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Lidar observations of stratospheric temperature above McMurdo Station, Antarctica
Journal of Atmospheric and TerrestrialPhysics, Vol. 58, No. 13, pp. 1391-1399, 1996
~ ) Pergamon
0021-9169(95)00164--6
Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0021-9169/96 $15.00+0.00
Lidar observations of stratospheric temperature above McMurdo Station, Antarctica G. Di Donfrancesco, ~* A. Adriani,2 G. P. Gobbi 2 and F. Congeduti2 ~ENEA, Arnb-Saf-Atmo,C. R. E. Casaccia, S. M. di Galeria, Rome, Italy; 2Istitutodi Fisica dell'Atmosfera CNR, CP 27, 00044, Frascati, Italy (Received 26 July 1995; accepted in revised form 4 August 1995)
Abstract--Stratospheric temperatures were measured by lidar at McMurdo station, Antarctica (78°S, 167°E) during two late spring months (September-October) in 1991 and 1992, and during the period March~3ctober in 1993 and 1994. The stratosphere was found to be quite active, with one major and several minor warmings occurring in 1993 and 1994, and showing the expected behaviour of a distinct region of high temperatures, formed in the polar mesosphere, descending with time and warming the stratopause region. A relative maximum of the stratopause temperature was observed in July 1994, and differences between two years in terms of the time development of average temperature in the different stratospheric layers and in terms of the average temperature variability over singlemonths are pointed out. Monthly mean temperature profiles determined from lidar observations are compared with a reference atmosphere (CIRA86). Fair agreement, with discrepancies less than + 4 K, in June, July and August in the middle stratosphere and just above the stratopause was found. Copyright © 1996 Elsevier Science Ltd
INTRODUCTION The stratosphere at polar latitudes is a very interesting region of our atmosphere. During the southern winter and spring, the slratopause temperature reaches a relative maximum in the polar region, contrary to what is expected from radiative arguments alone. Therefore, dynamical processes are necessary to explain the existence of this warmer stratopause (Hitchman et al., 1989). Moreover, it is well known that the lower stratosphere in the Antarctic winter is colder than in the Arctic winter and it is likely that this difference is due to a weaker effect of planetary waves in the Antarctic (Andrews et al., 1987). Since these problems about the: polar night stratosphere have not been completely elucidated, observational studies on the interannual variability of the polar winter temperatures in both hemispheres are required (Kanzawa, 1989). It is also of primary interest to determine the degree to which the structure of the polar vortex itself, with the associated transport properties, could be affected by the ozone-hole phenomenon. In fact, substantial ozone losses lead to decreased solar heating affecting
*ENEA, Amb-Saf-Atmo, CR Casaccia, Via Anguillarese 301, 00060 S. Maria di Galeria, Rome, Italy. Tel: +39 6 94186274. Fax: +39 6 94186266.
the stability and breakup of the Antarctic vortex during subsequent years. The Mahlman et al. (1994) calculations suggest that the presence of the ozone-hole over Antarctica could cause a temperature difference of - 8 K at low altitudes and + 6 K in the upper levels in the December stratosphere. Currently, two complementary methods, satellite and lidar remote sensing, are considered suitable for the observation of the middle atmosphere temperatures (Finger et al., 1993). The first one offers better coverage but lower accuracy and vertical resolution with respect to the second; which, however, is localized and can only perform nocturnal observations (Hauchecorne and Chanin, 1980; Jenkins et al., 1987). Frequent and considerable disturbances in the middle atmosphere temperature profiles are expected during the winter: temperature retrievals by satellites may be unreliable under certain conditions. Claud et al. (1993) observed, over Scandinavia, discrepancies between radiosonde temperature profiles and TOVS observations of about 5 K above 20 hPa. In general, the southern hemisphere circulation is less disturbed than in the north and less discrepancies have been observed (Koepken et al., 1995). Nevertheless, some significant differences could be expected in the presence of optically thick ice polar stratospheric clouds in the lower stratosphere. However, very few lidar stations in the polar regions
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have collected records long enough to constitute a basis for such studies of the stratosphere, particularly in Antarctica (Iwasaka et al., 1986). The aims of this paper are: to summarize our observations of stratospheric temperatures as obtained by a Rayleigh lidar at McMurdo Station (78°S, 167°E) during the 1991 and 1992 late winter and during all the 1993 and 1994 autumns, winters and springs; to point out the interannual variability; and to discuss systematic differences from the existing reference atmosphere (CIRA86) of a monthly average variability and climatology of the stratospheric temperatures. The analysis is restricted to medium- and long-term temperature variations. Therefore, short-term variations, like gravity waves, are filtered out by using weighted running averages; these will be discussed in a different paper.
INSTRUMENTATION The basic version of the lidar system was installed at McMurdo Station on Ross Island at 78°S and 167°E during the 1990 spring (Gobbi et al., 1991) in support of a balloon-borne counter experiment. Since the 1991 campaign, this system assumed primary importance in the study of polar stratospheric clouds and of the stratospheric temperature variability above McMurdo (Adriani et al., 1992). In 1992, the NdYAG laser emitting light pulses at the 532 nm wavelength, were increased in power (from 150 mJ to 250 mJ) and in fire rate (from 4 Hz to 10 Hz). The receiver is a 41.5 cm diameter Newtonian telescope with a field of view smaller than 1 mrad. The wavelength acceptance is reduced to a band of 0.15 nm around the laser wavelength by means of an interference filter. A 400 Hz mechanical chopper shuts off the photomultiplier during the time in which the atmospheric echo comes from the first 16-18 km, eliminating nonlinearity effects on the cooled photodetector. In such a way the noise is reduced to a few hundred pulses per second. The basic signal acquisition for temperature measurements is performed by means of a photoncounting chain located in the PC. Temperature is measured from 24 km up to 65 km. Standard observations last 30-60 minutes, with averages stored every 5 minutes. Measurements are necessarily restricted to night time since the daytime sky background noise is too high. In 1991 and 1992, the lidar operated at McMurdo Station in the period August~)ctober. In 1993 and 1994, the system operated during the Antarctic autumn, winter and spring in the period from 10 March to 10 October. In the period November-Feb-
Table 1. Number of observations employed in the analysis Month Mar Apr May Jun Jul Aug Sep Oct Total
1991
2 9 3 14
1992
1993
1994
Total
12 4 16
3 2 6 5 5 9 7 2 39
5 3 5 4 4 4 8 5 38
8 5 11 9 9 15 36 14 107
ruary, lidar data for temperature profiles were not available due to permanent daylight conditions at McMurdo. 107 night-time sessions of measurements were performed, with a mean of 13 temperature profiles per month, and for a total amount of about 60 hours. The seasonal coverage of measurements for each year is shown in Table 1. The method used for retrieving temperature profiles is the same as that described by Hauchecorne and Chanin (1980). Monthly average values of atmospheric pressure provided by the COSPAR International Reference Atmosphere (CIRA86) are employed as boundary conditions at the top of the lidar sounding. RESULTS
In our analysis, lidar profiles (one every 5 minutes) have been averaged along the whole period of measurement (0.5 to 1 hour) and vertically filtered with a resulting vertical resolution of approximately 6 km and an average temperature statistical error of 0.2 K at 25 km, 1.5 K at 40 km and 10 K at 55 km. W i n t e r temperature behaviours in 1993 a n d 1994
Large perturbations of temperature in the stratosphere during winter are called stratospheric warmings. The dynamical behaviour of the middle atmosphere during such a warming is known to follow a characteristic pattern of time development, as modelled and studied by various authors (see for example Andrews et al., 1987). There were, however, very few measurements at high latitude which extended to stratospheric altitudes on such occasions. The timeheight sections of the temperature profiles obtained by lidar during 1993 and 1994 are presented in Fig. 1. During 1993 the stratosphere was quiet with temperatures about 250-260 K in the upper stratosphere until mid-April when a warming was observed in the stratopause region. In the first part of May, a strato-
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spheric warming was detected with a temperature rise of 10 K in the region above 40 km, in coincidence with a cooling in the lower stratosphere. A deep cooling followed this first warming and the stratosphere reached a stable situation only 3 weeks later. A second warming appeared in mid-June with a consequent decrease of the 260 K line of about 6 kin; starting with this second event the whole region around the stratopause showed a general warming with an approximately isothermal shape. This shape survived for more than one month and was destroyed by the third and fourth wa.rmings at the beginning of August and September, respectively. These two last events were stronger and developed in a region of the stratosphere 10 km lower with respect to the first ones. A major warming followed at the beginning of October
in a wide part of the stratosphere (35-55 km) with a maximum of temperature of about 300 K near 45 km on 12 October. The 1994 winter was warmer above 40 km and the stratospheric warmings were stronger than in 1993, as is evident in Fig. lb. Moreover, the first minor warmings always appeared in the second half of April, but in a region of the stratosphere (60 kin) about 5 km higher than in 1993. After this first event, the following four minor warmings showed the same peculiarity of higher regions of interest and temperatures: in particular, the second one (mid-July) reached temperatures of 300 K around 55 km. This event lasted for weeks. No major warming was detected during 1994 up to 15 October, when lidar measurements stopped.
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For both years a temperature average for each profile in three height intervals (25-35 km, 35--45 km, 4555 km) was computed and plotted in Fig. 2. Each set of points was fitted by a polynomial curve of degree 3 in the period March-August. This approximation filters out all the temperature fluctuations except the seasonal behaviour. The shapes of these curves in 1993 and 1994 show strong differences: in the layers 25-35 km and 35-45 km at the beginning of the polar autumn, the 1993 average temperature was lower than the 1994 value and the cooling was less evident, with an averaged temperature minimum of 203 K at the
end of May and of 233 K at beginning of May for the two layers, respectively. These minima appear stronger in the 1994 data and delayed with respect to the 1993 ones: 199 K at middle of June and 228 K at middle of May. During the polar spring, this temperature difference (1994 colder than 1993) is maintained up to mid-September, when a sudden cooling strongly developed in 1993 in coincidence with a warming in 1994. The upper layer 45-55 km shows opposite behaviours: the minimum of the temperature is reached at the end of April for both years, but it is lower during 1993 and, moreover, the values during the year show reduced oscillations compared with 1994. An evident feature of both years is the time displacement of the temperature for the three layers: the minimum at 45-55 km occurs roughly two-three weeks earlier than at 35-45 km and six-seven weeks earlier than at 25-35 km. This is expected if, as discussed by Kanzawa (1989) and Hitchman et al. (1989) and modelled by Fisher et al. (1993) and Rosenfield et al. (1994), the warm polar stratopause observed during the winter is due to a gravity-wave-induced mean meridional flow converging over the pole, where it descends. The adiabatic heating associated with this intense downwelling into the strong polar confinement dominates the diabatic cooling in the thermodynamic equation and causes large departures from radiative equilibrium in a stratospheric region that descends during the winter as the westerly jet and easterly wave drag maxima descend. A peculiar climatological feature occurred during 1994 but was not evident in the 1993 data: the stratopause temperature reached a relative maximum during the polar night (July), exhibiting strong week-toweek variability, as shown in Fig. 3. The occurrence of such an event, sometimes indicated as "the separated polar winter stratopause" has not been completely elucidated and was explained by Hitchman et al. (1989) as a gravity-wave driven feature• Moreover, in the period April-August, the stratopause altitude and temperature values were higher (about 5 km and 10 K, respectively) in 1994 than in 1993 (Fig. 3) and no major warming was detected during 1994 before 15 October, contrary to the 1993 observations (Figs 1 and 2). Starting in April for both years, the time development of the temperature altitude profiles shows strong variations that lead to the peculiar features plotted in Fig. 4 for May 1994: a mesospheric inversion of temperature increases with time, warming the stratopause region that decreases its altitude; in a second phase the disturbance develops a peculiar feature of a split stratopause and occasionally, as in this
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case, a maximum of temperature at higher altitudes. This maximum is not necessarily the end of the upper stratospheric warming that develops further in a new similar feature. Such events occurred frequently, more often in 1994 than in 1993, in our data; and they lasted until the end of September. Observed interannual differences could reasonably be explained as the effects of an opposite quasibiennial oscillation (QBO) at 30 hPa over the equator during the years (Lait et al., 1989); although, in the extratropics, the signal should contain a 20 month period in addition to the 30 month equatorial QBO period (Tung and Yang, 1994). During QBO-easterly phase years, the Antarctic vortex is weaker and warmer and the propagation of gravity and planetary waves is enhanced (Poole et al., 1989; Kanzawa and Kawaguchi, 1990). The 30 hPa QBO wind phase persisted westerly only during the early winter 1993 and, starting in mid-July 1993, was easterly for the entire of 1994. In fact, our 1993 data show lower temperatures in the range 2535 km during the first half of the year (Figs 2 and 3). The presence of colder temperatures during the second half of 1994 could be explained as a result of lower values of solar activity during 1994 than in 1993; which could obscure an expected easterly QBO phase modulation of the polar stratospheric temperatures (Krzyscin, 1995).
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Seasonal variations of atmospheric temperatures are extensively modelled in terms of monthly means. One of the best available satellite climatologies of the middle atmosphere is the COSPAR International Reference Atmosphere, CIRA86 (Rees et al., 1990). It is therefore worthwhile comparing such a reference profile to monthly means obtained from our 107 temperature profiles (see Table 1) which are presented in Fig. 5 with l a limits of the average (dashed line). Contour plots of the monthly: average temperature are shown in Fig. 6a, and the differences with CIRA86 shown in Fig. 6b. Figures 5 and 6a reveal consistent inversions developing at approximately 60 km in April and May; these are not present in the CIRA86 reference temperatures. Considerable and systematic discrepancies occur in June and July in the range 30-40 km and in August in the range 40--50 km. Although the most considerable discrepancies occur in March and October, these are probably a result of our data set always starting in the second week of March and ending in the third week of October. Moreover, in April and May, the discrepancy above 45 km of the altitude is
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not significant due to the strong influence of the buildup phase of upper stratospheric warmings on the monthly averaged profiles. But no similar explanations were found in June, July and August. However, in most months and altitudes the agreement between the lidar data and CIRA86 is better than + 4 K (Fig. 6b) and CIRA86 profiles always fall inside the + 1ar range of monthly mean temperature variability of our data; which is reasonably good for the Antarctic polar region. A monthly me:m variability of our data for each year was calculated as one standard deviation of the averaged temperature profiles and plotted as a contour map in Fig. 7. The differences between 1993 and 1994 are immediately evident: during 1993 the mean variability of the stratosphere was less than 7 K, with peak values of about 9 K only in April and June-July in a small region (55-57 km and 42-44 km, respectively). On the contrary, the 1994 data show a mean variability of about 11 K but lower, in the region 3550 km during March and April, and rapidly increasing in the upper stratosphere in April, with peak values up to 20 K in June and 28 K in July (above 48 kin), as shown in Fig. 7. It is noticeable that just during the
most perturbed period (July 1994) two "quiet atmospheric layers" at about 46 and 33 km with almost no variations of temperature were observed (Fig. 7b). Similar stratospheric (and mesospheric) layers during warming events were frequently observed in winter at high latitudes in the northern hemisphere (Offerman et al., 1987; Hauchecorne et al., 1987) and they were tentatively interpreted as nodes of standing planetary waves (Plumb, 1982). A rapid downward transport of air from the mesospheric region into the stratospheric polar vortex of the southern hemisphere in winter and early spring is expected (Fisher et al., 1993). In fact, the observed atmospheric region of highest values of variability constantly descends during the winter, reaching the middle stratosphere in late spring. It is not surprising that similar behaviours were observed, in different years, by other authors in the mesopause region above the South Pole (Collins et al., 1994) due to the well known strong coupling between the upper stratosphere and mesosphere during the winter. CONCLUSIONS The lidar observations provide an interesting description of the temperature behaviour and varia-
G. Di Donfrancesco et al.
1398
A V R T E M P E R A T U R E V A R I A B I L I T Y - M c M u l d o 1993
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bility in the Antarctic stratosphere during 1993 and 1994. Differences between the two years have been pointed out: in 1994 a "separated winter stratopause" feature occurred and strong variability in the upper stratosphere was observed. In both years a split stratopause was observed during the build-up phase of the upper stratospheric warmings, with a higher occurrence in 1994. A major stratospheric warming was detected during early October 1993, but no major warming appeared in 1994 before 15 October when
our measurements stopped. Monthly averages showed fair agreement with a reference model (CIRA86), with evident discrepancies of about 10 K only during June and July in the range 30-40 km. A stratospheric region with high values of temperature variability was observed to descend during the winter from the stratopause region down to about 30 km altitude. Acknowledgements--work has been supported by Pro-
gramma Nazionale Ricerche in Antartide. Thanks are due to M. Viterbini for his technical assistance.
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1994
Instability of the distorted polar night vortex: a theory of stratospheric warmings. J. Atmos. Sci. 38, 25142531. CIRA (COSPAR International Reference Atmosphere) 1986. Adv. Space Res. 10(t2). Computations of diabatic descent in the stratospheric polar vortex. J. Geophys. Res. 99, 16677-16689. Global QBO in circulation and ozone. Part I: re-examination of observational evidence. J. Atmos. Sci. 51, 2699-2707.
~ ) Pergamon
0021-9169(95)00164--6
Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0021-9169/96 $15.00+0.00
Lidar observations of stratospheric temperature above McMurdo Station, Antarctica G. Di Donfrancesco, ~* A. Adriani,2 G. P. Gobbi 2 and F. Congeduti2 ~ENEA, Arnb-Saf-Atmo,C. R. E. Casaccia, S. M. di Galeria, Rome, Italy; 2Istitutodi Fisica dell'Atmosfera CNR, CP 27, 00044, Frascati, Italy (Received 26 July 1995; accepted in revised form 4 August 1995)
Abstract--Stratospheric temperatures were measured by lidar at McMurdo station, Antarctica (78°S, 167°E) during two late spring months (September-October) in 1991 and 1992, and during the period March~3ctober in 1993 and 1994. The stratosphere was found to be quite active, with one major and several minor warmings occurring in 1993 and 1994, and showing the expected behaviour of a distinct region of high temperatures, formed in the polar mesosphere, descending with time and warming the stratopause region. A relative maximum of the stratopause temperature was observed in July 1994, and differences between two years in terms of the time development of average temperature in the different stratospheric layers and in terms of the average temperature variability over singlemonths are pointed out. Monthly mean temperature profiles determined from lidar observations are compared with a reference atmosphere (CIRA86). Fair agreement, with discrepancies less than + 4 K, in June, July and August in the middle stratosphere and just above the stratopause was found. Copyright © 1996 Elsevier Science Ltd
INTRODUCTION The stratosphere at polar latitudes is a very interesting region of our atmosphere. During the southern winter and spring, the slratopause temperature reaches a relative maximum in the polar region, contrary to what is expected from radiative arguments alone. Therefore, dynamical processes are necessary to explain the existence of this warmer stratopause (Hitchman et al., 1989). Moreover, it is well known that the lower stratosphere in the Antarctic winter is colder than in the Arctic winter and it is likely that this difference is due to a weaker effect of planetary waves in the Antarctic (Andrews et al., 1987). Since these problems about the: polar night stratosphere have not been completely elucidated, observational studies on the interannual variability of the polar winter temperatures in both hemispheres are required (Kanzawa, 1989). It is also of primary interest to determine the degree to which the structure of the polar vortex itself, with the associated transport properties, could be affected by the ozone-hole phenomenon. In fact, substantial ozone losses lead to decreased solar heating affecting
*ENEA, Amb-Saf-Atmo, CR Casaccia, Via Anguillarese 301, 00060 S. Maria di Galeria, Rome, Italy. Tel: +39 6 94186274. Fax: +39 6 94186266.
the stability and breakup of the Antarctic vortex during subsequent years. The Mahlman et al. (1994) calculations suggest that the presence of the ozone-hole over Antarctica could cause a temperature difference of - 8 K at low altitudes and + 6 K in the upper levels in the December stratosphere. Currently, two complementary methods, satellite and lidar remote sensing, are considered suitable for the observation of the middle atmosphere temperatures (Finger et al., 1993). The first one offers better coverage but lower accuracy and vertical resolution with respect to the second; which, however, is localized and can only perform nocturnal observations (Hauchecorne and Chanin, 1980; Jenkins et al., 1987). Frequent and considerable disturbances in the middle atmosphere temperature profiles are expected during the winter: temperature retrievals by satellites may be unreliable under certain conditions. Claud et al. (1993) observed, over Scandinavia, discrepancies between radiosonde temperature profiles and TOVS observations of about 5 K above 20 hPa. In general, the southern hemisphere circulation is less disturbed than in the north and less discrepancies have been observed (Koepken et al., 1995). Nevertheless, some significant differences could be expected in the presence of optically thick ice polar stratospheric clouds in the lower stratosphere. However, very few lidar stations in the polar regions
1391
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G. Di Donfrancesco et al.
have collected records long enough to constitute a basis for such studies of the stratosphere, particularly in Antarctica (Iwasaka et al., 1986). The aims of this paper are: to summarize our observations of stratospheric temperatures as obtained by a Rayleigh lidar at McMurdo Station (78°S, 167°E) during the 1991 and 1992 late winter and during all the 1993 and 1994 autumns, winters and springs; to point out the interannual variability; and to discuss systematic differences from the existing reference atmosphere (CIRA86) of a monthly average variability and climatology of the stratospheric temperatures. The analysis is restricted to medium- and long-term temperature variations. Therefore, short-term variations, like gravity waves, are filtered out by using weighted running averages; these will be discussed in a different paper.
INSTRUMENTATION The basic version of the lidar system was installed at McMurdo Station on Ross Island at 78°S and 167°E during the 1990 spring (Gobbi et al., 1991) in support of a balloon-borne counter experiment. Since the 1991 campaign, this system assumed primary importance in the study of polar stratospheric clouds and of the stratospheric temperature variability above McMurdo (Adriani et al., 1992). In 1992, the NdYAG laser emitting light pulses at the 532 nm wavelength, were increased in power (from 150 mJ to 250 mJ) and in fire rate (from 4 Hz to 10 Hz). The receiver is a 41.5 cm diameter Newtonian telescope with a field of view smaller than 1 mrad. The wavelength acceptance is reduced to a band of 0.15 nm around the laser wavelength by means of an interference filter. A 400 Hz mechanical chopper shuts off the photomultiplier during the time in which the atmospheric echo comes from the first 16-18 km, eliminating nonlinearity effects on the cooled photodetector. In such a way the noise is reduced to a few hundred pulses per second. The basic signal acquisition for temperature measurements is performed by means of a photoncounting chain located in the PC. Temperature is measured from 24 km up to 65 km. Standard observations last 30-60 minutes, with averages stored every 5 minutes. Measurements are necessarily restricted to night time since the daytime sky background noise is too high. In 1991 and 1992, the lidar operated at McMurdo Station in the period August~)ctober. In 1993 and 1994, the system operated during the Antarctic autumn, winter and spring in the period from 10 March to 10 October. In the period November-Feb-
Table 1. Number of observations employed in the analysis Month Mar Apr May Jun Jul Aug Sep Oct Total
1991
2 9 3 14
1992
1993
1994
Total
12 4 16
3 2 6 5 5 9 7 2 39
5 3 5 4 4 4 8 5 38
8 5 11 9 9 15 36 14 107
ruary, lidar data for temperature profiles were not available due to permanent daylight conditions at McMurdo. 107 night-time sessions of measurements were performed, with a mean of 13 temperature profiles per month, and for a total amount of about 60 hours. The seasonal coverage of measurements for each year is shown in Table 1. The method used for retrieving temperature profiles is the same as that described by Hauchecorne and Chanin (1980). Monthly average values of atmospheric pressure provided by the COSPAR International Reference Atmosphere (CIRA86) are employed as boundary conditions at the top of the lidar sounding. RESULTS
In our analysis, lidar profiles (one every 5 minutes) have been averaged along the whole period of measurement (0.5 to 1 hour) and vertically filtered with a resulting vertical resolution of approximately 6 km and an average temperature statistical error of 0.2 K at 25 km, 1.5 K at 40 km and 10 K at 55 km. W i n t e r temperature behaviours in 1993 a n d 1994
Large perturbations of temperature in the stratosphere during winter are called stratospheric warmings. The dynamical behaviour of the middle atmosphere during such a warming is known to follow a characteristic pattern of time development, as modelled and studied by various authors (see for example Andrews et al., 1987). There were, however, very few measurements at high latitude which extended to stratospheric altitudes on such occasions. The timeheight sections of the temperature profiles obtained by lidar during 1993 and 1994 are presented in Fig. 1. During 1993 the stratosphere was quiet with temperatures about 250-260 K in the upper stratosphere until mid-April when a warming was observed in the stratopause region. In the first part of May, a strato-
Lidar observations of stratospheric temperature above McMurdo Station, Antarctica
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spheric warming was detected with a temperature rise of 10 K in the region above 40 km, in coincidence with a cooling in the lower stratosphere. A deep cooling followed this first warming and the stratosphere reached a stable situation only 3 weeks later. A second warming appeared in mid-June with a consequent decrease of the 260 K line of about 6 kin; starting with this second event the whole region around the stratopause showed a general warming with an approximately isothermal shape. This shape survived for more than one month and was destroyed by the third and fourth wa.rmings at the beginning of August and September, respectively. These two last events were stronger and developed in a region of the stratosphere 10 km lower with respect to the first ones. A major warming followed at the beginning of October
in a wide part of the stratosphere (35-55 km) with a maximum of temperature of about 300 K near 45 km on 12 October. The 1994 winter was warmer above 40 km and the stratospheric warmings were stronger than in 1993, as is evident in Fig. lb. Moreover, the first minor warmings always appeared in the second half of April, but in a region of the stratosphere (60 kin) about 5 km higher than in 1993. After this first event, the following four minor warmings showed the same peculiarity of higher regions of interest and temperatures: in particular, the second one (mid-July) reached temperatures of 300 K around 55 km. This event lasted for weeks. No major warming was detected during 1994 up to 15 October, when lidar measurements stopped.
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For both years a temperature average for each profile in three height intervals (25-35 km, 35--45 km, 4555 km) was computed and plotted in Fig. 2. Each set of points was fitted by a polynomial curve of degree 3 in the period March-August. This approximation filters out all the temperature fluctuations except the seasonal behaviour. The shapes of these curves in 1993 and 1994 show strong differences: in the layers 25-35 km and 35-45 km at the beginning of the polar autumn, the 1993 average temperature was lower than the 1994 value and the cooling was less evident, with an averaged temperature minimum of 203 K at the
end of May and of 233 K at beginning of May for the two layers, respectively. These minima appear stronger in the 1994 data and delayed with respect to the 1993 ones: 199 K at middle of June and 228 K at middle of May. During the polar spring, this temperature difference (1994 colder than 1993) is maintained up to mid-September, when a sudden cooling strongly developed in 1993 in coincidence with a warming in 1994. The upper layer 45-55 km shows opposite behaviours: the minimum of the temperature is reached at the end of April for both years, but it is lower during 1993 and, moreover, the values during the year show reduced oscillations compared with 1994. An evident feature of both years is the time displacement of the temperature for the three layers: the minimum at 45-55 km occurs roughly two-three weeks earlier than at 35-45 km and six-seven weeks earlier than at 25-35 km. This is expected if, as discussed by Kanzawa (1989) and Hitchman et al. (1989) and modelled by Fisher et al. (1993) and Rosenfield et al. (1994), the warm polar stratopause observed during the winter is due to a gravity-wave-induced mean meridional flow converging over the pole, where it descends. The adiabatic heating associated with this intense downwelling into the strong polar confinement dominates the diabatic cooling in the thermodynamic equation and causes large departures from radiative equilibrium in a stratospheric region that descends during the winter as the westerly jet and easterly wave drag maxima descend. A peculiar climatological feature occurred during 1994 but was not evident in the 1993 data: the stratopause temperature reached a relative maximum during the polar night (July), exhibiting strong week-toweek variability, as shown in Fig. 3. The occurrence of such an event, sometimes indicated as "the separated polar winter stratopause" has not been completely elucidated and was explained by Hitchman et al. (1989) as a gravity-wave driven feature• Moreover, in the period April-August, the stratopause altitude and temperature values were higher (about 5 km and 10 K, respectively) in 1994 than in 1993 (Fig. 3) and no major warming was detected during 1994 before 15 October, contrary to the 1993 observations (Figs 1 and 2). Starting in April for both years, the time development of the temperature altitude profiles shows strong variations that lead to the peculiar features plotted in Fig. 4 for May 1994: a mesospheric inversion of temperature increases with time, warming the stratopause region that decreases its altitude; in a second phase the disturbance develops a peculiar feature of a split stratopause and occasionally, as in this
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case, a maximum of temperature at higher altitudes. This maximum is not necessarily the end of the upper stratospheric warming that develops further in a new similar feature. Such events occurred frequently, more often in 1994 than in 1993, in our data; and they lasted until the end of September. Observed interannual differences could reasonably be explained as the effects of an opposite quasibiennial oscillation (QBO) at 30 hPa over the equator during the years (Lait et al., 1989); although, in the extratropics, the signal should contain a 20 month period in addition to the 30 month equatorial QBO period (Tung and Yang, 1994). During QBO-easterly phase years, the Antarctic vortex is weaker and warmer and the propagation of gravity and planetary waves is enhanced (Poole et al., 1989; Kanzawa and Kawaguchi, 1990). The 30 hPa QBO wind phase persisted westerly only during the early winter 1993 and, starting in mid-July 1993, was easterly for the entire of 1994. In fact, our 1993 data show lower temperatures in the range 2535 km during the first half of the year (Figs 2 and 3). The presence of colder temperatures during the second half of 1994 could be explained as a result of lower values of solar activity during 1994 than in 1993; which could obscure an expected easterly QBO phase modulation of the polar stratospheric temperatures (Krzyscin, 1995).
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Seasonal variations of atmospheric temperatures are extensively modelled in terms of monthly means. One of the best available satellite climatologies of the middle atmosphere is the COSPAR International Reference Atmosphere, CIRA86 (Rees et al., 1990). It is therefore worthwhile comparing such a reference profile to monthly means obtained from our 107 temperature profiles (see Table 1) which are presented in Fig. 5 with l a limits of the average (dashed line). Contour plots of the monthly: average temperature are shown in Fig. 6a, and the differences with CIRA86 shown in Fig. 6b. Figures 5 and 6a reveal consistent inversions developing at approximately 60 km in April and May; these are not present in the CIRA86 reference temperatures. Considerable and systematic discrepancies occur in June and July in the range 30-40 km and in August in the range 40--50 km. Although the most considerable discrepancies occur in March and October, these are probably a result of our data set always starting in the second week of March and ending in the third week of October. Moreover, in April and May, the discrepancy above 45 km of the altitude is
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not significant due to the strong influence of the buildup phase of upper stratospheric warmings on the monthly averaged profiles. But no similar explanations were found in June, July and August. However, in most months and altitudes the agreement between the lidar data and CIRA86 is better than + 4 K (Fig. 6b) and CIRA86 profiles always fall inside the + 1ar range of monthly mean temperature variability of our data; which is reasonably good for the Antarctic polar region. A monthly me:m variability of our data for each year was calculated as one standard deviation of the averaged temperature profiles and plotted as a contour map in Fig. 7. The differences between 1993 and 1994 are immediately evident: during 1993 the mean variability of the stratosphere was less than 7 K, with peak values of about 9 K only in April and June-July in a small region (55-57 km and 42-44 km, respectively). On the contrary, the 1994 data show a mean variability of about 11 K but lower, in the region 3550 km during March and April, and rapidly increasing in the upper stratosphere in April, with peak values up to 20 K in June and 28 K in July (above 48 kin), as shown in Fig. 7. It is noticeable that just during the
most perturbed period (July 1994) two "quiet atmospheric layers" at about 46 and 33 km with almost no variations of temperature were observed (Fig. 7b). Similar stratospheric (and mesospheric) layers during warming events were frequently observed in winter at high latitudes in the northern hemisphere (Offerman et al., 1987; Hauchecorne et al., 1987) and they were tentatively interpreted as nodes of standing planetary waves (Plumb, 1982). A rapid downward transport of air from the mesospheric region into the stratospheric polar vortex of the southern hemisphere in winter and early spring is expected (Fisher et al., 1993). In fact, the observed atmospheric region of highest values of variability constantly descends during the winter, reaching the middle stratosphere in late spring. It is not surprising that similar behaviours were observed, in different years, by other authors in the mesopause region above the South Pole (Collins et al., 1994) due to the well known strong coupling between the upper stratosphere and mesosphere during the winter. CONCLUSIONS The lidar observations provide an interesting description of the temperature behaviour and varia-
G. Di Donfrancesco et al.
1398
A V R T E M P E R A T U R E V A R I A B I L I T Y - M c M u l d o 1993
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bility in the Antarctic stratosphere during 1993 and 1994. Differences between the two years have been pointed out: in 1994 a "separated winter stratopause" feature occurred and strong variability in the upper stratosphere was observed. In both years a split stratopause was observed during the build-up phase of the upper stratospheric warmings, with a higher occurrence in 1994. A major stratospheric warming was detected during early October 1993, but no major warming appeared in 1994 before 15 October when
our measurements stopped. Monthly averages showed fair agreement with a reference model (CIRA86), with evident discrepancies of about 10 K only during June and July in the range 30-40 km. A stratospheric region with high values of temperature variability was observed to descend during the winter from the stratopause region down to about 30 km altitude. Acknowledgements--work has been supported by Pro-
gramma Nazionale Ricerche in Antartide. Thanks are due to M. Viterbini for his technical assistance.
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