Tropical Pacific decadal variability and ENSO modulation: Role of stochastic atmospheric forcing from the extratropics

Abstract:

The tropical Pacific exhibits decadal variability that extends into the extratropics, similar to the El Niño-Southern Oscillation (ENSO). Unlike the ENSO, in which equatorial ocean-atmosphere interactions cause distinct interannual variability and affect the extratropics through atmospheric teleconnections, the origin of tropical Pacific decadal variability (TPDV) is not well understood. It also remains unclear how TPDV interacts with the ENSO, despite their apparent relationships in observations and models. To address these issues, we conduct a series of studies using a climate model that reproduces important features of tropical Pacific climate. Based on the analysis of long control simulations with varying degrees of ocean-atmosphere coupling, we hypothesize that TPDV is primarily driven by stochastic atmospheric forcing from the extratropics. We further hypothesize that the stochastically-forced TPDV interacts with the ENSO and enhances the interhemispheric coherence of decadal variability. These hypotheses are tested through a suite of climate model experiments. When the model is forced with small surface heat flux anomalies associated with extratropical atmospheric variability, sea surface temperature and wind anomalies propagate into the equatorial Pacific through thermodynamic air-sea interactions, leading to a global anomaly pattern. The equatorward propagation is regulated by the latitudinal position of the intertropical convergence zone (ITCZ), and the northward displacement of the mean ITCZ makes the equatorial Pacific more strongly influenced by South Pacific than North Pacific atmospheric variability. The changes in the tropical Pacific mean state forced from the extratropics, in turn, alter the occurrence and evolution of ENSO events. In particular, ENSO-like TPDV affects the relative frequency of El Niño and La Niña events through changes in the interbasin SST contrast and surface winds over the western Pacific. Our results indicate close linkages of tropical Pacific mean climate and variability on different time scales, underscoring the importance of reducing model bias to further our understanding of TPDV and ENSO modulation.