Understanding Unseasonal Melt and Runoff from the Greenland Ice Sheet

Modeling and remote sensing studies have documented that the Greenland ice sheet has experienced longer melt seasons, increased melt area and runoff, and more negative mass balance in the recent decade compared to previous decades. In summer 2012, surface melt covered nearly the entire ice surface, likely for the first time in more than a century. Most of the melt and runoff from the inland ice occurs during the summer season, but several discharge events coinciding with ice sheet runoff production are documented in small pro-glacial streams outside of the regular melting season. What is unclear is the spatial extent to which melt from the ice margin and inland ice contributes to unseasonal discharge. For example, is this process limited to the small catchments where discharge observations has been made, or is it manifested over larger parts of the Greenland ice sheet? Unseasonal runoff events are identified in discharge observations made near Kangerlussuaq in Southwest Greenland between 2008 and 2013. Surface melt occurrence derived from spaceborne microwave remote sensing and surface mass balance, surface air temperature from the PROMICE and GCNet networks, and runoff from a regional climate model are used to ascertain the possible role of melt from the ice margin and inland ice. Regional atmospheric circulation anomalies, such as Greenland blocking episodes, are examined for their role in unseasonal runoff events. The frequency and intensity of Greenland blocking episodes since 2007 have been highly anomalous compared to the climatological record, resulting in exceptional melting of the Greenland ice sheet. Greenland blocks, particularly the stronger episodes, occur most frequently during boreal autumn and winter, which suggests a potential linkage to the development of unseasonal runoff events across the ice sheet. The multifaceted role of North Atlantic cyclones on the evolution of unseasonal melt/runoff events will also be investigated. As these cyclones track generally eastward near the southern tip of Greenland, the combination of southerly latent and sensible heat advection can directly induce melting, particularly across the southern half of the ice sheet. In addition, these cyclones can either initiate downstream ridge-building over Greenland or amplify a preexisting ridge, ultimately leading to an adiabatically-driven heating and melting of the underlying ice sheet.