Early-Holocene South Atlantic thermal structure and the control of South American Monsoons.

The early Holocene is characterized by the final collapse of North American ice sheets, thus reorganizing the freshwater budget in the North Atlantic. The increasing summer insolation is thought to accelerate Laurentide Ice Sheet melting, resulting in the freshwater outburst of the Lake Agassiz and rerouting of Canadian basin discharge. Those early-Holocene meltwater events are linked to shifts in South American monsoonal system, through a change in the South Atlantic Subtropical Dipole (SASD). Here, we propose a mechanism by which freshwater addition to the North Atlantic strengthens the South American monsoons through a shift of the SASD in early Holocene. Using Ekman dynamics and a 1.5 layer model approach, we derived the wind effect on the South Atlantic isothermal displacement in two early Holocene (9-7ka) simulations: Simulation of Transient Climate Evolution over the last 21,000 years (TraCE-21ka) and the Community Climate System Model Version 3.0 (CCSM3) run for the 8.2 ka. The South Atlantic Subtropical Dipole (SASD) index was used in order to evaluate the state of the monsoonal system and the AMOC index used to check the state of the bipolar seesaw. In both simulations, as AMOC index decays, i.e. overturning slows down, heat builds up in the South Atlantic and the southern trade winds system weakens. The 1.5 Layer isothermal displacement results show that weak trade winds push isotherms upwards, with highest isothermal displacement of up to 20m in western South Atlantic. The wind-driven isothermal uplift then disperse heat from west towards the eastern South Atlantic, pushing SASD towards negative values.

Hence, we propose that while AMOC decelerates, the weakening of southern hemisphere trade winds displaces the South Atlantic isothermal structure, and concentrates heat in northeast South Atlantic. Finally, a warmer northeast South Atlantic enhances evaporation and increase continental moisture delivery, strengthening the South American monsoons.