Disentangling the role of two prominent climate forcing factors in the large decrease of temperatures since the Eocene : a pCO2 drawdown and the Tibetan Plateau uplift

Two prominent forcing factors occurring during the Cenozoic are the Indian Asian collision and the atmospheric drawdown of carbon dioxide concentration from 4 to 1 PAL. Both of them have been intensively studied, but only a few studies were devoted to disantengling them and to explore their impact on the meridional ocean circulation. Indeed, there are some interactions between these two factors and other important features occurred during this period, especially concerning the geometry of straits (Tan et al., GRL 2022). In this study, we simulate, with a coupled GCM model (CESM version 1.0.5), the response to both of these factors with idealized boundary conditions. Using four long-lasting simulations with two different values of pCO₂ (4 and 1 PAL) mixed with the presence or absence of TP, we demonstrate that the ocean heat transport in North Pacific and Atlantic ocean is differently impacted by the uplift of the TP. Such a response has been pointed out by Su et al., Climate of the Past 2018 and depicts a large increase of AMOC and decrease of PMOC from Eocene to present-day, but in this study, they only used a pCO₂ of 1 PAL.

This last feature was a severe limitation to compare these simulations to data. Moreover, the sea-ice response played an important role, which would be undoubtedly reduced at a CO₂ concentration of 4 PAL. In this new study, we disentangle the effect of the pCO₂ decrease from 4 to 1 PAL and the uplift of the Tibetan Plateau. We pin-point the important result that, even with 4 PAL CO₂, the Tibetan Plateau uplift led to major changes of the meridional ocean circulation, including pronounced differences in North Pacific and North Atlantic.

Moreover, our simulation with present-day TP and 1 PAL corresponding to the pre-industrial and the other extreme simulation, no TP and 4 PAL corresponding to the early Eocene, can be, therefore, compared to data, especially over the northern hemisphere, for which the Pacific and Atlantic ocean model response is largely different. Thanks to the availability of data over North Atlantic, it is possible to show that the simulated cooling is in agreement with these reconstructions using different marine proxies. In contrast, over North Pacific, not enough sea surface temperature reconstructions (SST) are yet available over 30°N to assess the SST cooling inferred by the simulation. (Hollis, GMD 2019 ;   Lunt et al., Climate of the Past 2021).

In summary, this study claims for more data in North Pacific during the early Eocene. More importantly, it pin-points the important role of the Tibetan Plateau uplift on building a modern circulation in North Atlantic.