Assessment of the southern polar and subpolar warming in the PMIP4 Last Interglacial simulations using paleoclimate data syntheses

Abstract. Given relatively abundant paleo proxies, the study of the Last Interglacial (LIG, ~129-116 thousand years ago, ka) is valuable to understanding natural variability and feedback in a warmer-than-preindustrial climate. The Paleoclimate Modelling Intercomparison Project Phase 4 (PMIP4) coordinated LIG model simulations which focus on 127 ka. Here we evaluate 12 PMIP4 127-ka Tier 1 model simulations against four recent paleoclimate syntheses of LIG sea and air temperatures and sea ice concentrations. The four syntheses include 99 reconstructions and show considerable variations, some but not all of which are attributable to the different sites included in each synthesis. All syntheses support the presence of a warmer Southern Ocean, with reduced sea ice, and a warmer Antarctica at 127 ka compared to the preindustrial. The PMIP4 127-ka Tier 1 simulations, forced solely by orbital parameters and greenhouse gas concentrations, do not capture the magnitude of this warming. Here we follow up on previous work that suggests the importance of preceding deglaciation meltwater release into the North Atlantic. We run a 3000-year 128-ka simulation using HadCM3 with a 0.25 Sv North Atlantic freshwater hosing, which approximates the PMIP4 127-ka Tier 2 H11 (Heinrich event 11) simulation. The hosed 128-ka HadCM3 simulation captures much of the warming and sea ice loss shown in the four data syntheses at 127 ka relative to preindustrial: south of 40° S, modelled annual sea surface temperature (SST) rises by 1.3±0.6 °C, while reconstructed average anomalies range from 2.2 °C to 2.7 °C; modelled summer SST increases by 1.1±0.7 °C, close to 1.2–2.2 °C reconstructed average anomalies; September sea ice area (SIA) reduces by 40 %, similar to reconstructed 40 % reduction of sea ice concentration (SIC); over the Antarctic ice sheet, modelled annual surface air temperature (SAT) increases by 2.6±0.4 °C, even larger than reconstructed average anomalies 2.2 °C. Our results suggest that the impacts of deglaciation ice sheet meltwater need to be considered to simulate the Southern Ocean and Antarctic changes at 127 ka.