Current large-scale climatic conditions in Southern Peru and their influence on snowline altitudes

Abstract

Based on detailed and publicly available data, a map of annual precipitation has been constructed. This map indicates for most of the area above 4000m altitude precipitation exceeding 600mm/yr, with many areas receiving even more than 800mm/yr. These values exceed the precipitation data given in maps in the literature by 200-500mm/yr and indicate considerably wetter conditions than have so far been assumed. However, the general pattern shown in previous precipitation maps was confirmed: high precipitation at the eastern edge of the Andes decreases towards the Pacific, and precipitation in the Western Cordillera decreases from north to south. A correlation between the annual river discharge of rivers flowing into the Pacific and records for the El Niño Southern Oscillation (ENSO) was found only for parts of the period 1926 to 1971, leaving the question of a direct influence of ENSO on the precipitation in the Western Cordillera of Southern Peru unanswered. From annual mean temperatures for thirty-five stations above 3000m, the altitude of the 0°C isotherm above each station was calculated, locating it above 5000-5100m in the Eastern Cordillera and below 4900-4800m in the Western Cordillera. A snowline altitude map was constructed from data for the contour line altitude that halves the glacier area, representing the accumulation area ratio (AAR) of 0.5 for 931 glaciers from the Peruvian glacier inventory. It shows the lowest snowlines at 4900-5000m in the Eastern Cordillera and the highest snowlines at 5600m in the Western Cordillera, but it also indicates that the 5200m and 5300m snowline contours cross the Andes from the Eastern to the Western Cordillera, leading to similar snowline altitudes at the Nevado Sara Sara in the Western Cordillera as in parts of the Eastern Cordillera, despite lower amounts of annual precipitation. With the concept of a 'normalised snowline altitude' the influence of precipitation differences as a single factor on the snowline altitude can be demonstrated. Because the observed precipitation differences should lead to larger snowline altitude differences between the Eastern and Western Cordillera than seen in the snowline altitude map, a second factor has to be taken into consideration for the climatic interpretation of the snowline altitude that is able to compensate the decreasing precipitation. This factor is the altitude of the annual 0°C isotherm.