Geometrical Structure of Atmospheric Models and the Dynamical Basis of Weather Regimes and Blocking

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Using a quasi-geostrophic model with realistic topography, we had shown that chaotic itinerancy is the dynamical basis of weather regimes. Furthermore, long-period oscillations exist in each attractor or attractor ruin, which is the dynamical basis of low-frequency oscillations. This year, we have made these phenomena more realistic and have clarified another kind of low-frequency variability.Changing a parameter in the model to unstable side, one regime (one attractor ruin, hereafter referred to as X) enlarges, while other regimes become obscure. These latter regimes are considered to correspond to real weather regimes. Amplitudes of low-frequency oscillations in X become large, which well explains real low-frequency oscillations.EOFs 1 and 2 have dominant low-frequency variabilities. These correspond to real teleconnection patterns. The reason why these modes are dominant is as follows. Spatial patterns of EOF1 well coincide with those of the first eigenfunctions of the equation system linearized with respect to the time-mean state. The corresponding eigenvalues are very small. In other words, these patterns strongly respond to forcing, and the time changes are small.Thus, EOF1 is selected among many other patterns. This characteristic is guaranteed by the fact that geometrical structure around the time-mean state is "flat". It is also shown that this characteristic is not accidental but necessary.