Abstract
Abstract. The global and regional climate changed dramatically with the expansion of the Antarctic Ice Sheet at the Eocene–Oligocene transition (EOT). These large-scale changes are generally linked to declining atmospheric pCO2 levels and/or changes in Southern Ocean gateways such as the Drake Passage around this time. To better understand the Southern Hemisphere regional climatic changes and the impact of glaciation on the Earth's oceans and atmosphere at the EOT, we compiled a database of 10 ocean and 4 land-surface temperature reconstructions from a range of proxy records and compared this with a series of fully coupled, low-resolution climate model simulations from two models (HadCM3BL and FOAM). Regional patterns in the proxy records of temperature show that cooling across the EOT was less at high latitudes and greater at mid-latitudes. While certain climate model simulations show moderate–good performance at recreating the temperature patterns shown in the data before and after the EOT, in general the model simulations do not capture the absolute latitudinal temperature gradient shown by the data, being too cold, particularly at high latitudes. When taking into account the absolute temperature before and after the EOT, as well as the change in temperature across it, simulations with a closed Drake Passage before and after the EOT or with an opening of the Drake Passage across the EOT perform poorly, whereas simulations with a drop in atmospheric pCO2 in combination with ice growth generally perform better. This provides further support for previous research that changes in atmospheric pCO2 are more likely to have been the driver of the EOT climatic changes, as opposed to the opening of the Drake Passage.
Highlights
Global cooling and the significant expansion of glacial ice over Antarctica at the Eocene–Oligocene transition (EOT) ∼ 33.7 million years ago (Myr; Zachos et al, 2001; Coxall et al, 2005) would have potentially resulted in large but uncertain changes in the Southern Ocean and the climate of the high-latitude Southern Hemisphere (Bohaty et al, 2012; Passchier et al, 2013)
This suggests that the Drake Passage was open prior to the EOT and the late Eocene
The marked reduction in performance by HadCM3BL when the Drake Passage is either closed before and after the EOT or closed before but opens across the EOT supports the conclusions of Goldner et al (2014) that changes in ocean gateways around the EOT are not the best way to model the changes observed in the proxy record
Summary
Global cooling and the significant expansion of glacial ice over Antarctica at the Eocene–Oligocene transition (EOT) ∼ 33.7 million years ago (Myr; Zachos et al, 2001; Coxall et al, 2005) would have potentially resulted in large but uncertain changes in the Southern Ocean and the climate of the high-latitude Southern Hemisphere (Bohaty et al, 2012; Passchier et al, 2013). Numerous palaeoclimate modelling studies have shown that changes in Antarctic Ice Sheet extent, atmospheric pCO2 levels and palaeogeographic reconstruction around this period of the Earth’s history can all impact the modelled global and/or regional climate (Goldner et al, 2014; Knorr and Lohmann, 2014; Kennedy et al, 2015). All of these studies show some areas of warming in the Southern Ocean in response to the imposition of an Antarctic Ice Sheet in their models, but the different models find the warming to occur in different regions. Kennedy-Asser: Changes in the Southern Hemisphere through the EOT regions due to uncertainties in the model boundary conditions that could potentially be exaggerated due to incomplete model spin-up
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