4. A Propagating Global Biennial Wave in Covarying SLP and SST Anomalies
Now we display zonal wavenumber-frequency spectra (Bendat and Piersol, 1986, pages 361-424) from time-longitude diagrams of monthly NCEP SLP and 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 3a). This spectrum differs from that in Figure 2a by focusing on the frequency range of biennial signals ranging in period from 18 to 36 months. These resulting zonal wavenumber-frequency spectra (top, Figure 3a) display largest peaks in the right hand quadrant in both variables at 24 month period and 30 month period, indicating the dominance of eastward over westward phase propagation in these biennial signals. Zonal wavelengths ranging from 12,000-36,000 km, making these signals global in extent.
In Figure 3a we can see the eastward phase propagation from one ocean to the next occurred across maritime Indonesia; across Central America and through the Caribbean; and across tropical Africa. As with the global ENSO wave, 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 Allan, 1999).
We apply the CEOF methodology that partitions the propagating global biennial wave from the standing Quasi-Biennial Oscillation wave (White and Allan, 1999). We find global RMS variability in standing wave components of anomalous SLP (ST) to be smaller than (about the same) for corresponding propagating wave components. This difference in magnitude is reflected in reconstructed SLP and ST anomalies from 1981 to 1983 (Figure 3b), with standing wave component of SLP variability (top left, Figure 3b) displaying about half the amplitude of propagating wave component (bottom left, Figure 3b), and the standing wave component of ST variability (top right, Figure 3b) displaying about the same amplitude as the propagating wave component (bottom right, Figure 3b).
