Impact of Stochastically Forced Tropical Pacific Decadal Variability on the El Niño–Southern Oscillation

The analysis of a millennium-long control simulation of the Community Climate System Model version 4 (CCSM4) shows that the amplitude and other properties of the El Niño-Southern Oscillation (ENSO) vary significantly with the leading modes of tropical Pacific decadal variability (TPDV). These modes of TPDV are reproduced to some degree in the atmospheric component of CCSM4 coupled to a slab ocean model, suggesting the role of stochastic atmospheric forcing. To test the hypothesis that TPDV driven by stochastic atmospheric forcing modulates the ENSO, we conducted a set of CCSM4 experiments by imposing surface heat flux anomalies associated with the leading atmospheric modes over the South and North Pacific, particularly the Pacific-South American (PSA) and North Pacific oscillation (NPO) patterns. In response to the surface heat flux forcing, the model simulates changes in both the mean state of the tropical Pacific and the ENSO. In both the PSA and NPO experiments, the mean state changes resemble the observed interdecadal Pacific oscillation and are accompanied by changes in the relative frequency of El Niño and La Niña events. Preliminary analysis suggests that the occurrence of El Nino and La Nina events are controlled by the sea surface temperature gradient between the tropical Pacific and the other two tropical oceans. When the tropical Pacific is anomalously warm relative to the other two oceans, the associated westerly wind anomalies over the western equatorial Pacific facilitate the development of El Niño events. The opposite condition appears to facilitate the development of La Niña events. We are conducting additional CCSM4 experiments to isolate the direct impact of mean state changes on ENSO, as well as the impacts of other internal atmospheric modes.