Abstract
Abstract. The last interglacial, also known as the Eemian, is characterized by warmer than present conditions at high latitudes. This is implied by various Eemian proxy records as well as by climate model simulations, though the models mostly underestimate the warming with respect to proxies. Simulations of Eemian surface air temperatures (SAT) in the Northern Hemisphere extratropics further show large variations between different climate models, and it has been hypothesized that this model spread relates to diverse representations of the Eemian sea ice cover. Here we use versions 3 and 4 of the Community Climate System Model (CCSM3 and CCSM4) to highlight the crucial role of sea ice and sea surface temperatures changes for the Eemian climate, in particular in the North Atlantic sector and in Greenland. A substantial reduction in sea ice cover results in an amplified atmospheric warming and thus a better agreement with Eemian proxy records. Sensitivity experiments with idealized lower boundary conditions reveal that warming over Greenland is mostly due to a sea ice retreat in the Nordic Seas. In contrast, sea ice changes in the Labrador Sea have a limited local impact. Changes in sea ice cover in either region are transferred to the overlying atmosphere through anomalous surface energy fluxes. The large-scale spread of the warming resulting from a Nordic Seas sea ice retreat is mostly explained by anomalous heat advection rather than by radiation or condensation processes. In addition, the sea ice perturbations lead to changes in the hydrological cycle. Our results consequently imply that both temperature and snow accumulation records from Greenland ice cores are sensitive to sea ice changes in the Nordic Seas but insensitive to sea ice changes in the Labrador Sea. Moreover, the simulations suggest that the uncertainty in the Eemian sea ice cover accounts for 1.6 °C of the Eemian warming at the NEEM ice core site. The estimated Eemian warming of 5 °C above present day based on the NEEM δ15N record can be reconstructed by the CCSM4 model for the scenario of a substantial sea ice retreat in the Nordic Seas combined with a reduced Greenland ice sheet.
Highlights
The last interglacial, known as the Eemian, is often regarded as a possible analogue for future climate as it stands for the most recent period in the past characterized by a warmer than present-day climate
The goals of the study are as follows: (i) quantifying the atmospheric warming in and around Greenland related to uncertainty in the Eemian sea ice cover, (ii) determining whether a sea ice retreat in a particular region leads to a temperature signal recorded in Greenland ice cores such as NEEM, and (iii) understanding the key processes that link the climate in Greenland with the sea ice in adjacent areas
We show for the CCSM3 model that differences in the simulation of the lower boundary conditions explain most of the spread with respect to the EEM-PI atmospheric warming in the North Atlantic sector including Greenland
Summary
The last interglacial (ca. 129–116 ka), known as the Eemian, is often regarded as a possible analogue for future climate as it stands for the most recent period in the past characterized by a warmer than present-day climate. A warmer than present Eemian climate has been observed in various proxy records (CAPE Last Interglacial Project Members, 2006; Turney and Jones, 2010; Capron et al, 2014) and simulated in climate model experiments (e.g., Bakker et al, 2013; Nikolova et al, 2013; Lunt et al, 2013; Merz et al, 2014a). The models mostly fail to simulate a warming for winter, instead generating lower temperatures due to the decrease in winter insolation (Lunt et al, 2013). This leads to a disagreement between models and proxies in annual mean
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