Mid-Pliocene Atlantic Meridional Overturning Circulation simulated in PlioMIP2

Zhongshi Zhang,Xiangyu Li,Chuncheng Guo,Ning Tan,Gilles Ramstein,Daniel J Lunt,Qiong Zhang,Mark A Chandler,Alan M Haywood,Charles Williams,Stephen J Hunter,Gerrit Lohmann,Qiang Li,Christian Stepanek,Odd Helge Otterå,Camille Contoux,Ran Feng,Anna S Von Der Heydt,Deepak Chandan,Kerim H Nisancioglu,Linda E Sohl,Michiel Baatsen ,Esther C Brady ,Bette L Otto‐Bliesner ,Julia C Tindall ,W L Chan ,Julia E Weiffenbach ,W R Peltier ,Ayako Abe‐Ouchi

doi:10.5194/cp-17-529-2021

Abstract

Abstract. In the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2), coupled climate models have been used to simulate an interglacial climate during the mid-Piacenzian warm period (mPWP; 3.264 to 3.025 Ma). Here, we compare the Atlantic Meridional Overturning Circulation (AMOC), poleward ocean heat transport and sea surface warming in the Atlantic simulated with these models. In PlioMIP2, all models simulate an intensified mid-Pliocene AMOC. However, there is no consistent response in the simulated Atlantic ocean heat transport nor in the depth of the Atlantic overturning cell. The models show a large spread in the simulated AMOC maximum, the Atlantic ocean heat transport and the surface warming in the North Atlantic. Although a few models simulate a surface warming of ∼ 8–12 ∘C in the North Atlantic, similar to the reconstruction from Pliocene Research, Interpretation and Synoptic Mapping (PRISM) version 4, most models appear to underestimate this warming. The large model spread and model–data discrepancies in the PlioMIP2 ensemble do not support the hypothesis that an intensification of the AMOC, together with an increase in northward ocean heat transport, is the dominant mechanism for the mid-Pliocene warm climate over the North Atlantic.

Highlights

The mid-Piacenzian warm period was a recent period of sustained warmth in geological history, with the land–sea distribution, topography and levels of greenhouse gases being comparable to today (Dowsett et al, 2010, 2016; Haywood et al, 2010, 2016a)
Compared with the PlioMIP1 ensemble in which the Arctic gateways were kept open, all PlioMIP2 models forced with the PRISM4 reconstructions that consider the closed Arctic gateways simulate an intensification in the mid-Pliocene Atlantic Meridional Overturning Circulation (AMOC)
CCSM4, COSMOS, HadCM3, IPSL-CM5A-LR, MIROC4m and NorESM-L have all participated in both PlioMIP1 and Pliocene Modelling Intercomparison Project (PlioMIP) 2. These six models simulate an increase in the midPliocene AMOC maximum, which is larger in PlioMIP2 than in PlioMIP1, supporting the hypothesis that closed Arctic gateways are a requirement for the intensification of the mid-Pliocene AMOC

Summary

Introduction

The mid-Piacenzian warm period (mPWP; 3.264–3.025 Ma) was a recent period of sustained warmth in geological history, with the land–sea distribution, topography and levels of greenhouse gases being comparable to today (Dowsett et al, 2010, 2016; Haywood et al, 2010, 2016a). Because the PlioMIP1 simulations (Zhang et al, 2013a, b) did not support a stronger Pliocene AMOC (compared with the pre-industrial level) and an inferred enhancement of Atlantic northward ocean heat transport (OHT) suggested by proxies (Dowsett et al, 1992; Raymo et al, 1996), it was difficult to explain the reconstructed strong surface warming in the high-latitude North Atlantic during the mid-Pliocene. In the Arctic, the simulated annual mean SAT increases by 3.7–11.6 ◦C compared with the pre-industrial level, with a multi-model mean increase of 7.2 ◦C (de Nooijer et al, 2020). We investigate the simulated AMOC in PlioMIP2 in order to further address the question of whether an intensified AMOC and enhanced Atlantic OHT can explain the reconstructed North Atlantic–Arctic sea surface warming during the mPWP.

Introduction of models used in PlioMIP2

Simulated AMOC in PlioMIP2

Simulated Atlantic OHT in PlioMIP2

Simulated North Atlantic sea surface warming

Findings

Discussion and summary