Earlier Seasonal March of the East Asian Summer Monsoon in the Mid-Pliocene

Abstract

Abstract The mid-Pliocene (∼3.3–3 million years ago) has been considered as a potential analog for future climate, but the seasonal march of the East Asian summer monsoon (EASM)—a distinctive feature of the modern EASM characterized by a northward migration of the monsoon circulation and rainband from early to late summer—remains unclear in this regard. Here, based on model simulations mainly from the Pliocene Model Intercomparison Project Phase 2, we find that the seasonal march of the mid-Pliocene EASM is about two pentads earlier than today. It is accompanied by a northward shift of the monsoon circulation and rainband in July, resulting from both extratropical and tropical processes. For the extratropical path, greener vegetation in the midlatitudes enhances the surface warming, which reduces the meridional temperature gradient. The subsequent weakening and northward shift of the subtropical westerly jet lead to weaker zonal topographic Rossby waves, resulting in an anticyclone over East Asia. For the tropical path, exposure of the Sunda and Sahul shelves weakens the Walker circulation, which suppresses the Maritime Continent convection. The subsequent weakening of the local Hadley circulation increases precipitation in the tropical western North Pacific, which excites a Pacific–Japan meridional teleconnection with anticyclonic circulation over northern East Asia. These processes lead to a northward shift of the western Pacific subtropical high and hence the monsoon rainband. Our results unravel the dominant roles of vegetation and geography in the earlier seasonal march of the mid-Pliocene EASM, and its implications for potential future changes in the EASM’s seasonal march are discussed.