Speaker
Description
The importance of water vapor in arid and hyperarid regions motivates a comparative study in the Atacama and Namib deserts, a natural laboratory to test the role of topography and circulation in present-day climate. Based on ERA5 reanalysis evaluated by satellite and ground-based instruments, we analyzed the total column water vapor (TCWV) from seasonal to interannual scales offshore from these deserts. Separating TCWV into the contributions by the marine boundary layer (MBL) and the free troposphere, we found that along the Namib coast, moisture variability is strongly influenced by air mass transport aloft. During austral fall and winter, warm, dry air masses from the continent lower the MBL to just a few hundred meters, drying the region and reducing stratocumulus cloud cover by 30-60% compared to the Atacama coast. This occurs despite the stronger lower troposphere stability and colder sea surface temperatures on the South Atlantic coast relative to the Southeast Pacific. In contrast, the Atacama coast is largely shielded from continental air due to the Andes, resulting in a moister MBL, more frequent stratocumulus clouds, but a drier free troposphere enhancing the hyperaridity. However, the long-term dry conditions are occasionally interrupted by rainfall in Atacama’s hyperarid core. Of particular interest are the summer rainfall episodes, which have received less attention than the winter ones. To study precipitation, we complemented ERA5 with high-resolution simulations and surface observations. We found that ~75% of rain episodes in the hyperarid core between 1960—2020 are triggered by the transport of moisture from the tropical Pacific along the west coast of South America (Moist Northerlies). As moisture is transported above the MBL, the daily heating of the west slope of the Andes pumps the moist enriched air inland, leading to cloud formation, widespread rainfall, and embedded convection. Our results also show that this synoptic pattern has become more frequent, increasing TCWV summer mean ~1 kg m-2 decade-1, with a notorious increase in daily extreme water vapor values between 2011—2020. We hypothesize that this is linked to the summer Hadley cell expansion increasing the occurrence of Moist Northerlies via an upper-troposphere mechanism. High-quality water vapor information, ideally profiles, is needed to better determine long-term trends in moisture and to identify their causes and effects, e.g., the role of circulation and relation to changes in the stratocumulus clouds. We believe that analyzing the water cycle in two regions with similar climatological features but also distinct differences (Namib and Atacama) helps to better understand the response of the Hadley cell to climate change.
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