A model-tested North Atlantic Oscillation reconstruction for the past millennium

Area of Science:

• Paleoclimatology
• Atmospheric Science
• Climate Dynamics

Background:

• The North Atlantic Oscillation (NAO) significantly influences Northern Hemisphere climate and impacts critical sectors like agriculture and water management.
• Assessing long-term NAO variability is crucial for understanding climate predictability, yet past reconstructions are debated.
• Previous studies suggested sustained positive NAO during medieval times, linked to warmer European conditions, but this is contested.

Purpose of the Study:

• To develop a more robust, annually resolved reconstruction of the North Atlantic Oscillation (NAO) spanning the past millennium.
• To re-evaluate the NAO's low-frequency variability, particularly during the medieval period.
• To investigate the relationship between volcanic eruptions and subsequent NAO phases.

Main Methods:

• Compilation of 48 annually resolved proxy records from the Atlantic region.
• Development of an ensemble of multivariate regression models for NAO reconstruction.
• Validation of the reconstruction against six past-millennium climate simulations.

Main Results:

• The new NAO reconstruction outperforms a previous bi-proxy index.
• No persistent positive NAO conditions were found during the medieval period.
• Positive NAO phases were dominant in the thirteenth and fourteenth centuries.
• A positive NAO response was observed two years after major volcanic eruptions.

Conclusions:

• The medieval period did not experience persistent positive North Atlantic Oscillation (NAO) conditions.
• The thirteenth and fourteenth centuries were characterized by dominant positive NAO phases.
• Volcanic eruptions are followed by a positive NAO phase within two years, aligning with model and observational data.