Examples of Extended Empirical Orthogonal Function Analyses

An extended empirical orthogonal function analysis technique is described which expands a data set in terms of functions that are the best representation of that data set for a sequence of time points. The method takes advantage of the fact that geophysical fields are often significantly correlated in both space and time. Two examples of applications of this technique are given which suggest that it may be a useful tool for diagnosing the modes of variation of dominant sequences of events. In the first, an analysis of 300-mb relative vorticity, a fairly regular advection of the major features of the spatial patterns, is evident. Westward speeds of 0.3-0.4 m sec super(-) super(1) are inferred. The second example illustrates extended functions of tropical Pacific Ocean surface temperatures. The dominant function, which is associated with El Nino, shows a high degree of persistence over a 6-mo sequence. The second most important function suggests opposing variations in the influences of the North and South Pacific Equatorial Currents.