New insights into the Pacific Walker Circulation from an 800-year-long water isotope-based reconstruction ensemble 

The Pacific Walker Circulation (PWC) has a major influence on weather and climate worldwide. But our understanding of 1) its response to external forcings; and 2) its internal variability across timescales remain unclear. This is in part due to the length of the observational record, which is too short to disentangle forced responses from internal variability.  Here we assess the internal variability of the PWC as well as its response to the two largest external forcings of the Common Era: volcanic eruptions and anthropogenic forcing. We do this using a new annually-resolved, multi-method, palaeoproxy-derived PWC reconstruction ensemble spanning 1200-2000. The reconstruction is derived from 59 palaeoclimate proxy records, mostly from the Iso2k database of water isotope proxy records (Konecky et al., 2020). The basis for the reconstruction is previous work by Falster et al. (2021), demonstrating that global water isotope variability has a strong mechanistic link with the PWC via its major influence on the global water cycle. The PWC reconstruction ensemble comprises 4800 members that sample uncertainty from observational data, reconstruction method, and record chronologies.  We identify a significant PWC weakening in the 1-3 years following large volcanic eruptions, similar to the response seen in some climate models. However, we find no significant industrial-era (1850-2000) PWC trend relative to the preceding 650 years, which contrasts the PWC weakening simulated by most climate models. In fact, the strength of the PWC is not correlated with global mean temperature across timescales. We also find that the 1992-2011 PWC strengthening—previously attributed either to volcanic or anthropogenic aerosol forcing—was indeed anomalous, but not unprecedented as compared to the past 800 years. Hence it may have occurred due to decadal internal variability. The one place we did identify an industrial-era PWC change is in the power spectrum, where a post-1850 shift to lower-frequency variability suggests a subtle anthropogenic influence.  References: Konecky, B. L., McKay, N. P., Churakova (Sidorova), O. V., Comas-Bru, L., Dassié, E. P., DeLong, K. L., Falster, G. M., Fischer, M. J., Jones, M. D., Jonkers, L., Kaufman, D. S., Leduc, G., Managave, S. R., Martrat, B., Opel, T., Orsi, A. J., Partin, J. W., Sayani, H. R., Thomas, E. K., Thompson, D. M., Tyler, J. J., Abram, N. J., Atwood, A. R., Cartapanis, O., Conroy, J. L., Curran, M. A., Dee, S. G., Deininger, M., Divine, D. V., Kern, Z., Porter, T. J., Stevenson, S. L., von Gunten, L., and Iso2k Project Members: The Iso2k database: a global compilation of paleo-δ18O and δ2H records to aid understanding of Common Era climate, Earth Syst. Sci. Data, 12, 2261–2288, 2020. Falster, G., B. Konecky, M. Madhavan, S. Stevenson, and S. Coats: Imprint of the Pacific Walker Circulation in Global Precipitation δ18O. J. Climate, 34, 8579–8597, 2021.