Abstract
Abstract. The Hadley Centre Global Environment Model version 3 (HadGEM3) is the first coupled climate model to simulate an ice-free Arctic during the Last Interglacial (LIG), 127 000 years ago. This simulation appears to yield accurate Arctic surface temperatures during the summer season. Here, we investigate the causes and impacts of this extreme simulated ice loss. We find that the summer ice melt was predominantly driven by thermodynamic processes: atmospheric and ocean circulation changes did not significantly contribute to the ice loss. We demonstrate these thermodynamic processes were significantly impacted by melt ponds, which formed on average 8 d earlier during the LIG than during the pre-industrial control (PI) simulation. This relatively small difference significantly changed the LIG surface energy balance and impacted the albedo feedback. Compared to the PI simulation: in mid-June, of the absorbed flux at the surface over ice-covered cells (sea-ice concentration > 0.15), ponds accounted for 45 %–50 %, open water 35 %–45 %, and bare ice and snow 5 %–10 %. We show that the simulated ice loss led to large Arctic sea surface salinity and temperature changes. The sea surface temperature and salinity signals we identify here provide a means to verify, in marine observations, if and when an ice-free Arctic occurred during the LIG. Strong LIG correlations between spring melt pond and summer ice area indicate that, as Arctic ice continues to thin in future, the spring melt pond area will likely become an increasingly reliable predictor of the September sea-ice area. Finally, we note that models with explicitly modelled melt ponds seem to simulate particularly low LIG sea-ice area. These results show that models with explicit (as opposed to parameterised) melt ponds can simulate very different sea-ice behaviour under forcings other than the present day. This is of concern for future projections of sea-ice loss.
Highlights
Interglacials are periods of globally higher temperatures which occur between cold glacial periods (Sime et al, 2009; Otto-Bliesner et al, 2013; Fischer et al, 2018)
We examine the HADGEM3-simulated Last Interglacial (LIG) sea ice and upper ocean, in order to identify factors that contributed to LIG summer sea-ice loss
We have identified the key thermodynamic processes that led to the ice-free summer LIG Arctic as albedo feedbacks triggered by the radiative forcing
Summary
Interglacials are periods of globally higher temperatures which occur between cold glacial periods (Sime et al, 2009; Otto-Bliesner et al, 2013; Fischer et al, 2018). At high latitudes orbital forcing led to summertime top-of-atmosphere short-wave (TOA SW) radiation 60–75 Wm−2 greater for the LIG, compared to the pre-industrial (PI) period (Kageyama et al, 2021; Guarino et al, 2020b). This drove differences between the LIG and PI surface energy balance. Whilst the significance of these PI to LIG surface energy balance differences vary between climate models (Lunt et al, 2013; Otto-Bliesner et al, 2013), proxy records of the LIG indicate that mean Arctic summer land temperature was +4–5 K higher than the PI (CAPE members, 2006)
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