FIGURE 3

Time/longitude (Hovmöller) diagram of the total ENSO signal in sea-level height between approximately 145°E and 155°W. The diagram is based on spatial interpolation onto a regular grid of the SSA-filtered time series at eight stations slightly north of the equator (see text for details); latitude differences between stations are neglected. Contour interval is 20 mm (courtesy of Y. Sezginer-Unal).

Following the distinction made by Gene Rasmusson (Rasmusson et al., 1991) in the wind data, we also looked at the low-frequency mode (LFM) associated with ENSO (Figure 2, again in the color well). In that case, the amplitudes are even more tightly connected to the tropical Pacific and the west coast of the Americas. While a QBO-type oscillation does seem to be more global, this LFM is really concentrated in the tropical Pacific.

As for the spatial patterns, we took a group of eight stations just a little bit north of the equator in the western tropical Pacific and did a Hovmöller diagram (Figure 3). We wanted to look at the eastward-propagating signal in the quasi-biennial component of the zonal winds Dr. Rasmusson mentioned. We actually combined the two components, QBO and LFM. In this plot, which goes from 145°E to 155°W and includes data from 1950 to 1976, you can see that there is not so much a propagating as a standing signal, and actually there seems to be something of a nodal line about 175°W.

So I think that we are getting on with this business of trying to define the spatial modes of interannual and interdecadal variability, which obviously is more interesting than just looking at spectral peaks. And perhaps what we should be noticing is that the peaks are in the same places, not in different places, for various records.



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