Astroclimatology of ESO Sites	ESO WIDE REVIEW M. Sarazin, April 2000, 7/10

Historical Southern Oscillation Index, representative of Niño/Niña activity: after applying a 3 months moving average, the data set was arbitrarily re-scaled and an offset of 0.7 was added for comparison with seeing. The dashed line indicates the Niño/Niña transition, the strong El Niños of 1983, 1987, 1992 and 1998 are well visible. The monthly mean seeing measured at Paranal since the beginning of the VLT site survey has been overplotted with the same 3 months moving average.