The Effect of Foehn‐Induced Surface Melt on Firn Evolution Over the Northeast Antarctic Peninsula Rajashree Tri Datta 1,2,3,4 , Marco Tedesco 4,5 , Xavier Fettweis 6 , Cecile Agosta 7,8 , Stef Lhermitte 8 , Jan T. M. Lenaerts 9 , and Nander Wever 9 1 Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD, USA, 2 NASA Goddard Space Flight Center, Greenbelt, MD, USA, 3 The Graduate Center, CUNY, New York, NY, USA, 4 Lamont‐Doherty Earth Observatory of Columbia University, Palisades, New YorkNY, USA, 5 NASA Goddard Institute of Space Studies, New York, NY, USA, 6 Department of Geography, Université de Liège, Liège, Belgium, 7 CNRS, Institut des Géosciences de l'Environnement, Université Grenoble Alpes, Grenoble, France, 8 Department of Geoscience and Remote Sensing, Delft University of Technology, Delft, Netherlands, 9 Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, Boulder, CO, USA Abstract Surface meltwater ponding has been implicated as a major driver for recent ice shelf collapse as well as the speedup of tributary glaciers in the northeast Antarctic Peninsula. Surface melt on the NAP is impacted by the strength and frequency of westerly winds, which result in sporadic foehn flow. We estimate changes in the frequency of foehn flow and the associated impact on snow melt, density, and the percolation depth of meltwater over the period 1982–2017 using a regional climate model and passive microwave data. The first of two methods extracts spatial patterns of melt occurrence using empirical orthogonal function analysis. The second method applies the Foehn Index, introduced here to capture foehn occurrence over the full study domain. Both methods show substantial foehn‐induced melt late in the melt season since 2015, resulting in compounded densification of the near‐surface snow, with potential implications for future ice shelf stability. Plain Language Summary Surface melt and the ponding of water on the surface has been linked to recent ice shelf collapse in the northeast Antarctic Peninsula, which includes the Larsen C ice shelf, one of the regions in Antarctica that is most vulnerable to a changing climate. Melt can be caused either by high temperatures or by foehn winds, that is, a hot, dry wind on the downwind side of a mountain range. To determine when foehn winds occurred from 1982 to 2017, and how much surface melt they produced, we use two methods. The first method finds recurring patterns of melt on the northeast Antarctic Peninsula from both satellite observations and models and determines which patterns are produced by foehn conditions. The second method uses simulated atmospheric conditions to determine when and over how much surface area foehn conditions occur and then calculates the melt produced at the same time. Both methods find high levels of foehn‐induced melt after the summer melt season occurring since 2015, resulting in high‐density snow near the surface in regions where foehn winds are common. If similar conditions persist into the future, late‐season and autumn melt could have substantial ramifications for the health of the Larsen C ice shelf. 1. Introduction Surface melt on the northeast Antarctic Peninsula (NAP) impacts the mass balance of grounded ice as well as ice shelf stability (Barrand et al., 2013; Kunz et al., 2012; Scambos et al., 2004). Besides producing melt- water runoff on the grounded part of the NAP (Hock et al., 2009), surface melt can indirectly lead to ice loss through the process of ice shelf hydrofracture, whereby preexistent crevasses on floating ice shelves fill with accumulating meltwater, leading to the ice shelf disintegration and tributary glacier speedup and thinning (Glasser & Scambos, 2008; MacAyeal & Sergienko, 2013; Rott et al., 1998, 2011; Scambos et al., 2000, 2004; van der Veen, 1998; Vaughan & Doake, 1996; Weertman, 1973). The potential contribution to global sea level rise from tributary glaciers resulting from the removal of the Larsen C ice shelf (LCIS) has been esti- mated at <2.55 to 2,100 mm and <4.2 to 2,300 mm (Schannwell et al., 2018). The fate of surface meltwater, that is, meltwater in the first meter of the snowpack, depends on the condi- tions of underlying firn. Anomalously low accumulation, enhanced firn compaction, liquid water ©2019. American Geophysical Union. All Rights Reserved. RESEARCH LETTER 10.1029/2018GL080845 Key Points: • A long‐term record of foehn‐induced melt is calculated from a regional climate model via two methods • We introduce the Foehn Index which captures foehn intensity over space and time • Late‐season foehn‐induced melt between 2015 and 2017 produced compounded densification in the upper snowpack resulting in increasing runoff Supporting Information: • Supporting Information S1 Correspondence to: R. T. Datta, tri.datta@gmail.com Citation: Datta, R. T., Tedesco, M., Fettweis, X., Agosta, C., Lhermitte, S., Lenaerts, J. T. M., & Wever, N. (2019). The effect of Foehn‐induced surface melt on firn evolution over the northeast Antarctic peninsula. Geophysical Research Letters, 46. https://doi.org/10.1029/ 2018GL080845 Received 9 OCT 2018 Accepted 4 FEB 2019 DATTA ET AL. 1