Modulation of orbitally forced ENSO variation by Tibetan Plateau topography

Abstract

The modulation of the orbitally induced variation in the El Niño/Southern Oscillation (ENSO) by Tibetan Plateau (TP) topography was investigated with four fully coupled ocean–atmosphere model experiments under the modern boundary conditions. The modeled orbitally induced ENSO change was strongly affected by the presence of the TP. Specifically, in the experiments with orbital forcing at 6 ka in comparison to the counterpart experiments with modern orbital forcing, it was shown that the orbitally forced ENSO variability was slightly weakened by 3% in the presence of the TP but increased significantly by 13% in the simulation of the absence of the TP. This difference was primarily attributed to the anomalous zonal winds over the tropical central-to-eastern Pacific in response to the increased summertime incoming insolation, which was intrinsically related to the different changes in the Asian monsoon and tropical Pacific Walker circulation. Under the absence of modulation by the TP, the anomalous westerly winds dominated the equatorial Pacific and favored the development of a stronger El Niño event and, in turn, the further intensification of La Niña due to the stronger heat discharge during the recession of the stronger El Niño. In contrast, the simulation with the presence of the TP showed the enhanced easterly wind anomalies in the boreal summer over the tropical Pacific, which further reduced (increased) the magnitude of the El Niño (La Niña) event in the subsequent winter. These results suggest that the TP is a potentially crucial boundary condition for evaluating the changes in the ENSO dynamics on orbital scales during the Cenozoic.