Using re-analysis model data to analyse climate trends related to the astroclimatology of Paranal and La Silla

This paper consists of a summary of the results of two separate research exercises. In the first part, the relationship between astronomical night-time viewing statistics (Seeing and Photometric Night Fraction) from ESO Paranal-La Silla observatories and thirty-five ERA-40 and NCEP-NCEP re-analyses variables (available with the FriOWL software) were examined for correlation, using data from the re-analysis model gridpoint closest to the two observatories. The results confirm that the photometric night fraction datasets of both observatories are better described than are the seeing datasets. The strongest relationships are found at La Silla due to its more southerly, baroclinic location. Interestingly, the NOAA outgoing longwave radiation (OLR) dataset performs equally well, or better than its equivalent ERA40 re-analyses cloud cover dataset. In second part of this study, the FriOWL tool was used in an analysis of the climatology of northern Chile region, to help determine the cause of a deterioration in astronomical seeing conditions at Paranal from 1998 onwards. The results show that an exceptional increase in lOOOhPa geopotential height has occurred to the south-east of Paranal over the 1998-2007 period, when compared to the previous nine-year period. This increase is not replicated at the 700, 500 or 200 hPa levels, however. A shift south in the axis of near-surface geopotential heights from 21°S to 25-27°S has also occurred over the same period. Furthermore, there appears to be a link between phase shifts of the Interdecadal Pacific Oscillation, and atmospheric circulation anomalies over the Pacific region.