Precipitation projections for Spain by means of a weather typing statistical method

• A new method of precipitation downscaling with stochastic properties is developed.
• Compensated circulation series minimize the biases of the input atmospheric data.
• We find a 30% reduction of precipitation in Spain by the end of the century.
• Projected precipitation loss is not uniform in time and across Spain.
• Around 10% of precipitation loss can be saved in less emissive scenarios.

This study develops a weather typing stochastic method for the climatic prediction of rainfall in Peninsular Spain and the Balearic Islands during the 21st century. Cluster analysis is applied to the geopotential height fields with the purpose of classifying the atmospheric states into distinct daily circulation patterns. The same kind of analysis is performed on the rainfall distributions, obtaining the corresponding daily rainfall patterns. It is possible to establish a suitable association between each of the circulation types and the rainfall patterns. This circulation–rainfall link becomes the essence of our downscaling method, which will allow the “reconstruction” of the accumulated rainfall field from a generated sequence of daily rainfall patterns. This is done with the support of a statistical weather generator using the daily patterns of atmospheric circulation provided by different GCMs as input. The weather typing approach and the weather generator strategy interrelate in a novel and unique way different from any previous method. The downscaling method is first subjected to calibration, using reanalysis circulation as input, from which the optimal number of atmospheric and rainfall patterns to perform the projection is found. Later on the method is subjected to validation, using GCMs' daily atmospheric circulation as input, in order to test its robustness. The final part of the study focuses on the analysis and intercomparison of future precipitation projections for the 21st century under the A1B emission scenario from five different GCMs. A substantial drying of about 30% is foreseen at the end of the century in Spain compared to present, although with a nonuniform pattern in space and time as the century progresses.