A History of Infrared Extinction at CTIO, Chile, and a Possible Connection with the El Niño...

A History of Infrared Extinction at CTIO, Chile, and a Possible Connection with the El NiñoPhenomenonAuthor(s): Jay A. FrogelSource: Publications of the Astronomical Society of the Pacific, Vol. 110, No. 744 (February1998), pp. 200-209Published by: The University of Chicago Press on behalf of the Astronomical Society of the PacificStable URL: http://www.jstor.org/stable/10.1086/316119 .

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Publications of the Astronomical Society of the Pacific, 110:200–209, 1998 Februaryq 1998. Astronomical Society of the Pacific. All rights reserved. Printed in U.S.A.

A History of Infrared Extinction at CTIO, Chile, and a Possible Connectionwith the El Nino Phenomenon

Jay A. Frogel1

Department of Astronomy, The Ohio State University, 174 West 18th Avenue, Columbus, OH 43210; [email protected]

Received 1997 July 28; accepted 1997 November 15

ABSTRACT. Extinction coefficients and sensitivity values in the JHKL bandpasses measured on nearly 200clear nights of observing between 1978 and 1992 on the 1.5 m and the Blanco 4 m telescopes at Cerro TololoInter-American Observatory are presented and discussed. Analysis of these data shows the following: there areseasonal variations in both the extinction coefficients and sensitivity values that are qualitatively consistent withexpected variations in the amount of H2O in the atmosphere—relatively high in the summer months, lower inthe winter months. The linear correlation coefficients between most of these quantities are statistically significant.The yearly mean values of these quantities also show significant variability of a few hundredths of a magnitude.The correlations between these yearly means are again consistent with variations in the H2O content of theatmosphere. At least some of these longer term variations are closely correlated with quantitative measures ofthe strength of the atmospheric and oceanic El Nino/Southern Oscillation phenomenon.

1. INTRODUCTION

Unlike atmospheric extinction in optical passbands, whichis due primarily to molecular diffusion and to scattering byaerosols, both of which are smooth functions of wavelength(see, e.g., Burki et al. 1995; Lockwood & Thompson 1986),extinction in the near-infrared (near-IR) is due almost entirelyto molecular bands of H2O (primarily) and CO2 (secondarily).Many of the lines that make up these bands are saturated evenunder typical conditions; consequently, near-IR extinction canbehave in a complex manner (see the theoretical study by Man-duca & Bell 1979). Because of their origin, temporal variationsin the extinction coefficients may be coupled with variationsin the “sensitivity” in a filter bandpass, a quantity usually re-ferred to as the “photometric zero point” in optical photometry.

Between 1978 and 1992, I observed in the near-IR on ∼200photometric nights on the 1.5 m and the Blanco 4 m reflectorsat CTIO with the single-channel D3 InSb system. These ob-servations provide a uniform, long-term data base of extinctioncoefficients and sensitivity values that can be investigated forseasonal and secular variations. The effect on atmospherictransparency of the El Nino/Southern Oscillation (ENSO) phe-nomenon, which has a strong influence on global weather (Cane1986), can be investigated; so too can effects from volcaniceruptions, which are known to reduce atmospheric transmission

1 The author was a staff astronomer at CTIO when most of these observationswere made. CTIO is part of the National Optical Astronomy Observatories,which are operated by the Association of Universities for Research in As-tronomy, Inc., under cooperative agreement with the National ScienceFoundation.

in the visual (Burki et al. 1995; Lockwood & Thompson 1986;Thompson & Lockwood 1996). Sections 2 and 3 describe thedata and the calculation of monthly and yearly averages forthe extinction and sensitivity values. Section 3 also examinesdifferences between the two telescopes and other possible in-strumental effects. Section 4 addresses the main topic of thepaper—seasonal and secular variations in the data. Section 5gives a brief summary and some conclusions.

2. MEASUREMENT OF EXTINCTIONCOEFFICIENTS AND SENSITIVITIES

The data for this paper were obtained during ∼200 clearnights of observing at CTIO with a single-channel near-IRphotometer, D3, and with f/30 chopping secondaries. Two-thirds of the nights were on the 4 m Blanco reflector; theremainder were on the 1.5 m. Most of the observations weremade from 1978 through 1985, with only 16 nights of ob-serving from 1986 through 1992. Less than 10% of all the datawere obtained in June through September.

The observations were made in the JHKL (leff 5 1.25, 1.65,2.2, and 3.5 mm, respectively) bandpasses and with pairs offilters that measure stellar absorption due to CO2 and H2O inthe K bandpass (Aaronson, Frogel, & Persson 1978; Frogel etal. 1978; Elias et al. 1982). Transmission curves of filters sim-ilar to those used for these observations are in Figure 1 ofManduca & Bell (1979). The 2.00 mm filter, which measuresstellar H2O, is the filter most influenced by changes in terrestrialatmospheric transmission. Figure 2 of Ferriso, Ludwig, &Thompson (1966) compares H2O absorption coefficients at 300



HISTORY OF INFRARED EXTINCTION 201

1998 PASP, 110:200–209

Fig. 1.—(top) These extinction coefficients are the average values for eachobserving run based on values determined for each night within the run.Constant shifts have been applied to some of the variables for clarity of pres-entation. Typical 1 j uncertainties are 50.02 mag or less in all the values.(bottom) Sensitivities for colors are plotted for photometric nights on the CTIO1.5 and 4 m Blanco reflectors. A constant was added to the values for S(CO)for clarity of presentation. The sharp break observed near JD 2,431,000 insome of the quantities is due to changes in instrumental parameters, as dis-cussed in the text. Uncertainties in the plotted values are comparable in sizeor smaller than the symbols.

and 3000 K, temperatures appropriate for Earth and stellaratmospheres, respectively.

Usually, a dozen or more CIT/CTIO standards (Elias et al.1982) were observed on each night. Instrumental sensitivities,denoted by S, were determined from these data. The S-valuefor a filter is the average of true minus instrumental magnitudefor all standards observed on a night normalized to the sameintegration time and to one air mass, i.e., overhead. A morepositive value of S means greater transmission (atmosphere plusinstrument) or brighter instrumental magnitude. Aside fromtelescope size, S-values will vary with time as a result of seeing,atmospheric transparency, cleanliness of the optics, changes ingain of the electronics, etc. Except for the K band, the sensi-tivities are given for the colors, i.e., the difference in sensitivitybetween two bands, which should be independent of most ofthese first-order effects.

The extinction value for a filter, in units of magnitudes perair mass, measures the attenuation of starlight as a result ofatmospheric opacity. The air mass of an observation has theusual definition, viz., , where z is distance from the zenithsec zin degrees. The extinction in a color, e.g., E( ), is theJ 2 Ksimple difference in the coefficients for the two filters. Sincethe magnitudes and colors of the standards are known to anaccuracy ≤1%, extinction coefficients were determined by ob-serving, several times during a night, two standards in rapidsuccession whose air mass differed by ∼0.7 or greater, thuseliminating effects of time-variable transmission. If the night-to-night scatter in the coefficients during an observing run,defined as two or more consecutive nights on one telescope,was comparable to or less than those for a single night withmultiple determinations of the coefficients—a situation thatobtained for nearly all runs—the nightly values were averagedand used for the entire run. Typically, the scatter in the ex-tinction coefficients over an observing run was only slightlygreater than expected from uncertainties in the observations ofthe standards themselves, ≤0.02 mag for all magnitudes andcolors. In almost all cases, the need for two sets of extinctioncorrections during a run was associated with a significantchange in the weather, e.g., a large shift in night-time temper-ature or humidity accompanying passage of a frontal system.

3. PRESENTATION OF THE DATA

Figure 1 displays the average values of the extinction co-efficients for each observing run and the color sensitivities forevery photometric night of every run, respectively.

3.1. Is There a Difference between Telescopes?

There are no significant differences in the extinction coef-ficients for the two telescopes. Table 1 gives the global meansand standard deviations of these values. All these differencesare ≤0.01 mag. Even those that appear greater than the un-certainties in the means, , are still considerably less than1/2j/Nthe observed seasonal variations, and there was a seasonal bias



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TABLE 1Comparison of Results from the 4 m and 1.5 m Telescopes : Extinction Coefficients

ExtinctionCoefficients

(All Data)

1.5 m 4 m Combined

Number of Nights Mean j Number of Nights Mean j Mean j

. . . . . . .E(J 2 K) 70 0.011 0.018 125 0.018 0.014 0.015 0.016. . . . . .E(H 2 K) 70 20.029 0.011 125 20.030 0.010 20.030 0.010

E(K) . . . . . . . . . . . 70 0.081 0.016 125 0.087 0.019 0.085 0.018. . . . . .E(K 2 L) 62 20.061 0.021 105 20.071 0.026 20.067 0.025

E(H2O) . . . . . . . . 67 0.236 0.030 118 0.242 0.028 0.240 0.029E(CO) . . . . . . . . . 67 0.072 0.013 118 0.070 0.011 0.071 0.012

TABLE 2Comparison of Results from the 4 m and 1.5 m Telescopes: Sensitivities

Sensitivities(1978 –1984)

1.5 m 4 m

Number of Nights Mean j Number of Nights Mean j

. . . . . . .S(J 2 K) 63 0.64 0.12 100 0.58 0.12. . . . . .S(H 2 K) 63 0.47 0.04 100 0.47 0.05

S(K) . . . . . . . . . . . 63 16.91 0.57 100 18.90 0.50. . . . . .S(K 2 L) 58 1.45 0.10 82 1.47 0.09

S (H2O) . . . . . . . 61 20.58 0.08 96 20.56 0.08S(CO) . . . . . . . . . 59 20.60 0.03 96 20.59 0.03

in which telescope was used. Combined values are given inthe two rightmost columns of Table 1 and will be used in theremainder of this paper.

Table 2 presents the sensitivities for the two telescopes. Onlypre-1985 data were considered since, as is evident from Figure1, a big jump occurred in the sensitivities in 1985 because ofa change in D3. The difference in S(K) of mag1.99 5 0.09between the 1.5 m and 4 m telescopes is close to the expectedvalue of 2.13 mag. With the possible exception of S( ),J 2 Kthere does not appear to be any difference greater than theuncertainty in the means for the two telescopes. Therefore, thesensitivities measured on the two telescopes will be consideredtogether.

A final factor of interest is the state of the mirror coatings.The 4 and 1.5 m primaries were realuminized every other yearand every year, respectively. Deterioration of the coatings couldcause a reduction of sensitivity in the near-IR and an increasein the thermal background emission. For the former possibility,we can set an upper limit of 10%: The standard deviation ofthe difference in S(K) from one night to the next (except forlarge differences due to instrumental changes) was calculatedfor each telescope. Both of these values were ≤0.10 mag. Thissame quantity, calculated just for the nights on either side ofan aluminization (5 times on the 4 m, 8 times on the 1.5 m),was not significantly different.

3.2. Monthly and Yearly Averages

Monthly averages for the extinction coefficients were cal-culated from all observations on both telescopes. Average sen-sitivities use data only from 1984 and earlier because of the

large instrumentally caused jumps in the 1985 data. Thesemonthly averages are given in Tables 3 and 4 and illustratedin Figure 2. For each variable, the third line in the tables givesthe uncertainty in the mean. Tables 5 and 6 and Figure 3 presentyearly means for the extinction coefficients and sensitivity val-ues, respectively. All data from both telescopes are includedin calculating these means. For each variable the third line inthe tables gives the uncertainty in the mean. Because of thesmall numbers of observations in 1986 and 1987, these yearshave been combined, and similarly for 1991 and 1992. Sincethe extinction coefficients used for an observing run were av-erage values, to calculate the yearly averages, each night in arun was considered to have the average value used for the run.Thus, the values used on a long run in a year would have moreweight than those determined for a short run in the same year.

4. TEMPORAL VARIATIONS IN THE DATA

In this section I will argue that month-to-month and year-to-year changes seen in the extinction coefficients and the sen-sitivity values are primarily due to seasonal and secular changesin the H2O content of the atmosphere. I will also examine thepossibility that terrestrial events affect atmospheric transmis-sion in the near-IR.

4.1. Monthly Averages

The monthly averages for the extinction coefficients andsensitivities are presented in Tables 3 and 4 and Figure 2.Correlations that are significant at ≥95% are in Table 7. First,note that Table 1 shows that, with the exception of E(K 2 L)




1998 PASP, 110:200–209

TABLE 3Monthly Averages: Extinction Coefficients

Extinction Coefficients(All Data) Jan Feb Mar Apr May Jun Jul Sep Oct Nov Dec

:E(J 2 K)Number of nights . . . . . . 23 18 17 12 33 9 5 3 13 24 38Mean . . . . . . . . . . . . . . . . . . . 0.010 0.008 0.002 0.007 0.022 0.013 0.020 0.020 0.021 0.025 0.016

. . . . . . . . . . . . . . . . . . .1/2j/N 0.002 0.002 0.004 0.005 0.003 0.005 0.012 0.000 0.006 0.002 0.003:E(H 2 K)

Number of nights . . . . . . 23 18 17 12 33 9 5 3 13 24 38Mean . . . . . . . . . . . . . . . . . . . 20.034 20.028 20.036 20.029 20.027 20.028 20.028 20.030 20.022 20.030 20.029

. . . . . . . . . . . . . . . . . . .1/2j/N 0.002 0.002 0.003 0.003 0.002 0.001 0.007 0.000 0.002 0.002 0.002E(K):


. . . . . . . . . . . . . . . . . . .1/2j/N 0.002 0.004 0.005 0.003 0.002 0.002 0.005 0.000 0.009 0.003 0.003:E(K 2 L)


. . . . . . . . . . . . . . . . . . .1/2j/N 0.005 0.005 0.010 0.003 0.003 0.010 0.002 0.000 0.006 0.004 0.005E(H2O):


. . . . . . . . . . . . . . . . . . .1/2j/N 0.003 0.008 0.004 0.006 0.005 0.006 0.007 0.000 0.008 0.005 0.004E(CO):


. . . . . . . . . . . . . . . . . . .1/2j/N 0.001 0.004 0.001 0.002 0.002 0.003 0.010 0.000 0.003 0.002 0.002

TABLE 4Monthly Averages: Sensitivities

Sensitivity Values(1978–1984) Jan Feb Mar Apr May Jun Jul Sep Oct Nov Dec

:S(J 2 K)Number of nights . . . . . . 23 18 15 10 21 6 5 3 13 13 36Mean . . . . . . . . . . . . . . . . . . . 0.62 0.50 0.60 0.70 0.65 0.50 0.52 0.78 0.59 0.68 0.59

. . . . . . . . . . . . . . . . . . .1/2j/N 0.02 0.03 0.04 0.02 0.03 0.01 0.02 0.01 0.04 0.01 0.02:S(H 2 K)


. . . . . . . . . . . . . . . . . . .1/2j/N 0.01 0.00 0.01 0.02 0.01 0.00 0.00 0.01 0.02 0.02 0.01:S(K 2 L)


. . . . . . . . . . . . . . . . . . .1/2j/N 0.01 0.02 0.02 0.08 0.02 0.01 0.01 0.02 0.04 0.01 0.02S(H2O):


. . . . . . . . . . . . . . . . . . .1/2j/N 0.01 0.03 0.01 0.01 0.02 0.01 0.01 0.02 0.02 0.01 0.01S(CO):


. . . . . . . . . . . . . . . . . . .1/2j/N 0.01 0.01 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.01 0.01

and E(H2O), the dispersion in each extinction index when allthe data are considered together is less than 0.02 mag. AlthoughMay–September are the wettest months at CTIO with the high-est relative humidity, they are actually the driest in terms ofcolumn density of H2O above the observatory, since these

months are also the coldest. An increase in the H2O contentof the atmosphere will make instrumental values of ,J 2 K

, and blue (because atmospheric H2O has a greaterH 2 K K 2 Leffect on L than on K and on K than on J or H) and the H2Oand CO indices red or large (because atmospheric H2O affects



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Fig. 2.—(top) Monthly averages for the extinction coefficients from Table3. The inner error bars (invisible for many of the points) are the uncertaintiesin the mean values plotted. The longer error bars show the variances of themeans. No data were obtained in the month of August. (bottom) Monthlyaverages for the sensitivity values from Table 4. The inner error bars (invisiblefor many of the points) are the uncertainties in the mean values plotted. Thelonger error bars show the variances of the means. No data were obtained inthe month of August.

the 2.00 and 2.36 mm filters more strongly than it does the 2.20mm continuum filter used for both indices). An increase inatmospheric H2O will also cause instrumental K magnitudes tobe measured fainter, or more positive. Changes in sensitivitieswill be of opposite sign to the instrumental magnitudes andcolors, while algebraic changes in the extinction coefficientswill be in the same sense as the instrumental magnitudes andcolors.

The two indices with the largest overall dispersion, E(K 2and E(H2O), also show the largest seasonal variations of theL)

kind one might expect at CTIO. Figure 2 (top) shows thatE(H2O) is smallest during the southern hemisphere’s winter,while is reddest. Table 7 shows that linear correla-E(K 2 L)tions in the expected sense, and significant at ≥95%, are indeedpresent between most of the monthly averaged extinction co-efficients. Figure 4 illustrates some of the significant correla-tions between these monthly means as well as the near con-stancy of .E(H 2 K)

As noted above, one would expect variations in the sensi-tivities to correlate with variations in the extinction coefficientsbut with opposite sign. They might be more difficult to detect,however, since sensitivity values will also be affected by evensmall changes in instrumental configuration. Nonetheless, Table7 reveals that such correlations do exist. Thus, these data dem-onstrate the existence of seasonal variations in both overallatmospheric transmission and in the dependence of that trans-mission on air mass. The latter variation does not necessarilyhave to follow from the former if the absorption bands re-sponsible for overall atmospheric transmission were all purelysaturated ones.

4.2. Yearly Averages

Figure 3 and Tables 5 and 6 present yearly averages for theextinction coefficients and sensitivity values. As with themonthly averages, we want to know if there are changes inthese values due to changes in Earth’s atmosphere. First, withthe possible exception of , there are no systematicE(K 2 L)secular changes over a time span of 14 years in the extinctioncoefficients. This is deduced from the fact that except for

the variations in the extinction coefficients over thisE(K 2 L)time span can be fitted by straight lines with slopes≤ . A fit to the data, though, yields a0.001 5 0.001 E(K 2 L)slope of 2 , a 3 j result. However, exclusion of0.003 5 0.001either the first or last datum reduces this to the 2 j level, makingits significance doubtful.

Are the year-to-year changes in the extinction coefficientsreal, since several of the values differ by 3 j or more from oneor both of their neighbors? The most extreme example is thedrop in E(H2O) for 1984 compared to 1983, a 7 j difference.At the same time there is a 4.5 j increase in , and aE(K 2 L)7 j decrease in . The signs of these changes are consistentE(K)with that expected from a decrease in atmospheric H2O opacity.




1998 PASP, 110:200–209

TABLE 5Yearly Averages: Extinction Coefficients

Extinction Coefficients 1978 1979 1980 1981 1982 1983 1984 1985 1986/1987 1991/1992

:E(J 2 K)Number of nights . . . . . . 12 35 27 32 19 14 22 18 10 6Mean . . . . . . . . . . . . . . . . . . . 0.025 0.003 0.021 0.015 0.003 0.025 0.028 0.014 0.006 0.033

. . . . . . . . . . . . . . . . . . .1/2j/N 0.002 0.003 0.002 0.003 0.001 0.003 0.001 0.004 0.003 0.002:E(H 2 K)

Number of nights . . . . . . 12 35 27 32 19 14 22 18 10 6Mean . . . . . . . . . . . . . . . . . . . 20.025 20.037 20.027 20.029 20.034 20.024 20.019 20.033 20.040 20.017

. . . . . . . . . . . . . . . . . . .1/2j/N 0.002 0.002 0.002 0.002 0.001 0.001 0.001 0.002 0.003 0.002E(K):


. . . . . . . . . . . . . . . . . . .1/2j/N 0.000 0.004 0.001 0.002 0.003 0.001 0.003 0.004 0.009 0.004:E(K 2 L)


. . . . . . . . . . . . . . . . . . .1/2j/N 0.000 0.004 0.003 0.006 0.007 0.004 0.002 0.001 0.004 0.007E(H2O):


. . . . . . . . . . . . . . . . . . .1/2j/N 0.005 0.005 0.004 0.006 0.004 0.005 0.004 0.006 0.010 0.023E(CO):


. . . . . . . . . . . . . . . . . . .1/2j/N 0.000 0.002 0.001 0.002 0.003 0.002 0.001 0.000 0.003 0.000

TABLE 6Yearly Averages: Sensitivities

Sensitivity Values 1978 1979 1980 1981 1982 1983 1984 1985 1986/1987 1991/1992

:S(J 2 K)Number of nights . . . . . . 12 35 27 32 19 16 22 18 10 6Mean . . . . . . . . . . . . . . . . . . . 0.67 0.73 0.62 0.44 0.48 0.63 0.69 20.10 20.07 20.02

. . . . . . . . . . . . . . . . . . .1/2j/N 0.02 0.01 0.02 0.01 0.01 0.02 0.01 0.01 0.01 0.02:S(H 2 K)


. . . . . . . . . . . . . . . . . . .1/2j/N 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.01:S(K 2 L)


. . . . . . . . . . . . . . . . . . .1/2j/N 0.02 0.04 0.01 0.01 0.01 0.01 0.01 0.01 0.20 0.01S(H2O):


. . . . . . . . . . . . . . . . . . .1/2j/N 0.01 0.01 0.01 0.02 0.02 0.01 0.02 0.01 0.03 0.04S(CO):


. . . . . . . . . . . . . . . . . . .1/2j/N 0.01 0.00 0.00 0.00 0.01 0.01 0.01 0.00 0.01 0.01

The upper part of Table 7B shows that three pairs of extinctioncoefficients are correlated at ≥95% level, although none of theminclude . Thus, it would seem that some of the year-E(K 2 L)to-year changes in the mean extinction coefficients are due tochanges in Earth’s atmosphere, most likely the H2O content,but that these changes do not show any long-term pattern.

The existence of significant correlations between the ex-tinction coefficients and the sensitivity values averaged year-by-year further support the conclusion that there are nonsea-sonal changes in atmospheric transparency with a timescale of*1 yr. These correlations are listed in the middle right sectionof Table 7 and illustrated in Figures 5 and 6. All these cor-



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Fig. 3.—(top) Extinction coefficients averaged for the years indicated withuncertainties in the mean values. Data are from Table 5. (bottom) Sensitivitiesaveraged for the years indicated with uncertainties in the mean values. Dataare from Table 6. Individual values may be seen in Fig. 1(bottom).

Fig. 4.—Table 7 shows that significant correlations exist between themonthly means of E(H2O) and the other extinction coefficients except for

. This figure illustrates these correlations between the monthly meanE(H 2 K)values and also shows the simple, unweighted, least-squares regression linefor each. Error bars are the uncertainties in the mean values as given in Table3. Note the shift between the left- and right-hand axes.

relations are in the sense expected given the effects of H2O onthe filter bandpasses.

4.3. Correlations with the El Nino/Southern Oscillation(ENSO) Phenomenon

An ENSO event results from the coupled behavior of thewind pattern over the tropical Pacific Ocean and the temper-

ature distribution of the water in the Ocean’s surface layers.An excellent review of ENSO is given by Cane (1986).2 Theaspects of ENSO events of importance for this investigationare the significant elevation of sea surface temperatures off thewestern coast of South America and the accompanying en-hancement of rainfall over these coastal regions. A very strongENSO event occurred during 1982/1983.

If there were a connection between the ENSO phenomenonand atmospheric extinction and transmission in the near-IR,one would expect the H2O extinction measured at CTIO to goup at times corresponding to a strong El Nino due to the overallincrease in atmospheric water content in the eastern Pacific.Two quantitative measures of the ENSO phenomenon that canbe compared with the near-IR extinction and sensitivity valuesare (a) the southern oscillation index (SOI), which measuresthe departure of the pressure difference between Tahiti and

2 Most recent work including current observations and predications can befound at the comprehensive World Wide Web site http://www.pmel.noaa.gov/toga-tao/el-nino/home.html.




1998 PASP, 110:200–209

Fig. 5.—Similar to Fig. 4, except correlations between the yearly meansare illustrated. Again note the shift between the left- and right-hand axes.

Fig. 6.—Similar to Fig. 5. Note the difference scales for the bottom leftand top right.

Darwin, Australia, from a predefined normal monthly mean;(b) the NINO112 index, which is the standardized anomaly(monthly means having again been subtracted) in the averagesea surface temperature measured between latitudes 0–10S andlongitudes 80W–90W.3

Figure 7 plots four of the normalized extinction values foreach observing run against the SOI and NINO112 indices.The normalization forces the mean value for each extinctioncoefficient to be zero and then scales them so that all have adispersion similar to that of NINO112. The signs of E(K 2

were reversed to emphasize the similarity of all four ex-L)tinction indices. The correlations between SOI, NINO112, andthe extinction coefficients are given at the bottom of Table 7for p-values ≤0.10. Correlations involving andE(J 2 K)

are all *0.7. This is easily understood forE(H 2 K) E(H 2since the effects of changing the atmospheric H2O contentK)

is nearly the same in both the H and K filters (see Fig. 7). It

3 These are available from ftp://nic.fb4.noaa.gov/pub/cac/cddb/indices/nino.

is less easily understood for but most likely is dueE(J 2 K)to a combination of significantly larger uncertainties in deter-mining and a relatively small overall variation in thisE(J 2 K)coefficient.

The values of the SOI near 23 and of NINO112 between3 and 4 occurred during the 1982/1983 ENSO event and arethe most extreme values recorded in a century of record keep-ing. Unfortunately, during a 1 yr period that included the timewhen these indices were at their extrema, I had only two CTIOobserving runs. Nevertheless, the correlations exhibited in Fig-ure 7 indicate that the ENSO phenomenon has a measurableimpact on observing conditions in the near-IR via increasedatmospheric H2O content. This conclusion is further supportedby the fact that the sensitivity of the H2O index, S(H2O), cor-relates with both SOI and NINO112 at more than the 99%level.

Both the NINO112 and SOI indices also have real excur-sions to large negative and positive values, respectively (seeFig. 7, but more readily apparent in the full data sets for theseindices). These excursions, indicative of atmospheric and oce-anic conditions opposite those associated with an ElNino—exceptionally cold waters off the South American coastand particularly strong easterly trades across the tropical Pa-cific—are referred to as La Nina (Cane 1986). Figure 7 suggeststhat La Ninas are associated with below normal atmosphericH2O content.

Near the start of the 1982/1983 ENSO event, the Mexicanvolcano El Chichon had two major eruptions (1982 March 23and April 4). About 150 days after these eruptions the extinctionat V increased by nearly 0.04 mag at the European Southern



208 FROGEL

1998 PASP, 110:200–209

Fig. 7.—Average extinction values for each run normalized to zero meanand with dispersions close to that of the NINO112 index as functions of boththe NINO112 and SOI indices. The NINO112 index is an indicator of howhigh above normal for any given month the sea surface temperature is between0–10S and 80W–90W. The SOI index is the departure from normal of thedifference in barometric pressure between Tahiti and Darwin, Australia. Thetime period covered in 1978 to 1992. The dashed lines are the regressionsolutions for each of the four extinction coefficients. For NINO112 as theindependent variable, the slopes, in the order indicated by the legend, are0.327, 0.234, 0.355, and 0.238. For SOI as the independent variable, the slopes,in the same order, are 20.359, 20.601, 20.358, and 20.326.

TABLE 7Tests for Correlations

A. Monthly Means

Pair of Variables p-Levels

E(H2O) and . . . . . . .E(J 2 K) 20.00E(H2O) and E(K) . . . . . . . . . . . . 10.02E(H2O) and . . . . . . .E(K 2 L) 20.02E(H2O) and E(CO) . . . . . . . . . . 10.01

and E(CO) . . . . . . . .E(J 2 K) 20.03and E(K) . . . . . . . . . .E(K 2 L) 20.04

S(H2O) and S(CO) . . . . . . . . . . . 10.04S(H2O) and . . . . . . . .E(J 2 K) 10.01S(H2O) and E(K) . . . . . . . . . . . . 20.02S(H2O) and . . . . . . .E(K 2 L) 10.04S(H2O) and E(H2O) . . . . . . . . . 20.00S(H2O) and E(CO) . . . . . . . . . . 20.03

and . . . . . .S(K 2 L) E(J 2 K) 20.01

B. Yearly Means


1978–1992:and . . . . . .E(J 2 K) E(H 2 K) 10.00

E(H2O) and E(K) . . . . . . . . . . . . . 10.05E(H2O) and E(CO) . . . . . . . . . . . 10.01

1978–1984:S(H2O) and . . . . . . . .S(K 2 L) 20.00S(CO) and . . . . . . . . .S(H 2 K) 20.00

and . . . . . .S(H 2 K) E(K 2 L) 20.01S(H2O) and E(H2O) . . . . . . . . . . 20.01S(H2O) and E(CO) . . . . . . . . . . . 20.02

and E(H2O) . . . . . . . .S(K 2 L) 10.05

C. With ENSO Indices


SOI and E(K) . . . . . . . . . . . . . . . . . . 0.029SOI and E(H2O) . . . . . . . . . . . . . . . 0.000SOI and . . . . . . . . . . . . .E(K 2 L) 0.032SOI and E(CO) . . . . . . . . . . . . . . . . 0.05NINO112 and E(K) . . . . . . . . . . . 0.015NINO112 and E(H2O) . . . . . . . . 0.010NINO112 and . . . . . .E(K 2 L) 0.10NINO112 and E(CO) . . . . . . . . . 0.09

Observatory on La Silla, Chile (Burki et al. 1995) and tookmore than 1000 days to decay back down to its normal value.Unlike the near-IR extinction, though, the normal extinction atV, which is also observed to be seasonally variable, is dueprimarily to scattering and molecular diffusion (Burki et al.1995). Burki et al. (see also Lockwood & Thompson 1986)demonstrated that the “volcanic aerosols” from the two erup-tions resulted in a considerably flatter wavelength dependencein the optical but still was due primarily to scattering rather

than absorption. Lockwood & Thompson (1986), based on op-tical extinction measurements at CTIO in the 1960s, point outthat the latitude of a volcano will probably be a strong deter-mining factor in the degree of enhancement of visible extinctionat a given observing site. The present near-IR data are notadequate to address the question of whether the recent eruptionsalso affected extinction in the near-IR. Given that the majorsource of this extinction is H2O, and given the correlation be-tween H2O extinction and the ENSO indices, it is doubtful thatany effect due to volcanoes would be detected even with muchbetter data sampling at the time of the eruptions. Neither Burkinor Lockwood & Thompson tested their optical data for var-iations due to the 1982/1983 ENSO event, but such variationsseem unlikely from the graphs in their papers.

Finally, the sharp drop in and (Fig. 3,S(J 2 H) S(H 2 K)




1998 PASP, 110:200–209

bottom, and Table 6) clearly shows the change to a doubletfield lens for D3 as well as a change from a Santa BarbaraResearch Corporation InSb detector to one by Cincinnati Elec-tronics. These “drops” in sensitivity were accompanied by evenlarger reductions in detector noise so that the S/N ratio of ameasurement actually increased for a source of given bright-ness and fixed integration time.

5. SUMMARY AND CONCLUSIONS

Since most of the atmospheric extinction in the near-IR canbe attributed to absorption by H2O (see, for example, any stan-dard atmospheric transmission curves; also Manduca & Bell1979), one expects that if some of the observed month-to-month and year-to-year variations in the extinction coefficientsand sensitivity values are intrinsic, then these variations shouldbe correlated with one another. Table 7 shows that this is thecase, since many of these quantities are linearly correlated witha significance ≥95%. Most of these involve the H2O index,which, because of the 2.00 mm filter used in measuring it, isexpected to show the greatest variation with atmospheric H2O.Furthermore, the signs of the correlations are all as expectedif H2O is the primary agent for varying atmospheric transmis-sion in the near-infrared. Almost all other pairings of extinctioncoefficients and sensitivity values on either a monthly or yearlybasis produced correlations with a significance considerablyless than those tabulated.

The correlations observed between the yearly means for ex-tinction and sensitivity strongly suggest that there are real long-term changes in the near-IR transmission properties of the at-

mosphere. Some of these changes are correlated with eventssuch as El Nino and La Nina, which are different aspects ofthe more general ENSO phenomenon. The present data cannotinvestigate any possible effects of volcanic eruptions on near-IR extinction.

Finally, I comment on the use of mean extinction coefficientsfor data reduction. Depending on the accuracy desired and thedifference in air mass between objects and standards, Table 1shows that on average, mean coefficients can yield acceptableresults. Table 3 and Figures 2 (top) and 4 show, though, thata reduction in observational uncertainty by up to several hun-dredths of a magnitude can be achieved by taking into accountthe month when the observations were made. There is no sub-stitute, however, for determining the extinction coefficients foreach observing run; Table 5 and Figures 1 (top), 3 (top), and5 show that even monthly means vary from year to year. Fur-thermore, global phenomenon such as ENSO will systemati-cally affect the values.

Much of the observing that resulted in the data for this paperwas carried out with the collaboration of Jay Elias. The ableassistance of the many CTIO telescope operators and the entireCTIO TELOPS crew played an essential role in 15 years ofdata gathering. I thank Oscar Saa (CTIO) for supplying mewith useful information, Lonnie Thompson and Keith Hen-derson (Byrd Polar Research Center, OSU) for acquainting mewith quantitative measures of the ENSO phenomenon, and mycolleague Kris Sellgren for a critical reading of the manuscript.My research at OSU was supported in part by NSF grantAST92-18281.

REFERENCES

Aaronson, M., Frogel, J. A., & Persson, S. E. 1978, ApJ, 220, 442Burki, G., Rufener, F., Burnet, M., Richard, C., Blecha, A., & Bratschi,

P. 1995, A&AS, 112, 383Cane, M. A. 1986, Ann. Rev. Earth Planet. Sci., 14, 43Elias, J. H., Frogel, J. A., Matthews, K., & Neugebauer, G. 1982, AJ,

87, 1029Ferriso, C. C., Ludwig, C. B., & Thomson, D. T.1966, J. Quant.

Spectrosc. Radiat. Transfer, 6, 241

Frogel, J. A., Persson, S. E., Aaronson, M., & Matthews, K. 1978,ApJ, 220, 75

Lockwood, G. W., & Thompson, D. T. 1986, AJ, 92, 976Manduca, A., & Bell, R. A. 1979, PASP, 91, 848Thompson, D. T., & Lockwood, G. W. 1996, Geophys. Res. Lett., 23,

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A History of Infrared Extinction at CTIO, Chile, and a Possible Connection with the El Niño...

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