Zonal jet structure and the leading mode of variability

An observational analysis of the Northern Hemisphere circulation during winter reveals that the leading mode of variability depends on longitude. In particular, the first EOF of the zonal-mean circulation differs over the Atlantic and Pacific sectors. These results provide motivation for a series of model runs where a simple GCM is used to investigate the effects of the zonal jet structure on the leading mode of variability in the Northern Hemisphere. Model results indicate that the leading mode of variability depends on the distance between the eddy-driven and subtropical jets. When the jets are well separated, the leading mode of variability describes latitudinal shifting of the eddy-driven jet. However, when the two jets are nearly collocated, pulsing of the combined jet dominates the variability. This change coincides with a weakening of the positive feedback between the eddies and zonal flow anomalies. These results provide a possible explanation for the reduced amplitude of the Northern Annular Mode in the Pacific sector relative to that in the Atlantic sector during Northern Hemisphere winter.