North Atlantic Oscillation imprints in the Central Iberian Peninsula for the last two millennia: from ordination analyses to the Bayesian approach

Abstract

[eng] The climate variability of the Iberian Peninsula (IP) can be explained in terms of relatively few large-scale atmospheric modes, such as the North Atlantic Oscillation (NAO), the East Atlantic (EA) and the Scandinavian (SCAND) patterns. The present-day IP climatology clearly show that the NAO is the most prominent mode, especially in winter. However, the recent investigations have highlighted that other climate modes play a key role in both modulating the NAO-climate relationship and controlling certain meteorological parameters, although little is known about the past evolution of these climate interactions. Furthermore, there is a reasonable understanding of the past NAO evolution in the northern and the southern IP, but almost no information is available in the Central IP. Within this framework, the main aim of this PhD thesis is to characterize the impacts of the NAO on the Central IP over the last 2,000 years. The conceptual lake model formulated to understand the present-day influence of the NAO on the limnological evolution of Peñalara (2016 m asl) and Cimera (2140 m asl) alpine lakes (Iberian Central Range, ICR) was established using Pearson's correlation coefficients between seasonal data of the NAO index, climatic data (i.e., precipitation and air temperature data) and ice phenology records from both lakes. The results suggest that the effects of the NAO are only reflected in the thawing process via the air temperature and the insulating effect of snow accumulation on the ice cover. An altitude component is evident in our survey because the effects of the NAO on Peñalara Lake are restricted to winter, whereas for higher Iberian alpine lakes (i.e., Redon Lake, Pyrenees), the effects extend into spring. A latitudinal component is also clear: in northern Europe, the NAO signal is primarily reflected in lake ice phenology via the air temperature, whereas our results confirm that in southern Europe, the strong dependence of both precipitation and temperature on the NAO determines the importance of these climatic variables for lake ice cover. The past NAO impacts on the Central IP were determined by the multi-proxy characterization of the sediments of Peñalara and Cimera lakes using ordination statistical analyses. This approach was used to reconstruct the intense runoff events, the lake productivity and the soil erosion in the Cimera Lake catchment and to interpret these factors in terms of temperature and precipitation variability in the ICR for the last two millennia. The spatio-temporal integration of this reconstruction with other Iberian reconstructions was employed to identify the main climate drivers over this region. During the Roman Period (RP; 200 BC – 500 AD), the generally warm conditions and the E–W humidity gradient in the IP indicate a dominant interaction between a negative NAO phase and a positive EA phase (NAO-–EA+), whereas the opposite conditions during the Early Middle Ages (EMA; 500 – 900 AD) indicate a NAO+–EA- interaction. The dominantly warm and arid conditions during the Medieval Climate Anomaly (MCA; 900 – 1300 AD) and the opposite conditions during the Little Ice Age (LIA; 1300 – 1850 AD) in the IP indicate the interaction of the NAO+–EA+ and NAO-–EA-, respectively. Furthermore, the higher solar irradiance and fewer tropical volcanic eruptions during the RP and MCA may support the predominance of the EA+ phase, whereas the opposite conditions during the EMA and LIA may support the predominance of the EA- phase. In addition, evidence of African dust inputs in these lakes could denote a coupled displacement between the Intertropical Convergence Zone and the NAO during the study period. Finally, a Bayesian random walk-modularised model was formulated to quantitatively reconstruct the evolution of the NAO impacts in the ICR (NAOICR) for the last two millennia using the raw chemical element profiles obtained from the Cimera Lake sediments using an X-Ray-Fluorescence Avaatech® core scanner. The obtained quantitative values of the NAOICR were in accordance with previously reconstructed precipitation and temperature conditions. In addition, the comparison of the NAOICR with other NAO approaches show that the local impact of the NAO can also display global aspects of this climate mode and that this impact reconstruction could

[spa] La variabilidad climática en la Península Ibérica (PI) puede ser explicada por un reducido número de modos atmosféricos como la Oscilación del Atlántico Norte (siglas en inglés NAO), el Atlántico Este (siglas en inglés EA) o el Escandinavo, aunque la NAO es el predominante, sobre todo en invierno. La construcción de un modelo conceptual basado en los coeficientes de correlación de Pearson entre el índice de la NAO, datos climáticos (temperatura, precipitación y nieve) y registros de la cubierta de hielo de lagos alpinos (Peñalara, 2016 m.; Cimera, 2140) situados en el Sistema Central Ibérico (SCI) ha permitido entender los efectos actuales de la NAO en estos lagos. Los resultados sugieren que dichos efectos sólo se reflejan en el proceso de deshielo de la cubierta a través de la temperatura del aire y del efecto aislante de la nieve acumulada en ella durante el invierno. A mayor altitud, este efecto se extiende hasta primavera y a mayor latitud este efecto depende principalmente sólo de la temperatura. El análisis multiproxy de los sedimentos lacustres de estos lagos ha permitido realizar una reconstrucción climática en el SCI para los últimos dos mil años. La integración espacio-temporal de dicha reconstrucción con otras reconstrucciones en la PI ha permitido determinar los principales factores climáticos que actuaron durante ese periodo. Las interacciones entre NAO y EA, la variabilidad solar y la actividad volcánica y quizás el movimiento acoplado entre la Zona de Convergencia Intertropical y al NAO fueron probablemente las principales causas de dicha variabilidad climática. Finalmente, el desarrollo de un modelo matemático basado en estadística bayesiana ha permitido reconstruir cuantitativamente el impacto de la NAO en el SCI (NAOSCI) para este periodo usando datos geoquímicos obtenidos mediante el análisis de fluorescencia de rayos X de los sedimentos de Cimera. A pesar de que el modelo no pudo ser validado por correlación cruzada, los valores de NAOSCI están en concordancia con la reconstrucción climática previamente obtenida. La comparación de NAOSCI con otras reconstrucciones de NAO sugiere que este impacto regional podría mostrar aspectos globales de este modo climático y por tanto, con algunas mejoras podría considerarse un índice regional para toda la PI.