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Weather and Climate Dynamics An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/wcd-2020-16
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/wcd-2020-16
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 28 Apr 2020

Submitted as: research article | 28 Apr 2020

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This preprint is currently under review for the journal WCD.

A Lagrangian Analysis of the Dynamical and Thermodynamic Drivers of Greenland Melt Events during 1979–2017

Mauro Hermann, Lukas Papritz, and Heini Wernli Mauro Hermann et al.
  • Institute for Atmospheric and Climate Science, ETH Zürich, Zurich, Switzerland

Abstract. In this study, we systematically investigate the dynamical and thermodynamic processes that lead to 77 Greenland melt events affecting high-elevated regions of the Greenland Ice Sheet (GrIS) in June–August (JJA) 1979–2017. For that purpose, we compute 8-day 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 favourable 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 vapour, 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 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 identified mechanisms that cause extensive melt over the GrIS, 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 melt and so global sea-level rise.

Mauro Hermann et al.

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Mauro Hermann et al.

Mauro Hermann et al.

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Latest update: 29 May 2020
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Short summary
We find, by tracing backward in time, that air masses causing extensive melt of the Greenland Ice Sheet originate from further south and lower altitudes than usual. Their exceptional warmth further arises due to ascent and cloud formation, which is special compared to near-surface heatwaves in the midlatitudes or the Central Arctic. The atmospheric systems and transport pathways identified here are crucial in understanding and simulating the atmospheric control of the ice sheet in the future.
We find, by tracing backward in time, that air masses causing extensive melt of the Greenland...
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