Regional drought modes in Portugal are identified applying the Principal Component Analysis (PCA) and Varimax rotation to the Standardized Precipitation Index (SPI) computed on different time scales at grid points regularly distributed over the country. The high-resolution (0.2 degree, approximately 20 km) gridded precipitation dataset provided by the Portuguese Meteorological Institute, which is based on a dense network of 400 quality-controlled stations covering the period 1950–2003, is used in the present study. The analysis aims to identify the spatial modes of drought variability in Portugal as well as to investigate their stability as a function of the SPI time scale used for monitoring the different kinds of drought. The identified spatial modes consist of two sub-regions with independent climate time variability, and they appear to remain relatively stable when the SPI time scale is varied from 1- to 36-month. As it is reasonable to expect, the wider extension over latitude than over longitude of the country (37–42N, 7–9W), controls the north-south character of the regionalization. Moreover, for long time scales it was found a third sub-region in central-eastern Portugal characterized by a prevailing downward trend in the SPI time series; this deserves further checks against observations and/or other precipitation datasets. The regionalization of drought herein proposed appears of particular interest for a rationale water resources management in the major Portuguese river basins (i.e., Douro, Tagus and Guadiana)
