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Abstract

The Atacama Desert is well known for its arid and hyper-arid conditions; nevertheless, in the coastal Atacama, where the extended low stratocumulus clouds cover over the southeast Pacific Ocean meets the Coastal Cordillera it produces a highly dynamic advective marine fog. This fog is a major feature of the local climate, which provides humidity and freshwater to this hyper-arid environment. Along the fog belt, fragile and highly endemic ecosystems have developed various strategies for adaptation that take advantage of fog water inputs, and therefore are sensitive indicators of change in fog and low clouds. At the same time, the fog belt covers an area with high potential for fog water collection, which could be used for a number of purposes, including human consumption and low-scale economic activities. However, currently there is little known about the dynamics between fog and climate in the coastal Atacama. For example, we still lack a basic understanding of the long-term spatiotemporal dynamics of this extensive coastal fog, how climate change has affected it in the recent past, is affecting it at present and how it will change in the near future. These are important questions, as understanding cloud variability over the coastal Atacama will provide insights into the regional, tropical Pacific atmospheric-oceanic system, and therefore the long-term climate controls on coastal biodiversity in the Atacama, as well as the potential of fog water as an alternative fresh water resource. In this research is presented an integrated approach of scales of analysis, based on time series analysis of satellite data collected between 1995 and 2017 and a local network of instruments that measure atmospheric variables, including a new ground oriented fog observation system (GOFOS) created for this project. Based on detailed knowledge of the seasonal and daily cycles of the fog and low clouds, as well its spatial distribution, frequency presence, inter-annual variability, trends and vertical dynamics allow to enhance understanding of the fog climate and the factors involved in its spatiotemporal changes. Results show that for the month of September there was a weak positive trend in fog presence over the ocean and onshore areas (below 1.000 m a.s.l.) during the study period; however, those areas close to the coast with elevations above 1.000 m a.s.l. showed a weak negative trend in fog presence, demonstrating a decrease in the altitude of the thermal inversion layer over the years. At the same time, the El Niño Oceanic Oscillation (ENSO) apparently exerts significant influence on the inter-annual variability in fog and low clouds in the coastal Atacama, as there is a possible causal-link between ENSO anomalies in the Niño 3.4 region and fog and low cloud frequency presence (FFP). ENSO phases (+/-) generate opposing seasonal fog conditions: i) ENSO (+) produces an increase in FFP during summer, and a decrease in winter; ii) ENSO (-) produces a decrease in FFP during summer, and an increase in winter. Furthermore, linear regression indicates that ENSO anomalies explain ~50% of the variance in inter-annual FFP over the ocean and near shore areas during summer. In the winter/early spring, regression indicates that ENSO anomalies explain a ~47% of FFP variation over the ocean and ~66% in onshore areas. At the same time, results indicate that during the winter/early spring, FFP trends are apparently under a Pacific decadal signal (La Niña-like), whose influence can only be confirmed with a dataset that covers a longer time span. Results also demonstrate that the annual daily cycle shows a strong inverse correlation between FFP and the altitude of the thermal inversion layer, with lower (higher) altitudes related with higher (lower) FFPs, and this pattern holds from sunset to sunrise (from early morning to the afternoon). Finally, spatiotemporal identification and characterization of fog and low clouds along the coast of Atacama, allows to precisely determine the areas with higher or lower fog presence, as well as the seasonal and daily trends (horizontal and vertical) in fog presence. Understanding spatiotemporal changes in fog and low clouds may allow to link these changes with recent geographic changes in fogdependent ecosystems, allowing for a better understanding of the existing connection between the atmosphere and biosphere, which is vital to comprehend both past and future climatic changes. Such understanding of fog patterns will also allow the identification of sites with greater potential for fog water collection, and to forecast changes in fog patterns that will affect fog harvesting in the future, in a region where growing demand for water resources requires alternative and sustainable sources.