Variabilidad espacio-temporal de grandes aludes en el Pirineo Oriental según la estructura del manto nivoso y la circulación atmosférica

[spa] La tesis versa sobre los regímenes de aludes en el Pirineo Oriental (Pirineo de la Comunidad Autónoma de Catalunya). Está estructurada en tres artículos publicados en revistas de impact factor y consta de un cuarto capítulo, como artículo en preparación. Los resultados obtenidos abarcan desde la probabilidad de ocurrencia de aludes en la zona de estudio, los marcos sinópticos desencadenantes de aludes, tanto a escalas sinóptica como de baja variabilidad temporal, regionalización de tipo de manto nivoso y modelización de las variaciones que puede sufrir al manto en escenarios de aumento de temperatura del aire de hasta 3 °C debido al calentamiento global. Se concluye que la probabilidad de ocurrencia anual de un episodio o más de grandes aludes en el Pirineo de Cataluña es del 64%. Pero la frecuencia es muy variable de una región a otra, siendo máxima en el Aran-Franja Norte de la Pallaresa con un 44% y un mínimo del 3% en el Perafita-Puigpedrós. Disminuye de oeste hacia este y de norte a sur, pero vuelve a aumentar en el extremo oriental con un 25%. Estas diferencias entre regiones se deben a que las situaciones sinópticas que desencadenan grandes aludes no tienen los mismos efectos en todas las regiones. Se identifican seis patrones sinópticos desencadenantes de grandes aludes mediante un análisis de componentes principales. La situación más frecuente es la advección del NW, le sigue el paso de vaguadas, depresión aislada en el sur de la Península Ibérica, baja fría, advección del SW y dorsal anticiclónica sobre el Mediterráneo occidental. La mayor parte de estas situaciones son propias de fases negativas de la Oscilación del Atlántico Norte (NAO). Se demuestra una correlación negativa entre un índice acumulativo de valores mensuales negativos de la NAO y la actividad de grandes avalanchas (-0.7), que es máxima en los sectores de la Pallaresa y de la Ribagorçana-Vall Fosca y en el extremo oriental. En la zona oceánica no se observa ninguna correlación con la NAO ni con la Oscilación del Mediterráneo Occidental (WeMO) y los episodios de aludes suelen coincidir con fases ligeramente positivas de la NAO. A partir de un análisis de medidas de posición de variables de la estructura del manto nivoso, se ha identificado un patrón de manto nivoso propio de la zona oceánica del Pirineo y otro de la zona mediterránea. El manto oceánico tiene una mayor proporción de granos de fusión que el mediterráneo, mientras que éste presenta unos índices de inestabilidad por capas débiles internas superior y mayor proporción de granos no persistentes que el oceánico. Mediante un análisis de modos combinados de percentiles de temperatura y precipitación, se observa que en la zona oceánica en los 10 días previos a actividad de grandes aludes y antes de un accidente aumenta notablemente el número de días fríos-húmedos, mientras que en la zona mediterránea, predominan los días fríos-secos antes de actividad de aludes. En los últimos 40 años están aumentando el número días cálidos-secos (del orden de un día por año). A partir de la simulación de balances energéticos y de masa del manto nivoso, se observa que en condiciones de un aumento de la temperatura media del aire del orden de 3 °C el espesor de nieve máximo anual disminuiría entre un 15-22% y el período de innivación se reduciría entre 15-25 días. Las orientaciones soleadas sufrirían unas condiciones de fuerte restricción de nieve respecto las umbrías. Hay una serie de conclusiones de carácter climático, como el uso de índices del manto nivoso para avanzar en la clasificación nivoclimático del Pirineo y otras de carácter aplicado en la predicción, como el uso de un catálogo de situaciones sinópticas desencadenantes de aludes y el uso de índice de inestabilidad del manto nivoso en el análisis diario del peligro de aludes.[eng] This thesis deals with avalanche regimes in the Eastern Pyrenees (Catalan Pyrenees). It is structured in three articles published in impact factor journals and consists of a fourth chapter, as an article in preparation. The results cover from the probability of avalanches occurrence, the synoptic avalanche patterns, the regionalization of snowpack structures and modeling changes in the snowpack due to the global warming. It is concluded that the probability of an annual occurrence of one episode or more of large avalanches in the Pyrenees of Catalonia is 64%. Frequency variability is very high among regions. It decreases from west to east and from north to south, but again increases in the easternmost part. Regional differences are due to the fact that the synoptic situations that trigger avalanches do not have the same effects in all regions. Six synoptic patterns of large avalanches are identified through principal component analysis. Most of these situations are typical of negative phases of the North Atlantic Oscillation (NAO). A negative correlation between a cumulative index of negative monthly values of NAO and activity of large avalanches (-0.7) is found. In the oceanic zone there is no correlation with NAO or Western Mediterranean Oscillation (WEMO) and avalanche episodes usually coincide with slightly positive phases of NAO. It has been identified an oceanic snowpack pattern and a Mediterranean one. The oceanic one has a higher proportion of melting grains than the Mediterranean, while the latter presents higher instability indexes by internal weak layers and a higher proportion of non - persistent grains than the former. In the oceanic zone, in the 10 days prior to activity of large avalanches and before an accident the number of cold-wet days increase notably, whereas in the Mediterranean zone, the cold-dry days prevail before avalanche activity. In the last 40 years, the number of warm-dry days has dramatically increased. Under conditions of an increase in the average air temperature of the order of 3 °C the maximum annual snow thickness would decrease by 15-22% and the snow on the ground period would be reduced by 15-25 days. There are conclusions both of a climatic nature and of applied uses on avalanche forecasting

Characterizing major avalanche episodes in space and time in the twentieth and early twentyfirst centuries in the Catalan Pyrenees

With the aim of better understanding avalanche risk in the Catalan Pyrenees, the present work focuses on the analysis of major (or destructive) avalanches. For such purpose major avalanche cartography was made by an exhaustive photointerpretation of several flights, winter and summer field surveys and inquiries to local population. Major avalanche events were used to quantify the magnitude of the episodes during which they occurred, and a Major Avalanche Activity Magnitude Index (MAAMI) was developed. This index is based on the number of major avalanches registered and its estimated frequency in a given time period, hence it quantifies the magnitude of a major avalanche episode or winter. Furthermore, it permits a comparison of the magnitude between major avalanche episodes in a given mountain range, or between mountain ranges, and for a long enough period, it should allow analysis of temporal trends. Major episodes from winter 1995/96 to 2013/14 were reconstructed. Their magnitude, frequency and extent were also assessed. During the last 19 winters, the episodes of January 22-23 and February 6-8 in 1996 were those with highest MAAMI values,followed by January 30-31, 2003, January 29, 2006, and January 24-25, 2014. To analyze the whole twentieth century, a simplified MAAMI was defined in order to attain the same purpose with a less complete dataset. With less accuracy, the same parameters were obtained at winter time resolution throughout the twentieth century. Again, 1995/96 winter had the highest MAAMI value followed by 1971/72, 1974/75 and 1937/38 winter seasons. The analysis of the spatial extent of the different episodes allowed refining the demarcation of nivological regions, and improving our knowledge about the atmospheric patterns that cause major episodes and their climatic interpretation. In some cases, the importance of considering a major avalanche episode as the result of a previous preparatory period, followed by a triggering one was revealed

Análisis de la temperatura en alta montaña: reciente descenso térmico invernal en el Pirineo oriental y su relación con la circulación atmosférica. Resultados preliminares

Ponencia presentada en: V Congreso Internacional de la Asociación Española de Climatología celebrado en Zaragoza del 18 al 21 de septiembre de 2006.[ES]El presente trabajo apunta la posible finalización del aumento de las temperaturas medias registrado a mediados del siglo XX en zonas de alta montaña del Pirineo Oriental, a través de del análisis de las temperaturas mínimas invernales. Los resultados indican incluso una oscilación térmica negativa apreciable y ponen de manifiesto su relación con una inflexión en la evolución del índice de la Oscilación del Atlántico Norte (NAO) desde 1990 hasta 2006.[EN]In this communication, an analysis of winter mean minimum temperatures at high elevation sites of the Eastern Pyrenees is carried out, indicating a likely disruption of the warming detected in mid-twentieth century. The results also detect a noticeable negative oscillation, in close relation to the inflection of the North Atlantic Oscillation index highlighted from 1990 until 2006

Characterizing major avalanche episodes in space and time in the twentieth and early twentyfirst centuries in the Catalan Pyrenees

With the aim of better understanding avalanche risk in the Catalan Pyrenees, the present work focuses on the analysis of major (or destructive) avalanches. For such purpose major avalanche cartography was made by an exhaustive photointerpretation of several flights, winter and summer field surveys and inquiries to local population. Major avalanche events were used to quantify the magnitude of the episodes during which they occurred, and a Major Avalanche Activity Magnitude Index (MAAMI) was developed. This index is based on the number of major avalanches registered and its estimated frequency in a given time period, hence it quantifies the magnitude of a major avalanche episode or winter. Furthermore, it permits a comparison of the magnitude between major avalanche episodes in a given mountain range, or between mountain ranges, and for a long enough period, it should allow analysis of temporal trends. Major episodes from winter 1995/96 to 2013/14 were reconstructed. Their magnitude, frequency and extent were also assessed. During the last 19 winters, the episodes of January 22-23 and February 6-8 in 1996 were those with highest MAAMI values,followed by January 30-31, 2003, January 29, 2006, and January 24-25, 2014. To analyze the whole twentieth century, a simplified MAAMI was defined in order to attain the same purpose with a less complete dataset. With less accuracy, the same parameters were obtained at winter time resolution throughout the twentieth century. Again, 1995/96 winter had the highest MAAMI value followed by 1971/72, 1974/75 and 1937/38 winter seasons. The analysis of the spatial extent of the different episodes allowed refining the demarcation of nivological regions, and improving our knowledge about the atmospheric patterns that cause major episodes and their climatic interpretation. In some cases, the importance of considering a major avalanche episode as the result of a previous preparatory period, followed by a triggering one was revealed