The February 2010 Arctic Oscillation Index and its stratospheric connection

The atmospheric dynamics during the occurrence of the extreme negative value of the Arctic Oscillation Index in February 2010 are investigated using meteorological fields from ERA-Interim and ERA-40 reanalysis data. The study focuses on the possible causes of this anomalous value, finding that it was forced by a geopotential anomaly that propagated downward from the stratosphere to the troposphere in association with a major sudden stratospheric warming event which occurred at the end of January. An analysis of the dynamics of this warming is also developed, together with a comparison with past similar events. Synoptic and spectral properties of the geopotential fields are analyzed, and the time series of the 'Baroclinic Activity Index' are computed, finding an abrupt increase of the middle latitude baroclinic activity immediately after the central date of the warming event. A possible interpretation of this feature is proposed. Copyright (C) 2012 Royal Meteorological Society