Trends and extremes of drought indices throughout the 20th century in the Mediterranean

Abstract

Abstract. Average monthly precipitation, the original Palmer Drought Severity Index (PDSI) and a recent adaptation to Europe, the Self Calibrated PDSI (scPDSI) have been used here to analyse the spatial and temporal evolution of drought conditions in the Mediterranean during the 20th century. Monthly, seasonal and annual trends were computed for the period 1901-2000 and also for the first and second halves of this period. The statistical significance of trends was obtained with a modified version of the Mann-Kendall test that accounts for serial auto-correlation. The results show a clear trend towards drier conditions during the 20th century in most western and central Mediterranean regions, with the exceptions of northwestern Iberia and most of Turkey that reveal an increase of moisture availability. A Generalized Extreme Values (GEV) analysis was applied to the maximum and minimum regional values of scPDSI, with results pointing towards a significant decline of absolute extreme values in central areas (Italy and Balkans) and a less clear picture emerging in western (Iberia) and eastern (Turkey) realms.The inter-annual variability of the scPDSI index series is shown to be more realistic than the corresponding PDSI version, fitting better the drought episodes sequence and magnitude described in the literature for each sub-region. We assess the decadal and inter-annual variability of the scPDSI for each sub-domain and evaluate the role played by the major teleconnection patterns, and by several sea surface temperature (SST) anomalies. The main driver of scPDSI in western and central Mediterranean areas is the winter North Atlantic Oscillation (NAO) pattern that is also relevant during the following spring and summer seasons with anti-correlation values below −0.60. The second most important mode corresponds to the Scandinavian Pattern that is significantly assoCorrespondence to: R. M. Trigo (rmtrigo@fc.ul.pt) ciated to the scPDSI between winter and summer over central Mediterranean (correlation values around 0.50). Finally, the teleconnection and SST analysis has allowed us to calibrate a stepwise regression model, enabling the forecasting of summer drought conditions six months in advance. The final model obtained is capable of reproducing the observed scPDSI time series fairly well, with a correlation coefficient of 0.79 (0.77 after cross-validation) and a significant gain over climatology (SS c =59%), while the corresponding result against persistence is more modest (SS p6 =11%).

Precipitation Trends over Slovakia in the Period 1981–2013

“…The results indicate that the temporal patterns in the variables have not been uniform and that hydrological variables may accentuate trends and patterns existing in the meteorological variables, which act as inputs to the hydrological cycle. In recent years, a number of studies were published focusing on the evolution of precipitation, drought conditions, and moisture availability in the Mediterranean area during the 20th century [27][28][29]. Sousa et al [29] used average monthly precipitation figures to analyze the spatial and temporal evolution of drought conditions in the Mediterranean area during the 20th century.…”

“…In recent years, a number of studies were published focusing on the evolution of precipitation, drought conditions, and moisture availability in the Mediterranean area during the 20th century [27][28][29]. Sousa et al [29] used average monthly precipitation figures to analyze the spatial and temporal evolution of drought conditions in the Mediterranean area during the 20th century. Monthly, seasonal, and annual trends in precipitation were computed for the period 1901-2000 and for the first and second halves of this period.…”

Drought Trends in the Iberian Peninsula over the Last 112 Years

“…The drought indices do not present significant outliers, but they are autocorrelated, and so it is not advisable to test the statistical significance of the trend of these time series with the standard Mann-Kendall test [22]. This caveat is also present in series derived with moving sums and moving averages, and thus it is necessary to use a more advanced test that can deal with the autocorrelation such as the modified Mann-Kendall test [62].…”

“…Different sources of data have been used to compute drought indicators including observations from meteorological stations [26][27][28], gridded datasets [22,29], and reanalysis datasets [30]. Several authors have provided evidences of an increasing number of droughts forced by higher temperatures and enhanced dry conditions [28,31,32].…”

“…The systematic analysis of drought evolution and severity has been traditionally performed through the computation of the Palmer Drought Severity Index (PDSI) [19] or of the multiscalar Standardised Precipitation Index (SPI) [20]. More recently, a new formulation of the not so generalist PDSI, the so-called self-calibrated PDSI (scPDSI), was developed and used successfully to assess droughts over the Mediterranean region [21,22]. As different systems and regions respond to 2 Advances in Meteorology drought at very diverse temporal scales, some of the abovementioned drought indices present some caveats considering that they are unable to produce a multiscale outcome.…”