An NAO Climatology in Reanalysis Data with the Use of Self-organizing Maps

The North Atlantic Oscillation (NAO) is a basic variability mode of the Northern Hemisphere, long recognized to be significantly affecting the weather and climate of Europe. During the last decades, an eastward shift of the NAO centers of action along with a strengthening of the NAO index and a tendency to more positive values, have been noticed. Therefore, further studying and understanding of the changing NAO pattern is crucial. Here, we attempt to examine the different spatial and temporal characteristics of the NAO in NCEP/NCAR reanalysis data for the period 1971-2000, using Self-organizing Maps (SOMs). Winter daily anomalies of 500 hPa geopotential height over the broader European area are used. According to the results, NAO occupies different locations throughout the years, a fact that has to do with displacements of both the Icelandic Low and the Azores High, and is also related to the predominance of NAO positive and negative phases.