The crucial role of vegetation cover in shaping the dipole pattern of East Asian summer monsoon changes during the late Pliocene warm period

Abstract

The late Pliocene warm period is the most recent geological warm period with atmospheric CO₂ concentrations similar to today's (∼405 ppm) but higher than the modern reference pre-industrial period (280 ppm), featured distinct vegetation cover. Both CO₂ concentrations and vegetation cover influence global and regional climate, but their relative roles in the East Asian summer monsoon (EASM) are not well understood. In this study, we assess the impacts of vegetation change and CO₂ enhancement on the EASM during the late Pliocene through sensitivity experiments. Compared to the pre-industrial period, the EASM intensified with a meridional dipole pattern in summer precipitation change over East China during the late Pliocene. Changes in vegetation cover played a major role in shaping the dipole pattern of EASM variation, while elevated CO₂ alone played a minor role. The late Pliocene vegetation caused a dipole pattern in surface albedo changes, increasing at 50–65°N and decreasing at 30–50°N, thereby altering the regional thermal structure across the Asian continent. As a result, tropospheric warming was more pronounced at mid-latitudes, primarily driven by albedo feedback. The vegetation-induced adjustment of thermal structure over East Asia enhanced the summer monsoon and increased northward water vapor transport through diabatic heating modification. This process likely contributed to the dipole pattern of summer precipitation over East Asia, influenced by the secondary circulation due to the poleward shift of the westerlies. Our experiments highlight the crucial role of vegetation in EASM variation pattern during the late Pliocene and underscore the importance of the vegetation feedback in a future warming world.