A Lagrangian Analysis of the Dynamical and Thermodynamic Drivers of Greenland Warm Events during 1979-2017

The acceleration of mass loss from the Greenland Ice Sheet (GrIS) since the late 1990s was primarily driven by increased surface melt, partly concentrated in single extreme melt events. A textbook example was observed around 12 July 2012 (EV69), when almost the entire GrIS was melting, including Summit Station at 3216 m in the dry inland plateau. The melt event coincided with strong meridional air mass transport towards the GrIS followed by a Greenland blocking, which resembles the anomalous synoptic pattern that became increasingly frequent in the North Atlantic region in the last two decades. Here, we investigate the atmospheric forcing of extraordinary melt periods by assessing the main dynamical and thermodynamic processes that cause so-called warm events. We present an ERA-Interim-based climatology of 77 Greenland warm events affecting the high accumulation area in 1979-2017. These events became longer and more frequent during the study period. With Lagrangian backward trajectories started from the lowermost ~500 m above the GrIS, we identify transport from a climatologically warmer region as key process for Greenland warm events. With an about 15° latitude more southerly air mass origin and subsidence-induced adiabatic warming, this process is twice as important as diabatic heating to the final warm anomaly over North and East Greenland. In South and West Greenland, major contributions come from ascending air masses and in the Southwest, warm events are dominated by orographically induced latent heating as opposed to transport. 70-85% of all warm events go along with a Greenland blocking, which we suggest induces additional melt via modulations of the surface energy budget not captured by our Lagrangian methodology. We further qualify the importance of the concurrent US Great Plains heatwave for EV69. In contrast to previous studies, we find that air masses arriving over the GrIS during EV69 mostly originated from the Canadian Arctic, Newfoundland and the subtropical North Atlantic, which were partly also anomalously warm. Given the relevance of atmospheric blocking for Greenland warm events and their link to anomalously warm summers, it is crucial, as we argue, to better understand the modification of inter-annual climate variability by climate change in the North Atlantic region, to more accurately predict future GrIS mass loss. Also, more research on cloud radiative effects is needed to capture the GrIS-wide variability of surface energy budget anomalies during Greenland blocking