A Lagrangian analysis of the dynamical and thermodynamic drivers of large-scale Greenland melt events during 1979–2017

Abstract

Abstract. In this study, we systematically investigate the dynamical and thermodynamic processes that lead to 77 large-scale melt events affecting high-elevation regions of the Greenland Ice Sheet (GrIS) in June–August (JJA) 1979–2017. For that purpose, we compute 8 d kinematic backward trajectories from the lowermost ∼500 m above the GrIS during these events. The key synoptic feature accompanying the melt events is an upper-tropospheric ridge southeast of the GrIS associated with a surface high-pressure system. This circulation pattern is favorable to induce rapid poleward transport (up to 40∘ latitude) of warm (∼15 K warmer than climatological air masses arriving on the GrIS) and moist air masses from the lower troposphere to the western GrIS and subsequently to distribute them in the anticyclonic flow over north and east Greenland. During transport to the GrIS, the melt event air masses cool by ∼15 K due to ascent and radiation, which keeps them just above the critical threshold to induce melting. The thermodynamic analyses reveal that the final warm anomaly of the air masses is primarily owed to anomalous horizontal transport from a climatologically warm region of origin. However, before being transported to the GrIS, i.e., in their region of origin, these air masses were not anomalously warm. Latent heating from condensation of water vapor, occurring as the airstreams are forced to ascend orographically or dynamically, is of secondary importance. These characteristics were particularly pronounced during the most extensive melt event in early July 2012, where, importantly, the warm anomaly was not preserved from anomalously warm source regions such as North America experiencing a record heat wave. The mechanisms identified here are in contrast to melt events in the low-elevation high Arctic and to midlatitude heat waves, where adiabatic warming by large-scale subsidence is essential. Considering the impact of moisture on the surface energy balance, we find that radiative effects are closely linked to the air mass trajectories and enhance melt over the entire GrIS accumulation zone due to (i) enhanced downward longwave radiation related to poleward moisture transport and a shift in the cloud phase from ice to liquid primarily west of the ice divide and (ii) increased shortwave radiation in clear-sky regions east of the ice divide. Given the ongoing increase in the frequency and the melt extent of large-scale melt events, the understanding of upper-tropospheric ridges over the North Atlantic, i.e., also Greenland blocking, and its representation in climate models is crucial in determining future GrIS accumulation zone melt and thus global sea level rise.

Greater Greenland Ice Sheet contribution to global sea level rise in CMIP6

“…7 , we cannot assess from our data how much of the increase in cloud optical thickness is due to a phase change from mostly ice-containing clouds to mixed-phase or liquid clouds 9 , 16 , 17 . In addition, recent studies using lagrangian moisture tracking also point towards that a notable proportion of humidity is advected to the GrIS on the synoptic-scale and not locally from the surrounding ocean 46 , which will have to be evaluated in more detail in future studies.

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Local and Remote Atmospheric Circulation Drivers of Arctic Change: A Review

“…Research efforts during the era of Arctic amplification have placed specific attention on atmospheric blocking and associated moisture transport over Greenland due to its impact on the surface mass balance (SMB) of the Greenland Ice Sheet (GrIS). Greenland blocking events encourage surface melt through a variety of mechanisms that will be discussed below and, as such, are strongly associated with increased GrIS surface melt and runoff over synoptic to decadal timescales (Hanna et al, 2013;Häkkinen et al, 2014;Hermann et al, 2020). This amplified runoff from the ice sheet can impact the climate system in several ways, including through contributions to global sea level rise.…”

“…From a synoptic perspective, Greenland blocking events encourage melt by transporting moist, potentially warm air from unusually low latitudes poleward within the amplified ridge of the blocking anticyclone (Oltmanns et al, 2019;Steinfeld and Pfahl, 2019;Hermann et al, 2020), causing widespread, positive surface temperature anomalies across the ice sheet (Hanna et al, 2014;McLeod and Mote, 2016) (Figure 2). Advection from lower latitudes is greatest along the upstream side of the ridge.…”

Reconciling different methods of high‐latitude blocking detection

“…This is potentially linked to the enhanced poleward intrusion of warm and moist air towards Greenland induced by the blocking anticyclone, leading to the local intensification of downward infrared radiation and increased short-wave radiation in clear-sky regions (Woods et al, 2013; Woods and Caballero, 2013; Figure 10. Note that the mean daily seasonal cycle was subtracted from the daily mean timeseries to construct timeseries of daily mean anomalies of geopotential height at 500 hPa Lee et al, 2017;Hermann et al, 2020). Arctic anticyclones, which often result from extratropical cyclones injecting extratropical air masses with low potential vorticity into the Arctic upper troposphere, are of key importance for influencing the variability of the overall summertime Arctic sea-ice melting (Papritz and Wernli, 2018).…”