**16. Quasi-biennial oscillations in** τ**p and zonal wind in the LS region**

144 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

**15. Periodic variation of** τ**p in the LS region** 

In addition to the general increasing trend, the values of τp in Figure 21 shows a seasonal cycle with winter maximum and summer minimum modulated by a long-period oscillation. These oscillations could primarily be due to the influence of large-scale atmospheric waves. The time series data of τp is spectrum analyzed to bring out the characteristics of the prevailing periodic variations. Before subjecting the data to spectral analysis, the linear trend is removed from the original data. The residual part is Fourier analyzed and the resulting amplitude spectra for different latitude bands (0–15°N, 0–15°S, 15–30°N and 15– 30°S) are presented in Figures 22a and 22b. These amplitude spectra reveal the presence of a strong annual component (~12 months) along with a quasi-biennial component (~30 month) both in the equatorial and off-equatorial regions [88]. The spectral amplitude of QBO is as strong (significant) as that of annual oscillation (AO). Figure 22c shows the amplitude spectrum obtained from the lidar derived values of τp (at Gadanki). Even though Figure 22c shows more significant peaks in the short-period regime, the spectral amplitude is more pronounced for semi-annual (SAO) and annual (AO) components. This spectrum also shows a secondary peak around 46 months followed by troughs at 23 and 92 months. The spectral amplitude for 30 month periodicity is larger than those at the troughs on either side of this secondary peak. Though the period for the peak amplitude (46 months) is much larger than that expected for the stratospheric QBO, on the basis of the inference derived from Figures 22a and 22b, as well as owing to the fact that the spectral amplitude at 30 months is not a minimum, the characteristics of the 30 month periodicity is examined in the later part to

**Figure 22.** Amplitude spectra of τP in the altitude region18–28 km during the period 1998–2005 derived from the time series of zonal mean monthly average τP in this altitude region obtained from SAGE‐II data for (a) the equatorial region and (b) the off-equatorial-region, along with (c) that obtained from

lidar data at Gadanki

For this study the high resolution Radiosonde data of zonal wind in the lower stratosphere from an equatorial station (where the quasi-biennial oscillation, QBO, signature is expected to be maximum), Singapore (1°22″N, 103°55″E), obtained from Web site http://www.geo.fuberlin.de/en/met/ag/strat/produkte/qbo/ are used. The time series data of monthly mean zonal wind at Singapore at 20 hPa and 30 hPa levels during the study period are presented in Figure 23. This figure shows that the quasi-biennial oscillation in zonal wind (QBOU) is in the easterly phase during the 1998, 2000–2001, 2003 and 2005. The westerly phase during 1999 and the easterly phase during 2000–2001 are relatively broad. Different phases of QBOU are indentified from this time series to study the influence of QBOU in lower stratospheric aerosols.

**Figure 23.** Time series of monthly mean zonal wind at Singapore at 20 and 30 hPa levels during the period 1998-2005. E1, E2, E3, and E4 are the periods in which QBOU was in its easterly phase, and W1, W2, and W3 are those in which QBOU was in its westerly phase.

## **16.1. Latitude variation of** τ**p in LS region over the Indian longitude sector in two different phases of QBOU**

The latitude variation of τp in the altitude range 21–28 km in the band 0–30°N (averaged for every 5°) over the Indian longitude sector (70–90°E) is examined during the consecutive easterly and westerly phases of QBOU in 1998 and 1999 separately. Figure 24 shows the latitudinal variation of τp during these two phases of QBOU. This mean τp is obtained by averaging the particulate optical depth in individual months when the QBOU phase has reversed completely (the wind speed has reached its highest value). While the value of τp is

relatively high during the westerly phase of QBOU in the equatorial region up to 15°N, relatively high values are observed during the easterly phase of QBOU in latitudes beyond 15°N. The lidar derived τp is relatively low during the easterly phase of QBOU compared to that during its westerly phase. This is in good agreement with that from the latitude variation of τp for these 2 years derived from SAGE-II data.

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completes approximately 3.5 cycles, with two easterly (E1 and E2) and two westerly (W1 and W2) phases during the period 1998–2002 and two easterly (E3 and E4) and one westerly (W3)

**Figure 25.** Latitude variation of zonal mean τp in the altitude region 21–28 km from 30°S to 30°N obtained from SAGE-II during the alternate easterly and westerly phases of QBOU during the (a) absolute quiet period (1998–2002) and (b) mildly disturbed period (2003–2005). Vertical bars show the

During the former half (very quiet period), the mean τp in the equatorial region shows a general enhancement during westerly phase of QBOU (W1 and W2) while in the offequatorial region of northern hemisphere it shows an enhancement during the easterly phase of QBOU [88]. When the stratospheric QBOU was in its westerly phase the mean values of τp in both the hemispheres decreases steeply with increase in latitude on either side of equator. During the easterly phase of QBOU, though the value of τp decreases with increase in latitude beyond 15°N, it is fairly uniform in the equatorial region (15°S to 15°N) with a bite-out over the equator. During the latter half (mildly disturbed period), the mean τ<sup>p</sup> decreases rapidly on either side of equator with increase in latitude in both the phases of QBOU. Figure 25b shows a general enhancement in τp during W3 period in the equatorial and off-equatorial regions of both the hemispheres compared to that during the E3 period. However, during E4, τp in general is relatively large in the equatorial and northern hemispheric off-equatorial region compared to that during E3 and W3. Note that, in the above analysis two consecutive easterly and westerly cycles of QBOU are considered and the inferences arrived based on the latitudinal structure of τp in these two phases. The inference may apparently be contradicting if one considers the other two pairs, westerly of the first pair and easterly of the next (eg W3 and E4), particularly during the mildly disturbed phase of 2003–2005 (Figure 25b). This is mainly due to the fact that the background τp has a strong

phase during the mildly disturbed period of 2003–2005.

corresponding standard error

**Figure 24.** Latitude variation of mean τp in the altitude region 21–28 km obtained from SAGE-II averaged over the longitude sector 70°–90°E during the easterly phase (E1) and westerly phase (W1) of QBOU in 1998 and 1999, respectively (vertical bars represent the standard error). The open star represents the value of τ<sup>P</sup> during the easterly phase of QBOU (1998), and the red star represents that during the westerly phase (1999) for the altitude region 21–28 km obtained from lidar at Gadanki (marked on the X- axis by a vertical arrow), the scale of which is shown on the right-hand side of the plot.

The value of τp shows a pronounced decrease with latitude during westerly phase of QBOU compared to that during its easterly phase. Interestingly, during the easterly phase of QBOU, while the latitude variation of τp is relatively small in the equatorial region (0– 15°N), beyond 20°N it decreases sharply with increase in latitude. By examining the altitude profile of backscatter ratio obtained from lidar data at Mauna Loa at Hawaii (19.5°N, 155.6°W), Barnes and Hofmann [11] reported an enhancement of backscatter ratio (enhanced aerosol loading) in the altitude region 21–30 km during the easterly phase of QBOU. This feature agrees well with the above observation derived from the latitude variation of τp at Hawaii.

## **16.2. Latitude variation of zonal mean** τ**p in the LS region over the tropics in different phases of QBOU**

On the basis of the measured particulate loading in the stratosphere, out of the 8 year period considered for the present analysis, the period 1998–2002 was absolutely quiescent while the period 2003–2005 was mildly disturbed. The variation of zonal mean τp in the latitude region 30°S to 30°N (averaged for every 5°) in the alternative easterly and westerly phases of QBOU during the volcanically quiescent and mildly disturbed periods are presented in Figures 25a and 25b respectively. As seen from Figure 23 during the study period (8 years) the QBOU

completes approximately 3.5 cycles, with two easterly (E1 and E2) and two westerly (W1 and W2) phases during the period 1998–2002 and two easterly (E3 and E4) and one westerly (W3) phase during the mildly disturbed period of 2003–2005.

146 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

variation of τp for these 2 years derived from SAGE-II data.

arrow), the scale of which is shown on the right-hand side of the plot.

**different phases of QBOU** 

relatively high during the westerly phase of QBOU in the equatorial region up to 15°N, relatively high values are observed during the easterly phase of QBOU in latitudes beyond 15°N. The lidar derived τp is relatively low during the easterly phase of QBOU compared to that during its westerly phase. This is in good agreement with that from the latitude

**Figure 24.** Latitude variation of mean τp in the altitude region 21–28 km obtained from SAGE-II averaged over the longitude sector 70°–90°E during the easterly phase (E1) and westerly phase (W1) of QBOU in 1998 and 1999, respectively (vertical bars represent the standard error). The open star represents the value of τ<sup>P</sup> during the easterly phase of QBOU (1998), and the red star represents that during the westerly phase (1999) for the altitude region 21–28 km obtained from lidar at Gadanki (marked on the X- axis by a vertical

The value of τp shows a pronounced decrease with latitude during westerly phase of QBOU compared to that during its easterly phase. Interestingly, during the easterly phase of QBOU, while the latitude variation of τp is relatively small in the equatorial region (0– 15°N), beyond 20°N it decreases sharply with increase in latitude. By examining the altitude profile of backscatter ratio obtained from lidar data at Mauna Loa at Hawaii (19.5°N, 155.6°W), Barnes and Hofmann [11] reported an enhancement of backscatter ratio (enhanced aerosol loading) in the altitude region 21–30 km during the easterly phase of QBOU. This feature agrees well with the above observation derived from the latitude variation of τp at Hawaii.

**16.2. Latitude variation of zonal mean** τ**p in the LS region over the tropics in** 

On the basis of the measured particulate loading in the stratosphere, out of the 8 year period considered for the present analysis, the period 1998–2002 was absolutely quiescent while the period 2003–2005 was mildly disturbed. The variation of zonal mean τp in the latitude region 30°S to 30°N (averaged for every 5°) in the alternative easterly and westerly phases of QBOU during the volcanically quiescent and mildly disturbed periods are presented in Figures 25a and 25b respectively. As seen from Figure 23 during the study period (8 years) the QBOU

**Figure 25.** Latitude variation of zonal mean τp in the altitude region 21–28 km from 30°S to 30°N obtained from SAGE-II during the alternate easterly and westerly phases of QBOU during the (a) absolute quiet period (1998–2002) and (b) mildly disturbed period (2003–2005). Vertical bars show the corresponding standard error

During the former half (very quiet period), the mean τp in the equatorial region shows a general enhancement during westerly phase of QBOU (W1 and W2) while in the offequatorial region of northern hemisphere it shows an enhancement during the easterly phase of QBOU [88]. When the stratospheric QBOU was in its westerly phase the mean values of τp in both the hemispheres decreases steeply with increase in latitude on either side of equator. During the easterly phase of QBOU, though the value of τp decreases with increase in latitude beyond 15°N, it is fairly uniform in the equatorial region (15°S to 15°N) with a bite-out over the equator. During the latter half (mildly disturbed period), the mean τ<sup>p</sup> decreases rapidly on either side of equator with increase in latitude in both the phases of QBOU. Figure 25b shows a general enhancement in τp during W3 period in the equatorial and off-equatorial regions of both the hemispheres compared to that during the E3 period. However, during E4, τp in general is relatively large in the equatorial and northern hemispheric off-equatorial region compared to that during E3 and W3. Note that, in the above analysis two consecutive easterly and westerly cycles of QBOU are considered and the inferences arrived based on the latitudinal structure of τp in these two phases. The inference may apparently be contradicting if one considers the other two pairs, westerly of the first pair and easterly of the next (eg W3 and E4), particularly during the mildly disturbed phase of 2003–2005 (Figure 25b). This is mainly due to the fact that the background τp has a strong increasing trend during this half in addition to the periodic variations. This can override the periodic variations associated with QBOU. However, this contradiction is totally absent in the former half (1998–2002) when the background τp was fairly steady. During this period τ<sup>p</sup> in the equatorial region is high in both the westerly phases (W1 and W2) compared to that during the easterly phases E1 and E2, irrespective of sequence of pairs considered. This shows that the influence of QBOU in τp can be clearly delineated only during the very quiescent volcanic periods, when the background stratospheric aerosol loading remains in its steady background level.

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the equatorial and off -equatorial regions separately. The mean profiles thus obtained for the latitude region 0–10°N and 20–30°N (averaged zonally, as well as along the respective latitude bands) are shown in Figures 26b and 26c, respectively. Over the equatorial regions the altitude profile of aerosol extinction coefficient in the altitude region 22–27 km during the easterly phase of QBOU are consistently lower than that during the subsequent westerly phase. In the off-equatorial region, the aerosol extinction coefficient in the altitude region 25–32 km is found to be relatively low during the westerly phase. It may be noted that the earlier studies also have shown that aerosol extinction in the lower stratosphere can be influenced by the phase of the quasi biennial oscillation [11,28,29,95] in this region even though the difference in the nature of the latitudinal dependence over the equatorial and

Association between the altitude structure of QBO in particulate extinction (QBOa) and QBOU for the entire study period is examined by subjecting the time series of zonal mean monthly average αP (obtained from SAGE-II) and zonal mean zonal wind, U (obtained from NCEP reanalysis), at each altitude bin from 20 to 30 km to Fourier analysis after removing the linear trend. To illustrate the QBO features, the spectral components corresponding to the periods other than 22-45 month are removed from the respective trend removed time series data using the corresponding amplitudes and phases. The altitude structure of the residual amplitudes thus obtained for αp and U over the equatorial and off-equatorial regions are presented in Figure 27. The left side panels show the residual amplitude for αp and the right side panels the same for zonal wind. Both these panels show clearly the presence of a QBO. While the phase of the biennial oscillation in wind shows a clear downward propagation with time as expected, the phase of QBOa changes many times with increase in altitude. This change in the phase of QBOa with altitude is analogous to that reported for the stratospheric ozone [96] and was

**Figure 27.** Amplitude spectra of QBO in αp (from SAGE-II) and zonal wind (from NCEP) for the altitude region 20 to 30 km showing the time history of the biennial component for the period 1998 -

off-equatorial regions were not addressed in detail.

attributed to the secondary meridional circulation (SMC).

2005 for the equatorial and off-equatorial regions.

## **16.3. Altitude structure of** α**p in the lower stratosphere in different phases of QBOU**

Figure 26a shows the mean profile of αp in the altitude region 18–28 km obtained from lidar data at Gadanki for the two periods, January to September 1998 and November 1998 to September 1999, when the QBOU was in its easterly and westerly phases, respectively. Figure 26a clearly shows that the value of αp is consistently larger during the westerly phase of QBOU than the corresponding values during its easterly phase. This shift in the altitude profile toward a higher value side during the westerly phase is not due to the influence of any trend because the mean level of the stratospheric particulate optical depth in the 0°– 15°N region remains fairly the same during the period 1998–2002 and an increase in this level occurred only after 2002 (Figure 21).

For a more detailed study on the effect of QBOU on stratospheric aerosols, the zonal mean altitude profile of αp in different phases of QBOU during the study period is examined for

**Figure 26.** Altitude profiles of mean extinction from lidar at Gadanki for the E1 and W1 phases of QBOU for the period 1998-1999, along with the altitude profiles of zonal mean extinction from SAGE-II during the four alternate phases of QBOU from 1998-2005; averaged over the latitude regions (a) 0°N–10°N and (b) 20°N–30°N, representative of equatorial and off-equatorial region

the equatorial and off -equatorial regions separately. The mean profiles thus obtained for the latitude region 0–10°N and 20–30°N (averaged zonally, as well as along the respective latitude bands) are shown in Figures 26b and 26c, respectively. Over the equatorial regions the altitude profile of aerosol extinction coefficient in the altitude region 22–27 km during the easterly phase of QBOU are consistently lower than that during the subsequent westerly phase. In the off-equatorial region, the aerosol extinction coefficient in the altitude region 25–32 km is found to be relatively low during the westerly phase. It may be noted that the earlier studies also have shown that aerosol extinction in the lower stratosphere can be influenced by the phase of the quasi biennial oscillation [11,28,29,95] in this region even though the difference in the nature of the latitudinal dependence over the equatorial and off-equatorial regions were not addressed in detail.

148 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

its steady background level.

level occurred only after 2002 (Figure 21).

**QBOU**

increasing trend during this half in addition to the periodic variations. This can override the periodic variations associated with QBOU. However, this contradiction is totally absent in the former half (1998–2002) when the background τp was fairly steady. During this period τ<sup>p</sup> in the equatorial region is high in both the westerly phases (W1 and W2) compared to that during the easterly phases E1 and E2, irrespective of sequence of pairs considered. This shows that the influence of QBOU in τp can be clearly delineated only during the very quiescent volcanic periods, when the background stratospheric aerosol loading remains in

**16.3. Altitude structure of** α**p in the lower stratosphere in different phases of** 

Figure 26a shows the mean profile of αp in the altitude region 18–28 km obtained from lidar data at Gadanki for the two periods, January to September 1998 and November 1998 to September 1999, when the QBOU was in its easterly and westerly phases, respectively. Figure 26a clearly shows that the value of αp is consistently larger during the westerly phase of QBOU than the corresponding values during its easterly phase. This shift in the altitude profile toward a higher value side during the westerly phase is not due to the influence of any trend because the mean level of the stratospheric particulate optical depth in the 0°– 15°N region remains fairly the same during the period 1998–2002 and an increase in this

For a more detailed study on the effect of QBOU on stratospheric aerosols, the zonal mean altitude profile of αp in different phases of QBOU during the study period is examined for

**Figure 26.** Altitude profiles of mean extinction from lidar at Gadanki for the E1 and W1 phases of QBOU for the period 1998-1999, along with the altitude profiles of zonal mean extinction from SAGE-II during the four alternate phases of QBOU from 1998-2005; averaged over the latitude regions (a) 0°N–10°N and

(b) 20°N–30°N, representative of equatorial and off-equatorial region

Association between the altitude structure of QBO in particulate extinction (QBOa) and QBOU for the entire study period is examined by subjecting the time series of zonal mean monthly average αP (obtained from SAGE-II) and zonal mean zonal wind, U (obtained from NCEP reanalysis), at each altitude bin from 20 to 30 km to Fourier analysis after removing the linear trend. To illustrate the QBO features, the spectral components corresponding to the periods other than 22-45 month are removed from the respective trend removed time series data using the corresponding amplitudes and phases. The altitude structure of the residual amplitudes thus obtained for αp and U over the equatorial and off-equatorial regions are presented in Figure 27. The left side panels show the residual amplitude for αp and the right side panels the same for zonal wind. Both these panels show clearly the presence of a QBO. While the phase of the biennial oscillation in wind shows a clear downward propagation with time as expected, the phase of QBOa changes many times with increase in altitude. This change in the phase of QBOa with altitude is analogous to that reported for the stratospheric ozone [96] and was attributed to the secondary meridional circulation (SMC).

**Figure 27.** Amplitude spectra of QBO in αp (from SAGE-II) and zonal wind (from NCEP) for the altitude region 20 to 30 km showing the time history of the biennial component for the period 1998 - 2005 for the equatorial and off-equatorial regions.

## **17. QBO in** α**p and secondary meridional circulation**

To study the altitude structure of the QBOa, the time series of zonal-averaged monthly mean aerosol extinction coefficient at different altitudes (at 0.5 km interval) are subjected to Fourier analysis after removing the linear trend. This analysis of αp at different levels showed a prominent peak in its amplitude corresponding to the periodicity of QBO (30 month) and AO (12 month). Figure 28 shows the altitude profile of the amplitude and phase of QBOa from 18 to 32 km in the equatorial (0–15°N and 0–15°S) and off-equatorial (15–30°N and 15–30°S) regions. In general, the amplitude of QBOa decreases with increase in altitude (as that observed in the raw aerosol extinction data). Over the equatorial region, the altitude variation of QBOa amplitude (Figures 28b and 28c) shows three prominent peaks in the region 18–22 km, 23– 27 km and 28–32 km with peak amplitude centered around 20 km, 25 km and 30 km, respectively. This feature is remarkably symmetric about the. equator, in both the hemispheres. Another interesting feature to be noted is that the phase remains fairly constant around these peaks. In the equatorial region, the phase of QBOa around 25 km is ~15 months and that at around 30 km and 20 km are ~28 and 0 months, respectively. Thus the phase difference of QBOa in the upper and lower regime with respect to that at 25 km is around 13 and 15 months, which corresponds to 156° and 180°, respectively (for the 30 month periodicity one month in phase corresponds to 12°). This shows that the QBOa around 25 km is almost out of phase with that in the upper (28–32 km) and lower (18–22 km) regime [88].

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amplitude of QBOa is significant in the lower and upper altitude regions with a minimum value around 24.5 km in the northern hemisphere and around 21 km in the southern hemisphere. While the amplitude of QBOa shows a broad maximum in the altitude region 24–28 km with a peak around 26 km in the southern part, correspondingly it shows a

Over the equatorial region, the observed features of QBOa in the three altitude regions could be attributed to the influence of the secondary meridional circulation (SMC) induced by the vertical shear of QBO in stratospheric zonal wind. Through a detailed analysis of the QBO modulation of the meridional wind in the stratosphere, Ribera et al. [87] demonstrated the existence of discrete zones of meridional wind convergence and divergence over the equator. These convergence (divergence) zones during the westerly (easterly) phase of QBOU are located at the lower and upper limits of the maximum zonal wind shear and maximum temperature anomaly layers. The existence of convergence and divergence zones, which are directly related to the rising and sinking motions, forms two circulation cells

To examine the cycle-to-cycle variation of the QBO in aerosol extinction coefficient and its phase structure at the three altitude regions, the monthly mean values of αp at the central altitudes (of the three regions) 20 km, 25 km and 30 km for the equatorial region 0–15°N and 0–15°S are estimated during the period 1998 to 2005 and this data is subjected to wavelet analysis (after de-trending). Contour plots in Figure 29 shows the time history of the

**Figure 29.** Wavelet spectra of zonal averaged monthly mean aerosol extinction at 20, 25, and 30 km for

the equatorial region 0°N–15°N and 0°S–15°S during the period 1998–2005.

(SMC) in the consecutive vertical levels, quasi-symmetric about the equator.

maximum around 29 km in the northern part.

Over the off-equatorial region the altitude structure of the amplitude and phase of QBOa are found to be different from that over the equatorial region. Figures 28a and 28d show that the

**Figure 28.** Altitude structure of the QBO amplitude and phase in aerosol extinction for the altitude region 18–32 km in the equatorial region [(c) 0°N–15°N and (b) 0°S–15°S] and off-equatorial region [(d) 15°N–30°N and (a) 15°S–30°S].

amplitude of QBOa is significant in the lower and upper altitude regions with a minimum value around 24.5 km in the northern hemisphere and around 21 km in the southern hemisphere. While the amplitude of QBOa shows a broad maximum in the altitude region 24–28 km with a peak around 26 km in the southern part, correspondingly it shows a maximum around 29 km in the northern part.

150 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

km) regime [88].

15°N–30°N and (a) 15°S–30°S].

**17. QBO in** α**p and secondary meridional circulation** 

To study the altitude structure of the QBOa, the time series of zonal-averaged monthly mean aerosol extinction coefficient at different altitudes (at 0.5 km interval) are subjected to Fourier analysis after removing the linear trend. This analysis of αp at different levels showed a prominent peak in its amplitude corresponding to the periodicity of QBO (30 month) and AO (12 month). Figure 28 shows the altitude profile of the amplitude and phase of QBOa from 18 to 32 km in the equatorial (0–15°N and 0–15°S) and off-equatorial (15–30°N and 15–30°S) regions. In general, the amplitude of QBOa decreases with increase in altitude (as that observed in the raw aerosol extinction data). Over the equatorial region, the altitude variation of QBOa amplitude (Figures 28b and 28c) shows three prominent peaks in the region 18–22 km, 23– 27 km and 28–32 km with peak amplitude centered around 20 km, 25 km and 30 km, respectively. This feature is remarkably symmetric about the. equator, in both the hemispheres. Another interesting feature to be noted is that the phase remains fairly constant around these peaks. In the equatorial region, the phase of QBOa around 25 km is ~15 months and that at around 30 km and 20 km are ~28 and 0 months, respectively. Thus the phase difference of QBOa in the upper and lower regime with respect to that at 25 km is around 13 and 15 months, which corresponds to 156° and 180°, respectively (for the 30 month periodicity one month in phase corresponds to 12°). This shows that the QBOa around 25 km is almost out of phase with that in the upper (28–32 km) and lower (18–22

Over the off-equatorial region the altitude structure of the amplitude and phase of QBOa are found to be different from that over the equatorial region. Figures 28a and 28d show that the

**Figure 28.** Altitude structure of the QBO amplitude and phase in aerosol extinction for the altitude region 18–32 km in the equatorial region [(c) 0°N–15°N and (b) 0°S–15°S] and off-equatorial region [(d) Over the equatorial region, the observed features of QBOa in the three altitude regions could be attributed to the influence of the secondary meridional circulation (SMC) induced by the vertical shear of QBO in stratospheric zonal wind. Through a detailed analysis of the QBO modulation of the meridional wind in the stratosphere, Ribera et al. [87] demonstrated the existence of discrete zones of meridional wind convergence and divergence over the equator. These convergence (divergence) zones during the westerly (easterly) phase of QBOU are located at the lower and upper limits of the maximum zonal wind shear and maximum temperature anomaly layers. The existence of convergence and divergence zones, which are directly related to the rising and sinking motions, forms two circulation cells (SMC) in the consecutive vertical levels, quasi-symmetric about the equator.

To examine the cycle-to-cycle variation of the QBO in aerosol extinction coefficient and its phase structure at the three altitude regions, the monthly mean values of αp at the central altitudes (of the three regions) 20 km, 25 km and 30 km for the equatorial region 0–15°N and 0–15°S are estimated during the period 1998 to 2005 and this data is subjected to wavelet analysis (after de-trending). Contour plots in Figure 29 shows the time history of the

**Figure 29.** Wavelet spectra of zonal averaged monthly mean aerosol extinction at 20, 25, and 30 km for the equatorial region 0°N–15°N and 0°S–15°S during the period 1998–2005.

amplitudes for different periodicities (ranging from 2 to 45 months) in αp for the study period. The spectral characteristics at the respective altitudes are similar in both the hemispheres. The annual component repeats coherently in all the three altitudes indicating that they follow the same pattern in the entire altitude range 18–32 km. But the temporal pattern of QBO differs. When the QBO in αp is in its positive phase (maximum amplitude) in the lower (20 km) and upper (30 km) regimes, it is in the negative phase (minimum amplitude) in the middle regime (25 km). The QBOa at 25 km is out of phase with that in the upper and lower regimes during the entire study period. However, the amplitudes of AO and QBO in αp at 30 km are relatively small in the first 40 months compared to rest of the period. This analysis confirms the temporal repeatability of QBOa phase structure in these three altitude regions inferred from the mean αp at different altitudes averaged for the entire period of analysis shown in Figure 28. The phase of QBOU (Figure 23) matches with that of QBOa in the middle regime (25 km) while it is in opposite phase with those in the lower (20 km) and upper (30 km) regime.

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to 30°N and the altitude profiles of particulate backscatter, particulate extinction and volume depolarization for the same period derived from lidar data at Gadanki are used for this purpose. The lidar data are used mainly to study the influence of local features such as STCs, local convection etc in the properties of particulates in the UTLS region while the satellite data are used to study the spatial features in the tropical UTLS from 30°S to 30°N. A comparison of aerosol extinction from SAGE-II with that from lidar on a profile basis as well as on monthly mean basis over a small geographical grid size (6° in latitude and 13° in longitude) centered at Gadanki showed a very good agreement between the two in the

Lidar studies show a significant increase is particulate scattering in the upper troposphere with a high value of δ over the Indian subcontinent during the ASM period when the tropospheric convective activity is the highest. This feature is clearly associated with particle formation due to homogenous/heterogeneous nucleation and condensation and subsequent freezing to form non-spherical ice crystals (associated with STC). The particulate backscatter coefficient in the LS region is maximum during winter and minimum during the ASM period. The winter peak is closely associated with the increase in vertical mass flux in

Occurrence of STCs in the UT region is very common in this geographical region. The frequency of occurrence of STC is the largest during the ASM period, when most of the observed STCs are optically dense and geometrically thick. The most favoured altitude for STC is 14-16 km. Most of the observed STCs are optically and geometrically thin. Thin STCs are generally observed at higher altitudes, very close to the cold point. Most of the dense STCs are found to be associated with organized convection while the thin STCs could be of *in situ* origin. The values of δ in these clouds vary from 0.03 to 0.6 with low values occurring more frequently than high values. Though cloud depth (thickness) generally varies from 0.4 to ~4.0 km, in majority of the cases it is less than 1.7 km. While the cloud optical depth, in general, increases with increase in cloud temperature, the depolarization shows a decrease. For values of temperature < 198 K, the cloud thickness and depolarization show a sharp decrease. These clouds contribute significantly to the particulate scattering in the UT region. High value of βP observed in the UT region during the ASM period is mainly contributed by the STC particulates. They also contribute to scattering in the lower stratosphere, very close to the cold point, mainly through the penetration of particles from these STCs across the tropopause aided by the upward propagating inertia-gravity waves. This feature, which is characteristic for the Indian monsoon region, is almost absent in the southern hemisphere where the occurrence of organized very deep convection is minimal. However, the Brewer-Dobson circulation plays a significant role in the transport of UT particles in to the lower

The convective activity prevailing in the troposphere significantly influences the microphysical properties of particulates in the UTLS region. The particulate scattering and optical depth in the UT region shows a general decrease with increase in latitude on either

major features such as mean altitude structure as well as the mean annual pattern.

conjunction with the tropical upwelling and Brewer Dobson circulation.

stratosphere during the winter season.

Studies by Fadnavis and Beig [96] on the influence of SMC induced by QBO in the spatiotemporal variations of ozone over the tropical and subtropical regions however showed maximum amplitude of the QBO signal in ozone concentration at the two pressure levels, 30 hPa and 9 hPa, where the QBO manifests in opposite phase. This feature is in good agreement with the amplitude and phase of the QBO signal observed in aerosol extinction coefficient at 25 km and 30 km. From Figures 28b and 28c it can be seen that while the maximum amplitude of QBO signal in aerosol extinction coefficient centered around 20 and 30 km coincides with SMC cells centered at ~80 hPa and ~9 hPa as illustrated by Plumb and Bell [98] and Punge et al. [99], the maximum QBO signal in aerosol extinction coefficient in the altitude region 23–27 km centered around 25 km coincides with the level of maximum zonal wind shear (centered ~24 hPa) where the divergence and convergence zones (circulation cells) intersect. Similarly, Dunkerton [100] studied the QBO anomalies in ozone, methane and water vapor and observed a double peak structure in their amplitudes. Numerical simulations of QBO anomalies also showed influence of SMC in modulating the distribution of tracers leading to the formation of two peaks in their number density near 24 and 32 km [101-103]. However, the observed maximum amplitude of QBO in αp centered around 20 km is not reproduced in these model simulations.

## **18. Summary**

The altitude structure of aerosol extinction in the tropical UTLS region and its variability in different time scales is examined with particular stress for the Indian longitude sector, using the data from a dual polarization lidar (at 532 nm wavelength) located at Gadanki along with data obtained from SAGE-II onboard ERBS. Organized convection associated with the Asian summer monsoon (ASM) and highly dynamic ITCZ makes this region unique in the global scenario. Altitude profiles of particulate extinction for a period of eight years from 1998-2005 (volcanically quiescent period) obtained from SAGE-II in the latitude region 30°S to 30°N and the altitude profiles of particulate backscatter, particulate extinction and volume depolarization for the same period derived from lidar data at Gadanki are used for this purpose. The lidar data are used mainly to study the influence of local features such as STCs, local convection etc in the properties of particulates in the UTLS region while the satellite data are used to study the spatial features in the tropical UTLS from 30°S to 30°N. A comparison of aerosol extinction from SAGE-II with that from lidar on a profile basis as well as on monthly mean basis over a small geographical grid size (6° in latitude and 13° in longitude) centered at Gadanki showed a very good agreement between the two in the major features such as mean altitude structure as well as the mean annual pattern.

152 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

around 20 km is not reproduced in these model simulations.

km) and upper (30 km) regime.

**18. Summary** 

amplitudes for different periodicities (ranging from 2 to 45 months) in αp for the study period. The spectral characteristics at the respective altitudes are similar in both the hemispheres. The annual component repeats coherently in all the three altitudes indicating that they follow the same pattern in the entire altitude range 18–32 km. But the temporal pattern of QBO differs. When the QBO in αp is in its positive phase (maximum amplitude) in the lower (20 km) and upper (30 km) regimes, it is in the negative phase (minimum amplitude) in the middle regime (25 km). The QBOa at 25 km is out of phase with that in the upper and lower regimes during the entire study period. However, the amplitudes of AO and QBO in αp at 30 km are relatively small in the first 40 months compared to rest of the period. This analysis confirms the temporal repeatability of QBOa phase structure in these three altitude regions inferred from the mean αp at different altitudes averaged for the entire period of analysis shown in Figure 28. The phase of QBOU (Figure 23) matches with that of QBOa in the middle regime (25 km) while it is in opposite phase with those in the lower (20

Studies by Fadnavis and Beig [96] on the influence of SMC induced by QBO in the spatiotemporal variations of ozone over the tropical and subtropical regions however showed maximum amplitude of the QBO signal in ozone concentration at the two pressure levels, 30 hPa and 9 hPa, where the QBO manifests in opposite phase. This feature is in good agreement with the amplitude and phase of the QBO signal observed in aerosol extinction coefficient at 25 km and 30 km. From Figures 28b and 28c it can be seen that while the maximum amplitude of QBO signal in aerosol extinction coefficient centered around 20 and 30 km coincides with SMC cells centered at ~80 hPa and ~9 hPa as illustrated by Plumb and Bell [98] and Punge et al. [99], the maximum QBO signal in aerosol extinction coefficient in the altitude region 23–27 km centered around 25 km coincides with the level of maximum zonal wind shear (centered ~24 hPa) where the divergence and convergence zones (circulation cells) intersect. Similarly, Dunkerton [100] studied the QBO anomalies in ozone, methane and water vapor and observed a double peak structure in their amplitudes. Numerical simulations of QBO anomalies also showed influence of SMC in modulating the distribution of tracers leading to the formation of two peaks in their number density near 24 and 32 km [101-103]. However, the observed maximum amplitude of QBO in αp centered

The altitude structure of aerosol extinction in the tropical UTLS region and its variability in different time scales is examined with particular stress for the Indian longitude sector, using the data from a dual polarization lidar (at 532 nm wavelength) located at Gadanki along with data obtained from SAGE-II onboard ERBS. Organized convection associated with the Asian summer monsoon (ASM) and highly dynamic ITCZ makes this region unique in the global scenario. Altitude profiles of particulate extinction for a period of eight years from 1998-2005 (volcanically quiescent period) obtained from SAGE-II in the latitude region 30°S Lidar studies show a significant increase is particulate scattering in the upper troposphere with a high value of δ over the Indian subcontinent during the ASM period when the tropospheric convective activity is the highest. This feature is clearly associated with particle formation due to homogenous/heterogeneous nucleation and condensation and subsequent freezing to form non-spherical ice crystals (associated with STC). The particulate backscatter coefficient in the LS region is maximum during winter and minimum during the ASM period. The winter peak is closely associated with the increase in vertical mass flux in conjunction with the tropical upwelling and Brewer Dobson circulation.

Occurrence of STCs in the UT region is very common in this geographical region. The frequency of occurrence of STC is the largest during the ASM period, when most of the observed STCs are optically dense and geometrically thick. The most favoured altitude for STC is 14-16 km. Most of the observed STCs are optically and geometrically thin. Thin STCs are generally observed at higher altitudes, very close to the cold point. Most of the dense STCs are found to be associated with organized convection while the thin STCs could be of *in situ* origin. The values of δ in these clouds vary from 0.03 to 0.6 with low values occurring more frequently than high values. Though cloud depth (thickness) generally varies from 0.4 to ~4.0 km, in majority of the cases it is less than 1.7 km. While the cloud optical depth, in general, increases with increase in cloud temperature, the depolarization shows a decrease. For values of temperature < 198 K, the cloud thickness and depolarization show a sharp decrease. These clouds contribute significantly to the particulate scattering in the UT region. High value of βP observed in the UT region during the ASM period is mainly contributed by the STC particulates. They also contribute to scattering in the lower stratosphere, very close to the cold point, mainly through the penetration of particles from these STCs across the tropopause aided by the upward propagating inertia-gravity waves. This feature, which is characteristic for the Indian monsoon region, is almost absent in the southern hemisphere where the occurrence of organized very deep convection is minimal. However, the Brewer-Dobson circulation plays a significant role in the transport of UT particles in to the lower stratosphere during the winter season.

The convective activity prevailing in the troposphere significantly influences the microphysical properties of particulates in the UTLS region. The particulate scattering and optical depth in the UT region shows a general decrease with increase in latitude on either

side of the equator with a well pronounced summer-winter contrast. However, organized convection during the ASM period enhances the particulate loading in the UT in the northern latitudes even beyond 25°N. While the particulate optical depth in the 18–21 km region (lowest part of the stratosphere) is relatively low in the equatorial region, it shows an increase in the off-equatorial region mainly due to this enhancement in particulate concentration above the cold point, particularly over the Indo-Gangetic Plain, during this period. At a higher altitude (21–30 km) it shows a different pattern, with high values near the equator and low values in the off-equatorial region. This confirms the existence of a stratospheric aerosol reservoir. This spatial distribution could be attributed to horizontal advection in the lower regime (rapid transport from near equatorial region to higher latitudes) as well as lofting to higher altitudes over the equatorial region (B-D circulation).

Distribution of Particulates in the Tropical UTLS over

the Asian Summer Monsoon Region and Its Association with Atmospheric Dynamics 155

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Spectral analysis of zonal mean particulate optical depth in the stratosphere (18-32km) revealed the existence of a strong QBO both in the equatorial and off-equatorial regions. The phase of the QBO signal in particulate extinction (QBOa) around 25 km is found to be in opposite phase with that in the upper (28-32 km) and lower regime (18-22 km), illustrating the existence of a secondary meridional circulation (SMC) produced due to vertical shear of QBO phase in zonal wind (QBOU). While the particulate optical depth in the lower stratosphere is relatively large during the westerly phase of QBOU in the equatorial region, relatively high values are observed during the easterly phase of QBOU in the off-equatorial region. During the westerly phase of stratospheric QBOU, the mean particulate optical depth rapidly decreases with increase in latitude on either side of equator in both the hemispheres. During the easterly phase, this remains fairly steady between ±15° latitude, with a small bite-out around the equator, and decreases steadily for latitudes beyond 15°.
