\label{fig:eof}
Empirical Orthogonal Function (EOF) analysis of the monthly 500-hPa zonal streamfunction anomaly from the ERA-Interim reanalysis over the period 1979--2014. This is the most common method, variable and timescale used to investigate the PSA pattern and the data are presented as the correlation of the corresponding principal component with the original field.  The second and third EOF modes are degenerate according to the \citet{North1982} rule of thumb, which means the sample eignenvectors (i.e. EOF-2 and EOF-3) represent a random mixture of the true eigenvectors. Different filtering, datasets, time periods and EOF methodologies influence the location and magnitude of the anomaly centers slightly, but the overall structure of the EOF-1 mode and the degenerate nature of EOF-2 and EOF-3 are a consistent feature. Green lines indicate the search region of interest (the `PSA sector') and the percentage of variance explained is indicated for each EOF mode.