Middle atmosphere change through solar forcing

02731177/91 $0.00 + .50

Adv. Space Res. Vol. 11, No.3, pp. (3)9-(3)20, 1991 Printed in Great Britain.

1991 COSPAR

MIDDLE ATMOSPHERE CHANGE THROUGH SOLAR FORCING G. M. Keating* and C. Chen** *NASA Langley Research Center, Hampton, VA 23665, U.S.A. **ST Systems Corporation, Hampton, VA 23666, U.S.A.

ABSTRACT When attempting to isolate global middle atmosphere change associated with anthropogenic effects, it is crucial that the effects of natural atmospheric variations associated with solar ultraviolet changes be accurately taken into account. Recent studies concerning the response of the middle atmosphere to li-year solar variations, the relation between the li-year and 27-day responses, and the atmospheric response to 27-day solar variations are discussed. Strongest statistics are obtained on 27-day responses and these short-term responses can be used to understand and better estimate li-year responses. INTRODUCTION Of special importance when trying to isolate middle atmosphere change associated with possible anthropogenic effects is removal of natural atmospheric variations associated with solar ultraviolet (UV) changes. Direct isolation of the effects on the atmosphere of the li-year solar cycle would require an investigation of many solar cycles to be statistically significant but, unfortunately, the satellite data cover less than two solar cycles. Recent studies of possible 11-year variations in ozone and temperature are briefly reviewed here. A new approach for isolating and understanding solar forcing effects has recently been undertaken studying the statistically significant atmospheric response to variations associated with the 27-day rotation of the sun. In the upper stratosphere, the sensitivity of the atmospheric response to 27-day variations has been shown to be nearly as strong as to 11-year variations and thus statistically significant estimates can be made from the 27-day variations of the nature and magnitude of 11-year variations. The response of 03, HNO 3, NO2 and temperature to short-term solar UV variations have been isolated using various satellite data sets (Heath and Schlesinger, 1985/1/, Cille at al., 1984/2/, Keating et al., 1985/3/, Hood, 1986/4/, Keating et al,, 1986/5/; Chandra, 1986/6/; Keating et al., 1987/7/; Hood and Cantreli, 1988/8/; Keating et al., 1989/9/). The upper stratospheric 03 response is fairly well understood if temperature/ti’! effects are removed. However, there are unexplained short-term temperature/UV responses which may be related to dynamical heating. It is shown here that total column ozone has an unexpectedly strong response to the 27-day solar variations which may be linked to the unexpectedly strong long-term ozone variations recently detected in the lower stratosphere. In this paper, studies concerning the il-year responses, the relation between the il-year and 27-day responses, and the 27day responses of the atmosphere are discussed. 11-YEAR VARIATIONS Various theoretical modelers have calculated the response of the vertical structure of ozone to solar cycle and possible anthropogenic effects. Shown in Figure 1 (Brasseur, 1988/10/) is the calculated response of global mean ozone from the Brasseur at al. 2fl theoretical model to the solar cycle during the sun’s declining phase (1979-1986) and to anthropogenic trace gases over the same time interval. As may be observed, the two effects are comparable in magnitude and, in this instance, additive. On the other hand, the solar cycle component is positive during the recent increasing phase of the solar cycle resulting mostly in cancellation of the two effects. Shown in Figure 2 (Watson et al., 1988/11/) are the observed and theoretical changes in ozone

JASR 11:3-B

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G. M. Keating and C. Chen

THEORETICAL OZONE CHANGE DUE TO SOLAR AND ANTHROPOGENIC EFFECTS 1 ~7Q- 1966

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Fig. 1. 2-D Model calculation of percent ozone change between 1979 and 1986 averaged over all latitudes and seasons. Contributions of anthropogenic trace gases and solar effects are shown together with combined results (After Brasseur et al., 1988 /10/).

CHANGE IN 03 CONCENTRATiON VS. ALTITUDE 55

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Fig. 2. Percent changes in mid-latitude ozone profiles from 1979 to 1987. Differences based on SAGE I-Il (1979-80 vs. 1984-85) averaged between 20-50N latitude (triangles) and 20-SO’S latitude (squares); Umkehr data for northern mid-latitudes (1979 vs. 1986) (hatched Rreas); and Ozone Trends Panel model predictions (1979 vs. 1985) for this latitude band (After Watson et al. . 1988 /11/).

Middle Atmosphere Change

TABLE I

DATA

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Total Column Ozone Response to 11-Year Solar Cycle

TIME INTERVAL

OZONE RESPONSE* (1/100 F 10 UNITS)

INVESTIGATORS

Dobson

1970-1983

1.3

Oehlert,

Dobson

1970-1984

0.71 ±0.40

Reinseletal.,

Nimbus 7 SBUV

1978-1985

0.97 ±0.61

Reinseletal., 1988/14/

UMKEBR

1977-1987

1.62 ±0.84

Reinseletal., 1989 /15/

Dobson (53-64N)

1965-1986

1.8 ±0.6

Rowlandetal.,

1989/16/

Dobson (40-5rN)

1965-1986

0.8 ± 0.7

Rowlandetal.,

1989 /16/

Dobson (3O-39N)

1965-1986

0.1 ±0.6

Rowlandetal., 1989 /16/

Dobson (Global)

1960-1987

1.9

Angell, 1989 /17/

Dobson (N. Temporate)

1960-1987

1.5

Angell, 1989 /17/

Dobson (Tropics)

1960-1987

1.9

Angell, 1989 /17/

*Between 1979 and 1986 (solar 2) max. to mm.) F10 dropped 130 units (F10 unit equals to 10-22 Wm

1986 /12/ 1987/13/

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G. M. Keating and C. Chen

during the declining phase of the solar cycle. The observations show the difference between satellite based ozone measurements from SAGE II (1984-5) and SAGE I (1979-80) ozone measurements and also the difference between ground-based Umkehr ozone measurements between 1986 and 1979. The theoretical response is based on a set of models used by the ozone trends panel /11/ and is very similar to the independent solution shown in Figure 1 /10/. As may be seen, the theoretical response falls between the two sets of observations. It is important to emphasize that the anthropogenic component of the net observed change shown in Figure 2 cannot be isolated without making a large correction for the solar cycle component. Recent estimates of the total column ozone response to the 11-year solar cycle are shown in Table 1. Oehlert (1986/12/) found when studying Dobson data over .the interval 1970-1983 that unless he corrected for the solar component, he could not detect a statistically significant trend in the data. Since then, as indicated in the table, a number of other statisticians have corrected for the solar component in order to isolate trends in total column ozone. However, there is a wide range in their estimates of this response due principally to the very limited time series of good observations. Shown in Figure 3 are results from a study by Angell (1989/17/) of long-term variations in Dobson measurements of total column ozone in different latitude bands. The effects of the increasing ozone hole are clearly evident in the south polar data. On the bottom of the figure the variations in the combined global mean total ozone are shown to compare favorably with the 11-year variations in sunspot number. One of the advantages of studying the global mean is that dynamical effects somewhat cancel allowing better resolution of 11-year photochemical variations (Keating, 1981/18/). The long-term response of temperatures to solar forcing is understood less than the ozone response. Labitzke (1987/19/) has shown evidence that major stratospheric winter warmings have occurred during high solar activity when there were westerlies at the equator near 45 nib and during low solar activity when there were easterlies. This strong response to solar activity modulated by the quasibiennial oscillation in equatorial winds is not presently understood. During other seasons the QBO does not seem to play a role. RElATION BETWEEN 11-YEAR AND 27-DAY VARIATIONS The difficulty of accurately determining the response of the atmosphere to 11year solar variations is partially due to limited time series of good atmospheric data. In order to obtain convincing statistics on the existence of a cyclic oscillation, about 10 cycles generally need to be studied. Other problems are associated with instrument sensitivity drifts in satellite and ground-based instruments and correcting for possible climatological and anthropogenic variations when attempting to isolate solar effects. A new approach has recently been developed to study solar forcing and that is to study short-term variations related to the 27-day solar rotation. Using 27day variations, with only one year of satellite data the atmospheric response to more than 12 “solar cycles” can be obtained while obtaining data over as many li-year cycles would take more than a century. In most cases more than a year of satellite data is available allowing the study of even more “solar cycles.” Of course, studying 27-day variations eliminates the problems associated with long-term instrument drift and long-term climate and anthropogenic effects. Shown in Figure 4 is the character of solar UV variations in the 27-day time scale (Heath et al., 1984/20/). The ratio of the solar flux on July 20, 1980 to the average of fluxes on July 6 and August 4, 1980 is shown as a function of wavelength (in nanometers). The ratios show~1give some of the largest 27day variations observed during the early 1980’s. In general, relative variations increase with decreasing wavelength very similar to the case shown. At 205 nm the peak-to-peak variation is shown in Figure 4 to exceed 61. For comparison, estimates of the il-year variation at 205 tim range from 4 to 81. Thus the magnitude of 27-day solar UV variations can be comparable to 11-year variations. The 205 tim radiation has been generally chosen as the solar UV index in studies of the atmospheric response to short-term solar UV variations. This index is chosen because 205 tim radiation photodissociates 02 allowing production of 0~and the wavelength is short of the aluminum edge (208 ram) where solar variability decreases sharply.

Middle Atmosphere Change

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TOTAL OZONE CHANGE AND SOLAR ACTIVITY I

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Fig. 3. Smoothed annual percent changes in observed total column ozone (Dobson) for five climate zones and the world compared with sunspot number (After Angell, 1989 /17/).

SHORT-TERM SOLAR UV VARIABILITY

5,

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Wavelength A nm Fig. 4. Ratio of peak UV flux to average of proceeding and following minimum fluxes over a 27-day period as function of wavelength (After Heath et al., 1984 /20/)

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G. M. Keating and C. Chen

(%I%)

OZONE SENSITIVITY TO SOLAR UV

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WIth Temperature feedback

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03/ Fig. 5. Theoretical ozone sensitivity (percent increase of ozone for a 1 percent increase in 205 ram irradiance) to 13.5-day, 27-day and li-year solar cycles as a function of pressure (After Brasseur cC al., 1987 /10/).

RESPONSE OF 03 (LIMS) TO SHORT-TERM SOLAR tN VARIABILITY (SBW) ZONAL MEANS BETWEEN t 40° I I •

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(979 01W NUMBER Fig. 6. Percent residuals relative to 27-day running means of 205 ram solar UV irradiance and Nimbus 7 LIMS 2 nib ozone after empirical correction for temperature feedback. The ozone residuals represent averages between ±40’ latitude of the equator. The residuals are 5-day running means. A high correlation is shown between U’! variations and the ozone response (After Keating cC al., 1985 /3/).

Middle Atmosphere Change

Shown in Figure 5 is the theoretical sensitivity of ozone to solar UV variability on the 27-day and li-year time scales, as well as the sensitivity on the 13.5-day time scale (Brasseur et al., 1987 /21/). The percent increase in ozone for a 11 increase at 205 ram is shown as a function of pressure. Relative variations at other solar wavelengths are taken into account in the calculations and are essentially in accord with the previous figure. At a pressure level of say 2 mb the observed sensitivity determined on the 27-day time scale should be within 101 of the sensitivity on the 11-year time scale. As indicated in Figure 1, this is also an altitude where there should be strong anthropogenic effects. Thus it is clear how the statistically significant measurements of 27-day variations can be used to better estimate 11-year variations allowing isolation of possible anthropogenic effects. Of course, the observations of 27-day variations will actually be used to improve theoretical models of solar forcing on the 27-day time scale and those theoretical models can then estimate the difference between the 27-day and 11year sensitivities. A similar approach can be used in relating 27-day variations in temperature to li-year variations. 27-DAY VARIATIONS Shown in Figure 6 (from Keating et al., 1985/3/) is a clear example of the response of middle atmosphere ozone to solar UV variability associated with the rotation of the sun. The figure shows residuals relative to a 27-day running mean of two parameters: (1) 2 mb ozone averaged within 40” of the equator and (2) 205 ram solar irradiance. The ozone data, which have been corrected empirically for temperature feedback effects, were obtained by the Nimbus 7 LIMS instrument while the 205 ram solar irradiance data were simultaneously obtained by the NIMBUS 7 SBUV instrument. Increases in 205 ram irradiance result in the increased photodissociation of 0 from 02 with subsequent production of 03. The response of the middle atmosphere to the solar IJV variations is clearly detected. By studying residuals relative to a 27-day running mean possible effects of instrument drift and long-term climatological variations are removed. From a regression analysis, the percent change in ozone for a one percent increase in 205 nm irradiance can be determined. This parameter is referred to here as sensitivity. The two time series can also be shifted relative to one another to determine the response time giving the highest correlation between the two time series. Thus a sensitivity and response time can be determined at each pressure level. Shown in Figure 7 (from Keating et al., 1987/7/) are observed and predicted 0,/UV sensitivities as a function of pressure and approximate altitude. The sensitivities are shown for two Nimbus 7 ozone data sets obtained from the LIMS and SBUV measurements. Again, the sensitivities are expressed as the percent increase of ozone for a 1% increase of 205 ram solar irradiance. As may be identified, the two data sets, one in the IR and the other in the UV, give results which are generally consistent with one another. Also shown is the theoretical sensitivity of ozone assuming 27-day sinusoidal variations in the solar UV (Brasseur et al., 1987/21/). Solar UV variation as a function of wavelength are assumed to be essentially in accord with the relative variations shown in Figure 2. The results from the i-D time-dependent theoretical model are in fair agreement with observations. Thus there appears to be a fairly good understanding of the response of ozone vertical structure to solar UV variability when temperature feedback effects are removed. It is generally assumed that the ozone/UV response is weakened with negative temperature feedback effects. However, temperature feedback actually strengthens the ozone response. This occurs because the response times of stratospheric temperatures to solar UV variability are much longer than predicted by theory. This is discussed in some detail by Keating et al. (1987/7/) and by Brasseur et al. (1987/21/). Shown in Figure 8 is the sensitivity of total column ozone to 205 ram variability as determined from analysis of NIMBUS 7 TOMS total column ozone data obtained from 1979-1981. In this analysis the spring/fall data were combined (March, April, May, September, October, and November) and analyzed as a function of latitude. An analysis by Brasseur and DeRudder, using their lD time-dependent model, indicates net response times of total column ozone to solar UV variations of 3 to 4 days which is in general accord with low latitude observations. However, the observed total column ozone sensitivities are much stronger than the value predicted in the 1D theoretical. model. Furthermore, the sensitivity becomes even stronger at higher latitudes. Sensitivities with uncertainties exceeding 30% were excluded.

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0. M. Keating and C. Chen

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OBSERVED AND PREDICTED 031UV SENSITIVITY WITHOUT TEMPERATURE FEEDBACK dIMS: 11/78—5/79 oSBUV: 12/78

0.5

10/82

—

rasseur et aI., 50 1986 1.0

-

Pressure, mb 2.0

I—•0—4 -

Approx. altItude, km 40

5.0~

10.0. 0

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.10

~

.20

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.30

.40

(O3~03)1031(1205

30 .60

.50

1205)l’1205

Fig. 7. Observed and predicted ozone/U’! sensitivity (percent increase in ozone for 1 percent increase in 205 ram irradiance) without temperature feedback as futaction of pressure and approximate altitude (After Keating et al.,, 1987 /7/).

PERCENT INCREASE IN TOTAL COLUMN OZONE FOR ONE-PERCENT 205-NM INCREASE TOMS/UV RESPONSE 1979 - 1981

FALL/SPRING 0.76 0.72 0.68 0.64

0.60 0.56 0.52 0.48

•

KEATh4O ~ a~ IOBSESVATIONS)

0. 32

0.00~ 0

20

40

60

80

LATITUDE, deg Fig. 8. Observed and predicted total column ozone/U’! sensitivity (percent increase in total ozone for 1 percent increase in 205 ram irradiance) combining the spring and fail months as a function of latitude. Observations of shortterm variations are based on Nimbus 7 TOMS total ozone data and SBUV 205 ram solar measurements from 1979-1981. The theoretical calculation is based on a study by Brasseur and DeRudder using the lD Brasseur et al. model.

Middle Atmosphere Change

Shown in Figure 9 are total column ozone sensitivities for the winter (December, January and February in the Northern Hemisphere and combined with June, July, and August in the Southern Hemisphere) and summer (June, July, August in the Northern Hemisphere combined with December, January and February in the Southern Hemisphere) periods. Again, sensitivities where uncertainties exceeded 30% have been omitted. In this case all the sensitivities again exceed the lD theoretical value. The unexpectedly strong total ozone/UV sensitivities shown in Figures 8 and 9 appear to be in accord with the increased ozone depletions measured at the lowest altitudes by SAGES 1 and 2 (see Figure 2) during the period of decreasing solar activity. Thus these strong mid latitude ozone depletions in the lower stratosphere may be related to solar activity variations. There is evidence now from Dobson data of stronger depletions in total column ozone in winter than other seasons (Rowland et al., 1989/16/, Watson et al., 1988/11/). These new trends should now be reevaluated taking into account the latitudinal/seasonal variations in the total column ozone sensitivity to solar U’! variability. From analysis of the four years of highest quality Nimbus 7 SAI4S temperature data, the temperature response to short-term solar UV variability has also been determined (Keating et 81., 1987/7/, Hood and Cantrell, 1988/8/). The approach is similar to that used in isolating the ozone response. Shown in Figure 10 is the change in temperature per IX increase in 205 ram irradiance (Keating et al., 1989/9/) as a function of latitude combining data from all months. Maximum mean sensitivity in the stratosphere occurs near 1.5 mb. According to theory (Brasseur et al., 1987/21/), temperature sensitivity to 11-year variations at 1.5 mb is within 10 percent of the sensitivity to 27-day variations. If 205 ram irradiance varies about 8% over the solar cycle, the results from Figure 9 would indicate that the temperature near the equator would vary by slightly more than 1K. In a recent study by Gelman et al. (1990 /22/) of equatorial temperatures derived from the series of NOAA operational satellites from 1978-1989 they estimate long-term solar and trend components. Their estimated solar cycle component of equatorial temperatures near these pressures is consistent with the temper~ture response to short-term solar IN variations given in Figure 10. Other studies of 27-day responses in the stratosphere have included analyses of ground based ozone data (Danilin et al., 1989/23/), analyses of planetary waves (Ebel et al., 1986/24/) analyses on lidar measurements of temperature (Chanin and Keckhut, 1990/25/) and analysis of the nitric acid response (Keating et al., 1986/5/). Studies of 27-day responses in the mesosphere include analyses of the ozone response (Keating et al., 1987/7/, Eckman, 1986/26/, and Aiken and Smith, 1986/27/) and detection of the temperature response (Keating et al., 1987/7/, Chanin and Keckhut, 1990/25/). CONCLUSIONS The following may be concluded concerning changes in the middle atmosphere through solar forcing: 1.

Accurate determination of solar forcing effects is crucial for early detection of anthropogenic effects.

2.

Limited evidence of solar forcing on the 11-year time scale is becoming available including the response of temperature and total column ozone. However, in most cases the time series are too short for accurate determinations.

3.

On the other hand, the response variations can be determined to year of data allows analysis of effects of instrument drift and variations can be removed.

4.

In the upper stratosphere it can be shown that the change of ozone and temperature on the 27-day time scale should be within 10% of the change over the 11-year time scale per unit change in solar IN variations. At lower altitudes a theoretical correction can be made to account for differences in time constants. Thus 27-day variations can be used to better estimate li-year variations.

of the middle atmosphere to 27-day solar good statistical accuracy because even one over 12 “solar cycles.” In addition, unrelated long-term climatological

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0. M. Keating and C. Chen

PERCENT

INCREASE IN TOTAL COLUMN OZONE

FOR ONE-PERCENT 205-NM INCREASE TOMS/IJV RESPONSE 1979 - 1981 WINTER

0.57 0.54

SUMMER

0.51. 0.48 0.45

0.42 ~ 0.39 PI~ 0.36 ~i:~ 0.33 0.30 0.27 0.24

KEATING & CHEN (OBSERVATIONS)

BP.ASSEUR & DCRUDDER (THEORY)

0.03

—

0.00 80

60

40

20

0

LATITUDE,

20 deg

40

60

80

Fig. 9. Same as Figure 8 but for winter (December, January and February in Northern Hemisphere combined with June, July. August in Southern Hemisphere) and summer data compared to same 1D theoretical model.

SAMS

YEARS I

-

4

-5

~

____

___

LATITUDE, deg

Fig. 10. Observed temperature/IN sensitivity (degrees K change for a 1 percent increase in 205 ram irradiance) as a function of latitude and pressure. Based on all four years of highest quality Nimbus 7 SAMS data (After Keating et al., 1989 /9/).

Middle Atmosphere Change

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5.

The response of ozone vertical structure, total column ozone and temperature to short-term solar IN variations have been determined. The ozone vertical structure response is in fair accord with theory when temperature feedback effects are removed. The response times of stratospheric temperature to solar IN variability are unexpectedly long. The total column ozone/lilY response is much stronger than estimated by a 1D photochemical model. This latter effect may help explain the large decreases of ozone occurring during decreasing solar activity in the lower stratosphere as observed by a comparison of SAGE 1 and 2 data.

6.

It is crucial that more emphasis be placed on empirical and theoretical studies of the response of the atmosphere to short-term solar variations so that the li-year variations can be better estimated and understood which in turn can allow earlier detection of predicted and unexpected anthropogenic effects. The temperature responses to solar forcing have been identified as the most difficult to understand. REFERENCES

1.

D. F. Heath and B. M. Schlesinger, Global Responses of Stratospheric Ozone to Ultraviolet Solar Flux Variations, in: Atmos~hericOzone. Proceedinas of the Ouadrerania]. Ozone Symuosium. Halkidiki. Greece, eds. C. S. Zerefas and A. Chazi, D. Reidel, Hingham, Mass 1985, p. 666.

2.

J. C. Gille, C. M. Smythe and D. F. Heath, Observed ozone response to variations in solar ultraviolet radiation, Science 225, 315 (1984).

3.

G. M. Keating, G. P. Brasseur, J. Y. Nicholson III and A. DeRudder, Detection of the response of ozone in the middle atmosphere to short-term solar ultraviolet variations, GeoDhys. Rca. Lett, 12, 449 (1985).

4.

L. L. Hood, Coupled stratospheric ozone and temperature responses to short-term changes in solar ultraviolet flux: an analysis of Nimbus 7 SBUV and SlIMS data, J. Geoohvs. Res., 91, 5264 (1986).

5.

G. H. Keating, J. Nicholson III, G. Brasseur, A. DeRudder, U. Schmailzl and H. Pitts, Detection of stratospheric HNO 3 and NO2 response to shortterm solar ultraviolet variability, Nature, 322, 43 (1986).

6.

S. Chandra, Solar and dynamically induced oscillations in the stratosphere, J. Geophys. Res., 91, 2719 (1986).

7.

G. M. Keating, H. C. Pitts, G. Brasseur and A. DeRudder, Response of middle atmosphere to short-term solar ultraviolet variations: 1. Observations, J. Geot,hvs. Res., 92, 889 (1987).

8.

L. L. Hood and S. Cantrell, Stratospheric ozone and temperature responses to short-term solar ultraviolet variations-reproducibility of low-latitude response measurements, Annales Geo~hysicae, 6, 525 (1988).

9.

G. M. Keating, M. C. Pitts and G. Brasseur, Recent detection of the response of the middle atmosphere to short-term solar ultraviolet radiation, in: Ozone in the Atmosphere, eds. R. D. Bojkov and P. Fabian, A. Deepak Pubi. Co. 1989, p. 375.

10.

G. Brasseur, M. Hitchman, P. C. Simon, and A. DeRudder, Ozone reduction in the 1980’s: A model simulation of anthropogenic and solar perturbations, Geotahys. Res. Lett., 15, 1361 (1988).

11.

R. W. Watson and Ozone Trends Panel, M. J. Prather and Ad Hoc Theory Panel, and M. J. Kurylo and NASA Panel for Data Evaluation, Present State of Knowledge of the Upper Atmosphere 1988: An Assessment Report, NASA RP-ET1 w448 173 m483 173 lSBT

J.2.Q~ (1988). An update through 1983,

~L..

12.

G. W. Oehlert, Trends in Dobson total ozone: Georahvs. Rca., 91, 2675 (1986).

13.

G. C. Reinsel, G. C. Tiao, A. J. Miller, D. J. Wuebbles, P. S. Connel, C. L. Mateer and J. L. DeLuisi, Statistical analysis of total ozone and stratospheric Umkehr data for trends and solar cycle relationship, L.. Geophys. Rca., 92, 2201 (1987).

.

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14.

C. C. Reinsel, G. C. Tiao, S. K. AIm, K. Pugh, S. Basu, J. J. DeLuisi, C. L. Mateer, A. J. Miller, P. S. Connell and B. J. Wuebbles, An analysis of the 7-year record of SBUV satellite ozone data: Global profile features and trends in total ozone, J. CeoDhys. Rca., 93, 1689 (1988).

15.

C. C. Reinsel, C. C. Tiao, J. L. DeLuisi, S. Basu and K. Carriere, Trend analysis of aerosol - corrected Umkehr ozone profile data through 1987, ~L GeoDhva. Rca., 94, 16,373 (1989).

16.

F. S. Rowland, N. R. P. Harris, R. D. Bojkov and P. Bloomfield, Statistical error analysis of ozone trends - winter depletion in the Northern Hemisphere, in: Ozone in the AtmosDhere, eds. R. D. Bojkov and P. Fabian, A. Deepak Publishing Co. 1989, p. 71.

17.

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