Seasonal evolution differences of east Asian summer monsoon precipitation between Bølling-Allerød and younger Dryas periods

The rapid warming trend during the last deglaciation triggered significant global climate instabilities due to a complex non-linear response of the climate system to the gradual increase in insolation over the northern hemisphere. Although climate impacts can be detected globally, major regional imprints such as seasonal evolution and changes of the East Asian Summer Monsoon (EASM) during the last deglaciation are still poorly constrained due to a lack of comprehensive proxy data. In this study, we compare how the extreme climate shifts are linked to changes in EASM precipitation over China between the unusually warm Bølling-Allerød (BA) interstadial and the following strong cooling of the Younger Dryas (YD) stadial. Our analysis is based on the transient atmosphere-ocean simulations of TraCE-21ka, in addition to new results from high-resolution simulations of the CESM1 model for the BA and YD time slices. We find that the earlier onset and stronger intensity of the EASM in the BA interstadial lead to more precipitation in early summer (May–June) but drier conditions during mid-summer (July–August) over Southern China compared to a stadial climate during the YD episode. For Northern China, we find the opposite response. The insolation change in spring and the forced response of the atmospheric system are thought to be responsible for these differences. Relative to the YD episode, the hemispheric temperature gradient during the BA period is enhanced due to the asymmetric warming between the two hemispheres, leading to an intensified northward equatorial cross flow. Combined with a stronger sensible heating of the Tibetan Plateau in spring and the related earlier northward shift of the westerly jet, the early onset of the EASM is triggered. The latent heat release, which is accompanied by the onset of the EASM and the sudden increasing precipitation over Southern China in early summer, contributes to the westward shift of the Western Pacific Subtropical High (WPSH) and eastward movement of the South Asia High (SAH) in mid-summer. Under the above conditions, Southern China experiences a hot and dry climate, while Northern China receives more precipitation. Additionally, the La Niña-like pattern of the equatorial Pacific also partly contributes to the strong EASM in the warm period by influencing the WPSH location and Pacific-North American (PNA) teleconnection pattern.