Impacts of Mid‐Pliocene Ice Sheets and Vegetation on Afro‐Asian Summer Monsoon Rainfall Revealed by EC‐Earth Simulations.

The impact of mid‐Pliocene boundary conditions on Afro‐Asian summer monsoon (AfroASM) rainfall is examined using the fully coupled Earth System Model EC‐Earth3‐LR. Our focus lies on the effects of varying CO2 concentration, diminished ice sheets and vegetation dynamics. We find that the enhanced AfroASM rainfall is predominantly caused by the "warmer‐gets‐wetter" mechanism due to elevated CO2 levels. Additionally, the ice sheet, similar in size to that of the mid‐Pliocene era, creates several indirect effects. These include sea ice‐albedo feedback and inter‐hemispheric atmosphere energy transport. Such influences result in the southward shift of Hadley circulation and formation of Pacific‐Japan pattern, leading to reduced rainfall in North African and South Asian monsoon regions but increased rainfall in East Asian monsoon region. Interestingly, while dynamic vegetation feedback has a minimal direct effect on AfroASM rainfall, it significantly influences rainfall in the mid‐high latitudes of the North Hemisphere by enhancing water vapor feedback. Plain Language Summary: The mid‐Pliocene (3.3–3.0 Ma) was a pre‐Quaternary warming period with similar high CO2 level as we have today. The period is often considered as an analog for future greenhouse driven climate projections. Previous studies indicate that the Afro‐Asian summer monsoon (AfroASM) rainfall was more abundant then. However, the underlying mechanisms of relative contributions of boundary conditions on increased AfroASM rainfall remains to be clarified. Here, we use an Earth System Model, performed nine simulations to study the separate effects of CO2 level, ice sheets size, and vegetation dynamics on AfroASM rainfall patterns during the mid‐Pliocene. We found that (a) A rise in CO2 concentration during the mid‐Pliocene enhanced AfroASM rainfall through "warmer‐gets‐wetter" mechanism. (b) The dynamics of the ice sheet during this period, including both melting and distribution changes, contributed to more pronounced warming in South Hemisphere than the north due to ice‐albedo feedback. This change affected global atmospheric circulation, leading to decreased rainfall in North Africa and South Asia, but increased rainfall in East Asia. (c) We observed that during the mid‐Pliocene, vegetation expanded and move further north in the North Hemisphere. This increases summer rainfall at Northern mid‐high latitudes through local water vapor feedback. Key Points: Elevated CO2 levels during mid‐Pliocene increase Afro‐Asian summer monsoon rainfall mainly due to the "warmer‐gets‐wetter" mechanismMid‐Pliocene ice sheets changes favor to reduce North African/South Asian monsoon rainfall and increase East Asian monsoon rainfallDynamic vegetation feedback has less effect on Afro‐Asian monsoon, but more on mid‐high latitudes rainfall [ABSTRACT FROM AUTHOR]