This extratropical pattern is similar to that observed by Tanimoto et al. [1993] for the North Pacific Ocean and by Zhang et al. [1997] for the entire Pacific Ocean for sub-ENSO frequencies.
The covarying spatial pattern of the EOF mode in winter SLP (Figure 4a, bottom) displays nearly uniform sign for the weights over the extratropical northern hemisphere oceans, with maximum magnitudes near 45°N. As such, maximum interdecadal SLP weights in extratropical Pacific and Atlantic basins are displaced 5° to 10° latitude poleward of the SAFZ between 35°N and 40°N. They also overlie the maximum zonal gradient of interdecadal SST anomalies between central and eastern extratropical oceans seen in the top panel of Figure 4a.
We refine these results by applying the EOF analysis to interdecadal GISST SST anomalies and SIO winter SLP anomalies for the 40 years from 1955 to 1994 (Figure 4b). In these analyses we display the first two modes for both SST and SLP anomalies, together explaining nearly equal amounts of interdecadal variance (i.e., 89% for both GISST SST and SIO SLP) and displaying the evolution of interdecadal SST and SLP variability as real and imaginary components of the complex EOF [Preisendorfer and Mobley, 1988].
Time sequences of the first-mode EOF amplitudes display peaks near 1965 and 1985, displaced ahead by 3-5 years from those in the longer record (Figure 4a, top left), indicating slightly different spatial patterns than those occurring over the past century. The corresponding first-mode spatial pattern for SST (Figure 4b, left) is similar to that in Figure 4a, but interdecadal SST variability in the central ocean at 35｡? can now be observed to be distinct from that in the western ocean in both hemispheres. Nakamura et al. [1997] find this SST anomaly in the central extratropical North Pacific linked to the Subtropical Front east of Hawaii, but whether this has a symmetric counterpart in the extratropical South Pacific Ocean needs to be examined. Yet this spatial pattern reveals comparable magnitudes in northern and southern hemispheres associated with the reflection symmetry and between ocean basins associated with the translation symmetries.
The corresponding first-mode spatial pattern for anomalous SIO winter SLP (Figure 4b, left) displays the same translation symmetry in the extratropical northern hemisphere as those of anomalous NCAR winter SLP in Figure 4a, again with magnitudes in the North Pacific basin greater than those in the North Atlantic basin by a factor of 2 or so. Moreover, fluctuations near the center of the lcelandic Low near 60°N in the North Atlantic basin occur out of phase with those in the North Pacific basin, indicative of regional North Atlantic interdecadal variability [Kushnir, 1994]. Here is added a reflection symmetry about the equator with extratropical westerly winds fluctuating in phase between 20°to 40° latitude in both hemispheres, consistent with Zhang et al. [1997]. A corresponding translation symmetry occurs in the southern hemisphere between the South Pacific and South Atlantic basins. In the tropical Pacific basin, relatively low SLP weights in the central-eastern ocean are associated with ageostrophic westerly wind anomalies in the vicinity of the warm pool, as demonstrated by Zhang et al. [1997].
Time sequences of the second-mode EOF amplitudes for SST and SLP display peaks near 1970 and 1990 (Figure 4b, top right) lagging by 5 years those in the first mode (Figure 4b, top left) and indicative of propagation in interdecadal SST and SLP variability. Comparing the spatial patterns of these first two SST modes finds extratropical SST weights in western-central SAFZ's in the first mode propagating eastward to the eastern boundaries of continents, there spreading north and south in the second mode.
