Global warming signature in observed winter precipitation in Northwestern Europe?

For 40 precipitation series in Northwestern Europe covering the period 1900 -1990, the question whether variability of winter (October to March) precip itation on all time scales longer than years can be explained by changes in circulation is investigated. This is done, for each time series, by applyi ng a linear statistical method (multi-regression) linking the winter precip itation to the coefficients of the leading 5 principal components (PCs) of the winter mean mean sea level pressure. Having determined the coefficients , the corresponding hindcasted time series is obtained by applying the mode l. The interannual variability of winter precipitation is linked to circula tion, This has been quantified by the multiple-correlation coefficient betw een the time series of observed and hindcasted values. Independence between these 2 time series is required for the validation to be meaningful. This is obtained by applying a cross-validation technique. The multiple-correlat ion coefficient is the largest in the Western Norway region. A comparison i s also made between the performance of this standard model and a 'reduced' model based on the link between precipitation and the North Atlantic Oscill ation (NAO) index only. This comparison shows that the performance is signi ficantly larger using 5 PCs than using I PC (the NAO), In contrast, the low est frequencies are not related to changes in the circulation. This shows u p as a systematic positive trend in the difference between the observed and hindcasted precipitation for the majority of series. A Monte Carlo test re veals that this result is unlikely to have occurred by chance. This is inte rpreted as a change in the physics of the climate system, due to enhanced g reenhouse forcing, to changes in sea surface temperatures in connection wit h some very low frequency mode, or to changes in land use enhancing the hyd rological cycle.