Climate change and precipitation trends in the northern Mediterranean

• Main Author: Fortuny, Didac
• Other Authors: [e-mail], false, Bladé, Ileana, true, Universitat de Barcelona. Departament d'Astronomia i Meteorologia
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Universitat de Barcelona 2015
• Subjects: Meteorologia; Meteorología; Meteorology; Canvis climàtics; Cambios climáticos; Climatic changes; Climatologia; Climatología; Climatology; Clima mediterrani; Clima mediterráneo; Mediterranean climate; Oscilación del Atlántico Norte; Oscil.lació de l'Atlàntic Nord; North Atlantic oscillation; Ciències Experimentals i Matemàtiques; 504; North Atlantic
• Online Access: http://hdl.handle.net/10803/323369

According to climate projections for the end of the twenty-first century, regional responses to global warming include a strong and widespread drying over the northern Mediterranean, particularly intense in summer. Many authors have reported observed negative precipitation trends in the last decades and have considered them evidences that the projected drying is already detectable. Because of the strong decadal variability of precipitation series, however, it is not straightforward to discern whether observed trends are attributable to an external signal or whether they are consistent with internal variability alone. In order to assess if precipitation series in the northern Mediterranean region are compatible with the presence of a persistent signal, we examine the sensitivity of trend estimations to the choice of time interval: since internal variability is expected to be random, the magnitude of natural precipitation changes varies greatly when estimated for intervals with different arbitrary initial and final years. Instead, series under the influence of a persistent signal exhibit trends of similar magnitude in a wide range of consecutive intervals. In this regard, our results indicate that only in winter and from mid-twentieth century onwards the evolution of precipitation series is consistent with what one would expect if a persistent signal was superimposed to internal variability. Trends for other seasons and periods, instead, are sensitive to the choice of time interval. In addition, we find that the recent observed winter drying trends are largely explainable by an upward trend in the North Atlantic Oscillation (NAO) observed in the last decades of the twentieth century. The role played by climate change and internal variability in the observed winter northern Mediterranean drying is examined using historical (forced) and pre-industrial (unforced) CMIP5 climate simulations. For area-averaged precipitation we find that neither historical nor pre-industrial simulations contain trends as strong as the ...