Asian-Pacific Oscillation index and variation of East Asian summer monsoon over the past millennium

To study the long-term variation of the East Asian summer monsoon (EASM), the Asian-Pacific Oscillation index (I-APO), representing a zonal thermal contrast between Asia and the North Pacific, is reconstructed over the past millennium. During the Little Ice Age (LIA), the variability of the reconstructed I-APO is closely linked to dry-wet anomalies in eastern China on the centennial scale. This correlation pattern is consistent with the observation during the current period, which suggests that the reconstructed I-APO may generally represent the centennial-scale variation of the EASM and rainfall anomalies over eastern China during the LIA.