Ian Fenty

Undercutting of marine‐terminating glaciers in West Greenland

Abstract Marine‐terminating glaciers control most of Greenlands ice discharge into the ocean, but little is known about the geometry of their frontal regions. Here we use side‐looking, multibeam echo sounding observations to reveal that their frontal ice cliffs are grounded deeper below sea level than previously measured and their ice faces are neither vertical nor smooth but often undercut by the ocean and rough. Deep glacier grounding enables contact with subsurface, warm, salty Atlantic waters (AW) which melts ice at rates of meters per day. We detect cavities undercutting the base of the calving faces at the sites of subglacial water (SGW) discharge predicted by a hydrological model. The observed pattern of undercutting is consistent with numerical simulations of ice melt in which buoyant plumes of SGW transport warm AW to the ice faces. Glacier undercutting likely enhances iceberg calving, impacting ice front stability and, in turn, the glacier mass balance.

Fast retreat of Zachariæ Isstrøm, northeast Greenland

Shrinking shelf and faster flow Zachariæ Isstrøm, a large glacier in northeast Greenland, began a rapid retreat after detaching from a stabilizing sill in the late 1990s. Mouginot et al. report that between 2002 and 2014, the area covered by the glaciers ice shelf shrank by 95%; since 1999, the glaciers flow rate has nearly doubled; and its acceleration increased threefold in the fall of 2012. These dramatic changes appear to be the result of a combination of warmer air and ocean temperatures and the topography of the ocean floor at the head of the glacier. Rising sea levels should continue to destabilize the marine portion of Zachariæ Isstrøm for decades. Science, this issue p. 1357 A large glacier in northeast Greenland is retreating rapidly as air and ocean warm. After 8 years of decay of its ice shelf, Zachariæ Isstrøm, a major glacier of northeast Greenland that holds a 0.5-meter sea-level rise equivalent, entered a phase of accelerated retreat in fall 2012. The acceleration rate of its ice velocity tripled, melting of its residual ice shelf and thinning of its grounded portion doubled, and calving is now occurring at its grounding line. Warmer air and ocean temperatures have caused the glacier to detach from a stabilizing sill and retreat rapidly along a downward-sloping, marine-based bed. Its equal-ice-volume neighbor, Nioghalvfjerdsfjorden, is also melting rapidly but retreating slowly along an upward-sloping bed. The destabilization of this marine-based sector will increase sea-level rise from the Greenland Ice Sheet for decades to come.

Observed thinning of Totten Glacier is linked to coastal polynya variability

Analysis of ICESat-1 data (2003-2008) shows significant surface lowering of Totten Glacier, the glacier discharging the largest volume of ice in East Antarctica, and less change on nearby Moscow University Glacier. After accounting for firn compaction anomalies, the thinning appears to coincide with fast-flowing ice indicating a dynamical origin. Here, to elucidate these observations, we apply high-resolution ice-ocean modelling. Totten Ice Shelf is simulated to have higher, more variable basal melting rates. We link this variability to the volume of cold water, originating in polynyas upon sea ice formation, reaching the sub-ice-shelf cavity. Hence, we propose that the observed increased thinning of Totten Glacier is due to enhanced basal melting caused by a decrease in cold polynya water reaching its cavity. We support this hypothesis with passive microwave data of polynya extent variability. Considering the widespread changes in sea ice conditions, this mechanism could be contributing extensively to ice-shelf instability.

BedMachine v3: Complete Bed Topography and Ocean Bathymetry Mapping of Greenland From Multibeam Echo Sounding Combined With Mass Conservation

Abstract Greenlands bed topography is a primary control on ice flow, grounding line migration, calving dynamics, and subglacial drainage. Moreover, fjord bathymetry regulates the penetration of warm Atlantic water (AW) that rapidly melts and undercuts Greenlands marine‐terminating glaciers. Here we present a new compilation of Greenland bed topography that assimilates seafloor bathymetry and ice thickness data through a mass conservation approach. A new 150 m horizontal resolution bed topography/bathymetric map of Greenland is constructed with seamless transitions at the ice/ocean interface, yielding major improvements over previous data sets, particularly in the marine‐terminating sectors of northwest and southeast Greenland. Our map reveals that the total sea level potential of the Greenland ice sheet is 7.42 ± 0.05 m, which is 7 cm greater than previous estimates. Furthermore, it explains recent calving front response of numerous outlet glaciers and reveals new pathways by which AW can access glaciers with marine‐based basins, thereby highlighting sectors of Greenland that are most vulnerable to future oceanic forcing.

Bathymetry data reveal glaciers vulnerable to ice‐ocean interaction in Uummannaq and Vaigat glacial fjords, west Greenland

Marine-terminating glaciers play a critical role in controlling Greenlands ice sheet mass balance. Their frontal margins interact vigorously with the ocean, but our understanding of this interaction is limited, in part, by a lack of bathymetry data. Here we present a multibeam echo sounding survey of 14 glacial fjords in the Uummannaq and Vaigat fjords, west Greenland, which extends from the continental shelf to the glacier fronts. The data reveal valleys with shallow sills, overdeepenings (> 1300 m) from glacial erosion, and seafloor depths 100-1000 m deeper than in existing charts. Where fjords are deep enough, we detect the pervasive presence of warm, salty Atlantic Water (AW) (> 2.5 degrees C) with high melt potential, but we also find numerous glaciers grounded on shallow (< 200 m) sills, standing in cold (< 1 degrees C) waters in otherwise deep fjords, i.e., with reduced melt potential. Bathymetric observations extending to the glacier fronts are critical to understand the glacier evolution.

Modeling of ocean‐induced ice melt rates of five west Greenland glaciers over the past two decades

Geophysical Research Letters RESEARCH LETTER 10.1002/2016GL068784 Key Points: • How does the ocean control the melting of Greenland glaciers? • Glaciers dominated by calving processes are less sensitive to ice-ocean interactions • The study provided a basis for quantifying ice-ocean interaction elsewhere around Greenland Supporting Information: • Supporting Information S1 Correspondence to: E. Rignot, erignot@uci.edu Citation: Rignot, E., et al. (2016), Modeling of ocean-induced ice melt rates of five west Greenland glaciers over the past two decades, Geophys. Res. Lett., 43, doi:10.1002/2016GL068784. Received 25 MAR 2016 Accepted 25 MAY 2016 Accepted article online 30 MAY 2016 Modeling of ocean-induced ice melt rates of five west Greenland glaciers over the past two decades E. Rignot 1,2 , Y. Xu 1 , D. Menemenlis 2 , J. Mouginot 1 , B. Scheuchl 1 , X. Li 1 , M. Morlighem 1 , H. Seroussi 1 , M. van den Broeke 3 , I. Fenty 2 , C. Cai 1 , L. An 1 , and B. de Fleurian 1 1 Department of Earth System Science, University of California, Irvine, California, USA, 2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA, 3 Institute for Marine and Atmospheric Research, Utrecht University, Utrecht, Netherlands Abstract High-resolution, three-dimensional simulations from the Massachusetts Institute of Technology general circulation model ocean model are used to calculate the subaqueous melt rate of the calving faces of Umiamako, Rinks, Kangerdlugssup, Store, and Kangilerngata glaciers, west Greenland, from 1992 to 2015. Model forcing is from monthly reconstructions of ocean state and ice sheet runoff. Results are analyzed in combination with observations of bathymetry, bed elevation, ice front retreat, and glacier speed. We calculate that subaqueous melt rates are 2–3 times larger in summer compared to winter and doubled in magnitude since the 1990s due to enhanced subglacial runoff and 1.6±0.3 ∘ C warmer ocean temperature. Umiamako and Kangilerngata retreated rapidly in the 2000s when subaqueous melt rates exceeded the calving rates and ice front retreated to deeper bed elevation. In contrast, Store, Kangerdlugssup, and Rinks have remained stable because their subaqueous melt rates are 3–4 times lower than their calving rates, i.e., the glaciers are dominated by calving processes. 1. Introduction The Greenland Ice Sheet has been experiencing an accelerating rate of mass loss over the past few decades from a combination of enhanced surface melt and increased glacier ice discharge [e.g., Rignot and Kanagaratnam, 2006; van den Broeke et al., 2009; Enderlin et al., 2014]. Widespread acceleration of its marine-terminating glaciers and fluctuations in the position of their calving margins has been attributed to the enhanced advection of warm subsurface ocean waters of subtropical origin at the ice-ocean boundary [e.g., Holland et al., 2008; Rignot et al., 2010; Straneo et al., 2010; Rignot et al., 2012; Bevan et al., 2012; Christoffersen et al., 2011]. Yet there has been no quantitative demonstration of the role of subaqueous melt in driving tide- water glaciers into a retreat, in part due to our poor knowledge of the magnitude of the subaqueous melt rates, their spatial pattern, horizontally and vertically, and their seasonal to interannual variability. One hypothesis is that subaqueous melting at the ice front has increased sufficiently in magnitude to counteract the advection of ice from upstream and force the glacier front to retreat. As the glacier retreats, the removal of grounded ice at the ice front will reduce basal resistance, which will enable glacier ice from upstream to speedup. Speedup will cause ice to thin from longitudinal stretching, which will be conducive to further retreat. The retreat rate will depend on the glacier geometry, especially the bed elevation below sea level and whether the bed ele- vation increases or decreases farther inland [e.g., Post et al., 2011]. When examining the details of the glacier response to ocean thermal forcing, large variations have been reported from one glacier to the next or within the same fjord system [e.g., Box and Decker, 2011; Moon et al., 2012], which suggests a complex interplay between subaqueous melting, iceberg calving, glacier geometry, and other factors [Benn et al., 2007]. ©2016. American Geophysical Union. All Rights Reserved. RIGNOT ET AL. Here we consider the recent history of five glaciers in central west Greenland for which we have quality data to reconstruct ocean-induced melt rates. We review their time evolution and response to atmospheric and oceanic forcings. We use three-dimensional, high-resolution simulations of ice melt into the ocean to model their horizontal, vertical, seasonal, and interannual variability. Model forcing is constrained using in situ oceanographic data, reconstructions of ice sheet runoff, novel bed topography, and bathymetry data. We compare changes in subaqueous melt rate through time with ice velocity at the ice front. We conclude on the impact of ice-ocean interaction on the glacier dynamics and in turn on glacier stability. MODELING OF GLACIER MELT IN GREENLAND

Global coupled sea ice-ocean state estimation

We study the impact of synthesizing ocean and sea ice concentration data with a global, eddying coupled sea ice-ocean configuration of the Massachusetts Institute of Technology general circulation model with the goal of reproducing the 2004 three-dimensional time-evolving ice-ocean state. This work builds on the state estimation framework developed in the Estimating the Circulation and Climate of the Ocean consortium by seeking a reconstruction of the global sea ice-ocean system that is simultaneously consistent with (1) a suite of in situ and remotely-sensed ocean and ice data and (2) the physics encoded in the numerical model. This dual consistency is successfully achieved here by adjusting only the model’s initial hydrographic state and its atmospheric boundary conditions such that misfits between the model and data are minimized in a least-squares sense. We show that synthesizing both ocean and sea ice concentration data is required for the model to adequately reproduce the observed details of the sea ice annual cycle in both hemispheres. Surprisingly, only modest adjustments to our first-guess atmospheric state and ocean initial conditions are necessary to achieve model-data consistency, suggesting that atmospheric reanalysis products remain a leading source of errors for sea ice-ocean model hindcasts and reanalyses. The synthesis of sea ice data is found to ameliorate misfits in the high latitude ocean, especially with respect to upper ocean stratification, temperature, and salinity. Constraining the model to sea ice concentration modestly reduces ICESat-derived Arctic ice thickness errors by improving the temporal and spatial evolution of seasonal ice. Further increases in the accuracy of global sea ice thickness in the model likely require the direct synthesis of sea ice thickness data.