Jolliffe I. Principal Component Analysis (2ed., Springer, 2002)(518s)

4.3. Spatial and Temporal Variation in Atmospheric Science 73Figure 4.1. Graphical representation of the coefficients in the second PC for sealevel atmospheric pressure data.to be so valuable that it is almost routine. For example, Craddock andFlood (1969) find PCs with ready interpretations for Northern Hemispheric500 mb geopotential surfaces, Craddock and Flintoff (1970) do the same for1000 mb surfaces and 1000–500 mb thickness, Overland and Preisendorfer(1982) interpret the first three PCs for data on spatial distributions of cyclonefrequencies in the Bering Sea, Wigley et al. (1984) discuss PCs forEuropean precipitation data, and Folland et al. (1985) find interpretablepatterns in PCs of worldwide sea surface temperature anomalies. Somepatterns recur in different data sets. For example, Figure 4.1 could beinterpreted as the North Atlantic Oscillation (NAO), which reflects thestrength of the zonal flow in the North Atlantic and neighbouring areas, asmeasured by the pressure difference between the Azores and Iceland. Thispattern, and a small number of others, notably ENSO (El Niño–SouthernOscillation), have been identified as major modes of climate variability indifferent parts of the world. They have been studied extensively (see, forexample, Ambaum et al. (2001) for a discussion of the NAO).