Northern Hemisphere Wintertime Variability in a Two-Level General Circulation Model. Part 1: Statistical Characteristics of Short and Long Time-Scale Disturbances

Abstract

The properties of atmospheric Northern Hemisphere wintertime variability, simulated by the Oregon State University two-level general circulation model are examined. Time series of the dependent variables and diabatic heating components are extracted from ten simulated Northern Hemisphere winters. Variance and covariance analyses are performed to determine the geographical distribution of the intensities and transport properties of eddies of high-frequency (periods between 2.5 and 10 days) and low-frequency (periods between 10 days and a season). In agreement with observations the simulated high-frequency fluctuations are caused by rapidly propagating, eastward-moving disturbances with structures that are consistent with baroclinic instability theory. The regions of strong high-frequency variability (storm tracks) and the associated transports of beat and momentum in the OSU model show a good correspondence with regions where the time-mean circulation has pronounced vertical shear and a weak gradient of absolute vorticity. Thus, discrepancies between the observed and simulated positions of the storm tracks appear to be related to systematic errors in the simulated time-mean circulation. like their observed counterparts, low-frequency fluctuations are found to be due to disturbances that am almost stationary in phase and show indications of eastward energy dispersion in qualitative agreement with the theory of Rossby-wave dispersion on a sphere. The patterns of low-frequency variability of the geopotential height field are in good agreement with observations except over the Atlantic Ocean.