Towards an improved understanding of Pacific Decadal Variability using paleoclimate reanalysis

The Pacific Decadal Variability (PDV) is the dominant mode of Earth System variability on multi-decadal timescales. Depending on the phase of the PDV, it can either accelerate or mitigate the global warming trend. Additionally, PDV has significant societal impacts. It influences hazards across the Pacific region, including floods, droughts, and bushfires, as well as the two main ice sheets, resulting in substantial coastal impacts. Most studies investigating the drivers of PDV have focused on the instrumental period, concluding that PDV is primarily driven by atmospheric processes, with a relatively minor contribution from oceanic processes. However, the instrumental period may be too short to fully capture the low-frequency climate variability in the Pacific, particularly its associated oceanic processes. Moreover, this period is marked by large changes in external forcings, especially those resulting from anthropogenic greenhouse gas and aerosol concentrations. Here we present a novel, fully coupled paleoclimate reanalysis spanning the past 400 years. This reanalysis utilizes the Norwegian Climate Prediction Model (NorCPM), equipped with an Ensemble Kalman Filter data assimilation method. Originally developed for producing oceanic reanalysis with skills comparable to top-performing ocean reanalysis systems, we adapted NorCPM to incorporate hundreds of paleoclimate records, including coral, tree-ring, and ice-core observations, extending back four centuries. When compared with state-of-the-art atmospheric reanalyses and surface oceanic observations, our reanalysis demonstrates high skill across the Pacific domain. As a coupled model, it enables a detailed quantification of multi-decadal, two-way interactions between the ocean and atmosphere that drive the PDV. By comparing this fully coupled reanalysis with standalone simulations (i.e., those without ocean-atmosphere coupling), we quantify the contribution of these coupled interactions to the PDV. Finally, we present the impacts of long-lasting extreme PDV states on hydroclimate variability across the Pacific basin, providing new insights into the effects of PDV at regional and global scales.