Dependence of Eemian Greenland temperature reconstructions on the ice sheet topography

Abstract

Abstract. The influence of a reduced Greenland Ice Sheet (GrIS) on Greenland's surface climate during the Eemian interglacial is studied using a set of simulations with different GrIS realizations performed with a comprehensive climate model. We find a distinct impact of changes in the GrIS topography on Greenland's surface air temperatures (SAT) even when correcting for changes in surface elevation, which influences SAT through the lapse rate effect. The resulting lapse-rate-corrected SAT anomalies are thermodynamically driven by changes in the local surface energy balance rather than dynamically caused through anomalous advection of warm/cold air masses. The large-scale circulation is indeed very stable among all sensitivity experiments and the Northern Hemisphere (NH) flow pattern does not depend on Greenland's topography in the Eemian. In contrast, Greenland's surface energy balance is clearly influenced by changes in the GrIS topography and this impact is seasonally diverse. In winter, the variable reacting strongest to changes in the topography is the sensible heat flux (SHF). The reason is its dependence on surface winds, which themselves are controlled to a large extent by the shape of the GrIS. Hence, regions where a receding GrIS causes higher surface wind velocities also experience anomalous warming through SHF. Vice-versa, regions that become flat and ice-free are characterized by low wind speeds, low SHF, and anomalous low winter temperatures. In summer, we find surface warming induced by a decrease in surface albedo in deglaciated areas and regions which experience surface melting. The Eemian temperature records derived from Greenland proxies, thus, likely include a temperature signal arising from changes in the GrIS topography. For the Eemian ice found in the NEEM core, our model suggests that up to 3.1 • C of the annual mean Eemian warming can be attributed to these topographyrelated processes and hence is not necessarily linked to largescale climate variations.

Impact of ice sheet meltwater fluxes on the climate evolution at the onset of the Last Interglacial

“…Such scaling is in line with recent studies (e.g. van de Berg et al, 2013;Merz et al, 2014;Sjolte and Hoffmann, 2014;Steen-Larsen et al, 2014) that put in question the high temperature of the central estimate reconstructed from the NEEM record. Precipitation rates for ice sheet forcing vary percentage-wise as a function of the δ 18 O record.…”

The last interglacial climate: comparing direct and indirect impacts of insolation changes

“…Pfeiffer and Lohmann (2016) illustrate how a reduction of GrIS during LIG causes a strong local warming and weaker, more widespread regional warming, while ocean circulation changes drive substantial warming near the Antarctic coast. Merz et al (2014), however, find a very stable response of the large-scale circulation in their AGCM simulations with a variety of last interglacial Greenland ice sheet reconstructions. For Antarctica, previous studies indicate that reduction of the West Antarctic ice sheet could cause additional local warming over the Antarctic continent Holden et al 2010;Otto-Bliesner et al 2013).…”

Warm Greenland during the last interglacial: the role of regional changes in sea ice cover

“…The atmospheric circulation in the NH during Eemian winters is similar to present-day winters (Merz et al, 2014a). The dominant circulation in Greenland is a stationary high-pressure system, known as the Greenland anticyclone (Hobbs, 1945).…”

“…Another possibility is the climate effect related to modifications in the Greenland ice sheet topography as Greenland must have been smaller during the Eemian to conform with observed sea level high stands (Church et al, 2013). Depending on the actual ice sheet topography this can lead to an additional annual mean warming of up to 3.1 • C at pNEEM (altitude-corrected) as demonstrated in Merz et al (2014a). Hence, if a strong reduction in NordS sea ice coincided with a distinct retreat of the Greenland ice sheet, the full magnitude of the NEEM δ 15 N signal can be explained.…”

“…In contrast to the future year-round warming induced by rising greenhouse gas (GHG) concentrations, the Eemian warming, driven by anomalous orbital forcing, was mostly confined to the summer season and the extratropics. 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 also simulated in climate model experiments (e.g., Bakker et al, 2013;Nikolova et al, 2013;Merz et al, 2014a). However, model-data comparison studies have revealed rather poor agreement between simulations and data, with climate models generally underestimating the magnitude of warming inferred from proxy records Otto-Bliesner et al, 2013;Capron et al, 2014).…”