INVESTIGADORES
MILANA Juan Pablo
congresos y reuniones científicas
Título:
THE ASYMMETRY OF SNOWFALL ALONG THE ARID ANDES (27-33ºS) AND ITS RELATION WITH WIND TRANSPORT AND GLACIAR FORMATION
Autor/es:
JOSE ESPONOZA ABURTO; JUAN PABLO MILANA
Lugar:
Parana
Reunión:
Congreso; XVII Reunion Argentina de Sedimentologia y VIII Congreso Latinoamericano de Sedimentologia - Parana; 2021
Institución organizadora:
Asociacion Argentina de Sedimentología
Resumen:
It is widely known that glacier distribution along the central arid Andes segment (27-33ºS) is asymmetric, asc.80% of glaciers tend to preferentially face southern and eastern slopes. The southern orientation responds to theminimum amount of solar radiation that helps snow preservation and glacier formation, replicating the samebehaviour (inverted) observed at the Northern hemisphere. The eastern preferential orientation has beeninterpreted by most researchers as the result of the wind transport effect: Most snow-forming storms areassociated with the synoptic phenomena of polar fronts associated with strong winds directed to the East andSoutheast. However, snow-fall field data suggest there is more precipitation on the western Andean slope, whichis logical if we consider that the Andes may exert a shadow-mountain effect over all precipitation types. Then, wetried to tackle the problem of the validity of the snow-grain eastward wind transport, as the origin for the observedglacier tendency, as alternatively it could be the result of differential wind ablation. Alike the influence of sunposition on the southwards preferential orientation, wind can also strongly affect the snow melting rate. Thiseffect is portrayed by the significant ablative influence of wind on the two main turbulent fluxes that are used forcalculating the energy balance on snow surfaces, along with two main radiative fluxes. The effect of windremoving vapour molecules over the snow surface and mixing air layers with different temperatures directlyaffects the turbulent sensible- and latent-heat fluxes increasing evaporation and sublimation. As the arid Andes aresubject to strong and almost continuous westerlies, a permanent asymmetry of wind ablative processes should beexpected. To understand better the snow-fall asymmetry, a complete processing of the daily snow precipitationwas carried along the arid central Andes, separating the information in latitudinal belts of 1º along the N-S axis,and differentiated datasets at the water divide, that coincides with the Argentina-Chile international border alongthis Andes segment. For this analysis we used the product MOD10 generated from original daily images taken bythe sensor MODIS onboard Terra satellite of NASA. Different algorithms were used to eliminate cloudy imagesand weekly averages of clear-sky images were used for this analysis. The dataset was fragmented alonghypsometric belts of 100 m of altitude using the publicly available SRTM global DEM, and we selected two waysof representing it. The first is the graph of altitude vs. true snow covered area, while for the second we normalizedthe snow cover to the total area of each hypsometric belt over both opposite slopes. We generated graphs using thecomplete set of information of MODIS-Terra (c.21 years) for both slopes, from 27ºS until 33ºS, giving 12 graphs(6 latitude belts by 2 different slopes) with 21 curves for each year. The preliminary analysis shows discrepanciesbetween the traditional knowledge and the potential true dynamics. The first set of graphs shows very well theeffect of mountain shadow over snow precipitation, but only at lower altitudes. Indeed, the snow distribution inChile (for the latitude considered here) is significantly larger at lower altitudes, causing the entire curves to beslightly shifted to lower altitudes. For instance, the peak altitude belt for the 32º-33ºS belt is 3500 m for Chilewhile 4200 m for Argentina. To the north, it changes consistently so at the 29º-30ºS belt it becomes 4150 m forChile and 4400 m for Argentina, while in the northernmost belt, 32º-33ºS, the largest snow cover value occurs at4350 m in Chile and at 4750 m in Argentina. Besides the displacement of these entire curves that show quasiGaussian shapes, the shadow mountain effect is very well shown by the contrasting asymmetry of the curves: InChile the Gauss bell shows gentler slopes to the lower altitudes, while in Argentina it shows a sharp change at3000 m and a gentler slope to higher altitudes. However, these graphs are quite affected by the hypsometricdistribution of the Earth surface, and different structures affect this Andes segment from South to North as thePrecordillera starting at 33ºS, Sierras Pampeanas at 31ºS, Domeyko Cordillera and the Atacama/Puna plateau at28ºS. For instance, a secondary mode is created by the Domeyko Cordillera at 2900 m in the 32º-33ºS. The use ofnormalized graphs helps to understand the effect of these physiographic elements on the snow-fall asymmetry. Inthe 27º-28ºS belt, the curve shows maximum detachment near 4000 m (Chile 22% coverage, Argentina 2%)peaking at 4600 (29% and 12%) and unifying tendencies at 5600 m high. To the south these differences are lessbut significant in the lower altitudes, and unify tendencies near 4900 m. We conclude that the effect of mountainshadow is significant at lower altitudes, causing a significant tendency of lower-altitude snowfall in Chile vs.Argentina. However, at higher altitudes there is no visible effect of the water divide in the snow distributionsuggesting that the effect of West to East snow transport is not significant and would not be the reason for thepreferential eastward distribution of glacier orientation.