A record of the Southern Oscillation Index for the past 2,000 years from precipitation proxies

The El Nino-Southern Oscillation (ENSO) is a coupled ocean-atmosphere climate phenomenon in the tropical Pacific Ocean. The interannual climate variations have been shown to modify both the Hadley and Walker meridional and zonal atmospheric circulations, with strong impacts on global climate(1-3). Proxy-based reconstructions of the Southern Oscillation Index on a multi-decadal scale have shown that the strength and frequency of El Nino occurrences have varied over the past millennium(4-7). Here we compile reconstructions of precipitation(8-15) from regions that experience substantial ENSO variability to extend the multidecadal-scale Southern Oscillation Index to include the past 2,000 years. We find that the Medieval Warm Period (similar to AD 800-1300) was characterized by a negative index, which indicates more El Nino-dominated conditions, whereas during the Little Ice Age (similar to AD 1400-1850) more La Nina-dominated conditions prevailed. The Southern Oscillation Index we derive is significantly correlated with reconstructions of solar irradiance and mean Northern Hemisphere temperature fluctuations.