Observed and GCM-Simulated Westward-Propagating, Planetary-Scale Fluctuations with Approximately Three-Week Periods

Abstract

The structural characteristics and vorticity dynamics of westward-traveling patterns (WTP) in the troposphere are examined using the National Centers for Environmental Prediction?National Center for Atmospheric Research (NCEP?NCAR) reanalyses based on observations for the 1973?95 period, as well as the output from a 100-yr integration of a general circulation model (GCM) with a rhomboidal truncation at 30 wavenumbers and 14 vertical levels. An identical set of diagnostic tools, including progressive/retrogressive variance analysis, cross-spectra, and complex empirical orthogonal functions (EOFs), are applied to the reanalysis and GCM datasets for 300-mb height. These diagnoses all indicate that the WTP are most prominent during the cold season in the high-latitude zone extending westward from northwestern Canada to northeastern Siberia, with a typical period of ?22 days. Outstanding episodes are identified on the basis of the temporal coefficients of the leading complex EOF. Composite charts of the anomalous 300-mb height, sea level pressure, and 850-mb temperature fields at various phases of these events are constructed. The typical circulation changes accompanying the passage of the WTP are similar to those associated with well-known regional weather phenomena such as amplified pressure ridges over Alaska, cold air outbreaks over western North America and east Asia, and heavy snowfall over the Pacific Northwest. The occurrence of the WTP over the North Pacific is also characterized by notable changes in the spatial distribution and intensity of synoptic scale activity. The contributions of relative vorticity advection, planetary vorticity advection (the ?? effect?), and horizontal divergence to the vorticity tendency in various phases of the composite wave at 300, 500, and 850 mb are investigated. In the mid- and upper troposphere, the vorticity dynamics of the WTP is similar to that of free external Rossby waves, with the ? effect (which leads to westward propagation) being the dominant term, whereas the eastward advection of relative vorticity is less important due to the weak mean zonal flow in the Alaska?Siberia sector. Most of the essential characteristics of the observed WTP deduced from the NCEP?NCAR reanalyses are well reproduced by the GCM. The realism with which this phenomenon can be simulated in a model environment offers considerable promise for using the GCM as a tool for studying the impact of WTP on intraseasonal atmospheric variability in extended model experiments, and for assessing the dependence of the locality and activity level of the WTP on various states of the ambient circulation.