11.3.3.2 Mean Precipitation

A south-north contrast in precipitation changes across Europe is indicated by AOGCMs, with increases in the north and decreases in the south (Figure 11.5). The annual area-mean change from 1980 to 1999 to 2080 to 2099 in the MMD-A1B projections varies from 0 to 16% in NEU and from –4 to –27% in SEM (Table 11.1). The largest increases in northern and central Europe are simulated in winter. In summer, the NEU area mean changes vary in sign between models, although most models simulate increased (decreased) precipitation north (south) of about 55°N. In SEM, the most consistent and, in percentage terms, largest decreases, occur in summer, but the area mean precipitation in the other seasons also decreases in most or all models. More detailed statistics are given in Table 11.1. Increasing evaporation makes the simulated decreases in annual precipitation minus evaporation extend a few hundred kilometres further north in central Europe than the decreases in precipitation (Supplementary Material Figure S11.1).

Figure 11.5. Temperature and precipitation changes over Europe from the MMD-A1B simulations. Top row: Annual mean, DJF and JJA temperature change between 1980 to 1999 and 2080 to 2099, averaged over 21 models. Middle row: same as top, but for fractional change in precipitation. Bottom row: number of models out of 21 that project increases in precipitation.

Both circulation changes and thermodynamic factors appear to affect the simulated seasonal cycle of precipitation changes in Europe. Applying a regression method to five of the MMD simulations, van Ulden and van Oldenborgh (2006) found that in a region comprising mainly Germany, circulation changes played a major role in all seasons. In most models, increases in winter precipitation were enhanced by increased westerly winds, with decreases in summer precipitation largely due to more easterly and anticyclonic flow. However, differences in the simulated circulation changes among the individual models were accompanied by large differences in precipitation change, particularly in summer. The residual precipitation change varied less with season and among models, being generally positive as expected from the increased moisture transport capacity of a warmer atmosphere. In a more detailed study of one model, HadAM3P, Rowell and Jones (2006) showed that decreases in summer precipitation in continental and southeastern Europe were mainly associated with thermodynamic factors. These included reduced relative humidity resulting from larger continental warming compared to surrounding sea areas and reduced soil moisture due mainly to spring warming causing earlier snowmelt. Given the confidence in the warming patterns driving these changes, the reliability of the simulated drying was assessed as being high.

Changes in precipitation may vary substantially on relatively small horizontal scales, particularly in areas of complex topography. Details of these variations are sensitive to changes in the atmospheric circulation, as illustrated in Figure 11.6 for two PRUDENCE simulations that only differ with respect to the driving global model. In one, an increase in westerly flow from the Atlantic Ocean (caused by a large increase in the north-south pressure gradient) is accompanied by increases of up to 70% in annual precipitation over the Scandinavian mountains. In the other, with little change in the average pressure pattern, the increase is in the range of 0 to 20%. When compared with circulation changes in the more recent MMD simulations, these two cases fall in the opposite ends of the range. Most MMD models suggest an increased north-south pressure gradient across northern Europe, but the change is generally smaller than in the top row of Figure 11.6.

Figure 11.6. Simulated changes in annual mean sea level pressure (∆SLP), precipitation (∆Prec) and mean 10-m level wind speed (∆Wind) from the years 1961 to 1990 to the years 2071 to 2100. The results are based on the SRES A2 scenario and were produced by the same RCM (Rossby Centre regional Atmosphere-Ocean model; RCAO) using boundary data from two global models: ECHAM4/OPYC3 (top) and HadAM3H (bottom) (redrawn from Rummukainen et al., 2004).

Projections of precipitation change in Europe based on SD tend to support the large-scale picture from dynamical models (e.g., Busuioc et al., 2001; Beckmann and Buishand, 2002; Hanssen-Bauer et al., 2003, 2005; Benestad, 2005; Busuioc et al., 2006), although variations between SD methods and the dependence on the GCM data sets used (see Section 11.10.1.3) make it difficult to draw quantitative conclusions. However, some SD studies have suggested a larger small-scale variability of precipitation changes than indicated by GCM and RCM results, particularly in areas of complex topography (Hellström et al., 2001).

The decrease in precipitation together with enhanced evaporation in spring and early summer is very likely to lead to reduced summer soil moisture in the Mediterranean region and parts of central Europe (Douville et al., 2002; Wang, 2005). In northern Europe, where increased precipitation competes with earlier snowmelt and increased evaporation, the MMD models disagree on whether summer soil moisture will increase or decrease (Wang, 2005).