Atacama Climate: How El Niño Steers the Telescopes
Context
The Atacama Desert in northern Chile is the driest non-polar place on Earth, and it hosts a significant share of the world's largest ground-based telescopes. Nearby, the Extremely Large Telescope (ELT) with a 39-meter mirror is under construction and should start operations at the end of this decade. To plan observations months ahead, astronomers need to understand how temperature, humidity, and air turbulence vary. Previously, only seasonal variations were known, but there was no tool to forecast year-to-year changes linked to climate cycles like El Niño.
Methods
The authors collected weather data from three observatories at four sites: La Silla (2400 m), Paranal (2650 m), and two points on the Chajnantor plateau (5080 and 5150 m). They used temperature and humidity records from weather stations, as well as water vapor and astronomical seeing measurements from Paranal. For analysis, they applied the non-parametric Mann-Kendall test for trends and Pearson correlation for links with climate indices. El Niño and La Niña phases were defined using the Niño3.4 index: a deviation of sea surface temperature in the tropical Pacific of more than 0.75 degrees from the average.
Results
All sites show a temperature increase over the last decade, especially noticeable at high-altitude Chajnantor: minimum nighttime temperatures in autumn are rising at about 1 degree per decade. A clear link with El Niño emerges: during warm phases, the number of days with extremely high temperatures at sunset increases, while humidity drops. At Paranal, the correlation between the Niño3.4 index and the number of hot days reaches 0.45. Water vapor near the ground is lower during El Niño, which is favorable for infrared observations. However, astronomical seeing (image blurring by turbulence) remains stable: the median value of about 0.7 arcseconds does not change with years or between El Niño and La Niña phases. Only during strong La Niña does the spread of seeing increase somewhat, making conditions less predictable.
Implications
This work gives astronomers the first practical tool for long-term planning: El Niño phases can be used as a predictor of dry and warm conditions suitable for demanding infrared programs. Unlike neighboring studies that focused on individual parameters or specific events, this one for the first time combines multi-year records of temperature, humidity, and seeing from several high-altitude sites. This allows assessing the impact of climate cycles on the entire set of observing conditions, not just individual meteorological variables.
Future development
In the coming years, accumulating seeing data during strong El Niño and La Niña phases will help clarify whether La Niña indeed increases the spread of image quality. There is also a need to systematically study the effect of temperature differences between the telescope dome and the outside air on image quality. High-altitude observatory weather stations could become a valuable data source for climatologists, filling a gap in high-elevation observations.
Impact
The results will directly affect observation planning at ESO telescopes in Chile, including the future ELT. They are also useful for climatologists studying warming in the Andes and for engineers designing telescope cooling systems.
Next steps
Continue monitoring seeing and meteorological parameters at Paranal and Cerro Armazones, especially during strong El Niño and La Niña periods. Conduct a separate study on the influence of temperature differences inside and outside the dome on image quality.
Key open problems
For astronomy, a key unresolved question is the long-term stability of the astronomical climate under global warming. This work shows that seeing has not deteriorated so far, but rising temperatures at high altitudes could alter turbulence in the future. The question of how changes in atmospheric circulation affect cloudiness and humidity over the observatories remains open.
The Chajnantor plateau at 5000 meters is so dry that water vapor is barely detectable: the density of water vapor in the air is tens of times lower than at sea level. That is why millimeter-wave radio telescopes, which are most affected by water vapor, are being built there.
Key numbers
- rate of increase in minimum nighttime temperature in autumn at Chajnantor: 1 °C per decade
- correlation between Niño3.4 index and hot days at Paranal: 0.45
- median astronomical seeing at Paranal: 0.7 arcseconds
- temperature threshold at sunset for telescope cooling systems: 16 °C
- fraction of the year with sunset temperatures above 16 °C in recent years: 20–25%