Frequency Dependence of the Structure and Temporal Development of Wintertime Tropospheric Fluctuations—Comparison of a GCM Simulation with Observations

Abstract

The three-dimensional structure and temporal evolution of tropospheric fluctuations appearing on various time scales in observed and model-simulated atmospheres are investigated using cross-spectral analyses. The datasets examined include NMC analyses of the 500 mb height and sea level pressure fields for 18 winters, as well as a 12-winter simulation of the same fields by a 15-wavenumber general circulation model at GFDL. Statistically significant phase differences between 500 mb height fluctuations at selected centers of action and the corresponding fluctuations at all other grid points are displayed for various frequency bands using a vectorial format. Similar plots are constructed to elucidate the vertical phase structure in the middle and lower troposphere at individual grid points, as well as the propagation characteristics of the sea level pressure field in the vicinity of sloping terrain. It is demonstrated that these phase/coherence diagrams offer a useful alternative for quantifying the lead/lag relationships between different anomaly centers associated with some of the well-known teleconnection patterns. The spectral results presented here indicate that the spatial and temporal behavior of the Pacific/North American, Atlantic and Northern Asian Patterns, as documented in various recent studies, exhibit a notable frequency dependence in both real and model atmospheres. For periods of 27?80 days, the atmospheric variability over the Pacific and Atlantic Basins is organized in north?south oriented dipoles, with an almost 180° out-of-phase relationship between oscillations at the opposite poles. As attention is shifted to the 20- and 10-day time scales the north?south seesaw pattern gradually weakens, and all three teleconnection patterns mentioned above are characterized by successive downstream development from west to east of alternating troughs and ridges. Fluctuations with periods longer than 20 days acquire an equivalent barotropic structure over much of the northern oceans, with in-phase variations at 500 mb and at sea level. The eddy behavior undergoes still further changes as one considers the 4-day period band. The high frequency disturbances tend to be elongated in the meridional direction. The corresponding horizontal phase variations are indicative of continuous eastward propagation across the midlatitude oceans and northern Siberia. The vertical phase variations suggest a systematic transition from a distinctly baroclinic structure at the starting points of such cyclone tracks, to a more barotropic structure in regions farther east. The perturbations near the eastern and northern peripheries of the Tibetan Plateau are noted for their weak coherence in the vertical direction. Horizontal phase diagrams based on sea level pressure data reveal that the path of near-surface fluctuations tends to be aligned parallel to the local topographic contours in this region. Comparison between model and observational results indicates that the GCM examined here is capable of reproducing the frequency and geographical dependence of the principal modes of variability in the Northern Hemisphere wintertime circulation.