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Annular Modes in the surface pressure

For every simulation the leading variability patterns of the surface pressure are AM-like. Fig. 2 shows these patterns for run $[\Phi_s]/[Q_m]/[Q_c]$ and run $\Phi_s(\lambda)/Q_m(\lambda)/Q_c(\lambda)$. The explained variances of the EOFs are 16.8% and 23.3%. Especially in run $\Phi_s(\lambda)/Q_m(\lambda)/Q_c(\lambda)$(Figs. 2b,d), the variability pattern and the explained variance of 23.3% are comparable with results of Thompson and Wallace (1998, their Fig. 1, 22% explained variance). The main features of their EOF, besides the strong zonally symmetric component, are the localized action centres over the Pacific and the Atlantic. These features can also be found for run $\Phi_s(\lambda)/Q_m(\lambda)/Q_c(\lambda)$. For the runs with thermal wave forcing alone (not shown) the variability patterns have a stronger zonally symmetric character, whereas embedded longitudinal structures are present in the model runs with orographic wave forcing .

Figure 2: a) and b) Leading EOF of surface pressure for run $[\Phi_s]/[Q_m]/[Q_c]$ and run $\Phi_s(\lambda)/Q_m(\lambda)/Q_c(\lambda)$(contour interval 0.02, negative values shaded). c) and d) one-point-correlation maps for same runs (contour interval 0.1, negative values shaded). Reference point is $24^{\rm{o}}$O $28^{\rm{o}}$N, with regard to the NAO.
\begin{figure}\begin{center} \epsfig{figure=c:/eigene/physik/sparc/bilder/am.eps,width=13cm}\end{center} \end{figure}

The pattern correlation between EOFs and one-point correlation maps reaches 0.78 for run $[\Phi_s]/[Q_m]/[Q_c]$ and run $\Phi_s(\lambda)/Q_m(\lambda)/Q_c(\lambda)$. Nevertheless, the physical meaning of surface pressure-EOFs is open to question (Ambaum et al., 2000). The situation is different for stratospheric EOFs, as the next section demonstrates.

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