Mid-Holocene reductions of equatorial Atlantic Ocean variability and linkages to high-latitude Southern Atlantic forcing
Speaker: Dr. Gerald T. Rustic, Rowan University
Title: Mid-Holocene reductions of equatorial Atlantic Ocean variability and linkages to high-latitude Southern Atlantic forcing
Host: Xiaojing Du
Time: Wednesday, Oct 29, 1:30-2:30 PM
Location: Horizon Hall, Room 4014 (Email xdu5@gmu.edu for Zoom link)
Abstract: The mid-Holocene witnessed large-scale climatic changes throughout the global tropics. Ocean sediment records of dust and plant matter from off the African coast document the ending of the African Humid period and tropical Pacific proxy records show reduced variability of the El Niño Southern Oscillation (ENSO) and associated hydroclimate change. Is it possible that these phenomena are connected, and if so, through what mechanisms? A vital but previously unexplored component of the tropical climate system is the Atlantic Zonal Mode, or “Atlantic Niño”, which is most strongly expressed in the eastern equatorial Atlantic and closely tied to African hydroclimate. We investigate the Holocene history of the eastern tropical Atlantic by reconstructing the Atlantic Niño variability through geochemical analysis of individual foraminifera from tropical Atlantic Ocean sediments. Eastern tropical Atlantic variability is reduced during the mid-Holocene suggesting dynamical alterations of tropical Atlantic circulation, mirroring ENSO changes observed in the Pacific. We link these alterations to changes across the tropics to uncover a reorganization of the Walker circulation at the mid-Holocene. We compare these results to the output of a Holocene transient climate model simulation that includes vegetative and dust feedbacks. These model results closely align with our findings of reduced tropical Atlantic variability and support its interpretation as change to the Atlantic Niño. We find indications of local forcing involving the southward migration of the West African Monsoon, and also find linkages between tropical Atlantic variability and the wind fields and currents in the Southern Ocean. Taken together, the reconstructed reduction in eastern tropical Atlantic variability may be linked not only to conditions across northern Africa, but also to the southern high-latitudes. We find that the southern high latitude changes lead those in the tropical Atlantic. These findings implicate the Atlantic Ocean, and high-latitude forcing, as key players in the mid-Holocene reorganization of tropical Walker circulation.
