Abstract
Abstract. The Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) is an international collaboration to simulate the climate of the mid-Pliocene interglacial, corresponding to marine isotope stage KM5c (3.205 Mya), using a wide selection of climate models with the objective of understanding the nature of the warming that is known to have occurred during the broader mid-Pliocene warm period. PlioMIP2 builds on the successes of PlioMIP by shifting the focus to a specific interglacial and using a revised set of geographic and orbital boundary conditions. In this paper, we present the details of the mid-Pliocene simulations that we have performed with a slightly modified version of the Community Climate System Model version 4 (CCSM4) and the enhanced variant of the PlioMIP2 boundary conditions. We discuss the simulated climatology through comparisons to our control simulations and to proxy reconstructions of the mid-Pliocene climate. With the new boundary conditions, the University of Toronto version of the CCSM4 model simulates a mid-Pliocene that is more than twice as warm as that with the boundary conditions used for PlioMIP Phase 1. The warming is more enhanced near the high latitudes, which is where most of the changes to the PlioMIP2 boundary conditions have been made. The elevated warming in the high latitudes leads to a better match between the simulated climatology and proxy-based reconstructions than possible with the previous version of the boundary conditions.
Highlights
The mid-Pliocene warm period, 3.3–3 million years ago, was the most recent time period during which the global temperature was higher than present for an interval of time longer than any of the Pleistocene interglacials
These climatologies represent the average over 30 model years, which is the required averaging duration agreed upon at the PlioMIP2 workshop in Leeds in 2016
A comparison between our midPliocene simulations and our control simulations is not the same as comparing two simulations with identical boundary conditions save for the differences concerning the Bering Strait, as Hu et al (2015) have done, our analysis suggests that their results regarding the impact of the Bering Strait closure on the North Pacific oceanic heat transport (OHT) is a robust response by the climate system that persists even when there are other differences in boundary conditions
Summary
The mid-Pliocene warm period, 3.3–3 million years ago, was the most recent time period during which the global temperature was higher than present for an interval of time longer than any of the Pleistocene interglacials. The prevalence of widespread warming during this time has been inferred from proxy-based sea surface temperature (SST) reconstructions from a number of widely distributed deepsea sedimentary cores (Robinson et al, 2008; Lawrence et al, 2009; Dowsett et al, 2010; Fedorov et al, 2012). Surface air temperature (SAT) estimates are more difficult to reconstruct owing to the limited availability of land-based proxies that are useful for palaeothermometry. Rybczynski et al (2013) found mid-Pliocene plant fossils in peat deposits on Ellesmere Island (north of the Arctic Circle in the Queen Elizabeth archipelago), which led them to estimate that the local mean annual SAT during the mid-Pliocene was 18.3 ± 4.1 ◦C warmer than present, while summer temperature hovered around 14 ◦C. Some records available from high latitudes in the Northern Hemisphere provide an important perspective on the magnitude of warming in that region. Rybczynski et al (2013) found mid-Pliocene plant fossils in peat deposits on Ellesmere Island (north of the Arctic Circle in the Queen Elizabeth archipelago), which led them to estimate that the local mean annual SAT during the mid-Pliocene was 18.3 ± 4.1 ◦C warmer than present, while summer temperature hovered around 14 ◦C. Brigham-Grette et al (2013) studied lacustrine records from the arctic lake El’gygytgyn in NE arctic Russia on the basis of which they inferred that the summer temperature there was ∼ 8 ◦C warmer than today
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