QBO and solar cycle signals on total ozone at tropical brazilian stations

E. Echer and V.W.J.H.Kirchhoff

• Introduction
• Results and Discussion
• Conclusions
• References

FIGURES

• Figure 1
• Figure 2
• Figure 3

Long-term measurements of total ozone have been carried out by The National Institute for Space Research, INPE, at Natal (5.8^oS, 35.2^oW) and C. Paulista (23.1^oS,45W), since 1978 and 1974, respectively, using two Dobson spectrophotometers. A spectral analysis was applied on these time series, using the Multiple Taper Method, the Maximum Entropy Spectral Analysis and an Iterative Regression Model. The time series were the yearly averages, 12 months (12m) and  36 months (36m) running means, and 12m-36m time series. Using these time series, the main periodicities found were 2.45 and 10 years for Natal, and 2.45 and 8 years for C. Paulista, associated to the quasi-biennial oscillation (QBO) and to the 11 year solar cycle. The amplitudes of these periodicities were close to 1% of the averages. Using 36m the QBO signal was eliminated (r² = 0.01 between total ozone and the zonal wind index, for Natal) and the solar cycle signal could be more easily seen. It was observed that ozone at Natal has a variation in-phase with the sunspot number, while at C. Paulista an out-of-phase variation was observed, with a lead for ozone of about 2 years. To study the QBO signal  the difference between 12m and 36m time series was calculated, removing the longer periods. In this time series, the correlation between total ozone and zonal wind index was higher, positive at Natal and negative at Cachoeira Paulista

Introduction

The solar cycle on total ozone is about 1-2% per cycle, and ozone varies in phase with the sunspot number (Ruderman and Chamberlain, 1976; Brasseur and Solomon, 1986; Zerefos et al., 1997). The Quasi-Biennial Oscillation ­ QBO, was identified in ozone as related to the zonal wind oscillation of 26-30 months ((Funk and Graham, 1962), and it is in phase with the zonal wind at equatorial regions and has a variation out-of-phase in extratropical latitudes (Hilsenrath and Schlesinger, 1981; Zerefos et al., 1992; Hollandsworth et al., 1995; Kane et al., 1998).

Long-term measurements of total ozone have been carried out by The National Institute for Space Research, INPE, at Natal (5.8^oS, 35.2^oW) and Cachoeira (C.) Paulista (23.1^oS,45W), since 1978 and 1974, respectively, using two Dobson spectrophotometers. In this paper a spectral analisys is made on Dobson ozone data to study the solar cycle and the QBO signal on total ozone in these two tropical stations. In Table 1 are shown the geographical locations of the stations and the period of the data analyzed. <o:p</o:p Table 1: Observational sites  <o:p</o:p

 <o:p</o:p

Results and Discussion<o:p</o:p

The time series analyzed were the annual averages, the 12 months running average (12m), 36 months running averages (36m) and 12m-36m. Spectral analyse were made using the Multiple Taper Method (Thomson, 1982), the Maximum Entropy Spectral Analysis &shy; MESA (Ulrich and Bishop, 1975), and a Iterative Regression Model (Wolberg, 1967) to calculate amplitudes and phase of  periodicities. <o:p</o:p

Figure 1 shows the spectrum obtained with the MESA applied to the annual average time series.  It is observed that the more significant periodicities are 9.7 years (solar cycle), 2.36 years (QBO) and 4 years for Natal, and 13 and 7.3 years (solar cycle) and 2.4 (QBO) for   C. Paulista. <o:p</o:p

<o:p</o:p

Similar results were obtained  using Multiple Taper and Iterative Regression Methods. The solar cycle and the QBO cycle have been identified in the ozone time series, and the periodicities are: for QBO, 2.4 years for both stations; and for the solar cycle, near 10 and 8 years at Natal and C. Paulista. <o:p</o:p

The ozone average for the period of data analysed in this paper is 265.4 &plusmn; 4.3 DU for Natal and 269.3&plusmn; 6.8 DU for C. Paulista.  The relative amplitude of the solar cycle signal were  1.0% of the ozone average for 9 years at Natal  and 1.5% for 7.8 years at C. Paulista. The relative amplitude of the QBO signal was 1.2 % for 2.4 years at Natal and 1.4% at 2.4 years at C. Paulista. <o:p</o:p

Filtering the data with a given smoothing technique could result in a QBO or solar signal more clearly seen. Table 2 shows the correlation coefficients calculated between total ozone monthly averages, 12m, 36m and 12m-36m time series and the sunspot number and zonal wind index time series.

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Table 2 &shy; Correlation coefficients<o:p</o:p

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It is observed fromTable 2 that higher ozone and sunspot number correlations are observed at Natal than at C. Paulista, which  is in agreement with observations that the solar cycle is more easily seen at the equator (Zerefos et al., 1997). The 36m  time series has the higher correlation, because smoothing ozone data with these elliminates the  lowest periods. Ozone and QBO correlations are also higher for Natal, and at Natal the correlation is positive (r=+0.70, 12m-36m), and C. Paulista is negative (r=-0.58, 12m-36m), which is the pattern observed in equatorial and extratropical latitudes (Zerefos et al., 1992; Hollandsworth  et al., 1995; Kane et al., 1998).

In Figure 2 the 36m total ozone time serie and the sunspot number are shown for Natal and C. Paulista. It is observed that ozone and sunspot number have a variation very similar at Natal, and out of phase in C. Paulista. Cross-correlation spectral analysis have resulted in a lag of 0 years between ozone and sunspot number at Natal, and a lag of 2 years at C. Paulista (ozone leads sunspot number by 2 years).

In Figure 3 the zonal wind index at 30 hPa and the ozone anomaly time series  (12m-36m) are shown for Natal and C. Paulista. It is observed that at Natal ozone and zonal wind vary nearly in phase, and at C. Paulista a large out-of-phase variation is observed. Cross-correlation spectral analysis results in a lag between total ozone and zonal wind index of +2 months at Natal, and &shy;14 months at C. Paulista. So at Natal the ozone maximum is on average 2 months later than the zonal wind maximum, and at C. Paulista it has a maximum 14 months earlier than the zonal wind. These results are in agreement with those obtained by Hollandsworth et al. (1995), Kane et al. (1998), among others, using satellite data in latitudinal ranges. <o:p</o:p

<o:p</o:p

Conclusions<o:p</o:p

The solar cycle was identified at both stations, as having a period near 10 years and  an amplitude of 1% at Natal, and near 8 years and 1.5 % at C. Paulista. Total ozone at Natal has variation in phase, while at C. Paulista it leads sunspot number by two years.

The QBO signal: zonal wind is correlated in a positive way with total ozone at Natal, with a period of 2.4 years and amplitude of 1.2% and lag of +2 months, while at C. Paulista zonal wind and total ozone are anticorrelated, with QBO period in ozone of 2.4 years, 1.4% of amplitude and &shy;14 months of lag, ozone reaching maximum 14 months earlier than the zonal wind.

References

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