Comments on ‘Spatial structure of ultra‐low frequency variability of the flow in a simple atmospheric circulation model’ by I. N. James and P. M. James (October 1992, 118, 1211–1233.)

SUMMARY James and James (1992) examine internally generated interannual variability in a multi-level primitiveequation model. The authors claim that the variability in the global mean relative angular velocity is associated with fluctuations in the mid-latitude zonally-averaged zonal winds. We question this relationship, and present results from an aqua-planet GCM which indicate that, instead, the global mean relative angular velocity is primarily related to changes in the tropical zonally-averaged zonal winds. Furthermore, we also question whether the ‘ultra-low-frequency variability’ examined by James and James is not simply just ‘climate noise’. James and James (1992) (JJ hereafter) investigated the interesting problem of whether processes internal to the atmosphere can generate large-amplitude interannual variability. This is an important problem given the recent concern regarding the detection of global warming. They show that large-amplitude interannual changes to the wind and temperature fields may occur with fixed external forcing in their model, and their results imply that when examining interannual variability it may be important to consider internally generated processes in addition to those extensively studied such as carbon dioxide doubling or variations in the sea surface temperature. Numerical experiments carried out by JJ with a multi-level primitive-equation model revealed decadal variability in both the global-mean relative angular velocity and the principal components of the first EOF (EOFl) of the zonally-averaged zonal wind. The temporal properties of the globalmean relative angular velocity were examined by calculating the time-power spectrum of the coefficient of the gravest zonal-mean spherical harmonic, denoted (( 1 ,O), averaged over pressure. This analysis indicated that the variance increases with period and has its largest amplitudes on decadal timescales (Fig. 3(b) of JJ). The power spectrum of the principal components of EOFl also was shown to exhibit a red-noise spectrum with greatest amplitudes on decadal timescales (Fig. 6(a) of JJ). The power spectra for the principal components of the next several EOFs were either white or weakly red. Because of the similarity in the shape of power spectra for the f( 1.0) time series and the principal components of EOFl, JJ concluded that “most of the unexpected ultra-low frequency variability which was noted in the previous section is associated with the largescale structure described by EOFl” (note that the ultra-low frequency variability of the ‘previous section’ refers to that variability found for the c(1,O) time series). In this note, we question the validity of the relationship between the global mean relative angular velocity and EOFl claimed by JJ, and also argue that the ultra-low-frequency Variability observed by JJ is not ‘unexpected’ but is consistent with expectations from the concept of ‘climate noise’. 2. RESULTS AND DISCUSSION We have re-examined some of JJ’s findings using data from a Geophysical Fluid Dynamics Laboratory (GFDL) rhomboidal-30, nine-level, aqua-planet, general circulation model (GCM). The model was run for the equivalent of 2025 days with solstitial solar forcing and zonallyhomogeneous sea surface temperatures that are symmetric about the equator. The first two EOFs of the zonally-averaged zonal wind, using pentad averages, are shown in Fig. 1. As expected from the symmetry of the model’s forcing, these EOFs are very similar when reflected at the equator. The variance of the first two EOFs-EOF1 and EOF2-are 22% and 17%, respectively. We supposed that if this GCM were run for a longer period of time, then the cross-equatorial symmetry would improve. The structure of these two EOFs resembles that of the first of JJ’s EOFs (see their Fig. 4(a)). The power spectra of the principal components of EOFl and EOF2 are shown in Figs.