Volcanoes and ENSO over the past millennium

The controversial claim that El Ni (n) over tildeo events might be partially caused by radiative forcing due to volcanic aerosols is reassessed. Building on the work of Mann et al., estimates of volcanic forcing over the past millennium and a climate model of intermediate complexity are used to draw a diagram of El Ni (n) over tildeo likelihood as a function of the intensity of volcanic forcing. It is shown that in the context of this model, only eruptions larger than that of Mt. Pinatubo ( 1991, peak dimming of about 3.7Wm(-2)) can shift the likelihood and amplitude of an El Ni (n) over tildeo event above the level of the model's internal variability. Explosive volcanism cannot be said to trigger El Ni (n) over tildeo events per se, but it is found to raise their likelihood by 50% on average, also favoring higher amplitudes. This reconciles, on one hand, the demonstration by Adams et al. of a statistical relationship between explosive volcanism and El Ni (n) over tildeo and, on the other hand, the ability to predict El Ni (n) over tildeo events of the last 148 yr without knowledge of volcanic forcing.The authors then focus on the strongest eruption of the millennium (A. D. 1258), and show that it is likely to have favored the occurrence of a moderate-to-strong El Ni (n) over tildeo event in the midst of prevailing La Ni (n) over tildea-like conditions induced by increased solar activity during the well-documented Medieval Climate Anomaly. Compiling paleoclimate data from a wide array of sources, a number of important hydroclimatic consequences for neighboring areas is documented. The authors propose, in particular, that the event briefly interrupted a solar-induced megadrought in the southwestern United States. Most of the time, however, volcanic eruptions are found to be too small to significantly affect ENSO statistics.