The daily AO index is constructed by projecting the daily (00Z) 1000mb height anomalies poleward of 20°N onto the loading pattern of the AO. Please note that year-round monthly mean anomaly data has been used to obtain the loading pattern of the AO (Methodology). Since the AO has the largest variability during the cold sesaon, the loading pattern primarily captures characteristics of the cold season AO pattern.
The daily AO index and its forecasts using MRF and Ensemble mean forecast data are shown for the previous 120 days as indicated and they are normalized by standard deviation of the monthly AO index from 1979 to 2000. A 3-day running mean is applied for the forecast indices.