Understanding the characteristics and drivers of Pacific decadal variability over the last millennium

Pacific Decadal Variability (PDV) is known to drive global and regional climate and ecosystem changes. Understanding the spatiotemporal characteristics, processes, and drivers of these variations is therefore critical to improving our knowledge for near-term climate predictions. Although the instrumental period offers valuable insights into the drivers and the characteristics of PDV, it only offers a partial view. In contrast, the last millennium holds promises for a more comprehensive understanding of PDV. However, our knowledge of PDV over the last millennium remains incomplete: there are questions on the relative role of internal variability and external forcings on PDV that remain unresolved, and it remains unclear how accurately we can reconstruct PDV using available coral records over the last millennium. This study uses the Community Earth System Model Last Millennium Ensemble (CESM-LME) to address these shortcomings and advance our understanding of PDV over the last millennium. Using information theory metrics, we show that while there is a detectable spatial fingerprint from each forcing on decadal SST variations, their influence on large-scale coherent spatiotemporal patterns is small relative to internal variability over the last millennium. The influence of external forcing remains small even in recent decades despite the increasing anthropogenic forcing. Our results further indicate that there is potential to reconstruct PDV over the past $\sim$300 years reliably using available coral records. Overall, these results clarify the relative role of internal variability and external forcings on PDV and highlight pathways to improve our reconstruction of past PDV.