Coherent, Millennial-Scale Trends in δ18O: A Fingerprint of Pacific Walker Circulation Changes?

Changes to the large-scale atmospheric circulation are fundamental to climate variability and extremes, yet the mechanisms governing trends in circulation patterns on centennial to millennial timescales remain poorly understood. A challenge to date has been the difficulty of synthesizing paleoclimate information from multiple proxy types and source waters. The recent release of the PAGES Iso2k database represents a major advance in analysis capacity, given that it provides δ ¹⁸ O and δ ¹⁸ H records from marine and terrestrial archives spanning the past 2,000 years in a single, easily manipulated format. Here we combine Iso2k with the isotope-enabled Last Millennium Ensemble (iLME), a suite of 8+ isotope-enabled Community Earth System Model simulations covering the 850-2005 period, to understand the dynamics of circulation trends over the past 1000-2000 years. When smoothed trends are computed using decadally averaged data, a spatially coherent trend pattern featuring multiple centers of action is a persistent feature in both Iso2k and the iLME, with opposing isotopic trends in the Asian monsoon region and western North America. The iLME shows that the circulation pattern associated with this isotopic trend strongly resembles the Asian monsoonal branch of the Pacific Walker circulation over both the pre-industrial (850-1850) and 20th century, and the latter result is corroborated by analyses of observational products over the modern era. Together, the iLME/Iso2k synthesis suggests the presence of a monotonic, millennial trend in the Pacific Walker circulation which reverses in the 20^th century, potentially driven by long-term trends in global temperature. `Single-forcing' iLME experiments isolating individual external climate forcings then shed light on the role of natural and anthropogenic influences on these patterns.