Phase shifts in precipitation isotope seasonality across East Asia track monsoon dynamics

Abstract

Oxygen isotope records from various East Asian geological archives are vital tools for reconstructing past monsoon variations, yet their climatic interpretations remain the subject of considerable debate. While seasonality is inherently linked to monsoon patterns, a comprehensive understanding of precipitation δ¹⁸O (δ¹⁸O_p) seasonality across East Asia and its connection to monsoon activities has remained elusive. We present an integrative analysis combining observation-based monthly δ¹⁸O_p maps, isotope-enabled model simulations, and multi-year daily observations. Our results reveal three distinct modes of δ¹⁸O_p seasonality across East Asia, closely linked to the seasonal march and retreat of the monsoon. Specifically, the western Pacific subtropical high (WPSH) serves as a primary source of isotopically enriched vapor due to suppressed convection in this high-pressure system. In conjunction with the northward progression of WPSH from spring to summer, the δ¹⁸O_p peak first appears in southern East Asia and subsequently shifts toward northern regions. In contrast, the southward retreat of WPSH in late summer and autumn leads to a secondary δ¹⁸O_p peak. The low δ¹⁸O_p values observed between these peaks reflect the progression of monsoon convection. These findings demonstrate that δ¹⁸O_p faithfully captures the seasonal dynamics of the East Asian Monsoon, offering a pathway to resolve ongoing debates regarding the interpretation of speleothem δ¹⁸O records. Furthermore, these insights highlight the potential of utilizing spatially distributed paleo δ¹⁸O_p records to uncover the timing and structure of past East Asian Monsoon variability.