A Further Analysis of Internal Variability in a Perpetual January Integration of a Troposphere-Stratosphere-Mesosphere GCM

An analysis of the 1000-day dataset from a perpetual January integration of the Berlin Troposphere-Stratosphere-Mesosphere General Circulation Model is performed to clarify the basic nature of the internal variability in the complicated non-linear system and to understand a possible role of such variability in the intraseasonal and interannual variability of the real atmosphere. During the 1000 days, seven stratospheric sudden warming episodes occur without any clear periodicity; a power spectrum of zonal-mean zonal wind [U] is nearly red-noise. Frequency distribution functions of [U] and zonal-mean temperature [T] in the polar stratosphere are largely different from a Gaussian distribution. Based on [U] and the amplitude of zonal wavenumber 1 in geopotential height Z1 in the polar stratosphere, the 1000-day data are classified into three categories (C), (M) and (W) as follows: (C) strong [U] and small Z1; (M) moderate [U] and Z1; (W) weak [U] and large Z1, where the capitals C, M and W stand for a cold, moderate and warm polar stratosphere, respectively. Generally Category (C) persists much longer than Categories (M) and (W). Composite meridional sections of [U], [T] and Z1 show opposite extremes between (C) and (W) in the stratosphere and mesosphere. In Category (W), [T] in the polar region is much higher than the 1000-day average in the stratosphere but lower in the mesosphere, a common feature of sudden warming events. Composites in wave quantities such as the Eliassen-Palm (EP) flux and its convergence, as well as the transformed Eulerian mean meridional circulation, show a dynamically consistent picture of the time variation of the non-local effect of the wave driving in the lower stratosphere polar region; in Category (W) stronger downward motion is observed with larger convergence of the EP flux above. Composites of the EP flux and composite maps of the geopotential height field in the troposphere show some evidence of a vertical link between the troposphere and the stratosphere. In Category (W), baroclinic wave activity in the troposphere is a little weaker, while planetary waves show a pattern of wave train in high latitudes.