*Z*

_{500}) from 20°N to 90°N for the boreal winter months December, January, February (DJF). To emphasise the low-frequency variability patterns, a simple filtering is achieved by using monthly mean data. Further pre-processing steps include the calculation of anomaly fields by removing the mean seasonal cycle and linear trends. For the comparison of data sets, for example, calculation of Taylor diagrams, all model data were projected via linear interpolation to a common 2.5°×2.5° grid.

*j*th principal component (PC) of the time-series. The EOFs are the eigenvectors

*a*

_{ j }(t) are standardised (cf. Von Storch and Zwiers,

^{2}-test applied for the calculation of confidence levels. Torrence and Compo (

*Z*

_{500}fields from 1958 to 1999. Nine of the resulting 10 leading rotated patterns have been named according to similar patterns, given by the NOAA Climate Prediction Center (

*Z*

_{500}data ERA40, 1958–1999

**0.95**

**0.96**

**0.91**

**0.93**

**0.84**

**0.83**

**0.84**

**0.90**

**0.93**

*R*is the correlation coefficient,

*&sgr;*

_{ f }and

*&sgr;*

_{ r }are the standard deviations of the test field

*f*and the reference field

*r*, respectively. The above-defined skill score

*S*depends on

*R*

_{0}, the maximal attainable correlation which measures the potentially realisable statistical agreement between simulated and observed teleconnection patterns. Thus, the skill score definition takes into account the noise associated with internally generated unforced model variability.

*R*

_{0}has been selected according to the maximal values of the mean intra-ensemble correlations for each pattern, given in

*R*→1 the skill score approaches unity (

*S*→1). Skill decreases to zero for

*R*→ −1 or model variance approaches zero or infinity.

*S*in eq. (

*R*

_{0}according to

*K*and larger has been estimated not only over the tropical Pacific, but also over the North Pacific and North Atlantic (

*u*at 250 hPa for the Atlantic (−120°–60°W, 20°–90°N) and Pacific (150°–240°W, 20°–90°N) sector separately. The first three columns of Figs.

*u*-PC1/PAC-

*u*-PC2) and in the strength of the two jets (Regression onto ATL-

*u*-PC2/PAC-

*u*-PC1).

*u*-PCs at each height (as introduced in Wettstein and Li, submitted to J. Climate). The most pronounced vertical structures are found for the NAO and PNA. For the NAO pattern, the profile of the explained variance revealed a maximum in the lower troposphere just above the boundary layer and decreasing values above. For the PNA pattern, the profile of the explained variance revealed a clear maximum in the upper troposphere at the jet core height (about 250–200 hPa).

*u*-PC is supposed to have its maximum in the lower troposphere. Otherwise, a characteristic profile of explained variance describing variability of the subtropical jet should have its maximum at the jet core height in the upper troposphere (see also the argumentation in Wettstein and Li, submitted to J. Climate). The profiles of explained variance shown in the top rows of Figs.

*u*-EOF1) and in the strength of the Atlantic jets (ATL-

*u*-EOF2). The simulated centres of action are found at similar locations as the observed ones, resulting in high pattern correlations between observed and simulated Atlantic zonal wind variability patterns (0.89 to 0.97 at 500 hPa and 0.85 to 0.97 at 250 hPa). Over the Pacific, the simulated zonal wind variability patterns display changes in the strength (PAC-

*u*-EOF1) and in the position (PAC-

*u*-EOF2) of the Pacific jets in the upper to middle troposphere. Again, the simulated centres of action are found at similar locations as the observed ones, resulting in high pattern correlations between observed and simulated Pacific zonal wind variability patterns (0.92 to 0.98 at 500 hPa and 0.88 to 0.97 at 250 hPa). The regression patterns of geopotential height fields at 500 hPa are very similar to the teleconnection patterns, underlining the close relation of the NAO/EA (PNA/WP) patterns to zonal wind variations over the Atlantic (Pacific).

*u*-PCs share the largest amount of their variance with the NAO-index. The general shape of the vertical profile is similar to that for the re-analysis data, displaying largest values of shared variance in the lower troposphere with a rather linear, though slight decrease throughout the troposphere. The second Atlantic

*u*-PCs share the largest amount of their variance with the EA-index in accordance with the re-analysis data, but larger differences in the shape of the vertical profile occur. Having in mind the mixed relationship of EA with eddy-driven and subtropical jet variability proposed by Wettstein and Li (submitted to J. Climate), which is rather sensitive to small changes in the location of the centres of actions of the related

*u*-EOF2, a greater variety of these profiles can be expected. The first Pacific

*u*-PCs show the largest values of explained variance with the PNA-index, but not all models reproduce the same shape of the vertical profile as the re-analysis with a maximum in the upper troposphere indicating a relation to subtropical jet variability. Considering the WP-index of the re-analysis data, the shape of the vertical covariance profile indicates a mixed relationship rather than the eddy-driven jet variability shown by Wettstein and Li (submitted to J. Climate).

*u*-EOFs and the regression patterns of the global geopotential height field at 500 hPa onto the sectoral zonal wind PCs at 250 hPa. The fourth and fifth column describe the skill scores for the comparison of the regression patterns (regression of geopotential height field at 500 hPa onto the first and second sectoral zonal wind PCs at 250 hPa) with the teleconnection pattern of the same model. These scores indicate, whether and how closely the distinct teleconnection pattern is related to the first or second dominant pattern of zonal wind variability for the considered model. The models have been sorted according to the skill score for the particular teleconnection pattern.

*u*-EOF1/ATL-

*u*-EOF2; PNA/WP connected with PAC-

*u*-EOF1/PAC-

*u*-EOF2.

*u*-EOF1 and PNA with PAC-

*u*-EOF1 are reproduced by the majority of the models. For the NAO, almost all of these models display vertical profiles of shared variance with the first Atlantic

*u*-PCs similar to the re-analysis indicating eddy-driven variability (see

*u*-variability and the teleconnection patterns display a much larger spread over the CMIP3 ensemble. The EA-ATL-

*u*-EOF2 and WP-PAC-

*u*-EOF2 relationships determined for the re-analysis data are reproduced only by about half of the models with large variations of the shape of the vertical profiles of shared variance between teleconnection indices and related

*u*-PCs (see

*u*-EOFs is related to the main teleconnection patterns in the same way as for the re-analysis data then the quality of simulated teleconnection pattern is largely determined by the quality of the simulated zonal wind variability patterns.