North–South Pacific Decadal Co-Variability over the last 500 years informed by a new paleo-reanalysis

Decadal variability in the Pacific plays an important role in shaping global climate and can modulate the expression of anthropogenic warming, as illustrated by its contribution to the early 21st-century global warming hiatus. The Interdecadal Pacific Oscillation (IPO) describes multi-decadal sea surface temperature variability across the Pacific, yet the processes driving its variability remain poorly understood. Although the Pacific Decadal Oscillation (PDO) and South Pacific Decadal Oscillation (SPDO) are often synchronized by tropical forcing, they are not always equal. Examining their covariance may provide critical insight into the mechanisms underlying IPO variability. In this study, we analyze a new paleo-based multivariate reanalysis of the Norwegian Climate Prediction Model (NorCPM) that assimilates annually resolved paleoclimate records from the past centuries while including transient external forcing. The dataset spans the 1500–2010 period, allowing us to overcome limitations of previous IPO studies associated with short observational records, especially in the Southern Hemisphere. The temporal evolution of PDO–SPDO covariance over the last five centuries is assessed using low-frequency component analysis to isolate the internal variability and examine how this relationship varies under differing backgrounds states. The results indicate that while the PDO and SPDO are predominantly in phase, their covariance is highly non-stationary, with periods of weakened or reversed coupling. Examining this covariance provides additional context for understanding how Pacific basin–scale interactions contribute to IPO variability.