3. A Propagating Global ENSO Wave in Covarying SLP and SST Anomalies
Now we compute zonal wavenumber-frequency spectra (Bendat and Piersol, 1986, pages 36 1-424) from time-longitude diagrams of monthly NCEP SLP and surface temperature (ST) anomalies (Kalnay et al., 1996; Reynolds and Marsico, 1993) extending around the globe at 10｡? for 48 years from 1950 to 1997 (top, Figure 2a). This latitude conforms approximately to the zonal average latitude of the ITCZ and the thermal equator following the path of maximum annual precipitation and SST around the globe in the tropics (Peixoto and Oort, 1992, page 166). These resulting zonal wavenumber-frequency spectra (top, Figure 2a) display largest peaks in the right hand quadrant indicating the dominance of eastward over westward phase propagation, with zonal wavelengths ranging from 12,000-36,000 km and periods ranging from 3 to 6 years.
In Figure 2a we can see eastward phase propagation of both variables from one ocean to the next occurred across maritime Indonesia; across Central America and through the Caribbean; and across tropical Africa. This occurs because the anomalous zonal winds associated with the SLP anomalies extend zonally at least a quarter the way around the globe, influencing SST anomalies in adjacent oceans that allow the coupled signal to continue propagating eastward across continents (White and Cayan, 1999).
We apply complex empirical orthogonal function (CEOF) methodology that partitions the propagating global ENSO wave from the standing Southern Oscillation wave (White and Cayan, 1999). We find global RMS variability in standing wave components of anomalous SLP and ST to be larger than for corresponding propagating wave components by a factor of 2 or so. This difference in magnitude is reflected in reconstructed SLP and ST anomalies from 1984 to 1989 (Figure 2b), with standing wave components (top, Figure 2b) displaying about twice the amplitude of propagating wave components (bottom, Figure 2b). The standing wave pattern in reconstructed anomalous SLP is similar to that of the Southern Oscillation (Berlage, 1957) with maximum amplitude in eastern South Pacific and eastern South Indian oceans near 20｡?, dominated by global zonal wavenumber 1. The standing wave pattern in reconstructed ST anomaly is similar to that associated with global ENSO pattern (Tourre and White, 1995) with maximum amplitude in eastern equatorial Pacific and western tropical Pacific oceans, dominated by global zonal wavenumber 2. The propagating wave in reconstructed SLP and ST anomalies is principally associated with global zonal wavenumber 2, traveling eastward around most of the tropical global ocean in 5 plus years, intensifying as it propagated into the tropical Pacific ocean.
