Wintertime Blocking and Mountain Forcing of the Zonally-Averaged Flow: A Cross-Spectral Time Series Analysis of Observed Data

Abstract

Nine winters of Northern Hemisphere observations are used for a statistical analysis of the relation between the mountain forcing of the zonally-averaged barotropic wind and blocking. Thereby, the temporal variability of the phenomenon ?blocking? is expressed by a blocking index lime series. It is demonstrated that the zonally averaged wind exhibits statistically significant negative anomalies at midlatitudes (40°N, 53°N) for the early blocking stage. The mountain drag is anomalously negative (corresponding to a slow-down of the zonal mean wind) at high (53°N, 70°N) and at middle (40°N, 53°N) latitudes. These significant negative mountain drag anomalies are found before the onset or at the very first stage of blocking. In terms of a signal-to-noise ratio, the strongest impact of the mountain drag on the zonal mean wind tendency occurs at the latitudes (30°N, 40°N), where the main mountain ranges of the earth are located.