The mean state of the tropical Pacific Ocean differed between the Medieval Warm Period and the Industrial Era

Changes in the El Niño Southern Oscillation (ENSO) over recent decades have been linked to anthropogenic greenhouse gas emissions; however, the sign, magnitude, and drivers of these variations during the Common Era are not yet well constrained. This study presents an 1800-year reconstruction of the ENSO mean-state derived from precipitation reconstructions based on lake sediments in East Asia and examines the long-term changes in the mean state and variance of the tropical Pacific. The Medieval Warm Period (MWP) was characterized by La Niña-like states with low variance, whereas El Niño-like states prevailed during the Little Ice Age (LIA) and the Current Warm Period (CWP) with high variance. Temperature changes caused by effective radiative forcing likely drove this response during the pre-industrial period, while greenhouse gas forcing likely accounts for variations in the tropical Pacific mean-state during the industrial period.

Shiwei Jiang, Xin Zhou, Julian P. Sachs, Zhibo Li, Luyao Tu, Yiyi Lin, Xuanqiao Liu, Anze Chen, Yanan Shen