Resilience and implications of an Antarctic monsoon during the Eocene

High latitude warmth during the Eocene greenhouse climate has posed many challenges for climate modelling studies. Recent improvements in both the proxy records and model simulations are bringing these closer together, particularly regarding the meridional temperature gradient. Yet, it remains difficult to understand the climatic conditions around the greenhouse-icehouse transition which involved the glaciation of Antarctica. How can we explain indications of ice near the Antarctic coast well before the transition, especially since Antarctic glaciation is thought to express strong hysteresis? How did Antarctica remain mostly ice-free and vegetated through large climatic swings during the Eocene? If Antarctic warmth was so resilient, which process was responsible for its eventual demise?

We consider a set of existing climate simulations for the middle-to-late Eocene (42-34Ma) using the CESM model (Baatsen et al. 2020)¹. The original set of simulations was expanded to include possible scenarios of orbital forcing, atmospheric composition, and the continental geometry. In addition, we look at the output from DeepMIP simulations for the early Eocene. Using these results, we make a detailed study of the Antarctic climate and find that most of the continent saw monsoonal conditions during the Eocene. Only a small corridor near the coast experienced perennially mild and wet conditions, explaining the presence of temperate to paratropical vegetation. Further inland, we see a rapid increase in temperature seasonality along with the appearance of a summer monsoon. Summertime warmth made most of the Antarctic continent a hostile place for any significant ice growth. Meanwhile, mountainous regions near the coast were suitable candidates for the formation of ice caps that may have grown substantially during cooler intervals.

Our simulations can explain seemingly contradictory indications from proxy records, as well as strong regional variations in the Antarctic climate. The monsoonal nature of this climate during the Eocene proves to be particularly resilient to the changes in external forcing considered here. Identifying the potential mechanism to break up the monsoonal regime and eventually lead to Antarctic glaciation remains the subject of ongoing work.

1. Baatsen, M., von der Heydt, A. S., Huber, M., Kliphuis, M. A., Bijl, P. K., Sluijs, A., & Dijkstra, H. A. (2020). The middle to late Eocene greenhouse climate modelled using the CESM 1.0.5. Climate of the Past, 16(6), 2573–2597. doi: 10.5194/cp-16-2573-2020.