Biological and Environmental Research - Earth and Environmental System Sciences
Earth and Environmental System Modeling
30 July 2020

Enriched East Asian Oxygen Isotope of Precipitation Indicates Reduced Summer Seasonality in Regional Climate and Westerlies


We analyze the climate changes associated with variation to the oxygen isotopic content of rainfall over East Asia, using a global climate model with water isotopes incorporated in it that is shown to accurately reproduce the seasonal cycle of both the atmospheric circulation and rainfall isotopes in that region.  Our results show unambiguously that years with enriched oxygen isotopes of rainfall across East Asia exhibit reduced summer seasonality across various climatic measures over that region; and vice versa for depleted years. In other words, peak summer is more muted in years when the rainfall isotope is enriched. Of particular note are the westerlies across Asia that seasonally migrate northwards across the Tibetan Plateau from winter to summer; in enriched years, the migration does not extend as far northwards.  


Our study confirms the role of the seasonality of the jet stream in precipitation variations in East Asia.  Our results provide a straightforward climate interpretation of fluctuations in the cave isotopic records of East Asian climate, that of reduced summer seasonality for enriched periods.    Moreover, our results point to the changing migration of the westerlies across the Tibetan Plateau as the root cause of the paleoclimate changes. The role of the westerly migration in the East Asian summer seasonality is emerging knowledge within contemporary studies of East Asian monsoon dynamics.  Our result thus links two disparate threads of East Asian climate research, the one being paleoclimate changes and the other being the modern-day climate dynamics.       


Cave records of East Asian paleoclimate record variations provide a window into how the climate of that region changes.  However, there is an outstanding question as to what the oxygen isotope variations of the records actually mean in terms of the regional climate changes.  By analyzing a model that simulates both the atmospheric circulation and water isotopes with fidelity, we show that there is a straightforward interpretation: enriched periods indicate reduced summer seasonality in both the atmospheric circulation and climate.  A pronounced circulation feature that is strongly tied to isotopes are the westerlies across the Tibetan Plateau: in enriched years, the westerlies do not migrate as far northwards across the Plateau. Our results points to how East Asian paleoclimate changes are manifested spatially and temporally and furthermore implicates the westerlies as the direct cause of said changes.  

John Chiang
University of California - Berkeley
Chiang, J, M Herman, K Yoshimura, and I Fung.  2020.  "Enriched East Asian Oxygen Isotope of Precipitation Indicates Reduced Summer Seasonality in Regional Climate and Westerlies."  Proceedings of the National Academy of Sciences 117(26): 14745-14750.