Maps of monthly self-calibrating Palmer Drought Severity Index (SC-PDSI) have been calculated for the period of 1901-2002 for Europe (35°-70°N, 10°W-60°E) with a spatial resolution of 0.5°ϫ 0.5°. The recently introduced SC-PDSI is a convenient means of describing the spatial and temporal variability of moisture availability and is based on the more common Palmer Drought Severity Index. The SC-PDSI improves upon the PDSI by maintaining consistent behavior of the index over diverse climatological regions. This makes spatial comparisons of SC-PDSI values on continental scales more meaningful.Over the region as a whole, the mid-1940s to early 1950s stand out as a persistent and exceptionally dry period, whereas the mid-1910s and late 1970s to early 1980s were very wet. The driest and wettest summers on record, in terms of the amplitude of the index averaged over Europe, were 1947 and 1915, respectively, while the years 1921 and 1981 saw over 11% and over 7% of Europe suffering from extreme dry or wet conditions, respectively.Trends in summer moisture availability over Europe for the 1901-2002 period fail to be statistically significant, both in terms of spatial means of the drought index and in the area affected by drought. Moreover, evidence for widespread and unusual drying in European regions over the last few decades is not supported by the current work.

[1] Potential evapotranspiration (PET) is one of the inputs to the Palmer Drought Severity Index (PDSI). A common approach to calculating PDSI is to use the Thornthwaite method for estimating PET because of its readily available input data: monthly mean temperatures. PET estimates based on Penman-type approaches are considered to be more physically realistic, but require more diverse input data. This study assesses the differences in global PDSI maps using the two estimates for PET. Annually accumulated PET estimates based on alternative Thornthwaite and Penman-Monteith, parameterizations have very different amplitudes. However, we show that PDSI values based on the two PET estimates are very similar, in terms of correlation, regional averages, trends, and in terms of identifying extremely dry or wet months. The reason for this insensitivity to the method of calculating PET relates to the calculations in the simple water balance model which is at the heart of the PDSI algorithm. It is shown that in many areas, actual evapotranspiration is limited by the availability of soil moisture and is at markedly lower levels compared to its potential value. In other areas, the water balance does change, but the quantity central to the calculation of the PDSI is, by construction, a reflection of the actual precipitation, which makes it largely insensitive to the use of the Thornthwaite PET rather than the Penman-Monteith PET. A secondary reason is that the impact of PET as input to a scaling parameter in the PDSI algorithm is very modest compared to the more dominant influence of the precipitation.Citation: van der Schrier, G., P. D. Jones, and K. R. Briffa (2011), The sensitivity of the PDSI to the Thornthwaite and Penman-Monteith parameterizations for potential evapotranspiration,

Moisture availability across Europe is calculated based on 22 stations that have long instrumental records for precipitation and temperature. The metric used is the self-calibrating Palmer Drought Severity Index (scPDSI) which is based on soil moisture content. This quantity is calculated using a simplified water budget model, forced by historic records of precipitation and temperature data, where the latter are used in a simple parameterization for potential evaporation. The precipitation and temperature records are updated to include the 2003 summer and all records, except for one, span at least 200 years, with the record for Kew going back to 1697.The Kew record shows a significant clustering of dry summers in the most recent decade. When all the records are considered together, recent widespread drying is clearly apparent and highly significant in this long-term context.By substituting the 1961-1990 climatological monthly mean temperatures for the actual monthly means in the parameterization for potential evaporation, an estimate is made of the direct effect of temperature on drought. This analysis shows that a major influence on the trend toward drier summer conditions is the observed increase in temperatures. This effect is particularly strong in central Europe.Based on the 22 scPDSI records, a gridded scPDSI dataset covering a large part of Europe has been constructed and compared to a recent high-resolution scPDSI dataset spanning the twentieth century only. We again observe that a major cause for the large areal extent of summer drought in the last two decades is high temperatures. Temperatures in the 12 months preceding and including the summer of 2003 explain an increase in the areas experiencing slightly dry (or worse) conditions of 11.1%.

Abstract. Climate proxy data provide noisy, and spatially incomplete information on some aspects of past climate states, whereas palaeosimulations with climate models provide global, multi-variable states, which may however differ from the true states due to unpredictable internal variability not related to climate forcings, as well as due to model deficiencies. Using data assimilation for combining the empirical information from proxy data with the physical understanding of the climate system represented by the equations in a climate model is in principle a promising way to obtain better estimates for the climate of the past.Data assimilation has been used for a long time in weather forecasting and atmospheric analyses to control the states in atmospheric General Circulation Models such that they are in agreement with observation from surface, upper air, and satellite measurements. Here we discuss the similarities and the differences between the data assimilation problem in palaeoclimatology and in weather forecasting, and present and conceptually compare three data assimilation methods that have been developed in recent years for applications in palaeoclimatology. All three methods (selection of ensemble members, Forcing Singular Vectors, and Pattern Nudging) are illustrated by examples that are related to climate variability over the extratropical Northern Hemisphere during the last millennium. In particular it is shown that all three methods suggest that the cold period Correspondence to: M. Widmann (m.widmann@bham.ac.uk) over Scandinavia during 1790-1820 is linked to anomalous northerly or easterly atmospheric flow, which in turn is related to a pressure anomaly that resembles a negative state of the Northern Annular Mode.