Abstract :
The effects of several methodological options in the application of principal component analysis (PCA) to gridded data
are examined for monthly sea level pressure anomalies over the Northern Hemisphere in the winter half-year. The options
include two grid-related ones, viz., the density of the grid and whether and howthe uneven areal distribution of gridpoints
is compensated for, and two PCA-related ones, viz., the selection of similarity matrix and rotation. The compensation
by a cosine weighting and by the use of a quasi-equal-area (QEA) grid has an almost identical effect if covariance matrix
is used; the principal components (PCs) based on a regular latitude/longitude grid differ considerably from those based
on the QEA grid and cosine-weighted data. The effect of a grid density is small, but recognizable for correlation-based
PCs. The PCs derived from a correlation and covariance matrix differ from each other, the difference being considerably
smaller for rotated solutions. Unrotated and rotated solutions differ from each other in the degree of simple structure
they possess and in their correspondence to correlation maps. The rotated PCs exhibit much stronger simple structure
and are more similar to the correlation maps, which suggests that in the interpretation of modes of variability, rotated
solutions should be preferred. This implies that in the description of the leading mode of the Northern Hemisphere
circulation variability, the sectorial view of the North Atlantic Oscillation should be preferred to the hemispheric view
of the Arctic Oscillation
