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CR 17:263-274 (2001)  -  doi:10.3354/cr017263

Global warming signature in observed winter precipitation in Northwestern Europe?

Torben Schmith*

Danish Meteorological Institute, Lyngbyvej 100, 2100 København Ø, Denmark

ABSTRACT: For 40 precipitation series in Northwestern Europe covering the period 1900-1990, the question whether variability of winter (October to March) precipitation on all time scales longer than years can be explained by changes in circulation is investigated. This is done, for each time series, by applying a linear statistical method (multi-regression) linking the winter precipitation 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 model. The interannual variability of winter precipitation is linked to circulation. This has been quantified by the multiple-correlation coefficient between 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-correlation coefficient is the largest in the Western Norway region. A comparison is also made between the performance of this standard model and a Œreduced¹ model based on the link between precipitation and the North Atlantic Oscillation (NAO) index only. This comparison shows that the performance is significantly larger using 5 PCs than using 1 PC (the NAO). In contrast, the lowest frequencies are not related to changes in the circulation. This shows up as a systematic positive trend in the difference between the observed and hindcasted precipitation for the majority of series. A Monte Carlo test reveals that this result is unlikely to have occurred by chance. This is interpreted as a change in the physics of the climate system, due to enhanced greenhouse forcing, to changes in sea surface temperatures in connection with some very low frequency mode, or to changes in land use enhancing the hydrological cycle.

KEY WORDS: Global warming · Climate change detection · Precipitation · Europe · Hindcast

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