The daily indices of the jet stream latitude and strength are calculated using the approach described in Woollings et al., 2010 (https://doi.org/10.1002/qj.625). This is calculated first by smoothing the tropospheric (850 hPa) zonal wind speed in the North Atlantic sector (0-60°W, 15-75°N) using a 5-day moving window. The location and magnitude of the maximum value in this smoothed daily field indicate the latitude and strength of the jet stream respectively.

The daily weather type indices were generated by classifying daily mean sea level pressure (MSLP) (changes compared to the 1981-2000 average) into 30 weather regimes which capture the full range of circulation types affecting Europe (see Neal et al., 2016, https://doi.org/10.1002/met.1563). These 30 weather patterns are also provided as an amalgamated set of 8 weather patterns.

The winter NAO index is calculated as the seasonal mean (December-January-February) of the difference in sea level pressure between Iceland and Gibraltar (the Atlantic pressure gradient). These are calculated using the difference between the seasonal mean sea level pressure time-series between grid-boxes containing at Gibraltar (36.14N, 5.35E) and Iceland (65.07N, 22.7W).