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ABSTRACT: The main aim of the present study is to investigate further into the association between total ozone (TOZ) and 11-year solar cycle (SC) during the period 1979 - 2010 by employing satellite observations of TOZ made by Nimbus-7, Meteor-3, and Earth Probe Total Ozone Mapping Spectrometer (TOMS) and Ozone Monitoring Instrument (OMI) instrumentation. A statistically significant correlation between the annual mean TOZ over both hemispheres and sunspot number (SN) is found. On the contrary, focusing on the January and February mean monthly TOZ fluctuations from the equator to the high latitudes, of the Northern Hemisphere no association between TOZ and SN is derived. It is attributed to the existence of the quasi-biennial-oscillation (QBO) and the El Niño-Southern oscillation (ENSO) in TOZ time series. The latter oscillation is herewith expressed by the recently introduced Ozone ENSO Index (OEI). However, when considering the TOZ zonal means centred at 17.5°N and 27.5°N during the east phase years of QBO in the equatorial zonal wind at 50hPa, a statistically significant correlation between TOZ and SN reveals. It is an indication that the quasi-periodic fluctuations (i.e. QBO, ENSO) strongly contaminate the relationship between TOZ and solar activity. Plausible mechanisms are discussed, exploring the momentum flux (MF) measurements between 45°N and 75°N, in the time periods of the increased dynamical variability. The findings obtained point to the conclusion that the 11-yr solar cycle response in TOZ is caused by dynamical changes which are caused by solar activity. These are of crucial importance because solar radiation is a major driving force of the climate system.
Key words total ozone; zonal wind; solar activity
Several studies have shown that variations in the 11-year solar irradiance and subsequent ultraviolet absorption by ozone cause changes in temperature and wind in the upper stratosphere (Crooks and Gray, 2005; Alexandris et al., 1999; Kondratyev and Varotsos, 1996; Katsambas et al., 1997). These relatively weak direct changes could alter the atmospheric dynamics giving rise to indirect feedbacks on the lower atmosphere. The latter can be achieved via the upward propagation of planetary-scale waves through a change of the stratospheric mean circulation (i.e. Brewer- Dobson circulation) (Gernandt et al., 1995; Kodera and Kuroda, 2002; Tzanis and Varotsos, 2008; Cracknell and Varotsos, 1994, 1995; Efstathiou et al., 2003; Varotsos, 2002; 2005; Varotsos et al., 1994; Varotsos, 1989; 2004)
Matthes et al. (2010) indicated that the solar response in the upper stratosphere does not depend on the quasi-biennial oscillation (QBO) of equatorial wind. However, it is not the case in the middle to lower stratosphere where the solar response depends on the QBO phases. More specifically, during QBO east, the combination of production and advection resulted in the net ozone increase.
Lu et al. (2009) proposed some insights on the modulation of the 11-year solar cycle signals by the QBO especially in the Northern Hemisphere (NH) winter temperature and zonal wind. They have used daily ERA-40 Reanalysis and ECMWF Operational data for the period of 1958-2006 in order to examine the seasonal evolution of the QBO-solar cycle relationship at various pressure levels up to the stratopause. The results showed that the solar signals in the NH winter extratropics are indeed QBO-phase dependent, moving poleward and downward as winter progresses with a faster descent rate under westerly QBO than under easterly QBO.
Sitnov (2009) using TOZ data that were collected at 10 ground-based European stations during the period 1957 - 2007 investigated the effects of the QBO and 11-year solar cycle, manifesting in total ozone (TOZ). One of the results obtained by Sitnov (2009) is that solar activity modulates the phase of the QBO so that the quasi-biennial total ozone signals during solar maximum and solar minimum are nearly in opposite phase.
Titova and Karol (2010) having applied the method of discriminant analysis to the TOMS data of satellite sounding of TOZ in the March months of 1979-2008, attempted to make a new estimate of the TOZ field variability in the NH and inter-longitudinal regularities of its changes under the action of climatic variability. The effects of temperature variations in the polar stratosphere, El Niño -Southern Oscillation (ENSO) and QBO seemed to be comparable and reach 80 DU in some regions. Titova and Karol (2010) also proposed that the regions of TOZ variations and their location and dimensions change depending on the phases of QBO and ENSO. Three regions of increased TOZ-over Europe, Eastern Siberia, and the Pacific Ocean-are formed in years with a warm stratosphere. A compensating TOZ decrease takes place in the tropics and over Greenland. In the years of El Niño and the easterly QBO phase, the TOZ increases over Europe and drops over the central Pacific, as well as to the south from 45° N.
Ziemke et al. (2010) have recently established an ozone ENSO index (OEI) using TOZ data measured in tropical latitudes from Nimbus 7 TOMS, Earth Probe TOMS, NOAA SBUV, and Aura OMI satellite instruments from 1979 to the present. In more detail, OEI was developed by calculating first the western and eastern Pacific TOZ monthly mean averages and taking their difference. The combined Aura OMI and MLS ozone data confirmed that zonal variability in TOZ in the tropics imposed by ENSO events in the troposphere. The latter reveals that OEI can be calculated by considering TOZ instead of tropospheric TOZ.
Soukharev (1997) studying the TOZ monthly means, in months January to March between 1973 - 1995 on five stations in Northeastern Europe, indicated statistically significant correlations between the variations of TOZ in February/March and the SN during the different phases of QBO. Similar correspondence was established between the index of stratospheric circulation and SN considering the QBO phase. Assuming the correlations between the ozone and stratospheric circulation index, Soukharev (1997) concluded that a connection between solar cycle - QBO - ozone occurs through the dynamics of stratospheric circulation.
Varotsos (1989) studying the global TOZ, during the period 1958-1984, suggested that there was not any evident connection between TOZ and 10.7 cm solar flux (F10.7). However, when the data were separated according to the east or west phase of QBO in the equatorial stratosphere, it was derived that TOZ was correlated (anticorrelated) with the solar cycle, during the west (east) phase of QBO.
The main aim of this paper is to explore further the association between TOZ and solar activity, from the equator to the high latitudes in both Hemispheres over the last three solar cycles.
2. Data and analysis
QBO data used in the present paper were calculated at the NOAA Earth System Research Laboratory-Physical Science Division (NOAA/ESRL-PSD) from the zonal average of the 30mb zonal wind at the equator. Those data were computed from the NCEP/NCAR.
Additionally, the mean monthly sunspot numbers (SN) derived from the datasets of the National Geophysical Data Center (NGDC), during the period January 1749 - October 2009, were employed.
TOZ data set, was obtained from Nimbus-7, Meteor-3, and Earth Probe Total Ozone Mapping Spectrometer (TOMS) and Ozone Monitoring Instrument (OMI), covering the period 1979-2010 (with measurement gaps for several months of the years 1994, 1995 and 1996).
Momentum Flux (MF) measurements between 45°N and 75°N, through 1979 - 2010, obtained by the NASA Goddard Space Flight Center, were also used.
Finally, Ozone ENSO index (OEI) measurements obtained by the NASA Goddard Space Flight Center, Code 613.3 (Chemistry and Dynamics Branch), in the tropics during 1979 - 2010, were employed (Ziemke et al., 2010).
All time series presented in this study were normalized (the long-term mean subtracted and then divided by the standard deviation) and detrended.
3. Discussion and Results
Several studies argued that when the solar UV radiation is stronger, more ozone via the photolysis of O2 would be formed in the upper stratosphere, so that the maximum ozone level would occur at the maximum solar activity. Very recently, Haigh et al. (2010) have noticed that during the descending march of the most recent '11-year' solar cycle (2002-2009) a 4-6 times larger drop in the solar ultraviolet radiation (SUVR) occurred comparing with that predicted by the models. Haigh et al. (2010) suggested that this drop in SUVR was almost compensated by an unexpected increase at visible spectrum. More remarkably, they have also showed that these spectral anomalies have deduced a drop in stratospheric ozone below 45 km, and an increase above this layer.
Therefore, it is interesting to re-visit the investigation of the influence of the solar activity to the column ozone variability on a global and hemispheric basis.
3.1. The total ozone and solar cycle on a global and hemispheric basis
Along the lines above the 11-year solar cycle and the TOZ annual mean fluctuations over the globe, the NH and the SH, during the last three solar cycles are shown in Figure l (a), (b), (c), respectively. Inspection of Figure 1 shows that an apparent solar cycle signal is prominent in the TOZ data. To quantify this association, the correlation coefficients were calculated. These were derived statistically significant (at 95% confidence level) by using the non-parametric Spearman method.
This in-phase march of TOZ and solar activity is not surprising because it is quite consistent with the current understanding about the solar forcing in TOZ dynamics. According to this, the upper stratospheric ozone response (2-3% between solar minimum and solar maximum) is a direct radiative effect of heating and photochemistry. The lower stratospheric solar cycle in tropical ozone appears to be caused indirectly through a dynamical response to solar ultraviolet variations. However, the origin of such a dynamical response to the solar cycle is not fully understood (WMO, 2010).
3.2. The total ozone on the wintertime Northern Hemisphere and solar cycle
To get a better understanding of the afore-mentioned dynamical TOZ response, the investigation of the plausible relationship between TOZ and solar activity would be performed at the wintertime regime of the atmosphere. There is no doubt that during winter months, the solar cycle signal is weak compared to large atmospheric variations and the signal is therefore more difficult to extract (Labitzke and van Loon, 1988). In an attempt to further explore this problem, the fluctuations of the mean TOZ over the NH during January/February and the corresponding SN values during the period 1979 - 2010 are plotted in Figure 2(a).
The conclusion deduced from Figure 1(a) is that quasi-periodic components (2- 5 yrs) in the NH TOZ time series reduce remarkably the above mentioned correlation between TOZ and SN fluctuations. To investigate whether this contamination of the association of the TOZ and SN fluctuations by the QBO is a function of the solar activity, the method of running correlations was employed (Kodera, 1993). The results obtained are shown in Figure 2(b) where the running correlations (ri) for year i between the equatorial zonal wind at 50 hPa and the mean TOZ for January and February do not show an 11-y signal (Figure 2(b)). Therefore, the above-said contamination by the QBO of equatorial wind is independent of the solar cycle, disturbing any apparent association between TOZ and SN.
3.3. The latitudinal dependence of the association between the wintertime TOZ and solar cycle at the Northern Hemisphere
Next, the investigation of the possible association between the TOZ and SN is explored as a function of latitude. In this regard, Haigh (1994) have reported that due to the seasonality, the stratospheric ozone changes due to solar flux variation are largest at middle to high latitudes in the winter hemisphere. Figure 3(a-f) present the January / February mean TOZ and SN from the equator to the high latitudes, during 1979 - 2010. All these figures do not show any apparent correlation between TOZ and solar activity, due to the contamination by the quasi-periodic oscillations (QBO and ENSO) in the TOZ time series.
However, the solar response in the winter TOZ at 17.5°N and 27.5°N seemed to differ significantly under the two QBO phases.
Other studies have also identified solar influences on the strength and extent of the Walker circulation which is a cell circulation in the zonal and vertical directions in the tropical troposphere caused by differences in heat distribution between ocean and land. In this context, Meehl et al. (2008) and vanLoon et al. (2007) showed a strengthening of the Walker circulation, at years of the maxima of the 11-year solar cycle. It should be reminded that when the Walker cell weakens or reverses, an El Niño results, and when Walker cell becomes strong causes a La Niña.
3.4. The association between the wintertime TOZ and solar cycle at the Northern tropics; the role of the QBO and ENSO
In the following, the January / February mean TOZ and SN data were grouped according to the QBO phases of the equatorial zonal wind at 50hPa and were plotted against the OEI at 17.5°N and 27.5°N (Figures 4(a-d)).
During the west phase of QBO, a statistically significant anticorrelation between TOZ and OEI time series is apparent, resulting in a quasi periodic component that coincides with ENSO (Ziemke et al., 2010) and causes no correlation between TOZ and SN. On the other hand, during the east phase of QBO, TOZ time series exhibits the 11-year signal.
In the following, Figure 5(a) presents the February mean TOZ and SN at 17.5°N, during 1979-2010, while figures 5(b),(c) show the February TOZ and SN when the data were grouped in the west and east phase of QBO, respectively. Inspection of these figures shows an apparent correlation between TOZ and the 11-year solar cycle, during QBO east (statistically significant correlation at 95% confidence level). The ENSO component is apparent once more in the TOZ time series, when the data were grouped in the west phase of QBO and it is anticorrelated with OEI (Figure 5(b)).
To examine further the contribution of the QBO in the equatorial zonal wind at 50 hPa to the association between the February TOZ at 17.5°N and OEI the Figure 6(a) is shown. Figure 6(a) shows the statistically significant anticorrelation between OEI and TOZ, but no any association of TOZ with QBO. The latter can probably be explained by the fact that TOZ exhibits OEI and it is modulated by the temporal evolution of QBO maxima and minima. To give an insight to it, Figure 6(b) depicts the temporal evolution of the difference between the successive QBO maxima and [(max(i+1) - max(i)] and the temporal evolution of the difference between successive QBO minima [min(i+1) - min(i)] for year (i). The differences in the sequential maxima and the differences in the sequential minima of QBO demonstrate the ENSO signal.
3.5. The association between the wintertime TOZ and solar cycle at the Northern high latitudes; the role of the QBO and ENSO
Finally, in order to explore the role of the atmospheric dynamics to the relationship between the TOZ and solar cycle the interannual variability of the February mean momentum flux (MF) between 45°N and 75°N at 50hPa, during 1979 - 2010 was studied. Figure 7(a) illustrates the time series of MF and SN for February, while Figures 7(b,c) show the MF and SN when the data were grouped according to the QBO phase. According to Figure 7 (c), during the years of the east phase of QBO an apparent anticorrelation between MF and the 11-year solar cycle is observed. A plausible explanation is the fact that in winter months, the polar vortex is sensitive to equatorial wind. In this context, Salby and Callaghan (2000) have found that changes in the polar-night vortex are consistent with the solar signature observed in wintertime records of polar temperature that have been stratified according to the QBO of equatorial wind.
Another conclusion drawn from Figure 7 is that the increased dynamical variability occurs during the west phase of the equatorial QBO and the winter vortex is significantly weakened during solar maxima and western phase of QBO. The dynamical association between the equatorial stratospheric QBO at 30-70 hPa during austral late winter and spring and the solar dynamic parameters was very recently studied by Lu and Javris (2011).
In this study, a statistically significant correlation between the annual mean TOZ and SN over the globe, the northern and the southern hemisphere, through the period 1979 - 2010 was derived. The apparent 11-year signal in TOZ was obtained without any grouping of TOZ data (i.e. according to the QBO phases of the equatorial wind). Moreover, the study of the January / February mean TOZ and SN over the NH reveals that the quasi-periodic components in the TOZ time series, reduce noticeably the above-mentioned correlation between TOZ and 11-year solar cycle. In addition, no correlation was derived studying the January / February mean TOZ and SN from the equator to the high latitudes, due to the quasi-periodic components in the TOZ time series, stemming by QBO and ENSO.
Focusing on the January / February mean TOZ and SN at 17.5°N and 27.5°N, the TOZ time series contains an 11-year signal during the east QBO phase and an ENSO signal (expressed by the recently proposed Ozone ENSO Index) during the west QBO phase. The correlation between TOZ and the 11-year solar cycle, in the east phase of QBO becomes higher for February.
Finally, studying the February mean MF between 45°N and 75°N at 50hPa, during 1979 - 2010, a statistically significant anticorrelation between MF and the 11-year solar cycle is observed, when the data were grouped in the east QBO phase. The latter may be attributed to the increased dynamical variability (i.e. the disturbing factor) that occurs during the west phase of the equatorial QBO, as well as to the fact that the winter vortex is significantly weakened during solar maxima and QBO western phase. Therefore, the main conclusion drawn from the analysis above is that the solar cycle response in TOZ is caused by dynamical changes which are caused by solar activity.