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The Official Magazine of The OceTHE OFFICIALa MAGAZINEn ogOF THE OCEANOGRAPHYra SOCIETYphy CITATION Fenty, I., J.K. Willis, A. Khazendar, S. Dinardo, R. Forsberg, I. Fukumori, D. Holland, M. Jakobsson, D. Moller, J. Morison, A. Münchow, E. Rignot, M. Schodlok, A.F. Thompson, K. Tinto, M. Rutherford, and N. Trenholm. 2016. Oceans Melting Greenland: Early results from NASA’s ocean-ice mission in Greenland. Oceanography 29(4):72–83, https://doi.org/10.5670/oceanog.2016.100. DOI https://doi.org/10.5670/oceanog.2016.100 COPYRIGHT This article has been published in Oceanography, Volume 29, Number 4, a quarterly journal of The Oceanography Society. Copyright 2016 by The Oceanography Society. All rights reserved. USAGE Permission is granted to copy this article for use in teaching and research. Republication, systematic reproduction, or collective redistribution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The Oceanography Society. Send all correspondence to: [email protected] or The Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. DOWNLOADED FROM HTTP://TOS.ORG/OCEANOGRAPHY SPECIAL ISSUE ON OCEAN-ICE INTERACTION Oceans Melting Greenland Early Results from NASA’s Ocean-Ice Mission in Greenland By Ian Fenty, Josh K. Willis, Ala Khazendar, Steven Dinardo, René Forsberg, Ichiro Fukumori, David Holland, Martin Jakobsson, Delwyn Moller, James Morison, Andreas Münchow, Eric Rignot, Michael Schodlok, Andrew F. Thompson, Kirsteen Tinto, Matthew Rutherford, and Nicole Trenholm Photo taken from Greenland's southwestern coast- line in July and August 2015 during Phase 1 of the TerraSond/Cape Race bathymetry survey. From https://omg.jpl.nasa.gov/portal/gallery/2015-survey-p1 72 Oceanography | Vol.29, No.4 ABSTRACT. Melting of the Greenland Ice Sheet represents a major uncertainty in the ice sheet and to provide the diver- projecting future rates of global sea level rise. Much of this uncertainty is related to a sity of observations required so that local lack of knowledge about subsurface ocean hydrographic properties, particularly heat findings about ocean-ice interaction can content, how these properties are modified across the continental shelf, and about the be generalized across Greenland’s more extent to which the ocean interacts with glaciers. Early results from NASA’s five-year than 200 marine-terminating glaciers. Oceans Melting Greenland (OMG) mission, based on extensive hydrographic and Dynamic changes in the marine- bathymetric surveys, suggest that many glaciers terminate in deep water and are hence terminating glaciers of Greenland vulnerable to increased melting due to ocean-ice interaction. OMG will track ocean and Antarctica were cited by the 2007 conditions and ice loss at glaciers around Greenland through the year 2020, providing and 2013 Intergovernmental Panel on critical information about ocean-driven Greenland ice mass loss in a warming climate. Climate Change reports as being the larg- est source of uncertainty in sea level rise INTRODUCTION extreme northwest and northeast has projections (Lemke et al., 2007; Church Melting of the Greenland also been implicated in the calving and et al., 2013). NASA’s Oceans Melting Ice Sheet retreat of the Petermann, Humboldt, Greenland (OMG) mission will provide The acceleration of global mean sea level and Zachariæ Isstrøm Glaciers (Box and the first opportunity to quantify the rela- rise over the past several decades has Decker, 2011; Münchow et al., 2014; tionship between a warming ocean and been driven, in part, by increased melt- Mouginot et al., 2015). While it is very ice loss in Greenland by collecting obser- ing of the Greenland Ice Sheet (Nerem likely that a warming ocean is an import- vations from 2015 through 2020 that et al., 2010). Greenland ice mass loss has ant factor for Greenland Ice Sheet mass address each of these knowledge gaps. increased sharply since the 1990s due to loss, quantifying its contribution relative Specifically, our goal is improved under- a combination of increased ice discharge to other possible factors, such as atmo- standing of how ocean hydrographic and surface melting/runoff (Howat et al., spheric warming and increased surface variability around the ice sheet impacts 2007; Enderlin et al., 2014). Over the sat- melting, is a major outstanding research glacial melt rates, thinning, and retreat. ellite altimetry period (1992–present), challenge for the community (Truffer and OMG’s investigations into Greenland the rate of global mean sea level rise Motyka, 2016). ocean-ice sheet interaction will lead to has nearly doubled relative to its twen- Given the logistical challenges associ- new insights about past sea level changes tieth century average to 3.2 mm yr–1, ated with observing the ocean surround- and reduce the uncertainties in our future with ~0.75 mm yr–1 now attributable ing Greenland (sea ice, harsh weather, projections of sea level rise. to the net transfer of water from the iceberg-chocked fjords), it is unsurpris- melting of Greenland’s grounded ice ing that relatively few in situ ocean mea- OMG: A GREENLAND-WIDE (Shepherd et al., 2012; Church et al., surements have been made close to the EXPERIMENT 2013; Watkins et al., 2015). ice sheet (Straneo et al., 2012). The path- The OMG mission is guided by the over- Since 2003, Greenland Ice Sheet mass ways of ocean currents transporting heat arching science question: To what extent is loss has been greatest in its southeast and to Greenland’s glacial fjords are shaped the ocean melting Greenland’s glaciers from northwest parts where widespread gla- by seafloor geometry (Gladish et al., below? To address this question, the mis- cial acceleration has also been observed 2015a,b), yet large gaps exist in obser- sion formulated the following approach: (Moon et al., 2012). Glacier accelera- vations of seafloor shape and depth. 1. Observe the year-to-year changes in tion has been attributed to several fac- Recent progress has been made in mon- the temperature, volume, and extent of tors, including reduced basal and lateral itoring large-scale ice volume and mass warm, salty Atlantic Water on the con- sidewall resistive stresses following ter- changes (e.g., van den Broeke et al., 2009; tinental shelf. minus retreat and reduced ice mélange McMillan et al., 2016) but not at the spa- 2. Observe the year-to-year changes in back stress (Vieli and Nick, 2011; Joughin tial resolutions required to characterize the extent and height of all marine- et al., 2012). Both glacier front retreat and changes in individual glaciers. Despite terminating glaciers. diminished ice mélange (a mix of ice- increased interest in better understanding 3. Observe submarine topography bergs and sea ice) may be attributed to the processes linking ocean warming and (bathymetry) to reveal the complex increased submarine melting and calving ice sheet mass loss, observations of ocean network of underwater troughs and following the warming of ocean waters on temperatures and changes in glaciers near canyons connecting glacial fjords with Greenland’s continental shelf and prox- ocean-ice interfaces are available for only the continental shelf. imate offshore basins beginning in the a small number of fjord systems. More 4. Investigate the relationship between late 1990s (Holland et al., 2008; Straneo detailed surveys are needed to under- the observed variations in Atlantic et al., 2013; Straneo and Heimbach, 2013). stand basic processes such as how water Water properties and glacier extent Warming ocean waters in Greenland’s masses are modified as they move toward and height. Oceanography | December 2016 73 Airborne eXpendable conductivity, masses: cold, fresh water that is rela- Subglacial discharge is primarily sourced temperature, and depth (AXCTD) sur- tively light and of Arctic origin (hereafter from accumulated surface melt water veys will be conducted in September of Polar Water), and warm, salty water that from across glacial drainage basins. each year to observe interannual vari- is denser and originates in the subtropics Because it is relatively light, the outflow ations in the hydrographic proper- (hereafter Atlantic Water). During its rises quickly along the ice face in the ties of Atlantic Water on the shelf, and transit in the boundary currents around form of a turbulent plume. As it ascends, an interferometric radar will provide Greenland, warm Atlantic Water progres- the plume may entrain warm Atlantic annual measurements of the extent and sively loses most of its heat through lat- Water, enhancing the rate of ocean heat height of marine-terminating glaciers. eral mixing and exchange with the rela- transfer to the ice and thereby increas- Bathymetric observations will be made tively colder offshore basins (Rignot et al., ing subaqueous melting (Motyka et al., using ship-based sonars and airborne 2012a; Straneo et al., 2012; Kawasaki 2003; Jenkins, 2011; Sciascia et al., 2013; gravimetry. Statistical analysis of these and Hasumi, 2014). Xu et al., 2013). data and a suite of variable-resolution At select locations, deep (>500 m) sub- While the North Atlantic is argu- data- assimilating ocean general circula- marine canyons also known as cross-shelf ably the most studied region of the tion models will be utilized to make new troughs cross the shallow (<200 m) con- world ocean, previous research has quantitative estimates of ice sheet mass tinental shelf. Conserving potential vor- mainly focused on processes in the off- loss as a function of ocean warming. ticity and essentially following contours shore basins such as deep convec- of constant depth, dense Atlantic Water tion and dense water mass formation The Ocean diverges from the boundary current and (e.g., The Lab Sea Group, 1998; Pickart The Greenland Ice Sheet is surrounded is steered into these canyons where it et al., 2002). Argo profiling floats pro- by the North Atlantic subpolar gyre flows toward the ice sheet beneath the vide abundant temperature (T) and (which spans the Labrador and Irminger buoyant Polar Water layer.
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