Paleo-NorCPM: A framework toward fully coupled ocean-atmosphere paleoclimate reanalysis over the past centuries

Abstract. We present a new paleo reanalysis based on an adaptation of the Norwegian Climate Prediction Model (NorCPM), in which the Norwegian Earth System Model is equipped with an ensemble Kalman filter to assimilate hundreds of annually resolved proxy records, including tree rings, corals, and ice cores, over the last 500 years. First, an offline 30-member reanalysis is produced. This is followed by a dynamically coupled 10-member reanalysis obtained through atmospheric wind nudging from the offline reconstruction, allowing the ocean to respond to reconstructed atmospheric circulation variability. This approach enables an explicit representation of ocean dynamics and associated atmosphere-ocean feedback, which are typically indirectly represented in existing multi-century reanalyses. The offline reanalysis successfully reproduces large-scale atmospheric variability and hydroclimate variability over the 20th century across the tropics, mid-latitudes, and polar regions, and compares well with existing paleo reconstructions. The wind-nudged ensemble further captures ocean variability and exhibits more pronounced multi-decadal variability, highlighting the role of wind-driven ocean adjustment in shaping low-frequency climate variability. Challenges remain, particularly in the Southern Ocean where the wind-nudged ensemble shows strong sensitivity to wind variability. Nevertheless, these results demonstrate a promising pathway toward fully coupled ocean-atmosphere reanalyses spanning the past centuries, opening new opportunities to investigate the mechanisms underlying low-frequency variability and coupled feedback.