Pacific multidecadal (50–70 year) variability instigated by volcanic forcing during the Little Ice Age (1250–1850)

The Pacific decadal oscillation (PDO) is the leading mode of decadal climate variability over the North Pacific. However, it remains unknown to what extent external forcings can influence the PDO’s periodicity and magnitude over the past 2000 years. We show that the paleo-assimilation products (LMR) and proxy data suggest a 20–40 year PDO occurred during both the Mediaeval Climate Anomaly (MCA, ~ 750–1150) and Little Ice Age (LIA, ~ 1250–1850) while a salient 50–70 year variance peak emerged during the LIA. These results are reproduced well by the CESM simulations in the all-forcing (AF) and single volcanic forcing (Vol) experiments. We show that the 20–40 year PDO is an intrinsic mode caused by internal variability but the 50–70 year PDO during the LIA is a forced mode primarily shaped by volcanic forcing. The intrinsic mode develops in tandem with tropical ENSO-like anomalies, while the forced mode develops from the western Pacific and unrelated to tropical sea surface temperature anomalies. The volcanism-induced land–sea thermal contrast may trigger anomalous northerlies over the western North Pacific (WNP), leading to reduced northward heat transport and the cooling in the Kuroshio–Oyashio Extension (KOE), generating the forced mode. A 50–70 year Atlantic multidecadal oscillation founded during the LIA under volcanic forcing may also contribute to the forced mode. These findings shed light on the interplay between the internal variability and external forcing and the present and future changes of the PDO.