Strong Summer Atmospheric Rivers Trigger Greenland Ice Sheet Melt through Spatially Varying Surface Energy Balance and Cloud Regimes

Abstract

<jats:title>ABSTRACT</jats:title> <jats:p>Mass loss from the Greenland Ice Sheet (GrIS) has accelerated over the past two decades, coincident with rapid Arctic warming and increasing moisture transport over Greenland by atmospheric rivers (ARs). Summer ARs affecting western Greenland trigger GrIS melt events, but the physical mechanisms through which ARs induce melt are not well understood. This study elucidates the coupled surface–atmosphere processes by which ARs force GrIS melt through analysis of the surface energy balance (SEB), cloud properties, and local- to synoptic-scale atmospheric conditions during strong summer AR events affecting western Greenland. ARs are identified in MERRA-2 reanalysis (1980–2017) and classified by integrated water vapor transport (IVT) intensity. SEB, cloud, and atmospheric data from regional climate model, observational, reanalysis, and satellite-based datasets are used to analyze melt-inducing physical processes during strong, &amp;gt;90th percentile “AR90+” events. Near AR “landfall,” AR90+ days feature increased cloud cover that reduces net shortwave radiation and increases net longwave radiation. As these oppositely signed radiative anomalies partly cancel during AR90+ events, increased melt energy in the ablation zone is primarily provided by turbulent heat fluxes, particularly sensible heat flux. These turbulent heat fluxes are driven by enhanced barrier winds generated by a stronger synoptic pressure gradient combined with an enhanced local temperature contrast between cool over-ice air and the anomalously warm surrounding atmosphere. During AR90+ events in northwest Greenland, anomalous melt is forced remotely through a clear-sky foehn regime produced by downslope flow in eastern Greenland.</jats:p>

Extreme Greenland blocking and high‐latitude moisture transport

“…Extreme moisture transport, particularly through features called atmospheric rivers, can result in ice melt and mass loss as melt energy is provided by turbulent heat fluxes, driven by enhanced barrier winds that are in turn generated by a strong synoptic pressure gradient combined with an enhanced local temperature contrast between cool near‐ice air and anomalously warm surrounding air (Mattingly et al ., 2018; 2020). Furthermore, Greenland blocking, and in particular extreme Greenland blocking, has increased in frequency and magnitude between 1980–1999 and 2000–2019 (Table 1 and Figure 2).…”

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

“…The strong advection of heat and moisture during Greenland blocking plays a crucial role in augmenting melt over the steep margins of the ice sheet by mixing warm, moist air to the surface from above the stable boundary layer that typifies the near surface atmosphere, thus enhancing sensible and latent heat flux and increasing the net SEB (Duynkerke and van den Broeke, 1994;van den Broeke and Gallée, 1996;Mattingly et al, 2020). Turbulent heat flux associated with Greenland blocking has been shown to dominate the SEB response during intense melt across much of the ablation zone (Fausto et al, 2016a;Fausto et al, 2016b).…”

On the crucial role of atmospheric rivers in the two major Weddell Polynya events in 1973 and 2017 in Antarctica

“…Despite their rarity (i.e., around 12 major events per year in West Antarctica), ARs are a key factor in driving both surface melting on the major ice shelves of West Antarctica ( 4 ) and mass loss on the Greenland Ice Sheet because of enhanced downward LW radiation and turbulent heat fluxes ( 17 – 19 ). On the other hand, ARs can also contribute to local snow accumulation on the ice sheet ( 20 )—although sharp losses in surface mass balance caused by AR radiative forcing can locally exceed the moderate gain from snow accumulation during summer ( 3 , 4 ).…”