This occurs in the North and South Pacific Oceans (indicating a reflection symmetry in SST evolution about the equator), in the North Atlantic Ocean and the South Indian Ocean (indicating translation symmetries with SST evolution in the Pacific Ocean). In the Pacific Ocean, SST weights in the second mode along the subtropical eastern boundary in both hemispheres extend equatorward in two tongues that converge onto the equator.
To describe the evolution of interdecadal SST and winter SLP anomalies, we constructing animations of interdecadal GISST SST and SIO winter SLP anomalies over 11 years from 1974 to 1984 (Figure 4c). Over this 11-year record the entire tropical ocean from approximately 20｡? to 20｡? was cooler than normal for the period 1974 to 1976 and warmer than normal for the periods 1982 to 1984 (left, Figure 4c). In 1974 and 1984, patterns of SST variability display global reflection and translation symmetries discussed in connection with Figures 4a and 4b. The transition from tropical cool phase to tropical warm phase occurred over the 7 years from 1976 to 1982. By following this slow transition in the Pacific basin, maximum warm SST anomalies in the extratropics in both hemispheres in 1974 can be seen extending slowly eastward to the eastern boundary to 1978 and spreading equatorward from there into the tropics in both hemispheres from 1978 to 1980. Then, during 1980 to 1984, warm SST anomalies in the eastern subtropical and tropical oceans spread westward, filling the central and western tropical oceans with warm anomalies. This is consistent with the propagation inferred from the first two EOF modes for interdecadal SST anomalies in Figure 4b. Over the latter half of this transition period (i.e., 1980 to 1984), cool SST anomalies began developing along the western-central SAFZ of each ocean basin, achieving coolest values in 1984 at the same time that tropical warm SST anomalies achieved warmest values. During the latter part of this transition, warm SSI anomalies propagating equatorward from the South Pacific Ocean merged with those from the North Pacific at the ITCZ near 160｡? in 1980, achieving warmest values in 1984 upon their mutual extension into the western tropical North Pacific Ocean. In the Atlantic Ocean, warm extratropical SST anomalies from the North Atlantic Ocean reached the equator first, while in the Indian Ocean warm extratropical SST anomalies spread equatorward from the south reaching the equator at about the same time as in the South Pacific Ocean.
Interdecadal change in winter SLP anomalies during the transition period from 1976-1982 (right, Figure 4c) finds high SLP anomalies along the eastern SAFZ in the North Pacific, South Pacific, and North Atlantic basins propagating eastward and equatorward and fading in magnitude as they enter the tropical oceans, consistent with the propagation inferred from the first two EOF modes for interdecadal SLP anomalies in Figure 4b. These high extratropical SLP anomalies were replaced with low extratropical SLP anomalies propagating equatorward from high latitude in the northern hemisphere (i.e., near centers of the Aleutian and Icelandic Lows) to the central and eastern SAFZ in each ocean. During this transition period, equatorward propagation of high extratropical SLP anomalies from middle latitudes to the tropics occurred in conjunction with the equatorward propagation of warm SST anomalies, while equatorward propagation of low extratropical SLP anomalies from high latitudes to middle latitudes in the northern hemisphere occurred in conjunction with equatorward propagation of high latitude cool SST anomalies.
