A potential problem with extended EOF analysis of standing wave fields

The extended empirical orthogonal function (EEOF) analysis of signals conta ining a standing wave component is considered. It is shown using a simple a nalytical model that for a single time-lag EEOF analysis, two EEOF patterns describe the standing wave, and that their associated eigenvalues become d egenerate if the lag chosen is close to a zero of the sample autocovariance function of the standing wave time series. This degeneracy strongly affect s the relative magnitudes of the two sectors of each EEOF pattern and the s tructure of the associated spatial pattern. A set of EEOF analysis of varyi ng lags is performed on a tropical Pacific sea level pressure (SLP) dataset , known to be dominated by standing variance, and the predictions of the an alytical model are verified. As well, it is seen that at degeneracy there i s a reduction in the quality of the spatial patterns produced by the EEOF a nalysis.