Contribution of the coupled atmosphere–ocean–sea ice–vegetation model COSMOS to the PlioMIP2

Abstract

Abstract. We present the Alfred Wegener Institute's contribution to the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) wherein we employ the Community Earth System Models (COSMOS) that include a dynamic vegetation scheme. This work builds on our contribution to Phase 1 of the Pliocene Model Intercomparison Project (PlioMIP1) wherein we employed the same model without dynamic vegetation. Our input to the PlioMIP2 special issue of Climate of the Past is twofold. In an accompanying paper we compare results derived with COSMOS in the framework of PlioMIP2 and PlioMIP1. With this paper we present details of our contribution with COSMOS to PlioMIP2. We provide a description of the model and of methods employed to transfer reconstructed mid-Pliocene geography, as provided by the Pliocene Reconstruction and Synoptic Mapping Initiative Phase 4 (PRISM4), to model boundary conditions. We describe the spin-up procedure for creating the COSMOS PlioMIP2 simulation ensemble and present large-scale climate patterns of the COSMOS PlioMIP2 mid-Pliocene core simulation. Furthermore, we quantify the contribution of individual components of PRISM4 boundary conditions to characteristics of simulated mid-Pliocene climate and discuss implications for anthropogenic warming. When exposed to PRISM4 boundary conditions, COSMOS provides insight into a mid-Pliocene climate that is characterised by increased rainfall (+0.17 mm d−1) and elevated surface temperature (+3.37 ∘C) in comparison to the pre-industrial (PI). About two-thirds of the mid-Pliocene core temperature anomaly can be directly attributed to carbon dioxide that is elevated with respect to PI. The contribution of topography and ice sheets to mid-Pliocene warmth is much smaller in contrast – about one-quarter and one-eighth, respectively, and nonlinearities are negligible. The simulated mid-Pliocene climate comprises pronounced polar amplification, a reduced meridional temperature gradient, a northwards-shifted tropical rain belt, an Arctic Ocean that is nearly free of sea ice during boreal summer, and muted seasonality at Northern Hemisphere high latitudes. Simulated mid-Pliocene precipitation patterns are defined by both carbon dioxide and PRISM4 paleogeography. Our COSMOS simulations confirm long-standing characteristics of the mid-Pliocene Earth system, among these increased meridional volume transport in the Atlantic Ocean, an extended and intensified equatorial warm pool, and pronounced poleward expansion of vegetation cover. By means of a comparison of our results to a reconstruction of the sea surface temperature (SST) of the mid-Pliocene we find that COSMOS reproduces reconstructed SST best if exposed to a carbon dioxide concentration of 400 ppmv. In the Atlantic to Arctic Ocean the simulated mid-Pliocene core climate state is too cold in comparison to the SST reconstruction. The discord can be mitigated to some extent by increasing carbon dioxide that causes increased mismatch between the model and reconstruction in other regions.

Mediterranean heat injection to the North Atlantic delayed the intensification of Northern Hemisphere glaciations

“…Model results show that during KM5c the Mediterranean experienced 2-3°C warmer temperatures, shifting its hydrological balance into greater net loss of freshwater 46 . This is a similar climate development as predicted for the Mediterranean region until the year 2100 47,48 . Consequently, the lack of freshwater influx to the Mediterranean during KM5c probably boosted its salinities and with it MOW production (Fig.…”

Strengthening Atlantic Inflow Across the Mid‐Pleistocene Transition

“…The ocean model MPI‐OM (Marsland et al., 2003), including sea‐ice dynamics formulated using viscous‐plastic rheology (Hibler III, 1979), has a resolution of GR30 (3° × 1.8°) in the horizontal, with 40 uneven vertical layers. COSMOS has previously been used to investigate a range of paleoclimate phenomena, including the last millennium (Jungclaus et al., 2010), the Miocene warm climate (Hossain et al., 2020; Knorr & Lohmann, 2014), the Pliocene (Stepanek et al., 2020), internal variability of the climate system (Wei et al., 2012), Holocene variability (Wei & Lohmann, 2012), the Last Glacial Maximum (LGM) climate (Abelmann et al., 2015; Zhang et al., 2013) and glacial millennial‐scale variability (Gong et al., 2013; Maier et al., 2018; Zhang et al., 2014, 2017). This indicates that it is capable of capturing key features of different climate states and is thus a suitable climate model for this study.…”

Mid-Pliocene West African Monsoon rainfall as simulated in the PlioMIP2 ensemble

“…The mPWP is often used as an analogue for near-future climate change due to its similar-to-modern paleogeography and high concentrations of CO 2 in the atmosphere (Corvec and Fletcher, 2017;Dowsett et al, 2013;Sun et al, 2013), and both marine and terrestrial proxy reconstructions indicate a climate with higher sea surface and surface air temperatures than present (Dowsett et al, 2013;Salzmann et al, 2008). A model-data comparison using PlioMIP1 indicated that the models underestimated the high-latitude warming by up to 15 • C, while overestimating the low-latitude temperatures by 1-6 • C (Dowsett et al, 2013;Haywood et al, 2013;Salzmann et al, 2013).…”