Origins of Tropical Pacific Decadal Variability: Role of Stochastic Atmospheric Forcing from the South Pacific*

Abstract

Abstract Based on the analysis of multicentury–millennium integrations of an atmospheric model coupled to the ocean with varying degrees, it is argued that ENSO-like decadal variability is primarily driven by stochastic atmospheric forcing. In particular, the leading mode of internal atmospheric variability over the South Pacific, which projects onto the Pacific–South American (PSA) pattern, plays an important role in modulating the trade winds and sea surface temperature (SST) in the southeast tropical Pacific. Subsequent ocean–atmosphere interactions organize a basinwide SST anomaly pattern in the tropics, which in turn forces atmospheric Rossby waves into the extratropics, reinforcing the PSA pattern and inducing coherent decadal changes in the North Pacific. In the absence of ocean dynamics, equatorial SST variability is reduced and the North Pacific exhibits decadal variability independent of the tropical–South Pacific. The strong tropical–South Pacific linkage may be attributed to the equatorially asymmetric nature of tropical Pacific climate.