ENSO response to external forcing in CMIP5 simulations of the last millennium

- Robust influence of volcanic forcing on ENSO during past 1000-yr
- The response of ENSO to solar forcing is more likely to be weak
- Detection of GHGs variations signal in ENSO response is not significant.
- The responses of ENSO to TSI and GHGs radiative forcing are uncertain.

The El Niño-Southern Oscillation (ENSO) is dominant mode of interannual climate variability, but its response to external climate forcings remains uncertain. Past studies have limitations including the use of short datasets and the uncertainty contained in reconstructions or simulations of past ENSO variations. To improve our understanding on ENSO variations, it is important to examine its response to these forcings by using multi model simulations since they provide longer datasets and help improving the statistical significance of the results. In this study, Granger causality test is applied to investigate the influence of external forcings on ENSO by using past millennium simulations (period 850-1850 CE) of Coupled Model Intercomparison Project Phase 5 (CMIP5) models. The results show robust influence of volcanic forcing to ENSO during preindustrial times of last millennium. The response of ENSO to solar forcing is more likely to be weak. Detection of GHGs variations signal in ENSO response is not significant. However, this study also indicates that there are uncertainties in the responses of ENSO to solar forcing and GHGs radiative forcing. The possibility of the true causal connection between Total Solar Irradiance (TSI) or GHGs radiative forcing and ENSO cannot be rejected at 95% significance level.