[Source: NOAA Climate Prediction Center, http://www.cpc.ncep.noaa.gov/
The zonal wind indices are obtained by averaging the daily (00Z) zonal wind anomalies over the indicated area. Note that positive values of the 850 hPa zonal wind indices imply easterly anomalies.
The zonal wind indices and their forecasts using MRF and Enesmble mean forecast data are available for the previous 120 days and they are normalized by standard deviation using 1979-1995 base period. A 3-day running mean is applied for the forecast indices.
Wind anomalies are based on the 1979-1995 period.