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Dependence of slope lapse rate over the Greenland ice sheet on background climate

O. Erokhina,I. Rogozhina,M. Prange,P. Bakker,J. Bernales,A. Paul,M. Schulz

Published 2017 in Journal of Glaciology

ABSTRACT

Near-surface temperature is among the most important external forcings for ice-sheet models studies. It defines where and how much snow and ice is lost from an ice sheet through surface melt. In currently glaciated areas such as Greenland and Antarctica, an increasing number of near-surface temperature measurements are available from automatic weather stations (Steffen and others, 1996; Ahlstrøm and others, 2008; van As and others, 2014). Owing to the fact that ice sheets typically have smooth geometries and large horizontal dimensions, the overlying atmospheric conditions are characterized by smooth near-surface temperature fields, which are, to a great extent, modulated by ice-surface elevation. This enables a spatial interpolation between in situ measurements using an intrinsic linear relation between the surface elevation and near-surface temperature, which is commonly termed a slope lapse rate (here and in the following the slope lapse rate is defined to be positive, if the nearsurface temperature decreases with elevation) (Ritz and others in 1997; Fausto and others in 2009). A slope lapse rate indicates the rate of near-surface temperature decrease with elevation and generally differs from the free-air lapse rate or the rate of moist adiabatic cooling, which is normally within the range of 6–7°C km (Marshall and others, 2007; Fausto and others, 2009; Gardner and others, 2009). Analyses of existing observational data in Greenland have shown that slope lapse rates vary significantly through the year, with lower lapse rates recorded over melting glaciers close to the ice-sheet margin (Hanna and others, 2005; Stone and others, 2010). In glacial modeling studies, slope lapse rates (especially over the melting period) play a critical role, and are often used as tunable parameters (e.g. Gardner and others, 2009). The modeling experiments of Stone and others (2010) have demonstrated that an increase in the summer slope lapse rate by ∼1.6°C km leads to an increase of up to 50% in the modeled present-day volume of the Greenland ice sheet. The outcomes of stand-alone paleo ice-sheet simulations emphasize an even stronger influence of lapse rates on the modeled volumes of the former ice sheets, which depend strongly on how near-surface temperature forcing is modified during the inception, growth and decay of ice sheets. For example, Abe–Ouchi and others (2007) tested the sensitivity of the former Northern Hemisphere ice sheets to changes in slope lapse rates by 1– 2°C km and revealed that such changes triggered differences of 50–250% in the modeled ice volume during the Last Glacial Maximum (LGM). It is therefore important to understand whether the present-day estimates of slope lapse rates from ice-covered regions are appropriate for modeling ice sheets under climate conditions that are different from modern. Using observational data from four glaciers in the Canadian high Arctic and climate reanalyses, Gardner and others (2009) suggested a negative relation between near-surface lapse rates and lower-troposphere temperatures. Here we seek to extend their analysis to glacial-interglacial timescales by comparing the slope lapse rates that would arise from the two extreme climate states of the last glacial cycle, namely the Holocene and the LGM. For this purpose, we use the outputs of paleoclimate modeling experiments to derive the LGM, early Holocene, and preindustrial climate states and compare our model results with observation-based lapse rates. Our study focuses on the Greenland ice sheet, since it is an ideal target for such an analysis. Although the Greenland ice sheet was more extensive during the glacial period (in many places reaching the continental shelf break), changes in its geometry since the LGM can be considered minor compared with the regions that were entirely buried under continental-scale ice masses during the LGM and had undergone a complete deglaciation during Termination I (O’Brien and others, 1995; Johnsen and others, 1997; Rasmussen and others, 2006). Greenland can therefore serve as a reference region for a nearly one-to-one evaluation of the Holocene and LGM boundary layer conditions over ice-sheet surfaces.

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