Influence of ice sheet topography on Greenland precipitation during the Eemian interglacial

Abstract

Greenland precipitation and its relationship to the synoptic forcing has been studied for the last interglacial period (i.e., the Eemian) using a set of global climate simulations. We distinguish between precipitation changes due to the Eemian orbital forcing and responses to modifications in the Greenland ice sheet (GrIS) topography. Precipitation changes caused by orbital forcing alone are of moderate amplitude and are largely determined by large‐scale changes in moisture availability. In contrast, changes in GrIS topography lead to distinct precipitation anomalies over Greenland, while the effect on far‐field regions is negligible. The analysis of the simulations reveals the control of the GrIS topography on where moist air masses are orographically lifted and cause substantial precipitation. However, the general moisture availability and the moisture transport associated with typical weather situations remain unchanged in all simulations. A focal point of the study is precipitation at pNEEM, i.e., the suggested deposition site of Eemian ice archived in the North Greenland Eemian ice drilling project (NEEM) ice core. Eemian orbital forcing leads to an increase in summer precipitation at pNEEM, whereas changes in the GrIS topography can result in either increased or decreased precipitation. Transport routes prior to precipitation events at pNEEM show that moisture is predominantly advected from westerly to southerly directions as the GrIS acts as an impassable barrier for easterly moisture transport. One scenario of Eemian melting of northeastern Greenland, however, allows moist air masses from the Norwegian Sea to arrive at pNEEM. Consequently, this GrIS topography would result in transport‐related changes of Eemian wet‐deposited aerosol records.

How warm was Greenland during the last interglacial period?

“…Stronger increases in accumulation rate in the LIG associated with significantly warmer than preindustrial temperature were obtained in relation to a reduction of sea ice in the Nordic Seas (10 % increase in accumulation; Merz et al, 2016). Finally, it has been shown that the geometry of the Greenland ice sheet and topographic changes can lead to various local accumulation scenarios for the LIG at the upstream NEEM deposition site (Merz et al, 2014b): depending on the prescribed LIG ice sheet topography, the modeled accumulation rate at LIG can be 25 % lower to 13 % higher than the preindustrial accumulation rate. The lowest estimate is linked to a change in the trajectory of air mass to the NEEM deposition site, with an increased eastward origin.…”

“…When accounting for a reduced Greenland ice sheet and a retreat in sea ice cover in the Nordic Seas, atmospheric simulations can explain up to 5 • C annual mean warming with respect to the preindustrial period (Merz et al, 2014a(Merz et al, , 2016. Moreover, all LIG climate modeling studies cited above strongly enhance summer precipitation seasonality in Greenland, suggesting a summer bias for LIG δ 18 O ice and weaker annual mean change than the initial estimate of 8 ± 4 • C Merz et al, 2014b). As an example, if we use surface temperature and precipitation rate at a monthly resolution from the Norwegian Earth System Model (NorESM) at the NEEM LIG deposition site, we observe a simulated increase in summer temperature (accumulation rate) of 3.5 • C (7 mm month −1 ) and a decrease in winter temperature (accumulation rate) of 2 • C (3 mm month −1 ).…”

“…temperature and accumulation rate in Greenland i. Accumulation rate and temperature can be linked through thermodynamic laws and ice sheet topography, despite significant uncertainties associated with atmospheric transport characteristics that lead to regional variability (Kapsner et al, 1995;Merz et al, 2014b). In a first approximation, temperature and moisture content of an air mass are linked through saturation pressure ("P sat approach").…”

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

“…In the successor model, CCSM4, these biases have been substantially improved through changes in sea ice albedo and ocean overflow parameterizations (Gent et al, 2011). Further, CCSM4 shows in general good skill in simulating the present-day surface climate and atmospheric circulation in and around Greenland (Vizcaino et al, 2013;Merz et al, 2013Merz et al, , 2014b. Hence, we have good confidence in CCSM4's capability in representing the components of the North Atlantic and Greenland climate system that are of importance for this study, e.g., SAT, surface energy fluxes, surface winds, or precipitation.…”

“…For Greenland, most of the additionally available moisture precipitates above the steep margins of the ice sheet in the southeast, where the moist air masses are lifted and, consequently, cause orographic precipitation. The resulting maximum in winter precipitation in southeastern Greenland is a prominent feature in the North Atlantic winter climate (e.g., Tsukernik et al, 2007;Merz et al, 2014b) related to a local maximum in cyclone frequency in the area of the Icelandic low. Enhanced moisture availability in the NordS domain thus results in a precipitation increase in this specific Greenland region, with cyclones being the carrier.…”