Climatic setting

Along the 33°S parallel, annual average precipitation ranges from 459 mm in Valparaiso (33.02°S, 71.63°W, 41 m a.s.l.) on the Pacific coast to 356mm in Santiago (33.45°S, 70.70°W, 520 m a.s.l.) and 180 mm in Mendoza (32.89°S, 68.83°W, 769 m a.s.l.), on the Argentinian side of the Cordillera (Schwerdtfeger 1976). In this region, climatic seasonality is well defined, with dry summers and most of the precipitation occurring during the winter months. The south-western Pacific perturbations reach the mountains only during the winter, producing variable precipitation, which is always in the form of snow at altitude (Lliboutry 1965). During the summer, the weather is extremely dry and stable, characterised by the constant presence of the Pacific anticyclone over the region. In fact, less than 1% of the annual total precipitation is recorded during the December-February period. Frontal activity is infrequent, and precipitation, both on the mountain range and in the lee of the mountains, is mainly due to convective activity (Schwerdtfeger 1976).

The synoptic situation during fieldwork is summarised in Figure 3.2, which shows the surface sea level pressure over South America. As indicated by Lliboutry (1998), a belt of stationary high pressure extends across the Pacific Ocean west of South America, preventing intrusion of moisture-laden air masses to the continent. This stationary anticyclone is also responsible for a minimum in relative humidity over central Chile during the summer months. The same reanalysis data show a mean relative humidity of 35% on the western coast of Chile, at about 33° latitude south, the minimum for the southern hemisphere outside Antarctica. The solar radiation is very intense, with a daily average of over 400 Wm-2 for the

Individual Monthly Means slp millibars

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Figure 3.2 Surface sea level pressure averaged from December 2000 to February 2001. The image is a visualisation of NCEP/NCAR reanalysis data provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, from their web site at http://www.cdc.noaa.gov/. Courtesy NOAA

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NCEP GrADS image

Figure 3.2 Surface sea level pressure averaged from December 2000 to February 2001. The image is a visualisation of NCEP/NCAR reanalysis data provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, from their web site at http://www.cdc.noaa.gov/. Courtesy NOAA

Figure 3.3 Penitentes field on the middle section of the Loma Larga glacier, at about 4500 m a.s.l. The whole glacier above 4000 m is covered in these snow pinnacles, which make difficult the movement of mountaineers and researchers and alters the surface energy balance of the glacier. On the right photograph is a detail of penitentes about 2 m in height

Figure 3.3 Penitentes field on the middle section of the Loma Larga glacier, at about 4500 m a.s.l. The whole glacier above 4000 m is covered in these snow pinnacles, which make difficult the movement of mountaineers and researchers and alters the surface energy balance of the glacier. On the right photograph is a detail of penitentes about 2 m in height same period, the maximum for both hemispheres during the summer months excluding the South Pole.

The climatic regime of the Dry Central Andes is clearly different from that of subtropical Andes of Bolivia and Peru, further north, characterised by convective intrusions of moist air masses from the Amazon basin during the summer (Vuille et al. 1998). Here the ablation session is well defined and characterised by long periods of clear and stable weather. This climatic setting is responsible for the formation of a very peculiar ablation morphology, the snow penitentes, common to all the central Andes and to other dry high mountains such as the Pamirs (Lliboutry 1965, Kotlyakov and Lebedeva 1974).

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