Forced changes in the Pacific Walker circulation over the past millennium

Area of Science:

• Paleoclimatology
• Climate Dynamics
• Atmospheric Science

Background:

• The Pacific Walker circulation (PWC) significantly impacts global weather and climate.
• PWC responses to external forcings are poorly understood, with discrepancies between empirical data and climate models.
• Most climate models project a weakening of the PWC due to global warming.

Purpose of the Study:

• To reconstruct the PWC over the past millennium using a new ensemble of paleoproxy data.
• To assess the recent strengthening of the PWC (1992-2011) in the context of long-term variability.
• To investigate the drivers of PWC changes, including volcanic, anthropogenic aerosol, and internal variability forcing.

Main Methods:

• Development of an annually resolved, multi-method, paleoproxy-derived PWC reconstruction ensemble from 1200 to 2000.
• Statistical analysis of the PWC reconstruction to identify trends and anomalies.
• Comparison of reconstruction results with climate model simulations and historical forcing data.

Main Results:

• The 1992-2011 PWC strengthening, while anomalous, is not unprecedented over the past 800 years.
• Volcanic forcing is unlikely to be the primary driver of the recent PWC strengthening.
• No significant industrial-era (1850-2000) PWC trend was detected, contrasting with most climate model predictions.
• A shift towards lower-frequency PWC variability during the industrial era suggests a subtle anthropogenic influence.
• Volcanic eruptions were found to induce El Niño-like PWC weakening, consistent with model simulations.

Conclusions:

• The recent PWC strengthening may be attributed to anthropogenic aerosol forcing or natural variability.
• Current climate models may not fully capture the PWC's response to industrial-era changes.
• Paleoproxy reconstructions are crucial for understanding long-term climate dynamics and validating climate models.