Föhn winds on Larsen C ice shelf generate an unusually massive ice layer

Surface melt and ponding has been observed on many Antarctic ice shelves and is implicated in ice-shelf collapse through firn compaction and hydrofracture (van den Broeke, 2005). Ice-shelf surface meltwater can percolate into the firn, transferring heat to deeper layers by refreezing (Vaughan, 2008). This can lead to warming and densification to the point where firn air content approaches zero (Holland and others, 2011), potentially impacting ice dynamics and fracture toughness. Surface processes also contribute to the recent thinning observed on Larsen C ice shelf (LCIS; Pritchard and others, 2012; Holland and others, 2015). In northern LCIS, föhn winds provide extra sensible heat to drive surface melt (Luckman and others, 2014). The NERC MIDAS Project (Impact of Melt on Ice Dynamics and Stability: 2014–2017) aims to investigate the mechanisms of melt and ponding, and test their impact on the stability of the LCIS. In November 2014 we visited Cabinet Inlet in northern LCIS to install an ARGOS-enabled automatic weather station, drill a 100 m borehole, and determine ice layering and temperature using an optical televiewer (OPTV) and logged thermistor string. The OPTV log shows a ~40 m layer of refrozen meltwater perched above glacier ice presumably advected from beyond the grounding line 40 km away. The thermistor data show the ice to be many degrees warmer than mean atmospheric temperatures would lead us to expect. We infer the source of this unusual ice configuration and temperature profile as föhn-driven surface melt. In April 2015, well after normal mean surface temperature had fallen below freezing, several föhn wind events lasting a few days raised the temperature well above 0°C and, in MODIS satellite data, melt ponds were seen to form. We continue to investigate using numerical modelling the potential impact on ice-shelf stability of melt, warming and massive ice layers