ENSO as a mediator of the solar influence on climate

Using a climate model of intermediate complexity, we simulate the response of the El Niño-Southern Oscillation (ENSO) system to solar and orbital forcing over the Holocene. Solar forcing is reconstructed from radiocarbon production rate data, using various scaling factors to account for the conflicting estimates of solar irradiance variability. As estimates of the difference since the Maun-der Minimum range from 0.05% to 0.5% of the solar "constant", we consider these two extreme scenarios, along with the intermediate case of 0.2%. We show that for large or moderate forcings, the low-pass filtered east-west sea surface temperature gradient along the equator responds almost linearly to irradiance forcing, with a short phase lag (about a decade). Wavelet analysis shows a statistically-significant enhancement of the century-to-millenial scale ENSO variability for even a moderate irradiance forcing. In contrast, the 0.05%-case displays no such enhancement. Orbitally-driven insolation forcing is found to produce a long-term increase of ENSO variability from the early Holocene onwards, in accordance with previous findings. When both forcings are combined, the superposition is approximately linear in the strong scaling case. Overall, the sea surface temperature response is of the magnitude required, and persistent enough, to induce important climatic perturbations worldwide. The results suggest that ENSO may plausibly have acted as a mediator between the Sun and the Earth's climate. A comparison to key Holocene climate records, from the Northern Hemisphere sub-tropics and midlatitudes, shows support for this hypothesis.