Threshold of the volcanic forcing that leads the El Niño-like warming in the last millennium: results from the ERIK simulation
In order to examine the threshold of the volcanic forcing that leads to the El Niño-like warming, we analyze a millennium ERIK simulation (AD 1000-1850) forced by three external forcings including greenhouse gases, solar forcing and volcanic eruptions using the ECHO-G coupled climate model. It is found that there exists a threshold of the volcanic forcing above 15 W/m2 to lead the El Niño-like warming in the climate model. When the volcanic forcing is above this threshold forcing, then the intensity of the Inter-tropical Convergence Zone (ITCZ) is weakened and its position is shifted to the south. This might be associated with the processes of less evaporation in the subtropical cloudless region by a cooling due to the reduction of net surface shortwave radiation. Concurrently, a weakening of ITCZ is associated with a weakening of the trade winds and the subsequent Bjerknes feedback causes El Niño-like warming. Therefore, El Niño-like warming events can occur when volcanic eruption is above threshold forcing, implying that there exists a certain level of radiative forcing change which is capable of changing the state of tropical Pacific sea surface temperature. The last millennium simulation of Paleoclimate Modeling Intercomparison Project Phase 3 climate models also indicates that there may exist a threshold forcing to lead the El Niño-like warming, which has been also discussed in the present study.