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Natural Climate Variability on Decade-to-Century Time Scales
here, whereas K88 found that it was the leading mode of interannual variations of 500 hPa height in the ECMWF analyses. Kidson (1988a) has shown that this mode occurs on all time scales from intraseasonal to interannual.
The other mode found in both seasons represents an out-of-phase north-south variation of height between low and middle latitudes; it is shown in Figure 1b for winter. This mode, hereafter referred to as the low-latitude mode, explains about 13 percent of the variance in winter. Strong departures from zonal symmetry occur in the South American region, with the nodal line running almost parallel to the coast. More intraseasonal variation than interannual variation is found for this mode in winter, but in summer it is mainly associated with interannual variations.
Structures similar to this low-latitude mode have been identified by SK using station data and by Kidson (1988a) using zonal mean wind fields. This mode in the station data explains a larger fraction of variance than here, probably because of the more northerly extent of the analysis region in SK. K88 did not identify this mode, probably because his analysis using the covariance matrix of 500 hPa height variations is restricted to patterns that have large-amplitude anomalies at that height, whereas they are generally small in low latitudes.
MW presented a correlation analysis of the monthly zonal mean height and MSLP fields from their data set (refer to their Figures 1 and 2). They found structures in the zonal-mean fields that are similar to the zonally symmetric modes described here. The two leading modes for the zonal-mean fields, which explain about 70 percent of the variance, have see-saw structures over latitude bands similar to the high-latitude and low-latitude modes shown in Figure 1.
In addition to the zonally symmetric structures, wavelike patterns were also found in the PCA. They explain less variance and are less stable than the zonally symmetric modes, but are found in one-point correlation maps and composite analyses. In contrast to the NH wavelike modes, the modes found in the SH exert influence over hemispheric scales. They include zonal wave-number-three patterns and meridional wave trains in winter, and wave-number-four patterns or continent/ocean contrasts in summer.
Two stable zonal wave-number-three patterns were found for winter 300 hPa height; they are shown in Figure 2. The centers of action are located at around 60°S for the first version and about 50°S for the second version. The main difference between the two patterns is that one is shifted by about one quarter wavelength relative to the other, so that the extrema of one occur along the nodal lines of the other. The second version shows a stronger relationship (out-of-phase relative to the main centers) with lower latitudes, while the first version shows evidence of a weak
Rotated PC patterns of winter 300 hPa height in winter describing the two versions of the wave-number-three mode, as in Figure 1.
wavenumber-four structure at around 30°S. About 7 percent of the variance is explained by the first version, while the second explains about 5 percent of the variance. Together, wave-number-three patterns represent about 12 percent of the variance, a total similar to that of each of the zonally symmetric modes.
Using one-point correlation maps, MW identified a zonal wave-number-three pattern in the SH winter. The two orthogonal versions of the mode found here, with large amplitude at high latitudes, indicate that zonal wave-number-three