A history of infrared extinction at CTIO, Chile, and a possible connection with the El Nino phenomenon

Extinction coefficients and sensitivity values in the JHKL bandpasses measured on nearly 200 clear nights of observing between 1978 and 1992 on the 1.5 m and the Blanco 4 m telescopes at Cerro Tololo Inter-American Observatory are presented and discussed. Analysis of these data shows the following: there are seasonal variations in both the extinction coefficients and sensitivity values that are qualitatively consistent with expected variations in the amount of H2O in the atmosphere-relatively high in the summer months, lower in the 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 the atmosphere. At least some of these longer term variations are closely correlated with quantitative measures of the strength of the atmospheric and oceanic El Nino/Southern Oscillation phenomenon.