Summer and Autumn Insolation as the Pacemaker of Surface Wind and Precipitation Dynamics Over Tropical Indian Ocean During the Holocene: Insights From Paleoproductivity Records and Paleoclimate Simulations

Insolation is the engine of monsoon and Walker circulations over the tropical Indian Ocean. Here, we present Holocene coccolith-related net primary productivity (NPP) signals from two sediment cores retrieved in the wind-driven coastal upwelling systems off southern India and southern Sumatra. Upwelling-induced NPP is enhanced during summer and autumn and is a powerful tool to reconstruct atmospheric features at a seasonal scale. Our records indicate that during summer and autumn, westerly winds off southern India strengthened from the early-Holocene (EH) to late-Holocene (LH), while southeasterly winds off southern Sumatra strengthened from the EH to mid-Holocene (MH) and weakened from the MH to LH. Comparisons with previous paleoclimate records and simulations, allow us to confirm such wind patterns at a regional scale and identify distinct atmospheric features associated to insolation before and after the MH. From the EH to MH, as the insolation in the Northern Hemisphere weakens during summer and strengthens during autumn, the equatorial Indian Ocean is characterized by more vigorous Walker and monsoon circulations in summer and autumn, respectively. From the MH to LH, as the insolation weakens in the Northern Hemisphere during summer and over the equator during autumn, the equatorial Indian Ocean is influenced by a general reinforcement of the Walker circulation during both seasons, a feature that we relate to a modern negative IOD-like mode. The changes in wind result in increasing precipitation over Indonesia and India from EH to MH and over Indonesia from MH to LH as India is getting dryer. The aim of this study is to examine how monsoon and Walker circulations from the tropical Indian Ocean responded to changing insolation over the Holocene. Understanding their dynamics under varying boundary conditions is important for predicting future climate, as they shape precipitation of a region that sustains billions of people. First, we focus on reconstructing changes in seasonal surface wind intensity, based on net coccolith-related primary productivity signal from two sediment cores retrieved in the coastal upwelling systems off southern India and off southern Sumatra. When these upwellings occur during summer and autumn, nutrient-rich deep waters surface and enhance the primary productivity. Second, we use paleoclimate records and simulations to provide broad-scale atmospheric changes and test our assumptions. Our results show that westerly winds off southern India strengthened from early to late Holocene, while southeasterly winds off southern Sumatra strengthened from early to mid-Holocene and weakened from mid to late Holocene, thus implying distinct wind and precipitation dynamics patterns before and after mid-Holocene. These features appear to result in weaker and stronger precipitation over Africa and Indonesia from early- to mid-Holocene, respectively, and in a precipitation maximum over India during the mid-Holocene. Stronger zonal Walker circulation during summer from early- to late-HoloceneStronger meridional monsoon circulation during autumn from early- to mid-HoloceneStronger (weaker) precipitation over Indonesia (Africa) from early- to mid-Holocene and precipitation maximum over India in mid-Holocene