The BUV/Nimbus-4 total ozone data is analyzed with emphasis on the seasonal and interannual variability for the period April 1970 to April 1972. An objective analysis using a Fourier expansion shows the annual wave dominates at mid and high latitudes where the semiannual wave becomes significant in the tropics. A small interannual difference is detected and is most likely due to changes in the general circulation

Hilsenrath, E.

Schlesinger, B. M.

English

NASA Technical Reports Server

Paper No. 46
THE SEASONAL AND INTERANNUAL VARIABILITY OF TOTAL OZONE
AS REVEALED BY THE BUV NIMBUS-4 EXPERIMENT
Ernest Hilsenrath, Goddard Space Flight Center, Greenbelt, MD
Barry Schlesinger, Systems and Applied Sciences, Riverdale, MD
ABSTRACT
The initial study of the BUY/Nimbus-4 total ozone data
is directed toward an analysis of the seasonal and
interannual variability for the period April 1970 to
April 1972. An objective analysis using a Fourier
expansion shows the annual wave dominates at mid and
high latitudes where the semiannual wave becomes
significant in the tropics. A small interannual
difference is detected and is most likely due to
changes in the general circulation.
INTRODUCTION
The Nimbus-4 spacecraft was launched April 8, 1970 carry-
ing the Backscattered Ultraviolet (BUV) Spectrometer for measure-
ments of the columnar amount of ozone and the vertical distribu-
tion above 30 km. The BUV continued to operate until October 17,
1977 at which time it was turned off due to insufficient space-
craft power. The initial data was processed into ozone values
and reported by Krueger (1974) Ghazi (1976) as well as by others.
These early analyses used existing instrument calibrations, and
orbital and engineering data. In addition, these analyses ad-
justed the satellite measured total ozone values to Dobson using
a linear regression relationship developed from direct comparison
of BUV measured values and Dobson values. Because of the recently
recognized importance of the data, a task was initiated to repro-
cess the data and to continue processing the available data until
the time the instrument was turned off. Generally, the objectives
of the reprocessing activity were to: Establish a primary data
base with screened, Earth located, and calibrated data for con-
version to radiance values; improve the algorithms for processing
the primary data base of radiance values to ozone values; and
finally to validate the data and provide a consistent ozone data
base. Details on satellite coverage, data quality checks, algo-
rithm improvements, and validation tasks are being compiled and
will be published elsewhere.
An extensive analysis of the comparability of BUV with the
Dobson network has been conducted by Fleig, et al. (in prepara-
tion). This analysis shows from a comparison of nearly 4000
coincident measurements, using O0 code Dobson values and BUV
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overflight values within 2° of the station, that the average
difference changes by about l Dobson Unit (_0.3%) over the two-
year period studied here, indicating a high degree of stability
in the spaceborne measurement.
The first two years of total ozone data discussed here
covers the period April 1970 to April 1972 and contains about
300,000 total ozone values per year. The Nimbus-4 orbital param-
eters and the BUV instrument description are detailed in the Nim-
bus-4 User's Guide (1970) and not repeated here.. The BUV measure-
ment scheme has been previously described by Mateer et al. (1971)
and is briefly summarized below. Total ozone is derived from a
measurement of the solar irradiance, Fo, the backscattered
radiance, I, at 312.5 nm, 317.5 nm, and 339.8 nm and the effective
surface reflectance determined from the photometer. The measured
backscattered radiances are compared to those computed from 21
standard profiles sorted into three latitude zones compiled from
balloon and rocket data since the radiances are ozone profile
dependent. Any total ozone value can then be obtained from the
table of precomputed values containing the solar zenith angle
dependence, 8, and reflectance by interpolation. A description
of the standard profile compilation is given by Hilsenrath (1977).
This discussion of the total ozone is directed toward the
interpretation of its seasonal and interannual variability as
revealed by the spaceborne experiment over a two-year period. A
convenient way of depicting a time varying globally distributed
geophysical parameter is in terms of zonal means as a function of
time. This analysis is appropriate for total ozone since its
variability is dominated season and latitude. A rigorous analysis
of the seasonal and interannual variations detected by the BUV is
accomplished by a harmonic analysis of the zonal means as a
function of time where the first two components represent the
significant seasonal waves.
Time Latitude Cross Section
From ten degree wide latitude zones, a time latitude cross
section, depicted in figure la, is generated from the monthly
averaged daily zonal means. Total ozone is shown as contours in
20 Dobson Unit increments for the two year period (regions at
high latitudes in winter have no contours). Figure Ib immediately
below the cross section sharing the same absiscca is a calculation
of the mean global ozone value where this mean value is computed
for each month from the sum of the area weights of the zonal
means using extrapolation from lower latitudes to include the
polar night regions. (Note in proof, the values shown in this
figure are not final. Final processing indicate small changes in
absolute values, but the seasonal trends are essentially the same.)
The annual seasonal wave is clearly seen in the two hemi-
spheres. There is an ozone high and low at mid to high latitudes
in the spring and fall periods respectively in both years. How-
ever, there are distinctive differences in the seasonal trends for
each hemisphere.
278
I. In the Northern Hemisphere the spring maximum occurs
nearly simultaneously at mid and high latitudes with the maximum
value occurring near the pole with total ozone amounts of about
500 D.U. and a standard deviation of about + 50 Dobson Units.
2. In the Southern Hemisphere the spring maximum occurs
first in September at 65oS with ozone values substantially lower
than those in the Northern Hemisphere spring. The spring maximum
values occur about one month later in the polar regions. The
asymmetry Of the winter buildups in the two hemispheres can be
explained by the well known differences in the circulation fea-
tures in the two hemispheres as described by Newell, et ai.(1972),
DUtsch (1974) and others. In the Northern Hemisphere, eddy pro-
cesses associated with the intense winter planetary waves trans-
port ozone poleward from mid latitudes. Whereas in the Southern
Hemisphere the circulation is generally more zonal and poleward
transport is delayed.
3. In the tropical regions, the total ozone amounts and
the seasonal variation are considerably lower than at higher lati-
tudes. The average value is about 250 + lO D.U. The ozone mini-
mum is centered below the equator in ApFil and moves northward
as the year progresses. Also note the very rapid decrease in
the northern tropics, that appears to coincide with the Southern
Hemisphere spring maximum at 65oS in SeptembeG occurs in both
years.
Global Mean Ozone
A more rigorous description of the total ozone seasonal
variations for the two years will be qiven bv the harmonic analy-
sis and is discussed in the next section. However, at this point
the seasonal and interannual difference in the global mean ozone
amounts, shown in figure Ib, will be briefly discussed. In
general, the ozone is shown to decrease when comparing the same
months for the two year period. Analysis by Angell and Korshover
(1978) as well as others show a comparable decrease over the same
period using the Dobson network data. To some extent, however,
satellite data suffers from the same uncertainties as those com-
puted from the ground based data because of spatial and temporal
coverage biases. BUV makes no observations in the polar night and
in some cases coverage was not consistent from one year to the
next. The subject of long-term trends in global means as deter-
mined from satellite and ground networks is being studied else-
where (Heath, private communication) and is not pursued here.
The seasonal variation in the global mean, however, is
clear from figure lb. The global mean maximum occurs at the time
of the Northern Hemisphere spring maximum (April) where the mini-
mum occurs in December-January, after the Southern Hemisphere
spring maximum and prior to the Northern Hemisphere winter build-
up. The amplitude of the seasonal trend is about 20 D.U. (the day
to day variations in the global means is about l D.U.). The sea-
sonal amplitude presented here is about 5 times higher than that
reported by Keating (1978) in his analysis of 9 monthly mean
279
global ozone amounts determined from the Nimbus-4 IRIS. In addi _
tion, no correlation of the BUV measured mean values could be
found with solar activity associated with the I0.7 cm flux as was
reported in Keating's analysis of IRIS.
Harmonic Analysis
In order to more objectively characterize the _easonal
trends and the year to year differences in the global ozonm,a har-
monic analysis of the daily zonal mean data was performed. The
procedure will be discussed elsewhere (in preparation) but the
results are discussed below.
The annual wave or the first harmonic in the Fourier ex-
pansion is shown in figure 2a. The amplitude, given in Dobson
Units, is the value of the first Fourier coefficient and the per-
cent of the variance contributed by this harmonic is shown in the
portion of this figure. Both values are plotted as function of
latitude and both years are shown in the single figure to allow
for a direct comparison. It should be clear that the data shown
in this figure is a representation of information in figure la.
The important features are:
I. In the Northern Hemisphere the amplitude of the annual
wave increases with latitude. In addition, the variance is greater
than 90% from subtropical latitudes to the pole. These features
are essentially the same for the two years.
2. The amplitude of the annual wave is a minimum in the
tropics and is only a few Dobson Units. The variance is nearly
zero at the equator at least for the first year and the location
of the minimum amplitude shifts about I0 ° southward in the second
year.
3. In the Southern Hemisphere the amplitude of the annual
wave is a maximum at mid-latitude and decreases towards the South-
ern Hemisphere pole. The percent variance of the annual wave is
also very small near the pole indicating the south polar region
does not undergo a significant seasonal variation in total ozone.
At mid-latitudes there is a comparable I0 ° southward shift in the
location of the maximum amplitude. This could be the result of
year to year changes in the strength and location of the Hadley
cell circulation in the Southern Hemisphere. Interannual changes
in the strength of the Hadley cell have been demonstrated by Rosen
and Wu (1976) in the Northern Hemisphere.
The second harmonic of the Fourier expansion of the daily
zonal means provides information on the semiannual component in
the seasonal ozone trend. A semiannual component would be expected
because the Sun crosses the equator twice and a photochemical re-
sponse may be detected in the tropics. On the other hand, a pos-
sible correlation may exist with the semiannual oscillation in
the temperature and winds detected in the tropics in the upper
stratosphere. Though these phenomena are of great interest and
there may well be an ozone correlation found in a more detailed
analysis that that performed here, it will be shown below the
280
semiannual component detected in the satellite measured total
ozone for the two years studied here is the result of the inter-
hemispheric influence of the annual wave whose phases are dis-
placed by six months.
Figurei_depicts the semiannual component in the same for-
mat as the previous figure except the ordinate for both the ampli-
tude and the variance are half that of figure 2a. The important
features are as follows:
I. In mid-latitude for both years the semiannual wave is
small.
2. In the tropics this component becomes significant but
less so in the second year. From 0 to lO°S the amplitude of the
semi-annual component is comparable to the annual component and
the variance is about 40% for the first year. Clearly this fea-
ture should be associated with the location and strength of the
annual wave in the tropics, and further evidence of this associa-
tion will be given below.
3. in the Southern Hemisphere the semiannual oscillation
becomes important again and the amplitude is comparable to that of
the annual wave. However, the significance of this feature should
be regarded with some caution since nearly one-half of the values
appearing in the highest latitude zone are the result of extra-
polation from lower latitudes.
An additional solution to the harmonic analysis is the
phase; where for this study represents the time of the maximum
ozone value in each of the latitude zones for each harmonic com-
ponent. Figure 4 shows the time of the maximum ozone value as a
function of latitude for both the annual and semiannual waves for
each year. The ordinate is given in month as well as day of the
year. The significant aspects of the phase are summarized below.
It should be noted this result is consistent with what was seen
and interpreted from the time latitude cross section, figure I.
I. In the Northern Hemisphere the time of the maximum of
the annual wave occurs nearly simultaneously from mid to high
latitudes.
2. In the Southern Hemisphere after going through 6 month
phase shift at the equator the spring maximum occurs first at 40°S
in September and then spreads poleward and equatorward reaching a
maximum value in these regions in November.
3. With regard to the semiannual wave only the time of
the first maximum is plotted at latitudes where the variance is
greater than 2%. At low latitudes the first peak occurs about
April which is coincident with the Northern Hemisphere spring maxi-
mum of the annual wave. The second maximum of the semiannual com-
ponent occurs 6 months later or October which is coincident with
the maximum of the annual wave in the Southern Hemisphere sub-
tropics. This result does not exclude a possible response to the
solar equatorial crossing but it seems more likely that the semi-
annual wave detected at low latitudes in the first year is the
281
influence of the annual waves at higher latitudes in the two hemi-
spheres where the phases are six months apart.
Conclusion
A rigorous analysis of the seasonal and interannual vari-
ability of the total ozone measured by the BUV on Nimbus has
been initially performed to:
I. Provide information on the relative role of chemistry
and transport on the global distribution of ozone.
2. To help recognize transient phenomena in the strato-
sphere that may be important in Sun/weather relationships.
3. To better understand long-term trends associated with
solar cycles, other climate parameters, and man's activities that
may effect the environment.
This initial study has shown the BUV data for the period
processed between April 1970 and April 1972 has sufficient
accuracy and coverage to give a good description of the seasonal
variability and reveal some interannual differences in the total
ozone. These differences should eventually be correlated with
changes in the general circulation. This study provides a
starting point for ozone trend investigations from satellite
measurements that are important in climate and environmental
studies and a basis for studying the anomalous behavior of ozone.
Acknowledgment
The authors would like to acknowledge the remainder of the
Ozone Processing Team which was directed by Dr. Albert Fleig at
Goddard Space Flight Center. Special appreciation is given to
Dr. James Gatlin who retraced the BUV instrument performance
7 years after it was flown and to Dr. Ashok Kaveeshwar who organ-
ized and implemented the data processing schemes used to produce
the results given in this paper. Dr. Donald Heath is gratefully
acknowledged for his encouragement and useful discussions in the
interpretation of the data.
References
Angell, J. K., J. Korshover, 1978: "Global Ozone Variations: An
Update into 1976," Monthly Weather Review, Vol. I06, page 725-
737, May 1978.
D_tsch, H.U.: "The Ozone Distribution in the Atmosphere,"
Canadian Journal Chemistry, Vol. 52, No. 8, p. 1491-1504, 1974.
Ghazi, A., A. Ebel, D. F. Heath: "A Study of Satellite Observa-
tions of Ozone and Stratospheric Temperature During 1970-1971,"
Journal of Geophysical Research, Vol. 81, No. 30, pp. 5365-5376,
1976.
282
Hilsenrath, E., P. Dunn, C. Mateer, 1977: "Standard Ozone
Profiles from Balloon and Rocket Data for Satellite Theoretical
Model Input./' Contributions Presented at the Joint Assembly
CMUA Sessions IAOA/IAMAP, Seattle, Washington, August 1977.
Keating, G. M., 1978: "RelationshiP Between Monthly Variations
of Global Ozone and Solar Activity," Nature, 274.
Krueger, A. J., 1974: "Behavior bf Atmospheric Ozone Determined
from Nimbus Satellite Backscattered Data," Proceedings of the
International Conference on the Structure and Composition of
the Upper and Lower Atmosphere, Melbourne, Australia, IAMAP,
pp. 462-477, and NASA/GSFC X-Document 910-74-I01.
Mateer, C. L., D. F. Heath, A. J. Krueger, 1971: "Estimation of
Total Ozone from Satellite Measurements of Backscattered Ultra-
violet Radiances," Journal of Atmospheric Sciences, 28,
pp. 1307-1311.
Newell, R. E., J. U. Kidson, D. G. Vincent, G. J. Baer, 1972:
The General Circulation of the Tropica} Atmosphere and
Interactions with Extratropical Latitudes, MIT Press,
Cambridge, MA.
Nimbus Project, 1970: "The Nimbus-4 Users Guide," National Space
Sciences Data Center, Greenbelt, MD, 214 pages.
Rosen, R. D. and M. Wu, 1976: "Observational Study of the Inter-
annual Variability in Certain Features of the General
Circulation," Journal of Geophysical Research, 81, pp..6383-
6389.
283
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28B


The seasonal and interannual variability of total ozone as revealed by the BUV Nimbus-4 experiment

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