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While EOF analysis has been useful in highlighting the existence of the PSA pattern, it may not be the best tool for detailed climatological investigation. A particularly problematic shortcoming is that EOF analysis allows for only a crude representation of variations in wave phase (via the PSA-1 and PSA-2 modes, which are 90$^{\circ}$ out of phase), which makes it difficult to interpret characteristics such as the propagation of the pattern. This issue is further compounded by the degenerate \citep{North1982} nature of the PSA-2 mode \citep[e.g. Figure 1;][]{Mo2000}, which means the true second mode is some random combination of the apparent second and third modes.  In order to obtain more detailed information on variations in wave phase, existing studies of Rossby wave activity have tended to apply Fourier analysis along lines of constant latitude \citep[e.g.][]{Glatt2014}. More sophisticated methods have recently been considered for identifying/tracking non-zonal waves \citep[e.g.][]{Zimin2006,Souders2014}, however a key insight of the method developed here is that unlike the generalized case of all possible non-zonal propagation, analysis of the PSA pattern can make use of the fact that the waveform follows an approximate great circle path \citep{Hoskins1981}. By rotating the global coordinate system such that the equator (itself a great circle) traces the approximate path of the PSA pattern, we were able to simply apply Fourier analysis along the `equator' in the new zonal direction. Such grid rotation is commonly used in ocean modeling to avoid coordinate singularities caused by the convergence of meridians at the poles \citep[i.e. the grid is rotated to place the north pole over a continent; land;  e.g.][]{Bonaventura2012}, but has not previously been applied in the context of tropospheric planetary wave activity. This new approach to PSA pattern identification is described below, along with the other more general data analysis techniques used in the study. \subsection{Identification algorithm}