Two aspects of the principal oscillation pattern (POP) analysis are used to study the large-scale modes of the coupled atmosphere–ocean system. First, P0Ps can be considered as the normal modes of the system; one way of studying these normal modes is to estimate them from data. Second, a POP analysis can be viewed as a multivariate spectral analysis and the spectral characteristics of the modes are by-products of the POP analysis. Both aspects are studied using a combined dataset that includes both atmospheric (sea level pressure, 700-mb, and 200-mb zonal winds) and oceanic (sea surface temperature, Pacific sea level, and Pacific subsurface temperature) parameters.
Six joint modes of the coupled atmosphere-system are found in this study. For modes with small eigenvalues the atmosphere plays an important role. The associated oceanic anomalies appear to be generated by the anomalous atmospheric conditions. For the other modes, which have most of their power on much longer time scales, the ocean is more actively involved. Modes 4 and 5 describe decadal time scale variations. Mode 4 is characterized by changes in SST in all three tropical oceans, and in organized convection over the west Pacific. The results allow us to speculate that these tropical features might excite changes in the extratropical tropospheric and oceanic circulations. Mode 5 shows global-scale SST anomalies and large atmospheric anomalies in the Southern Hemispheric circulation. Mode 6 is the only oscillatory normal mode found in the coupled atmosphere-ocean system; it describes the quasi-cyclic behavior of the El Niñio-Southern Oscillation phenomenon.