Reorganization of the Northern Hemisphere westerlies at 41 Ma revealed by carbonate oxygen isotopes and precipitation data from the Tibetan Plateau

Abstract The Northern Hemisphere mid-latitude westerlies (NHMW) are a key component of the global climate system, and their modulation profoundly influences climate and precipitation patterns across the Northern Hemisphere. Previous studies have indicated that the uplift of the Tibetan Plateau could have triggered a reorganization of the NHMW, significantly altering hydroclimate in Asia and neighboring areas. However, the evidence supporting this link stems primarily from numerical models, lacking geological corroboration. Here, we present new evidence on this link based on middle to late Eocene carbonate oxygen isotope and pollen-based precipitation records from the Tibetan Plateau, as well as model simulations. Our results reveal a synchronous intensification of the southern branch of the NHMW but a weakening of the northern branch at ca. 41 Ma. This is indicated by a concurrent positive oxygen isotope (δ18O) shift and precipitation increase in the southeastern Tibetan Plateau, and a negative δ18O shift in the northeastern Tibetan Plateau. We suggest that full convergence of the Indian and Asian continents, and the resulting paleogeographic changes, may have catalyzed this NHMW reorganization, leading to an increased proportion of Tethys Ocean moisture to the southeastern Tibetan Plateau and a weakening of NHMW-derived recycled moisture to the northeastern Tibetan Plateau. Climate model simulations under varying topographic regimes further support the role of central Tibetan Plateau uplift in modulating the NHMW and influencing precipitation patterns in the southeastern and northeastern Tibetan Plateau regions.