“…The predicted seasonal mean PNA index is calculated based on the monthly mean 500 hPa geopotential height prediction in December, January, and February (DJF), that is, with a 2‐month lead time. To be consistent with previous studies (e.g., O’Reilly et al., 2017, 2020; Weisheimer et al., 2020), the PNA index is defined by using the pointwise method (Wallace & Gutzler, 1981), that is, a linear combination of the normalized 500 hPa geopotential height anomalies ( Z ) at the four active centers: PNA = 1/4[ Z (20°N, 160°W) − Z (45°N, 165°W) + Z (55°N, 115°W) − Z (30°N, 85°W)]. Consistent results are obtained if the PNA index is defined as the time series associated with the second leading rotated empirical orthogonal function pattern (Barnston & Livezey, 1987; Van Den Dool et al., 2000) of the 500 hPa geopotential height anomalies in the Northern Hemisphere.…”