UC Berkeley

The Holocene East Asian summer monsoon (EASM) was previously characterized as a trend toward weaker monsoon intensity paced by orbital insolation. It is demonstrated here that this evolution is more accurately characterized as changes in the transition timing and duration of the EASM seasonal stages (spring, pre-mei-yu, mei-yu, midsummer), and tied to the north-south displacement of the westerlies relative to Tibet. To this end, time-slice simulations across the Holocene are employed using an atmospheric general circulation model. Self-organizing maps are used to objectively identify the transition timing and duration of the EASM seasonal stages. Compared to the late Holocene, an earlier onset of mei-yu and an earlier transition from mei-yu to midsummer in the early to mid-Holocene are found, resulting in a shortened mei-yu and prolonged midsummer stage. These changes are accompanied by an earlier northward positioning of the westerlies relative to Tibet. Invoking changes to seasonal transitions also provides a more satisfactory explanation for two key observations of Holocene East Asian climate: the "asynchronous Holocene optimum" and changes to dust emissions. A mechanism is proposed to explain the altered EASM seasonality in the simulated early to mid-Holocene. The insolation increase over the boreal summer reduces the pole-equator temperature gradient, leading to northward-shifted and weakened westerlies. The meridional position of the westerlies relative to the Tibetan Plateau determines the onset of mei-yu and possibly the onset of the midsummer stage. The northward shift in the westerlies triggers earlier seasonal rainfall transitions and, in particular, a shorter mei-yu and longer midsummer stage.