Suchergebnisse

Abstract. Impacts upon vulnerable areas such as mountain ranges may become greater under a future scenario of adverse climatic conditions. In this sense, the
concurrence of long dry spells and extremely hot temperatures can induce environmental risks such as wildfires, crop yield losses or other problems,
the consequences of which could be much more serious than if these events were to occur separately in time (e.g. only long dry spells). The present
study attempts to address recent and future changes in the following dimensions: duration (D), magnitude (M) and extreme magnitude (EM)
of compound dry–hot events in the Pyrenees. The analysis focuses upon changes in the extremely long dry spells and extremely high temperatures that
occur within these dry periods in order to estimate whether the internal structure of the compound event underwent a change in the observed period
(1981–2015) and whether it will change in the future (2006–2100) under intermediate (RCP4.5, where RCP is representative concentration pathway) and high (RCP8.5) emission scenarios. To this end, we
quantified the changes in the temporal trends of such events, as well as changes in the bivariate probability density functions for the main
Pyrenean regions. The results showed that to date the risk of the compound event has increased by only one dimension – magnitude (including extreme
magnitude) – during the last few decades. In relation to the future, increase in risk was found to be associated with an increase in both the
magnitude and the duration (extremely long dry spells) of the compound event throughout the Pyrenees during the spring under RCP8.5 and in the
northernmost part of this mountain range during summer under this same scenario.

Abstract. In previous studies the Western Mediterranean Oscillation index (WeMOi) at
daily resolution has proven to constitute an effective tool for analysing
the occurrence of episodes of torrential precipitation over eastern Spain.
The western Mediterranean region is a very sensitive area, since
climate change can enhance these weather extremes. In the present study we
created a catalogue of the extreme torrential episodes (≥200 mm in 24 h) that took place in Catalonia (NE Iberia) during the 1951–2016 study
period (66 years). We computed daily WeMOi values and constructed WeMOi
calendars. Our principal result reveals the occurrence of 50 episodes (0.8 cases per year), mainly concentrated in the autumn. We confirmed a threshold of WeMOi ≤ −2 to define an extreme negative WeMO phase at daily resolution. Most of the 50 episodes (60 %) in the study area occurred on days presenting an extreme negative WeMOi value. Specifically, the most negative WeMOi values are detected in autumn, from 11 to 20 October, coinciding with the highest frequency of extreme torrential events. On comparing the subperiods, we observed a statistically significant decrease in WeMOi values in all months, particularly in late October and in November and December. No changes in the frequency of these extreme torrential episodes were observed between both subperiods. In contrast, a displacement of the extreme torrential episodes is detected from early to late autumn; this can be related to a statistically significant warming of sea temperature.

Mountain systems within the Mediterranean region, e.g., the Pyrenees, are very sensitive to climate change. In the present study, we quantified the magnitude of extreme precipitation events and the number of days with torrential precipitation (daily precipitation ≥ 100 mm) in all the rain gauges available in the Pyrenees for the 1981-2015 period, analyzing the contribution of the synoptic scale in this type of event. The easternmost (under Mediterranean influence) and north-westernmost (under Atlantic influence) areas of the Pyrenees registered the highest number of torrential events. The heaviest events are expected in the eastern part, i.e., 400 mm day−1 for a return period of 200 years. Northerly advections over the Iberian Peninsula, which present a low zonal index, i.e., implying a stronger meridional component, give rise to torrential events over the western Pyrenees; and easterly advections favour extreme precipitation over the eastern Pyrenees. The air mass travels a long way, from the east coast of North America, bringing heavy rainfall to the western Pyrenees. In the case of the torrential events over the eastern Pyrenees, the trajectory of the air mass causing the events in these areas is very short and originates in the Mediterranean Basin. The North Atlantic Oscillation (NAO) index has no influence upon the occurrence of torrential events in the Pyrenees, but these events are closely related to certain Mediterranean teleconnections such as the Western Mediterranean Oscillation (WeMO) ; The present study was conducted within the framework of the Climatology Group of the University of Barcelona (2017 SGR 1362, Catalan Government) and the Spanish CLICES project (CGL2017-83866-C3-2-R, AEI/FEDER, UE). M.L.-C. was awarded a pre-doctoral FPU Grant (FPU2017/02166) from the Spanish Ministry of Science, Innovation and Universities. R.S.-N. and J.M.C. are partially supported by the Government of Aragón through the "Program of research groups" (group H38, "Clima, Agua, Cambio Global, y Sistemas Naturales")