Low-frequency intraseasonal tropical-extratropical interactions

Abstract

Low-frequency (20-70 day) variability is examined during Northern Hemisphere (NH) winter based on seven years (1981-87) of European Centre for Medium Range Weather Forecasts initialized analyses. The dominant 200 mb zonal wind fluctuations in the Pacific sector, determined from an empirical orthogonal function (EOF) analysis, provide the baseline modes of atmospheric variability, which are related to fluctuations in other circulation parameters and outgoing longwave radiation (OLR). The composite circulation associated with the extreme phases of the zonal wind modes are examined for differences in forcing, wave propagation characteristics and stability. These results suggest that tropical convection in the western Pacific has a strong modifying influence on (extratropically forced) middle latitude low-frequency variability. The influence is primarily indirect via zonal wind changes which influence the propagation of waves originating in middle latitudes. -from Authors