Volcanoes and ENSO in millennium simulations: global impacts and regional reconstructions in East Asia

The impacts and cooperative effects of volcanic eruptions and ENSO (El Nio/Southern Oscillation) are analyzed in a millennium simulation for 800-2005 AD using the earth system model (ESM) ECHAM5/MPIOM/JSBACH subject to anthropogenic and natural forcings. The simulation comprises two ensembles, a first with weak (E1, five members) and a second with strong (E2, three members) variability total solar irradiance. In the analysis, the 21 most intense eruptions are selected in each ensemble member. Volcanoes with neutral ENSO states during two preceding winters cause a global cooling in the year after eruptions up to -2.5A degrees C. The nonsignificant positive values in the tropical Pacific Ocean indicate an El Nio-like warming. In the winter after an eruption, warming is mainly found in the Arctic Ocean and the Bering Sea in E2 warming extends to Siberia and central Asia. The recovery times for the volcano-induced cooling (average for 31 eruptions) vary globally between 1 and 12 years. There is no significant increase of El Nio events after volcanic eruptions in both ensembles. The simulated temperature and the drought indices are compared with corresponding reconstructions in East Asia. Volcanoes cause a dramatic cooling in west China (-2A degrees C) and a drought in East China during the year after the eruption. The reconstructions show similar cooling patterns with smaller magnitudes and confirm the dryness in East China. Without volcanoes, El Nio events reduce summer precipitation in the North, while South China becomes wetter; La Nia events cause opposite effects. El Nio events in the winters after eruptions compensate the cooling caused by volcanoes in most regions of China (consistent with reconstructions), while La Nia events intensify the cooling (up to -2.5A degrees C). The simulated and reconstructed drought indices show tripole patterns which are altered by El Nio events. The simulated impact of the Tambora eruption in 1815, which caused the year without summer of 1816 in Europe and North America and led to coldness and famines in the Chinese province Yunnan, depends crucially on the ENSO state of the coupled model. A comparison with reconstructed El Nio events shows a moderate cool climate with wet (in the south) and extreme dry anomalies (in the north) persisting for several years.