Monthly zonal indices (ZI:s) from January 1899 to August 1992 in a North Atlantic area (40° W-5° W) and a north European area (5° E-40° E) are analysed. The ZI:s are calculated as the sea-level pressure difference between latitudes 45° N and 65° N in a 5° by 5° gridded data set.Time series analysis show that the ZI:s for each area does not show the same evolutionary trends and that any distinct periods are hard to identify, except for a phase with low ZI for winters over the North Atlantic area during 1955–70. Overall positive linear trends (p=0.02) are present during summer in the North Atlantic area and during autumn in the European area. Since about 1970 the strength of the westerlies has increased markedly during winter and spring, especially in the North Atlantic area.Correlations between ZI:s for the eastern area and monthly average temperatures from two stations in southern Scandinavia show a distinct annual course with positive (≈ 0.7) correlation during December-March and negative correlation (≈ -0.6) during summer. In May and September, the correlations are close to zero, bringing the overall correlations in spring and autumn to positive values. Thus, an effect of increased zonality in southern Scandinavia is an increase in annual mean temperature. The correlation between ZI and temperature is shown to vary over time (especially for spring and autumn) indicating that the relationship between the strength of the zonal flow and temperature signal is not consistent.
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Jönsson et al. (1994) studied this question.