Abstract
The prominent Andes cordillera induces significant differences in climates between its eastern and western slopes. These climatic differences are largely reflected by contrasting vegetation and ice coverages but remain poorly documented. This study quantifies the abrupt changes of precipitation and cloud properties at both sides of the Andes south of 20°S by using surface daily precipitation and satellite (CloudSat and MODIS) data during the 2006-2016 period. Results show that the precipitation changes drastically and precipitating clouds can be of very different nature on each side of the Andes. In the tropical Andes (20°S-25°S), precipitation normally falls from a sole layer of thick stratiform and convective precipitating clouds during the warm semester, but the annual mean accumulation is about 10-100 times larger on the eastern than on the western slopes. A sole layer of low stratus clouds dominates over the Pacific coast, occasionally producing light rains, whereas high, thin, and non-precipitating clouds dominate most of the time over the continent. In the subtropical Andes (25°S-35°S), annual mean precipitation is similar on both sides, however, it falls from convective and stratiform precipitating clouds in the warm semester on the eastern slopes, and from stratiform precipitating clouds in the cold semester, mostly as frozen particles, on the western slopes. These different features on both slopes denote a climatic transition between the tropics and extratropics. In the extratropical Andes (south of 35°S), stratiform cloud types produces precipitation on both sides during all the year, but the annual mean precipitation and cloud frequency are enhanced on the western slopes and strongly reduced on the eastern slopes of the Andes. Cloud frequencies are higher than in the subtropics and evenly distributed as single- or multi-layers of low, middle and high clouds. Frozen particles become important in precipitating clouds over the mountains and on the lee side. These findings demonstrate the significant influence of the Andes cordillera on the climate all along southwestern South America, and constitute an excellent example of how the simple dependence of climate on latitude can be substantially altered by the topography.
Highlights
The long meridional extent of the Andes profoundly alter the frontal precipitation systems of the extratropical cyclones approaching from the southern Pacific Ocean (e.g., Smith and Evans, 2007; Garreaud et al, 2013), whereas the high Andean elevations in the tropics and subtropics strongly modulate local atmospheric circulation and moisture content that trigger convective precipitation systems (e.g., Falvey and Garreaud, 2005)
Surface daily precipitation and satellite data over the 2006–2016 period were first grouped in transects, according to their latitude and location in relation to the crest of the Andes, and used to explore the orographic effects on typical precipitation systems occurring in the tropics and extratropics
The polar-orbiting satellite data used here (CloudSata and Moderate Resolution Imaging Spectroradiometer (MODIS)) have the limitation of discontinuous measurements, which is partially solved by using a multiyear period (2006–2016) that includes hundreds to thousands passes over the Andes
Summary
The long meridional extent of the Andes profoundly alter the frontal precipitation systems of the extratropical cyclones approaching from the southern Pacific Ocean (e.g., Smith and Evans, 2007; Garreaud et al, 2013), whereas the high Andean elevations in the tropics and subtropics strongly modulate local atmospheric circulation and moisture content that trigger convective precipitation systems (e.g., Falvey and Garreaud, 2005). Rainy forests in the Chilean slopes of the Patagonian Andes contrast with the Argentinean dry slopes and the Patagonian steppe Despite these marked differences in vegetation induced by marked gradients in climate, a thorough assessment of the cross-barrier variations in precipitation and cloudiness has not been conducted yet. The objective of this study is to quantify the variations of precipitation, cloudiness, and cloud properties in cross-barrier direction of the Andes, from subtropical latitudes (20◦S) to the southern tip (55◦S) of the South American continent. Variations in these climatic variables strongly control water resources and insolation across the Andes.
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