JOURNAL
OF GEOPHYSICAL
RESEARCH, VOL. 90, NO. D5, PAGES 7967-7976, AUGUST 20, 1985
A Global Climatology of Total Ozone from the Nimbus 7 Total Ozone
Mapping Spectrometer
KENNETH P. BOWMAN AND ARLIN J. KRUEGER
Laboratory
for Atmospheres,
NASA GoddardSpaceFlight Center,Greenbelt,
Maryland
A globalclimatology
of totalcolumnozonehasbeencomputed
from4 yearsof dailyobservations
by
the total ozonemappingspectrometer
aboardthe Nimbus7 polar-orbitingsatellite.Observations
were
madeat localnoon;no observations
are availablein polarregionsduringthe polarnight.The orbital
data havebeenareaaveraged
onto a regular5ø by 5ø latitude-longitude
grid.The globalcoverage
is
more than 90% completeduringthe last 3 observingyears(October1979to September1982)and
approximately
70% complete
duringthefirstyear(October1978to September
1979).Globalmapsof the
temporalmeanand rms varianceand the amplitudeand phaseof the annualand semiannualharmonics
are presented.
Similaranalysesof the zonalmeanvaluesare givenalongwith a time-latitudesectionof
thezonal-mean
totalozone.Thisclimatology
shouldbe usefulbothfor validatingnumericalmodelsand
for inspiringtheoreticalanalyses.
INTRODUCTION
Becauseozone strongly absorbssolar ultraviolet (UV) radiation through photodissociation, it provides an internal
sourceof heat in the middle and upper atmosphereand forms
a crucial link between atmosphericradiation, chemistry,and
dynamics. For this reason alone the distribution of ozone in
the atmosphereis of interest,but ozone servesa secondimportant function: it protects living things at the earth's surface
from the harmful effects of UV radiation. In recent years,
concernhas arisenthat mankind may be affectingatmospheric
ozone by releasingphotochemicallyactive substances,such as
fluorocarbonsor nitrous oxides,into the atmosphere.Because
of the possiblehealth effects,this concernhas provided a very
practical reason for studying the chemical and dynamical processescontrolling ozone.
Observationsof the ozone distribution in the atmosphere
are needed for several purposes.The observed ozone distribution is used as an external parameter in some dynamical
and radiative models, and in the lower stratosphere,where
chemistryis lessimportant than transport, ozone may serveas
a tracer of atmosphericmotions. Model studiesof ozone itself
require thorough and accurate observations of ozone and
other chemically active speciesto verify that the models can
reproduce the existing distributions of those constituents.Experiments may then be performed with the models to determine the sensitivity of ozone to various perturbations. A
review of the current understandingof ozone transport and
the annual cycleof ozone can be found in Rood [1983].
Ozone has been observedfrom the ground for many years
with Dobson spectrophotometers,which measure the differential absorption of solar UV radiation at different wavelengths by the ozone [Dobson, 1931]. However, Dobson instrumentsare concentratedin northern hemisphereland areas
and provide poor global coverage.In recent years, many new
instruments have been developed to measure ozone both in
situ and remotely. The ability to provide global coverageand
the advantagesof using a singleinstrumenthave led to development of severalsatellite-bornedevicesfor measuringozone.
Hilsenrath et al. [1979], Hilsenrath and Schlesinger[1981],
and Tolson [1981] have analyzeddata from the backscattered
ultraviolet (BUV) instrumenton Nimbus 4. Becausethe BUV
was a nadir viewer(nonscanner),
samplingwas sufficientonly
to determine monthly and zonal means. The BUV data also
have many hiatuses.Nimbus 4 also carried an infrared interferometric spectrometer(IRIS), from which temperatureand
total ozone in the stratospherecould be estimated [Prabhakara et al., 1976]. Use of infrared wavelengthsallows ozone
retrievals at night, unlike the backscattered ultraviolet
method,but the inversionof IR radiancedata is considerably
more difficult than the BUV measurements.
The Nimbus
7 satellite carries two BUV
instruments:
the
solar backscatteredultravioletspectrometer(SBUV), a nadir
viewer,and the total ozonemappingspectrometer(TOMS), a
scanner.Vertical profiles of ozone and total column amounts
have been obtained from the SBUV [Nagatani and Miller,
1984; McPeters et al., 1984]. The TOMS instrument measures
only total ozonebut provideshigh spatialresolution.
This paper presentsa global climatology of total ozone
based on the 4 years of TOMS measurementsprocessedto
date. The global coverageand samplingfrequencyof this data
is superiorto anythingcurrentlyavailable.
METHODS
The TOMS
Data
Set
Nimbus 7 was launched in October 1978 in a near-polar,
sun-synchronousorbit and makes approximately 13.8 orbits
per day. TOMS observations
are madeon the ascendingnode
of the orbit at ground timesnear local noon.
The TOMS scansperpendicularto the orbit track to provide measurementsover the entire sunlit portion of the earth
once per day. Becausethe instrumentmeasuresbackscattered
solar radiation, no measurementsare available in regions of
polar night. The diameterof the field of view rangesfrom 50
km at nadir to 250 km in the farthest off-nadir
scan tracks.
Becausethe length of the scan across the orbital track is
• 3000 km, scan swaths from different orbits overlap in
middle and high latitudes, allowing multiple measurements
during 1 day.
The spectrometer
measures
the intensities
o• directand
Copyright 1985by the AmericanGeophysicalUnion.
backscattered solar ultraviolet radiation in six wavelength
channels.Two channelsfree of ozone absorption are used to
determine the effective background albedo. The remaining
Paper number 5D0365.
0148-0227/85/005D-0365505.00
7967
7968
BOWMANAND KRUEGER'GLOBALCLIMATOLOGY
OFTOTALOZONE
DAILY
GLOBAL
COVERAGE
100-
8O
20
0
0
1
i
i
2
3
4
TIME (YEARS FROM I OCT 78)
Fig. 1. Percentage
of thesurface
of theearth(whitearea)for whichdatais availablefor eachdayin the4-yearobserving
period.
ozone measurementshas shown the precision of the TOMS
retrievals to be better than 2% [Bhartia et al., 1984]. On the
average the TOMS measurementsare 6.6% smaller than
Dobson spectrophotometermeasurements.Most of the bias
appears to be causedby differencesin the ozone absorption
coefficientsused to calculate the ozone amounts. Processing
and calibration differencesexist between data years but are
four channelsare usedin pairs to provide two estimatesof the
total column ozoneby the differentialabsorptionmethod.The
algorithms used to compute the total ozone are describedin
Fleig et al. [1982] and addenda.
For this analysisof the large-scalefeaturesthe ozone data
were area averagedonto a regular 5ø latitude by 5ø longitude
grid from higher-resolutiongridded data [Huang and Lee,
1983]. For each day in the observingperiod, each 5ø by 5ø
grid cell was assignedan ozone value fl and a weight w. All
ozone values are in Dobson units (milliatmospherecentimeters). The weight is the area within each cell for which
measurementsare available on that day. If there were no good
less than 1%.
The observingyear for this study runs from October 1 to
September30. Four years of griddeddata are presentlyavailable (October 1978 through September1982).The fraction of
the globe for which measurementsare available on each day
measurements, w was set to zero, and fl was left undefined. in the 4-year period is shown in Figure 1. Only one day of
Ozone valueswere not interpolatedin either spaceor time. All data is available during October 1978. From October 31,
of the available data has been used in the analysesshown in
1978, through June 12, 1979, the TOMS was operated apthe following sections.
proximately 5 daysout of 6. During June 1979, the instrument
Intercomparison of the TOMS data with ground-based was inoperative for 6 consecutivedays (June 13-18). After
AVAILABILITY OF DAILY OBSERVATIONS (%)
90N
• I i I i i i
I
I
I
I
90N
50,=
60 -I
- 3O
-0
- 3O
6O
6O
50
90S
180W
I
•
•
I
I
•
i
i
i
150
120
90
60
30
0
30
60
90
90s
120
150
180E
Fig. 2. Distributionover the earth of the percentage
of daily observations
availablefrom the 4-yearobservingperiod.
Note the lossof data in high latitudesas a resultof the polar night and the regionof low coverageover the international
dateline.
BOWMAN
ANDKRUEGER:
GLOBAL
CLIMATOLOGY
OFTOTAL
OZONE
7969
TIME-MEAN TOTAL OZONE (DU)
TOMS
90N
ß
I
I
I
I
I
I
I
I
90N
60
6O
30
3O
0
0
30
30
60
60
90S
180W
I
i
i
I
I
I
i
I
I
!
I
150
120
90
60
30
0
30
60
90
120
150
90S
180E
GROUNDBASED
80N
80N
60
60
3O
30
3O
30
6O
60
80S
80S
180W
150
120
I
I
I
90
30
60
I
0
30
60
90
120
150
180E
Fig.3. TOMStime-mean
totalozone
f]0computed
fromequation
(2)forthe4-year
observing
period
(Dobson
units),
and the time-mean
total ozonederivedby London
andOltmans
[1978/1979]
fromground-based
Dobsonand M83
measurements
for the periodJuly 1957to June1975.
June19,whentheinstrument
beganto functionagain,it was dingthe data onto daily mapsby wholeorbits.The highest
operatedcontinuously.
Therefore
fromJuly1979to theendof data availabilityis in middle latitudes,where multiple
the datasetthe datacoverage
is veryhigh.The semiannual measurements
during 1 day are possibleand no polar night
periodicity
apparent
in Figure1 afterthefirstobserving
year occurs.
is causedby the lossof data duringthe alternating
polar
nightsin the two hemispheres.
Thespatialcoverage
of thedatafortheentire4-yearperiod
is shownin Figure2. The major systematic
featuresare the
decrease
in dataavailability
towardthepolescaused
by the
polar nightsand the oval regionof lowerdata availability
straddling
thedateline.
Thisovalfeatureis an artifactof grid-
Statistical
methods
Area-weighted
zonalmeansI-D] and globalmeansf) were
computedby usingthe standardformulafor weightedmeans'
[fl]j = Z wuflu/• wu
i
i
(la)
7970
BOWMANAND KRUEGER'GLOBALCLIMATOLOGY
OFTOTALOZONE
RMS DEVIATION OF DAILY OBSERVATIONS (DU)
•3N
I
I
I
I
I
I
I
I
I
I
I
30
30
0
0
<10
<10
30
30
90S
180W
1•
i
i
i
I
•
I
I
I
I
i
120
90
60
30
0
30
60
90
120
1•
90S
180E
Fig. 4. Rootmeansquare
deviations
ofthedailyvalues
oftotalozonefromthetimemean(Dobson
units).
and
ji
ji
wherei is the longitudeindexandj is the latitudeindex.
The time mean ozone amount and the amplitudes and
phasesof periodiccomponents
of the ozonevariationswere
data are missing.This assumption
is valid outsideof the polar
regions,
wheregenerallylessthan 10% of thedataaremissing.
In polarregionsthesystematic
lossof datain thewinterprobably producesmuchlargererrorsthan thosecausedby sampling,so error barshavebeenomittedpolewardof 70ø latitude.
RESULTS
estimated
by assuming
that thetimedependence
of fi couldbe
representedby
Global Fields
L
fl(tn)= flo + • fie COS
(aytn-- q•e)+ sn
(2)
g'=l
where n is time index, •' is the harmonic number, roe-- 2n•'/1
year,and Snis the residualerror.The residualerrorsare as-
A globalmap of the time-meantotal ozonerio is plottedin
Figure3. For comparison
the time-meantotal ozoneasinterpretedby London
andOltmans
[1978/1979]fromDobsonand
M83 observations
is reproducedin Figure 3.
The TOMS analysisshowsa numberof featuresthat differ
fromground-based
analyses
of total ozone.Thesedifferences
amplitudes
fit andphasesq>twerecalculated
with a weighted
mayhavea numberof sources:
(1) errorsin eithertheTOMS
least squaresmethod by minimizingthe total weighted instrumentor retrieval algorithm or in the Dobson/M83
sumed to have zero mean and to be uncorrelated in time. The
squarederror
measurements,
(2) subjective
errorsin the handanalysisof the
ground-based
measurements,
(3) sampling
errors,or (4) a real
changein the ozonecontentbetween
the observing
periodof
measurements
(1957-1975)and that of the
This curve-fittingapproachallows for the irregular distri- the ground-based
1982).Because
the results
bution of missingdata; as a resultthe weightedcosinebasis TOMS (October1978to September
EL2 = E WnSn2/EWn
n
n
functionsare not orthogonalover the domain tn. Therefore, of Bhartiaet al. [1984] indicatesmallrelativeerrorsbetween
addingnew harmonics
changes
the amplitudeand phaseof the TOMS and the Dobson/M83 measurements,source 1
the other componentsslightly.Harmonicshigher than the should not be the cause of the differencesseen in Figure 3.
errorsin the samedirectionmay remainin both
semiannualwere found to contributevery little to the total Systematic
data
sets.
Source
2 is verydifficultto judgeand is closelytied
variance,so resultspresentedare for L--- 2. The contribution
of higherharmonicswill be discussed
below.Outsidethe trop- to the poor spatialsamplingof the Dobson/M83network
(source3). Spatialand temporalsamplingerrors(source3)
shouldnot be a problemwith the TOMS. The possibilityof a
real changein the ozonecontentof the atmosphere
(source4)
by analysis
of theground-based
obsersemiannualharmonics.Anomaliespersistlongerin the tropics, canonlybedetermined
the entiretimeperiod1957-1982.
In viewof
which renders the assumptionthat the residuals are un- vationsspanning
ics the autocorrelation time of the residuals is much shorter
than the semiannual
period.Because
thesetimescalesare well
separated,
(2) shouldbe valid for estimatingthe annualand
correlated less realistic,but the annual and semiannualharmonicsstill appearto be well defined.
the known difficultiesof intercalibration and spatial coverage
with the Dobson and M83 networks, the TOMS instrument
Confidenceintervalsfor f•o, f• and •b• can be calculated shouldprovidea muchbetter estimateof the globaldistriwith standardformulas[Anderson,1971] by assumingthat no bution of total ozone.
BOWMAN
ANDKRUEGER:
GLOBALCLIMATOLOGY
OFTOTALOZONE
ANNUAL HARMONIC
AMPLITUDE
(DU)
7971
SEMIANNUAL HARMONIC
AMPLITUDE(DU)
90N
90N
0
90S
180W
--T-150
120
90
60
•OS
30
0
30
60
90
120
150
0
90S
180E
I
180W 150
120
90
60
30
Fig. 5a
30
30
30
30
30
30
I
90S
I
30 60 90 120 150180E
I
90S
180W 1•
I
i
120
I,
I
I
I
I
i 6•) 3OI 0I
3O
90
Fig. 5b
I
I
I
I
I
90
120
90S
150 180E
/
I , I
I
I 9•) 120
60
90N
150
90S
180E
Fig. 6b
FRACTION OF THE VARIANCEEXPLAINED(%)
90N
60
PHASE (MONTHS)
90N.
180W
150 120 9•3 6•) 310 0
30
Fig. 6a
PHASE (MONTHS)
90S
0
I
I
I
I
FRACTIONOF THEVARIANCEEXPLAINED
(%)
I
90N
30
90NI
'
'
'
'
'
'
'
I
•
'
'
'
'
IsON
I
I
90S
3O
0
0
0
30
30
30
90S i
180W
150
i
120
i 6•) i
90
30
i
0
I 6]3
30
90
120
150
180E
90S 90S
[
Fig. 5c
Fig. 6c
Fig. 5. (a) Amplitude
f)• (Dobsonunits),(b) phase•b•(months Fig. 6. (a) Amplitude•: (Dobsonunits),(b) phase&: (months
afterJanuary
1),and(c)fraction
ofthetotalvariance
explained
bythe afterJanuary1),and(c)fractionof thetotalvarianceexplained
by the
annual harmonic(%).
semiannualharmonic(%).
7972
BOWMAN AND KRUEGER: GLOBAL CLIMATOLOGY OF TOTAL OZONE
90N
I
zonal mean values.Onceagainthe largeridgesover the southern hemisphere oceans seen in the London and Oltrnans
[1978/1979] map are not presentin the TOMS map.
The rms deviationof the availabledaily measurements
from
the time mean is plotted in Figure 4. The varianceis minimal
just south of the equator and increasespoleward in both
hemispheres.The total variance is consistentlylarger in
middle and high latitudes of the northern hemispherethan in
3o
the southern.
Because the confidence
intervals
for the time
mean and the harmonics are proportional to the variance,
whichis nearlyzonallysymmetric,they have not beenplotted.
30
The 99% confidence interval for the time mean increases from
about + 0.4 DU (Dobson units) near the equator to + 2.5 DU
at 70ø latitude. Confidenceintervals for the harmonic amplitudes are +0.6 DU near the equator and +4 DU at 70ø.
Confidenceintervalsfor the phasesof the harmonicsare small
90S
I
I
I
I
JAN 1
APR 1
JUL 1
OCT 1
JAN 1
(• 10 days),exceptin a few regionswhere the amplitudeof the
harmonicsnearly vanishes(seebelow).
Fig. 7. Time-latitude
section
of the zonal-mean
totalozone
(Dobsonunits).The dailydatahavebeensmoothed
by computing The amplitude, phase, and fraction of the variance ex10-day means.
plainedby the annual harmonicf•x are plotted in Figure 5. In
the northern hemisphere,f•x increasesnearly uniformly away
The TOMS analysisis more nearly zonally symmetricin from the equator. There are two regions with large annual
middle latitudes than are the analysesof Londonand Oltrnans variations over the Sea of Okhotsk and the Canadian Arctic.
[1978/1979] or Wilcox et al. [1977, Figure 1]. The ridgesover Both are located coincident with maxima in the time mean
the oceansand central Asia and the correspondingtroughs ozone. The minimum of f• is located at about 10øS.The
over the continents are diminished in the TOMS map. The maximum of f• in the southernhemisphereis also co-located
TOMS map showsdistinct drops in total ozone over moun- with the maximum in f•o. There is a very low minimum in f•
tains, reflectingthe decreasein the atmosphericcolumn. The straddlingthe Antarctic peninsula.
The annual harmonic in the northern hemispherereachesa
three analyseshave a similar maximum over the Canadian
Arctic. The distinct ozone maximum over Europe observed maximumin late winter to early spring.The earliestmaximum
from the groundis not seenin the TOMS analyses.The Asian occurs where the amplitude is largest-•over the Sea of Okmaximum is presentin all three analysesbut differsin position hotsk. The phase of the annual harmonic increasessouthward
across the equator, so that maxima in the southern hemiand strength.
The southern hemisphereozone distribution is dominated spherealso occurin winter to early spring.Phaseis difficultto
by a large wave number 1 near 60øS,which is also the latitude determinenear the poles,but there appear to be large differ60
of the maximum
of the zonal mean. There is a minimum
over
ences between the annual harmonics
in the Arctic and Antarc-
tic regions.The annual harmonic explainsa large fraction of
but values south of • 70øS are unreliable because of the low
the varianceover much of the earth, especiallyin the northern
solar zenith anglesand the loss of data during the southern hemispheretropics.
hemispherewinter. This will be discussedfurther below for the
The amplitude, phase, and fraction of the variance exthe highest part of the Antarctic continent, not over the pole,
450
400
-
350-
300-
250
-
200
90S
I
i
I
I
i
60
30
0
30
60
90N
LATITUDE
Fig. 8. Time-meanzonal-meantotal ozone[fl]o (solidline) and [Q]o + the rms deviationsfrom the time-meanzonal
means(dashedlines)(Dobson units).
BOWMAN AND KRUEGER: GLOBAL CLIMATOLOGY OF TOTAL OZONE
15o
150
100
87'5øN
'••
0 •
,
"'• '
100
50
7973
50
• 150
0
',,875øS
-50
-100
-150
50
(
I
i
,oo
50
0
-50
0
-100
57.5øN.
-100
-150
1O0
'•'7!•
"'!"""
.••,.."
...
50
0
50
-]
,.
;.•
'•
-50
0j . ..,.•
,%.._6
.,5
ø
" :'"''
-50
:'
100
50
•
47.5øN
100
.......
'
,,._57
5øS
' " •
0
-50
,""-'
'
100
._.__•.•
'" '"'"
- •
•
50
-50
0
-100
ø3
-50
o
,
50
-50
Io
3z.os
25
50
i
I 7'5ø
S
27.5øS
0
0 •
,?'5ø.•,
,. , 0
-25
25
0
-25
JAN1
APR1
JU
1
OCT1
JAN1
25
•"•,•.,-...,•,L
•
•
...,..,;_.,._,
-•,
JAN 1
.,........
•
APR 1
0
,.•,,,,.•_.,.•
JU•L
1
-25
.
•
OCT 1
JAN 1
Fig. 9. Climatological
dailyzonal-meantotal ozonefor selected
latitudesand the fit to the data with two harmonics.
The
time mean has been removed.
degreessouth of the equator, and the minimum in the varianceoccursslightlysouthof that.
Hemisphericdifferences,especiallythose in high latitudes,
can be seenmore clearlyin Figure 9, in which the climatological daily ozonevaluesare plotted for selectedlatitudes.Superimposed are the fits achievedat each latitude with two harmonics. Time means have been removed. In northern high
latitudes the ozone peaks in late winter to early spring and
wind line. The total ozone shows no evidence for a maximum
then declinessteadilythrough the sunlit portion of the year.
in the amplitude of the semiannualharmonic in the tropics, Near the south pole the ozoneremainslow at the equinoxand
althoughsucha featurecouldoccurlocallyin the vertical.The risesabruptly about 1.5 monthslater. It then falls through the
maximum in the fraction of the varianceexplainedover Asia last half of the sunlit period. This springtimerise was identifrom
appearsto be associatedwith a real maximum in the ampli- fied by Dobson[1966] in ground-basedmeasurements
plainedby the semiannualharmonicare shownin Figure 6.
The amplitudef12is flat throughoutthe tropicsand generally
increasestoward the poles.At high latitudes the semiannual
harmonic becomes very unreliable because of the large
amount of missingdata and is probably largely an artifact of
the analysismethod. Hopkins [1975] has suggestedthat the
semiannualharmonic in the tropics results from the absorption of equatorward propagatingplanetary wavesat the zero
tude of the semiannual harmonic, but the maxima over the
Indian Oceanstretchingtoward the westand over the Weddel
Sea appear to be causedby the absenceof a strong annual
harmonic(Figure 5).
Zonal
Means
The annual evolution of the zonal mean ozone Ifil is shown
in Figure 7. The field has been smoothedslightly by com-
the Antarctic.
The fewnighttimemeasurements
availablefrom the Antarctic indicate a slight secondarypeak in May or June [Dobson,
1966; Chubachi,1984]. However, the exaggeratedpeak in the
fitted curvesseenduring the polar night in the southernhemisphereis causedby the large gap in the data and the abrupt
springtimerise.The annualcyclein the northernhemisphere
is much more easilyfit by two harmonics.
The structures of the annual and semiannual harmonics of
puting 10-day meansfrom the daily zonal means.This cross
the
zonal mean ozone and the 99% confidence intervals are
sectionis quite similar to that givenin Diitsch[1974] and the
BUV data from Nimbus 4 analyzedby Hilsenrathet al. [1979] shownin Figure 10. The percentageof the total varianceexand Tolson[1981].
plainedby eachharmonicis shownin Table 1 for the firstfour
At eachlatitude the daily, zonal mean valueswere expanded harmonics.Outsideof the south polar region,two harmonics
in temporalharmonicsby using(2). The time mean Ifil0 and seemto capturethe annualcycleof the zonal mean ozonevery
the rms deviationsof the daily valuesaround the time mean well. The spatial filtering of smaller scalefeaturesby zonal
are shownin Figure 8. The 99% confidenceintervalsare less averaginghas also removed much of the higher-frequency
than 1% of the time-mean zonal means at all latitudes. The
variability associatedwith planetary- and synoptic-scale
confidencelimits have beenomitted from Figure 8 becausethe waves. Therefore the annual and semiannual harmonics exprobablesystematicuncertaintiesare larger.The hemispheric plain a largerfractionof the varianceof the zonal meansthan
asymmetriesnotablein Figure 7 are also apparentin the time of the two-dimensionalgridded data.
The difference in the structure of the annual harmonic in
means. Both the means and the variances are larger in the
especiallypolewardof 50ø, is certainly
northern hemisphere.The tropical minimum occurs several the two hemispheres,
ANNUAL AND SEMIANNUAL
HARMONICS
THE ZONAL-MEAN
TOTAL OZONE
OF
AMPLITUDE
120
100
-
80-
60-
••SEMIANNUAL
40-
20-
0
90S
!
I
60
30
0
30
60
90N
LATITUDE
Fig. 10a
PHASE
JAN
1
OCT
1
ANNUAL
ß',- •'-
--SEMIANNUAL
APR1
JAN 1
90S
6O
I
I
30
0
30
6O
90N
LATITUDE
Fig. 106
FRACTION OF THE VARIANCE EXPLAINED
100
ANNUAL
+ SE
80-
60-
,ANNUAL
Z
40-
20
-
0
90S
I
I
I
I
I
60
30
0
30
60
90N
LATITUDE
Fig. 10c
Fig. 10. (a) Amplitude of the annual and semiannualharmonics[Q]x and [Q]2; (b) phaseof the annual and semiannual harmonics[•b]x and [•b]2' and (c) fraction of the total variance of the zonal means explainedby the annual
harmonic
and the sum of the annual and semiannual
harmonics.
7974
BOWMAN AND KRUEGER: GLOBAL CLIMATOLOGY OF TOTAL OZONE
real, as can be seenclearly in Figure 7. The harmonic amplitudesand phasesfrom TOMS resultsagreequite well with the
Nimbus 4 BUV resultsof Hilsenrath et al. [1979] and Tolson
[1981]. Together the annual and semiannual harmonics explain between 50% and 95% of the variance of the zonal
means.The third harmonic contributessubstantiallyonly in
high southern latitudes, where the sudden springtime rise
would produce a broad spectrum. The fourth harmonic is
essentiallynegligibleeverywhere.The lowest explained variance is found in the tropics,where the total varianceis small,
the annual and semiannual cycles are weak, and a contribution to the variancefrom the quasibiennialoscillationcould
be expected.
Global Means
Global meansare biasedby the absenceof measurementsin
the polar night, where ozonevaluesaverageabout 10% higher
than the global mean. Since the missing area comprises,at
most, 10% of the globe, the estimates of the global mean
7975
TABLE 2. Time-Mean and Annual and Semiannual Harmonics of
the Global-Mean
Harmonic
0
1
2
Total Ozone
Amplitude,
Phase,
Dobsonunits
monthsafterJanuary1
288.1
4.7
3.9
--
4.2
-- 5.2
DISCUSSION
The TOMS instrumenthas provided a valuable addition to
our knowledgeof the distributionof ozone in the atmosphere
and its annual cycle.Observationscontinue to be collectedat
the presenttime, and further analysisshould improve our understandingof the variability of ozone on both short (daily to
weekly)and multiyear time scales.The TOMS instrumentalso
resolvesfeaturesat spatial scalesmuch smaller than is possible
with SBUV instruments, both the one on Nimbus 7 and those
planned for the NOAA operational satellites. This should
values in Table 2 should be on the order of 1% low. Confiallow more detailed study of transport of ozone by eddies in
dence intervals are not included becauseof the systematic the lower stratosphere and an evaluation of the errors in
nature of the missingdata. The harmonicsare dominated by SBUV resultscausedby the poorer spatial and temporal samthe larger amplitude of the variations in the northern hemi- pling by the SBUV instrument.The latter questionis certainly
sphere.As a result the maximum of the global mean ozone important for long-term trend monitoring with the SBUV.
Sufficient measurements have now been collected to reduce
occurs •, 1 month after the vernal equinox, with a smaller
maximum near the autumnal equinox.
the sampling errors of the time mean and of the annual and
semiannual harmonics caused by short time-scale variability
to lessthan the accuracyof the instrument.Outside the polar
TABLE 1. PercentageVariance Explained by the Harmonics of the
regionsthe structureof the annual and semiannualharmonics
Zonal Means
is well determined.Becausethe large-scaleand low-frequency
Harmonic
features of the ozone are so well describedby the annual and
semiannualharmonics,the ability of numerical models to reLatitude
1
2
3
4
produce this behavior could be a good test of the models'
87.5
82.5
77.5
72.5
67.5
62.5
57.5
52.5
47.5
42.5
37.5
32.5
27.5
22.5
17.5
12.5
7.5
2.5
-2.5
-7.5
-12.5
-17.5
-22.5
-27.5
-32.5
-37.5
-42.5
-47.5
-52.5
-57.5
-62.5
-67.5
-72.5
-77.5
-82.5
-87.5
90.0
92.6
94.9
95.2
95.2
94.4
94.3
94.0
94.0
93.8
92.2
87.1
80.3
78.8
80.3
85.0
83.5
62.5
46.8
51.5
62.2
62.0
65.4
73.2
82.3
87.4
89.4
87.8
83.8
76.3
59.3
46.1
43.6
46.7
48.3
48.0
0.7
0.8
0.7
1.3
1.5
1.4
1.2
1.4
1.5
1.7
2.0
2.1
3.1
3.9
3.4
2.6
2.2
4.0
6.8
14.0
9.5
5.9
6.3
4.4
1.5
0.4
0.2
0.9
3.1
7.6
18.6
25.9
23.5
19.7
18.0
15.2
0.4
0.3
0.1
0.1
0.0
0.0
0.1
0.4
0.5
0.8
1.1
1.2
0.8
0.4
0.4
0.6
0.7
0.7
0.6
0.4
0.2
0.0
0.0
0.0
0.2
0.8
1.3
1.5
1.6
3.3
7.4
8.8
9.1
8.6
7.0
6.1
0.2
0.1
0.2
0.0
0.0
0.1
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.1
0.2
0.3
0.3
0.3
0.1
0.1
0.0
0.0
0.1
0.0
0.1
0.4
0.9
1.4
1.9
0.3
0.2
0.0
1.7
-0.4
accuracy.
Acknowledgments.G. R. North and D. Short contributed many
ideas and much encouragementto this work. K. Bowman was supported by a National ResearchCouncil Resident ResearchAssociateshipin the Climate and Radiation Branch of the Laboratory for
Atmospheres,NASA Goddard SpaceFlight Center.
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K. P. Bowman and A. J. Krueger, Code 613, Laboratory for Atmospheres,NASA Goddard SpaceFlight Center, Greenbelt,MD 20771.
(ReceivedJanuary 22, 1985;
revisedApril 22, 1985;
acceptedApril 23, 1985.)