Abstract
Abstract. In the Pliocene Model Intercomparison Project (PlioMIP), eight state-of-the-art coupled climate models have simulated the mid-Pliocene warm period (mPWP, 3.264 to 3.025 Ma). Here, we compare the Atlantic Meridional Overturning Circulation (AMOC), northward ocean heat transport and ocean stratification simulated with these models. None of the models participating in PlioMIP simulates a strong mid-Pliocene AMOC as suggested by earlier proxy studies. Rather, there is no consistent increase in AMOC maximum among the PlioMIP models. The only consistent change in AMOC is a shoaling of the overturning cell in the Atlantic, and a reduced influence of North Atlantic Deep Water (NADW) at depth in the basin. Furthermore, the simulated mid-Pliocene Atlantic northward heat transport is similar to the pre-industrial. These simulations demonstrate that the reconstructed high-latitude mid-Pliocene warming can not be explained as a direct response to an intensification of AMOC and concomitant increase in northward ocean heat transport by the Atlantic.
Highlights
The mid-Pliocene warm period is the most recent geological period in the past with global temperatures ∼ 2–3 ◦C warmer than present (Haywood and Valdes, 2004), corresponding to atmospheric greenhouse gas levels significantly above preindustrial levels (Seki et al, 2010)
All pre-industrial experiments simulate similar Atlantic salinity structures (Fig. 5) characterized by salty North Atlantic Deep Water (NADW) water produced between 20◦ N and 60◦ N in the North Atlantic; relatively fresh Antarctic Intermediate Water extending from the Southern Ocean northward into the South Atlantic at a depth of about 1000m; and Antarctic Bottom Water is produced in the Southern Ocean and fills the abyssal Atlantic Ocean
Most models simulate a shoaling of the Atlantic Meridional Overturning Circulation (AMOC) cell in the mid-Pliocene experiments
Summary
The mid-Pliocene warm period (mPWP; 3.264 to 3.025 Ma, Dowsett et al, 2012) is the most recent geological period in the past with global temperatures ∼ 2–3 ◦C warmer than present (Haywood and Valdes, 2004), corresponding to atmospheric greenhouse gas levels significantly above preindustrial levels (Seki et al, 2010). Since the mid-Pliocene palaeoenvironmental reconstructions were released (Dowsett et al, 1992, 1994, 1996, 1999), an increasing number of modelling studies have focused on understanding the mPWP climate (Chandler et al, 1994; Sloan et al, 1996; Haywood et al, 2000; Haywood and Valdes, 2004; Jiang et al, 2005; Yan et al, 2011) These models can account for the general pattern of warming in the reconstructed mPWP sea surface temperature (SST), there is no consistent pattern of a strong AMOC in the experiments (Haywood and Valdes, 2004; Yan et al, 2011). The paper is structured as follows: Sect. 2 reviews geological reconstructions of ocean circulation and climate during the mid-Pliocene; Sect. 3 compares the simulated climatological means of AMOC, northward ocean heat transport, and ocean stratification in the Atlantic; Sect. 4 discusses the model results in relation to the proxy data, and contains a summary
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