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Downscaling DEMETER seasonal ensemble precipitation forecasts in the tropics during "El Niño" episodes

During the last decade, seasonal ensemble forecast models have shown certain skill in the prediction of synoptic conditions (SST, etc.) during some El Niño episodes. Downscaling techniques allow adapting those forecasts to a regional scale, but a lower skill is expected for the resulting regional predictions. In this paper we use the reanalysis data provided by the multi-model ensemble project DEMETER to validate the skill of seasonal regional forecasts in different regions of Perú. This project provides forecasts from 1949 to 1999 for six European global coupled atmosphere-ocean models with different perturbation schemes (http://www.ecmwf.int/research/demeter/). This reanalysis provides a convenient framework to validate seasonal forecasts for different El Niño episodes. The downscaling method used in this paper is an implementation of an analog-based clustering technique which allows to efficiently combining different ensemble forecasts from different models into a single probabilistic forecast with an estimation of the associated predictability (based on the ispersion of the ensembles). As a result, we obtained skilful regional predictions for different El Niño episodes in two different areas of Perú: the coast (positive anomalies), and the central mountain range (negative anomalies).