The dipole index derived from NCEP reanalysis was calculated using the following steps:

• Step 1 500-hPa geopotential height anomalies were computed using the long-term (1950- 2017) mean.
• Step 2 For each month, the 500-hPa geopotential height anomaly at 70N and 157.5W was subtracted from the geopotential height anomaly at 42.5N and 75W.