms involved. The results given here support earlier work suggesting that the mean meridional circulation systems, particularly the BDC, are a3ected. The strong warming of the lower stratosphere over the tropics and subtropics during the maxima of the solar cycle in the east phase can only be explained with anomalous downwelling (adiabatic warming) which works against and weakens the BDC. This appears to be the case during the whole year, for the years in the east phase of the QBO. During the west phase the solar signal results in winter in warmer/weaker polar vortices over both polar regions during maxima of the SSC. Thus the sign of the anomalies is reversed during this time of year over high latitudes. Over the tropics and subtropics, the solar signal in the west phase is mostly weaker than in the east phase, but there are indications of a strengthening of the HC. The anomalies (height di3erences) between solar maxima and minima given in Figs. 5 and 6, show clearly for most months and in the constructed annual mean a maximum over the equator during the east phase of the QBO and a minimum during the west phase. This re:ects a weaker QBO during solar maxima in both phases. Acknowledgements I thank the members of the Stratospheric Research Group, FUB for professional support and Dipl. Met. Markus Kunze for doing the computations and graphics. The project was funded by the BMBF (Bundesministerium fPur Bildung und Forschung) within KIHZ, and by the EU in the project SOLICE (EVK2-CT-1999-00001). The 10:7 cm solar :ux data are from the World Data Center A, Boulder, Colorado. References Chandra, S., McPeters, R.D., 1994. The solar cycle variation of ozone in the stratosphere inferred from Nimbus 7 and NOAA 11 satellites. Journal of Geophysical Research 99, 20665–20671.
K. Labitzke / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1151 – 1157 Kalnay, E., Kanamitsu, R., Kistler, R., Collins, W., Deaven, D., Gandin, L., Iredell, M., Saha, S., White, G., Zhu, Y., Chelliah, M., Ebisuzaki, W., Higgins, W., Janowiak, J., Mo, K.C., Ropelewski, C., Wang, J., Reynolds, R., Jenne, R., Joseph, J., 1996. The NCEP/NCAR 40-year reanalysis project. Bulletin of the American Meteorological Society 77, 437–471. Kodera, K., Kuroda, Y., 2002. Dynamical response to the solar cycle. Journal of Geophysical Research 107 (D24), 4749. doi:10.1029/2002JD002224. Labitzke, K., 1987. Sunspots, the QBO, and the stratospheric temperature in the north polar region. Geophysical Research Letters 14, 535–537. Labitzke, K., 2002. The solar signal of the 11-year sunspot cycle in the stratosphere: di3erences between the northern and southern summers. Journal of the Meteorological Society of Japan 80, 963–971. Labitzke, K., 2003. The global signal of the 11-year solar cycle in the atmosphere: when do we need the QBO? Meteorologische Zeitschrift 12, 209–216. Labitzke, K., 2004. On the signal of the 11-year sunspot cycle in the stratosphere over the Antarctic and its modulation by the Quasi-Biennial Oscillation (QBO). Journal of Atmospheric and Solar Terrestrial Physics, in press. Labitzke, K., van Loon, H., 1988. Associations between the 11-year solar cycle, the QBO and the atmosphere. Part I: the troposphere and stratosphere in the northern hemisphere winter. Journal of Atmospheric and Terrestrial Physics 50, 197–206. Labitzke, K., van Loon, H., 1995. Connection between the troposphere and the stratosphere on a decadal scale. Tellus 47 (A), 275–286.
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Labitzke, K., van Loon, H., 1996. The e3ect on the stratosphere of three tropical volcanic eruptions. NATO ASI Series I, Vol. 42. Springer, Berlin, Heidelberg, New York, pp. 113–125. Labitzke, K., van Loon, H., 1999. The Stratosphere (Phenomena, History, and Relevance). Springer, Berlin, Heidelberg, New York, 179pp. Labitzke, K., van Loon, H., 2000. The QBO e3ect on the global stratosphere in northern winter. Journal of Atmospheric and Solar Terrestrial Physics 62, 621–628. Salby, M., Callaghan, P., 2000. Connection between the solar cycle and the QBO: the missing link. Journal of Climatology 13, 2652–2662. Salby, M., Callaghan, P., 2004. Evidence of the solar cycle in the general circulation of the stratosphere. Journal of Climatology 17, 34–46. Soukharev, B., Hood, L.L., 2001. Possible solar modulation of the equatorial quasi-biennial oscillation: additional statistical evidence. Journal of Geophysical Research 106, 14855–14868. van Loon, H., Labitzke, K., 1994. The 10–12 year atmospheric oscillation. Meteorologische Zeitschrift N.F. 3, 259–266. van Loon, H., Labitzke, K., 2000. The in:uence of the 11-year solar cycle on the stratosphere below 30 km: a review. Space Science Review 94, 259–278. van Loon, H., Meehl, G.A., 2003. A likely solar-cycle e3ect in the tropics in northern summer. Geophysical Research Letters, submitted.
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On the signal of the 11-year sunspot cycle in the stratosphere and its modulation by the quasi-biennial oscillation Karin Labitzke∗ Meteorologisches Institut, Freie Universitat Berlin, Carl-Heinrich Becker Weg 6-10 12165 Berlin, Germany
Abstract In earlier studies we examined the global structure and the size of the signal of the 11-year sunspot cycle (SSC) in the stratosphere and troposphere. For the correlations between the solar cycle and heights and temperatures at di3erent pressure levels we used mainly the whole data set, and only during northern winters, the years were strati5ed according to the phase of the quasi-biennial oscillation (QBO). This work is expanded here and it is shown that the QBO must be introduced throughout the year, because the solar signal is very di3erent in the respective phases of the QBO, particularly over the tropics and subtropics. The structure of the solar signal appears to indicate that the mean meridional circulation systems (Hadley and Brewer–Dobson Circulation) are in:uenced by the SSC, and stronger so during the east phase of the QBO. c 2004 Elsevier Ltd. All rights reserved. Keywords: 11-year solar cycle; Stratosphere; Quasi-biennial oscillation; Mean meridional circulation systems
1. Introduction We have shown in several publications (Labitzke, 1987; Labitzke and van Loon, 1988; van Loon and Labitzke, 1994, 2000) that there exists a strong signal of the 11-year solar cycle in the atmosphere during the late northern winters— but only if the data are strati5ed according to the phase of the QBO. If one uses the unstrati5ed data, no clear signal emerges. In winter, the arctic stratosphere is a region with large, dynamically induced temperature variations, and very cold, stable periods as well as warm, disturbed periods are observed. Because of this high variability, the identi5cation and explanation of the solar signal is very complex (Kodera and Kuroda, 2002). Therefore, this study concentrates at 5rst on the northern summer which is a dynamically less disturbed
∗
Tel.: +0049-30-8387-1166; fax: +0049-30-8387-1167. E-mail address: [email protected] (K. Labitzke).
season. The emphasis of this study is placed on the modulation of the solar cycle by the QBO.
2. Data and methods The NCEP/NCAR re-analyses (Kalnay et al., 1996) are used for the period 1968–2002. The re-analyses were less reliable earlier, i.e. before 1968, mainly because of the lack of radiosonde stations over the southern hemisphere, the lack of high reaching balloons in the early years and the scarce satellite information before 1979. But it should be mentioned that the inclusion of the early data yields similar but somewhat weaker results. As a measure of the SSC, the monthly mean values of the 10:7 cm solar :ux are used. The :ux values are expressed in solar :ux units: 1 s.f.u. =10−22 W m−2 Hz−1 . This is an objectively measured radiowave, highly and positively correlated with the SSC and with the UV part of the solar spectrum, which varies about 6–8% between solar maxima and minima (Chandra and McPeters, 1994).
c 2004 Elsevier Ltd. All rights reserved. 1364-6826/$ - see front matter doi:10.1016/j.jastp.2004.05.011
1152
K. Labitzke / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1151 – 1157
(a)
(b)
Fig. 1. (a), (left-hand side): Vertical meridional sections of the correlations between the 10:7 cm solar :ux and the detrended zonal mean temperatures in July; shaded for emphasis where correlations are above 0.5; (b), (right-hand side): The respective temperature di3erences (K) between solar maxima and minima, shaded where the correlations are above 0.5. Upper panels: all years; middle panels: only years in the EAST PHASE of the QBO; lower panels: only years in the WEST PHASE of the QBO (NCEP/NCAR re-analyses, 1968–2002)(Fig. 4 in Labitzke (2003)).
For the range of the SSC the mean di3erence of the 10:7 cm solar :ux between solar minima (about 70 units) and solar maxima (about 200 units) is used, i.e. 130 units. Any linear correlation can be represented also by a regression line with y = a + bx, where x in this case is the 10:7 cm solar :ux and b is the slope. This slope is used here, multiplied by 130, in order to get the di3erences between solar minima and maxima, as presented in this paper. The statistical signi5cance of the correlations cannot be determined with certainty because we have less than four solar cycles and the degrees of freedom are therefore limited. The QBO is an oscillation in the atmosphere which is best observed in the stratospheric winds above the equator, where the zonal winds are changing between east and west. The period of the QBO varies in space and time, but it lies on an average near 28 months at all levels (Labitzke and van Loon, 1999).
The QBO modulates the solar signal and it is therefore necessary to stratify the data into years when the equatorial QBO in the lower stratosphere (about 45 hPa) was in its west phase and years when it was in its east phase. 3. The solar signal during the northern summer We have shown that the correlations between stratospheric temperatures and heights are large during the northern summer, but that the signal is much stronger during the east phase of the QBO (Labitzke, 2003). Fig. 1a (left-hand side) shows for July, vertical meridional sections of the correlations between the zonal mean temperatures and the 11-year solar cycle, while Fig. 1b (right-hand side) gives the temperature di3erences between solar maxima and minima. The three panels show the results for all data (upper panel), for the years in the east phase of the QBO (middle
K. Labitzke / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1151 – 1157
panel) and for the years in the west phase of the QBO (lower panel). The di3erences of the size of the solar signal in the two di3erent phases of the QBO is striking and the result for all data is misleading. The structure of the signal hints to a connection to the mean meridional circulation systems, namely the Hadley Circulation (HC) in the troposphere and the Brewer–Dobson Circulation (BDC) in the stratosphere, as also reported by Soukharev and Hood (2001) and by Salby and Callaghan (2000, 2004). The two maxima of response in Fig. 1b for the east phase, at 30 hPa/25N and S, are probably an indirect e3ect of the direct warming observed in the upper stratosphere during solar maxima in connection with increased UV radiation and increased ozone, e.g., Chandra and McPeters (1994). They must be explained by vertical motions, i.e. adiabatic warming over the tropics and subtropics due to increased downwelling, which in turn means weaker upwelling, i.e. a weakening of the BDC. At the same time a weaker BDC leads to a colder polar stratosphere over the winter hemisphere. A realisation of this signal is given in Fig. 2 for the zonal mean 30-hPa temperatures at 25N and S in the form of scatter diagrams. The di3erence between the two phases of the QBO is very striking, as is the similarity of the solar signal between the two latitude regions which are 5500 km apart, and at one it is summer while it is winter at the other. Note, that the warming of the stratosphere after the volcanic eruptions of El Chichon (April 1982) and Pinatubo (June 1991) does not in:uence the correlations (Labitzke and van Loon, 1996). The third maximum of the correlations for the east phase in Fig. 1a directly over the equator at 70 hPa is likely connected with the QBO itself. It was shown in Labitzke (2003) that the 30-hPa height di3erences between solar maxima and minima in July are largest directly over the equator in the east phase of the QBO, implying during solar maxima an anomalous west wind, i.e. weaker east winds during solar maxima, (see discussion for Fig. 5). Similarly, in the west phase most of the time the height di3erences have a minimum during solar maxima, implying anomalous east winds, i.e. weaker west winds in solar maxima (cf. Fig. 5). The respective data of the QBO (winds over the equator) are given in Fig. 3. Again, the size of the solar signal in the east phase is surprisingly large: The di3erence of −14 m/s between solar maxima and minima is more than half of the total range of the data. The east phase of the QBO is generally connected with rising motion over the equator which results in low temperatures due to adiabatic cooling. The solar signal indicates in the east phase warming during solar maxima in the lower stratosphere over the equator (Fig. 1b) and this implies anomalous downwelling—against the BDC in the stratosphere and against the HC in the tropical troposphere. In the west phase of the QBO the correlation and the size of the solar signal in the QBO data are weaker, Fig. 3, and accordingly also the temperature di3erences,
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Fig. 2. Scatter diagrams of the zonal mean detrended 30-hPa temperatures (C) in July at 25N and 25S against the 10:7 cm solar :ux. Left: years in the EAST PHASE of the QBO (n = 16); right: years in the WEST PHASE (n = 19). The numbers indicate the respective years; r= correlation coeMcient; NT = temperature di3erences between solar maxima and minima (NCEP/NCAR re-analyses, 1968–2002).
Fig. 3. Scatter diagrams of the zonal wind (m/s) over the equator at (40 + 50 hPa)/2 in July (absolute values) against the 10:7 cm solar :ux. Left: years in the EAST PHASE of the QBO (n = 16); right: years in the WEST PHASE (n = 19). The correlation coeMcients (r) and the di3erences in the speed between maxima and minima are also given. The numbers indicate the respective years. Period: 1968–2002 (Data: QBO data set, FU Berlin).
Fig. 1b. But the connection to the meridional circulation system is consistent: A weaker QBO west phase during solar maxima (Fig. 3) is connected with weaker downwelling over the equator, i.e., an intensi5cation of the rising branch of the HC which results in adiabatic cooling above the tropopause, see also discussions in Labitzke and van Loon (1995) and van Loon and Meehl (2003). Here, at about 70 hPa, the temperature di3erence between solar maxima and minima is only about 0:2 K, in contrast to the east phase when it is above 2:5 K, Fig. 1.
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4. March of the solar signal through the year 4.1. 30-hPa temperatures As discussed above for July (Fig. 1), the correlations with the SSC and the connected temperature di3erences between solar maxima and minima are especially large at the 30-hPa level during the east phase of the QBO, maximizing over the subtropics, at about 25N and S. Therefore, this pressure level is chosen here for the discussion of the solar signal throughout the year. Fig. 4a shows again for the years in the east phase of the QBO (left-hand panels) that the correlations of the 30-hPa temperatures are largest during the northern summer, July/August, not only over the northern, but also over the southern hemisphere (top/left). The two regions of large correlations are connected with two regions of large, positive temperature di3erences (bottom/left) which imply anomalous downwelling, i.e. a weakened BDC (see discussions above). Over the northern hemisphere, the positive correlations extend towards the Arctic during the northern summer and autumn and remain con5ned to the subtropics (25–30S) over the southern (winter and spring) hemisphere, and over the southern middle latitudes weak negative correlations are observed. Over the Arctic, negative correlations are observed from autumn (October/November) through late winter (January/February), see e.g., Labitzke (1987) and Labitzke and van Loon (1988, 2000). The maxima of the positive correlations (NH and SH) move towards the tropics during the northern winter/southern summer. During the southern spring season positive correlations extend from the tropics to the Antarctic. For the years in the west phase of the QBO, Fig. 4b (right-hand panels), the structure of the solar signal is very di3erent and often opposite to the years in the east phase of the QBO, Fig. 4a. Over the northern hemisphere the maximum of the correlations is reached much earlier (April/May)(top/right). It is much weaker than during the east phase years and much more con5ned to the subtropics. During this time of the year a weak secondary maximum is observed over the southern subtropics. During the northern winter, positive correlations and a very large positive temperature response is observed over the Arctic (bottom/right). This re:ects the often described fact that during west phase winters a warmer (more disturbed) stratospheric polar vortex is observed in the maxima of the SSC (see, e.g., van Loon and Labitzke, 2000). This phenomenon maximizes during late winter. The stratospheric arctic warmings are connected with intense downwelling (adiabatic warming) over the Arctic and at the same time with upwelling (and connected adiabatic cooling) over the subtropics and tropics of both hemispheres (Labitzke, 2002; Salby and Callaghan, 2003). Therefore, compared to the east phase, the solar signal of the southern summer
is disturbed here during west phase winters in January/ February. Similar to the arctic polar region, there appears to exist a signal of a warmer and weaker polar vortex over the Antarctic during solar maxima in the west phase of the QBO, during the southern spring (September till November) which is the time of the Final Warmings over the Antarctic. And concurrently the correlations and temperature di3erences are negative from 30S towards the Arctic (September through December). This is the same hemispheric interaction as described before for the northern winters. But as the correlations are weak over the Antarctic, one has to be careful with an interpretation, although the correlations above 0.6 in October/November over 60S hint in the right direction. A more detailed analysis of the antarctic spring is under preparation (Labitzke, 2004). 4.2. 30-hPa heights For an easier comparison of the solar signal, i.e. di3erences between solar maxima and minima in the west and east phase of the QBO, respectively, Fig. 5 gives the zonal mean 30-hPa height di3erences separately for two-month means. The height of a pressure level is the result of the integrated temperatures below the respective pressure level, and in this presentation the integral of the temperature differences below 30-hPa. Again, during the east phase of the QBO, the size of the solar signal is impressively large over a wide range of latitudes throughout the year. And the di3erences are positive except for high latitudes during winter when they turn to negative values, while the west phase signal becomes strongly positive. During the east phase, the maximum of the height di3erences is situated directly over the equator most of the time, with lower values poleward, and therefore the anomalous winds connected with this structure are from the west. This implies for the QBO/east itself: weaker east winds during solar maxima, as discussed above for July with Fig. 3. This structure with the maxima of the height di3erences between solar maxima and minima directly over the equator exists from March through October, that is for 8 months. Only during the northern winter, a very weak anomalous height gradient exists. Therefore, one 5nds an in:uence of the solar cycle on the QBO for the whole year, as discussed by Soukharev and Hood (2001) and Labitzke (2003), (cf. discussion for Fig. 6). During the west phase of the QBO, most of the year the solar signal is much weaker than during the east phase. During the northern spring and summer (April through August) a clear signal exists from the northern subtropics to the Arctic, with a secondary maximum over the southern hemisphere. The positive height di3erences over the Arctic in late winter are again connected with the warmer arctic stratosphere in late winter during the west phase of the QBO in solar maxima. At the same time, the solar signal during the southern
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(a)
(b) Fig. 4. (a), EAST PHASE (top/left): 2-month running (DJ = December/January, etc.) correlations between the 10:7 cm solar :ux and the detrended zonal mean 30-hPa temperatures (n = 16); (bottom/left): temperature di3erences (K) between solar maxima and minima; (b), same as Fig. 4(a), but for the WEST PHASE, right-hand side panels (n = 19) (NCEP/NCAR re-analyses, 1968–2002).
Fig. 5. Meridional pro5les (60N–60S) of the detrended zonal mean 30-hPa height di3erences (gpm) between solar maxima and minima (2-month means); dashed: for the EAST phase, solid for the WEST Phase (NCEP/NCAR re-analyses, 1968–2002).
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of the QBO in the tropics with almost no signal directly over the equator in the west phase and a strong signal (almost 1 K) in the east phase (Fig. 6, top). 5. Conclusion
Fig. 6. (top), Meridional pro5les (80N–80S) of the Constructed Annual Mean 30-hPa temperature di3erences (K); 6 (bottom), same as 6 (top), but of the 30-hPa height di3erences (gpm); both arranged for the QBO WEST PHASE (solid lines) and QBO EAST PHASE (dashed lines), respectively.
summer is much reduced compared to the results obtained during northern summer. This is due to the dynamical interactions between high and low latitudes, as discussed above. Over the equatorial region the height di3erences show a structure completely opposite to the other phase of the QBO. A minimum of the height di3erences is found over the equator most of the year, implying anomalous east winds, that is a weaker QBO-west wind in solar maximum. Fig. 6 summarizes the di3erences discussed above in Fig. 5. As it is practically impossible to derive an annual mean for the QBO data, the data shown in Fig. 6 display a constructed annual mean of the di3erences between solar maxima and minima, where the data given in Fig. 5 are linearly averaged, and similarly so for the 30-hPa temperatures. The main features are clearly identi5ed: In the middle stratosphere the solar signal (di3erences between solar maxima and minima) is much stronger during the east phase of the QBO, from 60N till 50S. This is the fact for the 30-hPa temperatures and heights. The height di3erences (Fig. 6, bottom) have a clear maximum over the equator during the east phase, and a clear minimum during the west phase. These di3erences are connected with the described weaker QBO winds (in both phases) during solar maxima. The summarized solar signal in the 30-hPa temperatures shows nicely two maxima in the subtropics in both phases of the QBO, but large di3erences between the two phases
It is generally accepted that there exists a strong signal of the 11-year solar cycle in the atmosphere during the late northern winters—but only if the data are strati5ed according to the phase of the QBO. If one uses the unstrati5ed data, no clear signal emerges. The strati5cation of the data leads to a reduction of the number of years in each group. Therefore we avoided this before. But the results shown here indicate clearly that we cannot identify the size of the solar signal if we are using the undivided data, and therefore we cannot understand the mechanisms involved. The results given here support earlier work suggesting that the mean meridional circulation systems, particularly the BDC, are a3ected. The strong warming of the lower stratosphere over the tropics and subtropics during the maxima of the solar cycle in the east phase can only be explained with anomalous downwelling (adiabatic warming) which works against and weakens the BDC. This appears to be the case during the whole year, for the years in the east phase of the QBO. During the west phase the solar signal results in winter in warmer/weaker polar vortices over both polar regions during maxima of the SSC. Thus the sign of the anomalies is reversed during this time of year over high latitudes. Over the tropics and subtropics, the solar signal in the west phase is mostly weaker than in the east phase, but there are indications of a strengthening of the HC. The anomalies (height di3erences) between solar maxima and minima given in Figs. 5 and 6, show clearly for most months and in the constructed annual mean a maximum over the equator during the east phase of the QBO and a minimum during the west phase. This re:ects a weaker QBO during solar maxima in both phases. Acknowledgements I thank the members of the Stratospheric Research Group, FUB for professional support and Dipl. Met. Markus Kunze for doing the computations and graphics. The project was funded by the BMBF (Bundesministerium fPur Bildung und Forschung) within KIHZ, and by the EU in the project SOLICE (EVK2-CT-1999-00001). The 10:7 cm solar :ux data are from the World Data Center A, Boulder, Colorado. References Chandra, S., McPeters, R.D., 1994. The solar cycle variation of ozone in the stratosphere inferred from Nimbus 7 and NOAA 11 satellites. Journal of Geophysical Research 99, 20665–20671.
K. Labitzke / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1151 – 1157 Kalnay, E., Kanamitsu, R., Kistler, R., Collins, W., Deaven, D., Gandin, L., Iredell, M., Saha, S., White, G., Zhu, Y., Chelliah, M., Ebisuzaki, W., Higgins, W., Janowiak, J., Mo, K.C., Ropelewski, C., Wang, J., Reynolds, R., Jenne, R., Joseph, J., 1996. The NCEP/NCAR 40-year reanalysis project. Bulletin of the American Meteorological Society 77, 437–471. Kodera, K., Kuroda, Y., 2002. Dynamical response to the solar cycle. Journal of Geophysical Research 107 (D24), 4749. doi:10.1029/2002JD002224. Labitzke, K., 1987. Sunspots, the QBO, and the stratospheric temperature in the north polar region. Geophysical Research Letters 14, 535–537. Labitzke, K., 2002. The solar signal of the 11-year sunspot cycle in the stratosphere: di3erences between the northern and southern summers. Journal of the Meteorological Society of Japan 80, 963–971. Labitzke, K., 2003. The global signal of the 11-year solar cycle in the atmosphere: when do we need the QBO? Meteorologische Zeitschrift 12, 209–216. Labitzke, K., 2004. On the signal of the 11-year sunspot cycle in the stratosphere over the Antarctic and its modulation by the Quasi-Biennial Oscillation (QBO). Journal of Atmospheric and Solar Terrestrial Physics, in press. Labitzke, K., van Loon, H., 1988. Associations between the 11-year solar cycle, the QBO and the atmosphere. Part I: the troposphere and stratosphere in the northern hemisphere winter. Journal of Atmospheric and Terrestrial Physics 50, 197–206. Labitzke, K., van Loon, H., 1995. Connection between the troposphere and the stratosphere on a decadal scale. Tellus 47 (A), 275–286.
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Labitzke, K., van Loon, H., 1996. The e3ect on the stratosphere of three tropical volcanic eruptions. NATO ASI Series I, Vol. 42. Springer, Berlin, Heidelberg, New York, pp. 113–125. Labitzke, K., van Loon, H., 1999. The Stratosphere (Phenomena, History, and Relevance). Springer, Berlin, Heidelberg, New York, 179pp. Labitzke, K., van Loon, H., 2000. The QBO e3ect on the global stratosphere in northern winter. Journal of Atmospheric and Solar Terrestrial Physics 62, 621–628. Salby, M., Callaghan, P., 2000. Connection between the solar cycle and the QBO: the missing link. Journal of Climatology 13, 2652–2662. Salby, M., Callaghan, P., 2004. Evidence of the solar cycle in the general circulation of the stratosphere. Journal of Climatology 17, 34–46. Soukharev, B., Hood, L.L., 2001. Possible solar modulation of the equatorial quasi-biennial oscillation: additional statistical evidence. Journal of Geophysical Research 106, 14855–14868. van Loon, H., Labitzke, K., 1994. The 10–12 year atmospheric oscillation. Meteorologische Zeitschrift N.F. 3, 259–266. van Loon, H., Labitzke, K., 2000. The in:uence of the 11-year solar cycle on the stratosphere below 30 km: a review. Space Science Review 94, 259–278. van Loon, H., Meehl, G.A., 2003. A likely solar-cycle e3ect in the tropics in northern summer. Geophysical Research Letters, submitted.