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«1: Department of Geosciences, University of Fribourg, Switzerland 2: European Southern Observatory, Garching, Germany Abstract A study has been ...»

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Perturbations to astronomical observations at the European Southern Observatory’s very

large telescope site in Paranal, Chile: analyses of climatological causes

Martin Beniston, Paula Casals, and Marc Sarazin

1: Department of Geosciences, University of Fribourg, Switzerland

2: European Southern Observatory, Garching, Germany

Abstract

A study has been conducted to assess the reasons for a significant decrease in the astronomic

observing period since the Very Large Telescope of ESO (the European Southern Observatory) went into operation in 1998. Following a multi-year monitoring of meteorological parameters at the site of the ESO telescope in Paranal (northern Chile), the optimal climatic conditions observed there prior to the construction of the Very Large Telescope have not been as frequently recorded since.

In order to determine whether this region is being subjected to long-term changes in climate th consecutive to 20 century global warming, or whether the ENSO (El Niño/Southern Oscillation) event in the final years of the 1990s are responsible for this situation, climatological data from in situ measurements, upper-air soundings, analogical reconstructions of meteorological data to extend the records further back into the past, and large-scale re-analysis data have been used.

The results point towards a dominant role of ENSO in the current problems that astronomers face with reduced observation time.

1. Introduction Prior to setting up its Very Large Telescope (VLT) in Chile, the European Southern Observatory (ESO) undertook a multi-year climatological assessment of the Paranal site in terms of its suitability for astronomical observations (cloudiness, atmospheric turbidity, thermal turbulence, etc.). The site was deemed exceptional in terms of the quality and duration of conditions related to ground-based astronomical observations.

Following the construction and start of operations of ESO’s VLT in 1998, however, atmospheric conditions have been considerably different from those recorded during the measurement period earlier in the 1990s (ESO, 2001). These shifts in climatic patterns, which have over the last few years significantly reduced observational capability compared to that expected from the multi-year

climatological assessment, pose the following questions:

• Are these new climatic conditions totally anomalous and related to global warming?

Beniston et al., 2002 Revised version, May 2002 -1 Are they dominated by ENSO events?

• Are they part of a cycle that has already occurred in the past but which was not recorded during the 15-year observation period at Paranal?

This paper attempts to answer these questions based on data analysis. Because the time-series at the ESO site are too short for any meaningful climatological statistics to be compiled, a first step in this investigation was to obtain additional observational data from other sites in order to check the consistency between longer-term records and the short 15-year ESO data set.

Extending the data record helps provide key answers as to whether the current weather patterns are anomalous or not, against the backdrop of longer-term climate data.

Analysis from the NCAR-NCEP (National Center for Atmospheric Research/National Centers for Environmental Prediction) data also provides a synoptic overview to explain shifts in climatic conditions at Paranal. Such shifts include the circulation patterns that can favor or perturb the duration of astronomical observing periods.

This paper will thus report on the investigations based on the available climatological information;

following an overview of the data and the methodologies used, the possible causal mechanisms leading to perturbations in astronomical observing periods will be discussed in more detail.

2. Climate and data

Much of the climate of South America is influenced by robust surface pressure patterns and the migration of the inter-tropical convergence zone, which affects large areas of tropical South America. The southern part of the continent is affected by anticyclonic conditions that prevail over both the Atlantic and the Pacific, the thermally-induced low pressure system of northwestern Argentina, and the mid-latitudes westerlies. All of these circulation features interact strongly with the Andean mountain chain. The behavior of the Southern Oscillation is responsible for much of the climate variability observed at interannual scales on the continent. Changes in circulation th patterns observed during the 20 century are often associated with El Niño and La Niña events.

Indeed, over much of South America, atmospheric circulation patterns are generally more perturbed during El Niño than during La Niña years (Salles and Compagnucci, 1997).

Precipitation in northern Chile normally occurs during the winter months. However, during early stages of the warm phase of ENSO, precipitation tends to be above-average; nevertheless, the region remains essentially semi-arid to extremely arid (Compagnucci, 1991; Ruttland and Beniston et al., 2002 Revised version, May 2002 -2Fuenzalida, 1991). When El Niño conditions prevail, strong rainfall events tend to occur at low elevations along the coastal ranges of Chile. Because the arid zone is not adapted to heavy precipitation, these events are capable of triggering debris flows such as those that affected Santiago and much of the coastal zone during the El Niño events of 1991-1993 (Garreaud and Ruttland, 1996) and 1997. North of 40°S in Chile, streamflows are above normal during El Niño years (Waylen and Caviedes, 1990; Compagnucci and Vargas, 1998; Compagnucci, 2000), as a result of higher rainfall and, at the highest elevations in the Andes, snowfall amounts; the reverse is true during La Niña years. Compagnucci and Vargas (1998) note that the likelihood of dry conditions during La Niña in northern Chile is generally higher than the likelihood of wet conditions during El Niño, however.





Cerro Paranal is located at 24°40’S and 70°25’W at an altitude of 2,635 m above sea-level (Figure 1). It is located in a region that is characterized by a dry coastal zone bordering a cold ocean current, and a coastal escarpment that rises to about 1,500 m. The escarpment acts as a barrier to the inland incursion of the moist marine boundary layer, where persistent stratus and strato-cumulus clouds are adominant feature. Above the escarpment lie the crests of the coastal ranges, which are located to the west of the Andean chain proper. In this arid piedmont zone, moderate temperatures prevail throughout the year; precipitation increases in summer and with height, from a few mm at around 3,000 m height to over 400 mm/year on the high Altiplano (Ruttland and Ulriksen, 1979). Convective activity is enhanced at certain periods of the year as a result of a feature known as the “Bolivian Winter”, whereby a continental low-pressure pattern develops at latitudes located between 10°-30°S and centered on the 65°W meridian (van Loon et al., 1972). Upper-level tropical easterlies associated with the Bolivian low can occasionally lead to an advection of moisture that extends as far as the crests of the coastal ranges, down to altitudes of 2,500 m above sea level (Ruttland and Ulriksen, 1979).

Insert Figure 1 here

The time-series of available data at Paranal span the period from 1985 until present with, however, a number of gaps in the series that reflect occasional technical problems. In order to close these gaps, longer-term records have been used as will be discussed later, namely those from Antofagasta, located on the Pacific coast relatively close to Paranal.

The astronomy community uses a number of parameters to define the quality of atmospheric conditions favorable for astronomical observations. One such parameter that provides a measure of the quality of ground-based astronomical observations is termed seeing, which is the parameter that astronomers employ to describe the sky's atmospheric conditions. Seeing is the Beniston et al., 2002 Revised version, May 2002 -3effect of random variations of the index of refraction in the atmosphere on the quality of the image of an astronomic object; it is measured in arc-seconds. Because the atmosphere is in continual motion with changing temperatures, air currents and turbulence, weather fronts and dust particles combine to cause star images to shimmer (twinkle). If the stars are twinkling considerably, seeing conditions are poor, and the reverse is true when the star images are steady. The effects of poor seeing upon visual observation are, principally, small and erratic movements of the object and diffusion of its image. Atmospheric temperature and vertical stratification, in addition to turbulence, are thus the key elements that can influence seeing.

Insert Figure 2 here

At the Paranal site, there are several sets of missing data in the time series of the seeing parameter, as seen in Figure 2. However, several distinct periods can be identified, namely 1987and 1998-2001. A switch to much higher values of seeing occur after the beginning of 1998. As this is not a bias imposed by instrumental or site changes, it can be speculated that this shift in seeing has been triggered by a significant change in climatic regimes, as will be discussed later. Figure 2 also shows that, irrespective of variability on multiyear time-scales, there is also an annual cycle that is linked to differences in summer and winter regimes. These influence wind velocity and associated turbulence, as well as moisture and temperature characteristics at locations such as Paranal (see for example Garreaud and Battisti, 1999).

Insert Figure 3 here

The mean seeing for the period between April 1987 and August 1998 is 0.7. Figure 3 highlights the periods during which seeing is either above or below this mean value. In general, prior to 1996, conditions were considered excellent with occasional seasonal exceptions. However, since 1996, the seeing parameter has been systematically measured above 0.7. It has even exceeded

1.0 for several months since 1998; the highest value of the series was recorded in October 1998 (1.053). Even episodic improvements in 1999 and 2000 (0.759 in April 1999 and 0.808 in March 2000, for example) have been short-lived; the seeing parameter has not reverted to the low values characteristic of the pre-1996 period. Higher values of seeing are associated with a reduction in the quality of astronomical observations. Indeed, ESO considers that visibility deteriorated significantly from August 1998; a marginal amelioration was recorded in the latter part of the year 2000, with 0.712 in November 2000. Conditions have again deteriorated since then, however, with values exceeding 0.85 just 3 months later (February 2001).

Beniston et al., 2002 Revised version, May 2002 -4Because the meteorological observations at Paranal span a relatively short time frame (1985present), and because there are a number of gaps even in this short record, it is necessary to find an alternate method to close these gaps and to extend the Paranal record back in time. The main reason for undertaking this procedure is to allow an assessment of whether other periods in the past exhibit similar characteristics to those which have been causing problems for astronomical observations for the ESO VLT since the late 1990s.

Insert Figure 4 here

Analysis of the Paranal and Antofagasta records show that there is a synchronous behavior of the two time series, as seen in Figure 4; the correlation between the two sets of data is 0.77. This implies that climatic conditions at Paranal are also highly sensitive to ENSO conditions in the Pacific, despite the high elevation of the Paranal site and the fact that it is located well above the maritime boundary layer within which Antofagasta is embedded. It is thus legitimate and scientifically sound to use the Antofagasta temperature record as an analogy to reconstruct climate series at Paranal, taking into account the strong inversion that exists between sea-level and Paranal. Figure 5 provides an example for winter (May) given by the upper-air sounding at Antofagasta; it is seen that the inversion layer is systematically located between 800-1,200 m above sea-level and has a typical strength of 8°C. The two curves are provided for the best seeing conditions of the 1987-2001 period (May 1994) and worst seeing (May 1998). There is a difference of over 2°C between May 1998 and May 1994, which represents a systematic shift to cooler conditions at the time when seeing conditions are mediocre.

Insert Figure 5 here

Because temperature, thermal stratification, and its perturbation by turbulence are major determinants of seeing, an extension of the temperature record on the basis of the close correlation between Antofagasta and Paranal temperature data can help to identify possible periodicity and frequency of atmospheric disruptions of the quality of astronomical observations.

Other meteorological variables, such as cloudiness or water vapor content will also affect nighttime sky observations, but they play a lesser role in determining seeing, which is the parameter that ESO has identified as exhibiting the most significant deterioration in recent years.

Insert Figure 6 here The long-term trend in temperature at Antofagasta is very low, at +0.11°C per century, based on the 1951-2000 time series given in Figure 6; in statistical terms, the trend is not significant. The Beniston et al., 2002 Revised version, May 2002 -5rate of warming at Antofagasta is almost one order of magnitude lower than the global-average warming observed since 1900 (Jones et al., 1999). At Paranal itself, over the short period of observational record, the warming trend is even less significant (+0.02°C per century, based on the 15 years of data). Direct regional warming linked to global climatic change can, at least on the basis of the available evidence, be ruled out as a significant driving factor for changes in astronomical observation conditions for the short period of record at Paranal.



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