NOTES AND CORRESPONDENCE Analysis of Atlantic SST Variability Factoring Interbasin Links and the Secular Trend: Clarified Structure of the Atlantic Multidecadal Oscillation

Atlantic SST variability in the twentieth century is analyzed factoring the influence of natural SST variability in the Pacific basin and the secular change in global SSTs. The tropical and northern extratropical basins are analyzed together using the extended EOF technique, which permits extraction of the interannual and multidecadal modes in the pan-Atlantic basin in a single step. The leading mode of Pacific-uninfluenced SST variability is a multidecadal oscillation focused in the extratropical basin, with a period of ;70 yr. The mode differs from the conventional Atlantic multidecadal oscillation (AMO) in the near quiescence of the tropical–subtropical basin, highlighting the significant influence of the Pacific basin on this region in conventional analysis; as much as 45% of the regional variance resulting from the conventional AMO is due to this influence. The second and third modes capture the growth (east-to-west development) and decay (near-simultaneous loss of amplitudes) of interannual SST variability in the eastern tropical Atlantic. A nominal 4-yr evolution cycle is identified, but phase transitions are irregular. The fourth mode describes a north–south tripole with the mature-phase structure resembling the North Atlantic Oscillation’s (NAO’s) SST footprint in winter. The mode lags the NAO by two seasons. Modal evolution involves eastward extension of the main lobe (centered near the separation of the Gulf Stream) along with shrinkage of the oppositely signed two side lobes.