An examination of midlatitude power spectra: evidence for standing variance and the signature of El Niño

Low-frequency variability in the midlatitude winter circulation is investigated by computation of power spectra from 16 winters of Northern Hemisphere (NH) NMC data and 4 winters of Southern Hemisphere (SH) ECMWF data. Different techniques of partitioning the variance are compared. The results for the two hemispheres are similar in that a substantial standing variance with periods of 10 days and longer is found in both hemispheres. In the NH, this strong standing variance spans wavenumbers 2 through 4, while in the SH, it is quite prominent at wavenumber 3. The standing variance correspondsto wavenumbers exhibiting bimodal probability density distributions. Interhemispheric differences center around much stronger westward-traveling ultralong wave variance in the NH but stronger eastward traveling intermediate wavenumber variance in the SH. These interhemispheric differences in the traveling variance are likely an effect of the presence or absence of topographic forcing, as suggested by earlier GCM experiments. This implies that the rôle of the stationary topographic forcing is more significant for the traveling waves than for the standing waves. In addition, composite power spectra for El Niño, anti-El Niño and non-El Niño NH winters were constructed. The results indicate that the warm equatorial sea surface temperature (SST) anomalies of El Niño are accompanied by a strong composite spectral peak at wavenumber 7 with 5-day period in the midlatitude power spectra. Examination of individual winters reveals that this apparent difference does appear fairly consistently in all El Niño winters, but some control and anti-El Niño winters exhibit similar peaks. The most consistent difference between power spectra composed on the basis of SST anomalies is relatively stronger standing variance in the anti-El Niño composite and weak standing variance in the El Niño composite.