Universität zu Köln

Vicencio Veloso, José Miguel ORCID: 0000-0002-9464-0126 (2025). A comparative study of the atmospheric water cycle between the Atacama and the Namib desert. PhD thesis, Universität zu Köln.

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Abstract

The interplay between moisture, clouds, and rainfall in hyperarid environments drives local ecosystem evolution and impacts geomorphological features. The Atacama and Namib deserts, among the driest places on Earth, provide an ideal setting to investigate these processes due to their contrasting topographies and climatic influences. Despite extreme aridity, both deserts receive moisture via atmospheric water vapor transport, fog, and sporadic rainfall. However, the precise dynamics, variability, and long-term trends of these processes remain largely unexplored. This thesis examines the atmospheric water cycle in the present climate of the Atacama and Namib deserts, and identifies key similarities and differences. Understanding how moisture reaches these regions is crucial for assessing the present climate and reconstructing past climatic changes, offering insights into life evolution and surface processes. Given that human-induced global warming is already impacting temperature, moisture, and circulation patterns across the globe, studying the Atacama and Namib deserts can enhance our understanding of future climate scenarios in these hyperarid regions. Large-scale atmospheric circulation patterns distinctly influence both deserts. The Atacama, along South America’s western coast, is shaped by the Southeast Pacific subtropical anticyclone which maintains aridity through subsidence and coastal upwelling. On Africa’s southwestern coast, the Namib experiences similar arid-enhancing mechanisms but is influenced by different regional and continental climatic processes due to a different topographic configuration. While the Andes act as a formidable barrier to most of the zonal air mass exchange in the Atacama, the Namib experiences more frequent transport of air masses from the coast and the interior of the continent due to a more smoothed topography. While these aspects and their impact on moisture transport, cloud, and rainfall have been studied individually, an integrated comparative analysis can provide deeper insights into the atmospheric water cycle and the role of topography in influencing specific regional climate patterns. Using satellite-derived water vapor products and reanalysis, we found major differences in the seasonal, interannual, and vertical moisture variability. Moisture in the marine boundary layer (MBL) strongly depends on sea surface temperature and clouds, whereas the free troposphere (FT) is controlled by moisture transport. However, key differences emerge: in fall and winter, the Namib experiences easterly continental winds that bring warm, dry air, reducing MBL height, humidity, and cloud cover. In summer, these winds transport moisture, clouds, and rainfall from the continent to the coast. Conversely, the Atacama remains isolated due to the Andes which maintain stable southerly winds and stronger MBL mixing, reinforced by persistent stratocumulus clouds. The present-day climate conditions in Namib can also be seen as the one expected in a Paleo-Atacama with topographical characteristics resembling the present-day Namib (i.e., without the Andes), shedding light on the mechanisms of the hyperaridity onset in the Atacama during the last 20 million years. Our study highlights that the FT over the Atacama is significantly drier than over the Namib. This is due to persistent westerly winds that transport dry air offshore Atacama, whereas the Namib is consistently affected by easterlies, which bring continental moisture to the coast. Even though average moisture transport offshore the Atacama is weaker than offshore the Namib, we identified a previously unrecognized moisture transport mechanism along the west coast of South America, which brings tropical Pacific moisture to the Atacama, but only in summer and in the lowest levels of the FT (850 hPa). We further investigated this moisture transport mechanism using reanalysis, ground-based observations, and high-resolution weather simulations. We found a summer synoptic weather pattern that is hereby termed "Moist Northerlies" (MNs). MNs occur when an 850 hPa low-pressure system, located offshore Atacama, advects moisture from the lower FT in the tropical Pacific within a few days. This results in a significant increase in the total column water volume (TCWV), reaching levels comparable to those observed in the Amazon basin, typically exceeding 50 kg m$^{-2}$. The local circulation (the Rutllant cell) transports this moisture inland, triggering storms across the Atacama. The driving mechanism associated with the MNs is a southward-displaced Bolivian High and a weakened subtropical jet stream. Long-term rain gauge data show MNs account for 75% of rainy days in the Atacama, increasing in frequency during the neutral or cold phase of El Niño-Southern Oscillation (ENSO) and specific phases of the Madden-Julian Oscillation (MJO). Our findings challenge the conventional view that the Altiplano/Amazonian easterly intrusions primarily drive Atacama’s summer rainfall, instead highlighting the significant role of Pacific-sourced moisture. The increasing frequency of MNs in recent decades correlates with observed rainfall trends and the greening of the precordillera. This may be linked to human-induced climate change via Hadley cell expansion and the weakening of the upper tropospheric jet stream observed in recent decades. Additionally, wetter conditions in the Atacama during warmer periods in Earth's history (e.g., the Holocene and Pleistocene), typically associated with enhanced El Niño events, could also be partially influenced by the rising frequency of the MN regime—an ENSO-neutral or cold-induced mechanism that occurs within globally warmer climate phases. We also examined trends in stratocumulus clouds offshore of both deserts using satellite-derived products and reanalysis datasets. In the Southeast Pacific, stratocumulus cloud frequency has increased by 4% per decade, driven by a strengthening and poleward-shifting subtropical anticyclone, which enhances lower tropospheric stability, promotes free-tropospheric drying, and shifts the storm track poleward. The Andes further amplify these effects by limiting MBL-continent interactions, contributing to regional cooling trends of up to -0.2 K per decade along the Atacama coast. In contrast, stratocumulus cloud frequency trends in the South Atlantic are weaker. Despite the subtropical anticyclone's similar influence on interannual low-cloud variability, its long-term strengthening has produced different outcomes. Surface and low troposphere easterly winds are strengthening, transporting warm continental air into the coastal MBL, limiting stratocumulus formation while favoring localized stratus clouds along the Namib coast. In the rest of the South Atlantic, this warm continental air mixes with the oceanic MBL. It offsets the expected low-cloud increase by the strengthening of the anticyclone only, potentially explaining the contrasting cloud and temperature trends between the two regions. In conclusion, this thesis advances our understanding of the atmospheric water cycle in hyperarid environments through a comparative analysis of water vapor distribution, precipitation mechanisms, and cloud variability in the Atacama and Namib deserts. By addressing key gaps in the literature and using a comparative approach, this research advances knowledge of current climate dynamics while providing valuable insights into past and future climatic conditions in these extreme arid environments.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Vicencio Veloso, José Miguel jvicenc1@uni-koeln.de orcid.org/0000-0002-9464-0126 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-787620
Date:	2025
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Ehemalige Fakultäten, Institute, Seminare > Faculty of Mathematics and Natural Sciences
Subjects:	Natural sciences and mathematics Earth sciences
Uncontrolled Keywords:	Keywords Language Atacama UNSPECIFIED Namib UNSPECIFIED Water vapor UNSPECIFIED Water cycle UNSPECIFIED Climate change UNSPECIFIED South America UNSPECIFIED South Africa UNSPECIFIED Reanalysis ERA5 UNSPECIFIED Satellite UNSPECIFIED Comparative UNSPECIFIED
Date of oral exam:	22 May 2025
Referee:	Name Academic Title Crewell, Susanne Prof. Dr. Villa, Jordi Prof. Dr. Dunai, Tibor Prof. Dr.
Projects:	CRC1211 - Evolution at the Dry Limit
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/78762