Role of Stochastic Atmospheric Forcing in Tropical Pacific Decadal Variability and ENSO Modulation

The analysis of a millennium-long control simulation of the Community Climate System Model version 4 (CCSM4) suggests that the amplitude and other properties of El Niño-Southern Oscillation (ENSO) vary significantly on decadal-interdecadal time scales in association with changes in the tropical Pacific background state. Intriguingly, the patterns of tropical Pacific decadal variability (TPDV) related to the ENSO modulation in CCSM4 are reproduced to some degree in a control simulation of the atmospheric component (CAM4) coupled to a slab ocean model (SOM). In both CCSM4 and CAM4-SOM, the extratropical atmospheric circulation anomalies associated with TPDV project onto the leading modes of internal atmospheric variability over the North and South Pacific, namely, the North Pacific oscillation (NPO) and Pacific South-American (PSA) patterns. This result suggests that TPDV driven by stochastic atmospheric forcing interacts with the equatorial ocean dynamics and modulates ENSO. To test this hypothesis, we conduct a set of CCSM4 experiments by imposing surface heat flux anomalies associated with the NPO and PSA patterns over the North and South Pacific, respectively. The results show that the PSA experiment induces a basin-wide sea surface temperature (SST) anomaly pattern that resembles the observed interdecadal Pacific oscillation although the forcing is confined to the South Pacific. However, the corresponding wind anomalies induce negative feedback through oceanic processes, resulting in a weak response in the equatorial Pacific. The influence of the NPO pattern, on the other hand, is mostly confined to the North Pacific with small SST changes over the tropical Pacific. Both the NPO and PSA experiments show significant changes in the relative frequency of El Niño and La Niña events, in support of the hypothesized mechanism of ENSO modulation.