Downscaling summer rainfall in the UK from North Atlantic ocean temperatures

Annual series of three stochastic rainfall model parameters-the seasonal we t day amount (or intensity). the conditional dry-day probability (or dry-sp ell persistence). and the conditional wet-day probability (or wet-spell per sistence)-were examined using daily rainfall records for ten UK stations fo r the period 1901-1995. The purpose was first. to determine the extent to w hich these indices of summer (June-August) rainfall were correlated with em pirical orthogonal functions (EOFs) of summer North Atlantic sea surface te mperature (SST) anomalies: second. to evaluate the skill of EOFs of precedi ng winter (December-February) SSTs for summer rainfall forecasting and down scaling. Correlation analyses suggest that observed increases in summer dry -spell persistence since the 1970s coincided with positive SST anomalies in the North Atlantic. In contrast. wet-spell persistence and intensities wer e relatively weakly correlated with the same patterns. implying that the us e of SSTs is justifiable for conditioning occurrence but not intensity para meters. Furthermore, the correlation strengths were greater for EOFs of SST s than those reported for area-average SST anomalies, indicating that the p attern of SST anomalies conveys important information about seasonal rainfa ll anomalies across the UK. When EOFs of winter SSTs were used to forecast summer rainfall in Cambridge, the skill was once again greater for dry-spel ls than either wet-spells or intensities. However, even for dry-spells. the correlation with observations whilst statistically significant-was still r ather modest (r<0.4). Nonetheless. the results are comparable to previous i nvestigations of summer rainfall across Europe, and suggest that forecastin g skill (across the UK) originates from the predictability of the rainfall occurrence process.