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

• Step 1. Sea-level pressure (SLP) anomalies were computed using the long-term (1950- 2017) mean.


• Step 2. For each month, SLP anomalies were averaged across the rectangular region (Box 2) bounded by 40N and 50N and 50W and 65W. The resulting time series was then divided by its long-term standard deviation.


• Step 3 . For each month, SLP anomalies were averaged across the rectangular region (Box 1) bounded by 30N and 40N and 80W and 90W. The resulting time series was then divided by its long-term standard deviation.


• Step 4. For each month,the standardized SLP anomaly corresponding to Box 1 was subtracted from that of Box 2. In other words, ENA Index = SLP2 - SLP1, where SLP2 is the standardized SLP anomaly corresponding to Box 2 and SLP1 is the standardized SLP anomaly corresponding to Box 1.