Tidewater Outlet Glaciers Research Articles

Seven-year variation in glacier surface velocity at Narsap Sermia in Southwest Greenland

ABSTRACT Glacier velocity is a critical factor closely related to ice dynamics of the Greenland ice sheet (GrIS). We present variations of ice velocity using space-based synthetic aperture radar (SAR) observations from July 2013 to September 2020. We focus on a tidewater outlet glacier of Narsap Sermia (NS), located in southwest Greenland, which is experiencing significant ice loss. A time series of high spatiotemporal resolution maps of ice velocity was traced by using an offset tracking approach with seven-year SAR acquisitions to understand glacier dynamics in time. We observed seasonal variations showing a dramatic increase in late spring (May) and early summer (June) followed by a sharp decrease in late summer (September). The seasonal variation was confirmed through Seasonal and Trend decomposition using Loess (STL) analysis. Overall, the NS glacier experienced a gradual speed-up of ice velocities. The ice velocity increased by 0.1–0.3 km/year from 2015 to 2017, then after a temporary decrease in 2018, the velocity moved faster by 0.9 km/year in 2020. The speed of glacier movement accelerated as it approached the tidal terminus of the glacier, showing approximately 6.0 km/year in 2020. The ice speed in 2020 increased by 0.9 km/year at the terminus region compared with 2014. We also discuss the relationship between the glacier velocity and temperature data such as air, land surface, and sea surface temperatures.

Vertical Mixing in Stratified Fjords Near Tidewater Outlet Glaciers Along Northwest Greenland

AbstractVertical mixing of upper water masses in Arctic fjords is important for circulation and transport of nutrients and heat. However, the distribution of turbulent mixing is poorly known. Here we present hydrographic and microscale turbulence measurements from six fjords in Northwestern (69°N–75°N) Greenland. Water mass distributions showed the presence of warm bottom water of Atlantic origin in all fjord transects and a significant modification of upper water masses in fjords with tidewater outlet glaciers. Spatial and temporal distribution of turbulence in the fjords showed, in general, low rates of turbulent kinetic energy, ε (∼10−9 W kg−1), and strong stratification implies that vertical exchange in the upper ∼200 m was relatively weak. However, measurements within 2 km of the terminus of a tidewater outlet glacier showed values of ε and inferred turbulent diffusion coefficients >10−4 m2 s−1 that were about two orders of magnitude larger than elsewhere in the deep fjords. This elevated mixing corresponded to comparatively large heat and salt fluxes. These observations suggest that vertical mixing near tidewater outlet glaciers results in localized mixing hot spots, likely due to subglacial discharge and near‐glacial current shear, that contribute significantly to vertical exchange in the deep fjords.

Early spring subglacial discharge plumes fuel under-ice primary production at a Svalbard tidewater glacier

Abstract. Subglacial upwelling of nutrient-rich bottom water is known to sustain elevated summer primary production in tidewater-glacier-influenced fjord systems. However, the importance of subglacial upwelling during the early spring season has not been considered yet. We hypothesized that subglacial discharge under sea ice is present in early spring and that its flux is sufficient to increase phytoplankton primary productivity. We evaluated the effects of the submarine discharge on primary production in a seasonally fast-ice covered Svalbard fjord (Billefjorden) influenced by a tidewater outlet glacier in April and May 2019. We found clear evidence for subglacial discharge and upwelling. Although the estimated bottom-water entrainment factor (1.6) and total fluxes were lower than in summer studies, we still observed substantial impact on the fjord ecosystem and primary production at this time of the year. The subglacial discharge leads to a salinity-stratified surface water layer and sea ice formation with low bulk salinity and permeability. The combination of the stratified surface layer, a 2-fold higher under-ice irradiance due to thinner snow cover, and higher N and Si concentrations at the glacier front supported phytoplankton primary production 2 orders of magnitude higher (42.6 mg C m−2 d−1) compared to a marine reference site at the fast-ice edge. Reciprocal transplant experiments showed that nutrient supply increased phytoplankton primary production by approximately 30 %. The brackish-water sea ice at the glacier front with its low bulk salinity contained a reduced brine volume, limiting the inhabitable brine channel space and nutrient exchange with the underlying seawater compared to full marine sea ice. Microbial and algal communities were substantially different in subglacial-influenced water and sea ice compared to the marine reference site, sharing taxa with the subglacial outflow water. We suggest that with climate change, the retreat of tidewater glaciers in early spring could lead to decreased under-ice phytoplankton primary production. In contrast, sea ice algae production and biomass may become increasingly important, unless sea ice disappears first, in which case spring phytoplankton primary production may increase.

A Transient Coupled Ice Flow‐Damage Model to Simulate Iceberg Calving From Tidewater Outlet Glaciers

AbstractIceberg calving, the detachment of an ice block at the glacier front, is the main process responsible for the dynamic mass loss from the ice sheets to the ocean. Understanding this process is essential to accurately predict ice sheet response to the future climate. We present a transient multiphysics finite‐element model to simulate iceberg break‐off and geometry evolution of a marine‐terminating glacier. The model solves the coupled equations of ice flow, damage mechanics, oceanic melt, and geometry evolution on the same Lagrangian computational grid. A modeling sensitivity analysis shows that the choice of stress measure used for damage evolution strongly influences the resulting calving front geometries. Our analysis suggests that the von Mises stress measures produce the most realistic calving front geometry evolutions for tidewater glaciers. Submarine frontal melt is shown to have a strong impact on the calving front geometry. The presented multiphysics model includes all processes thus far shown to be relevant for the evolution of tidewater glaciers and can be readily adapted for 3‐D and arbitrary bedrock geometries.

Exceptional Retreat of Kangerlussuaq Glacier, East Greenland, Between 2016 and 2018

Kangerlussuaq Glacier is one of Greenland’s largest tidewater outlet glaciers, accounting for approximately 5% of all ice discharge from the Greenland ice sheet. In 2018 the Kangerlussuaq ice front reached its most retreated position since observations began in 1932. We determine the relationship between retreat and: (i) ice velocity; and (ii) surface elevation change, to assess the impact of the retreat on the glacier trunk. Between 2016 and 2018 the glacier retreated ∼5 km and brought the Kangerlussuaq ice front into a major (∼15 km long) overdeepening. Coincident with this retreat, the glacier thinned as a result of near-terminus acceleration in ice flow. The subglacial topography means that 2016–2018 terminus recession is likely to trigger a series of feedbacks between retreat, thinning, and glacier acceleration, leading to a rapid and high-magnitude increase in discharge and sea level rise contribution. Dynamic thinning may continue until the glacier reaches the upward sloping bed ∼10 km inland of its current position. Incorporating these non-linear processes into prognostic models of the ice sheet to 2100 and beyond will be critical for accurate forecasting of the ice sheet’s contribution to sea level rise.

Local Coastal Water Masses Control Heat Levels in a West Greenland Tidewater Outlet Glacier Fjord

AbstractFjords form the gateway between the open ocean and the Greenland Ice Sheet (GIS) and consequently play a crucial role for the stability of the Ice Sheet. Hydrographic observations, especially with seasonal resolution, from these fjords are limited making it difficult to assess linkages between the fjord, coastal water masses, and freshwater discharge from GIS. Here we present a decade‐long monthly hydrographic time series from a southwest Greenland fjord in direct contact with GIS. Our observations reveal significant temporal and spatial water mass variations related to coastal, glacial, and atmospheric dynamics. During winter, the fjord circulation is dominated by seasonal dense coastal inflows and the timing of these inflows determines intermediate and deepwater temperatures. During summer, runoff from GIS leads to a pronounced freshening of the fjord. In general, the fjord's seasonal circulation system damps the seasonal variation in temperature in the fjord. This leads to a seasonal temperature range in the intermediate layer in the inner part of the fjord that is half the observed range at the fjord entrance. Changes in mean water temperatures in the intermediate layer seem predominantly linked to local coastal water masses, where cold winter/warm summer events decrease/increase the mean water temperatures. Consequently, these events play an important role in heat transport toward glacier termini.

Dynamic changes in outlet glaciers in northern Greenland from 1948 to 2015

Abstract. The Greenland Ice Sheet (GrIS) is losing mass in response to recent climatic and oceanic warming. Since the mid-1990s, tidewater outlet glaciers across the ice sheet have thinned, retreated, and accelerated, but recent changes in northern Greenland have been comparatively understudied. Consequently, the dynamic response (i.e. changes in surface elevation and velocity) of these outlet glaciers to changes at their termini, particularly calving from floating ice tongues, is poorly constrained. Here we use satellite imagery and historical maps to produce an unprecedented 68-year record of terminus change across 18 major outlet glaciers and combine this with previously published surface elevation and velocity datasets. Overall, recent (1995–2015) retreat rates were higher than at any time in the previous 47 years (since 1948). Despite increased retreat rates from the 1990s, there was distinct variability in dynamic glacier behaviour depending on whether the terminus was grounded or floating. Grounded glaciers accelerated and thinned in response to retreat over the last 2 decades, while most glaciers terminating in ice tongues appeared dynamically insensitive to recent ice tongue retreat and/or total collapse. We also identify glacier geometry (e.g. fjord width, basal topography, and ice tongue confinement) as an important influence on the dynamic adjustment of glaciers to changes at their termini. Recent grounded outlet glacier retreat and ice tongue loss across northern Greenland suggest that the region is undergoing rapid change and could soon contribute substantially to sea level rise via the loss of grounded ice.

Linear response of east Greenland’s tidewater glaciers to ocean/atmosphere warming

Predicting the retreat of tidewater outlet glaciers forms a major obstacle to forecasting the rate of mass loss from the Greenland Ice Sheet. This reflects the challenges of modeling the highly dynamic, topographically complex, and data-poor environment of the glacier-fjord systems that link the ice sheet to the ocean. To avoid these difficulties, we investigate the extent to which tidewater glacier retreat can be explained by simple variables: air temperature, meltwater runoff, ocean temperature, and two simple parameterizations of "ocean/atmosphere" forcing based on the combined influence of runoff and ocean temperature. Over a 20-y period at 10 large tidewater outlet glaciers along the east coast of Greenland, we find that ocean/atmosphere forcing can explain up to 76% of the variability in terminus position at individual glaciers and 54% of variation in terminus position across all 10 glaciers. Our findings indicate that (i) the retreat of east Greenland's tidewater glaciers is best explained as a product of both oceanic and atmospheric warming and (ii) despite the complexity of tidewater glacier behavior, over multiyear timescales a significant proportion of terminus position change can be explained as a simple function of this forcing. These findings thus demonstrate that simple parameterizations can play an important role in predicting the response of the ice sheet to future climate warming.

Sea ice breakup and marine melt of a retreating tidewater outlet glacier in northeast Greenland (81\xb0N)

Rising temperatures in the Arctic cause accelerated mass loss from the Greenland Ice Sheet and reduced sea ice cover. Tidewater outlet glaciers represent direct connections between glaciers and the ocean where melt rates at the ice-ocean interface are influenced by ocean temperature and circulation. However, few measurements exist near outlet glaciers from the northern coast towards the Arctic Ocean that has remained nearly permanently ice covered. Here we present hydrographic measurements along the terminus of a major retreating tidewater outlet glacier from Flade Isblink Ice Cap. We show that the region is characterized by a relatively large change of the seasonal freshwater content, corresponding to ~2 m of freshwater, and that solar heating during the short open water period results in surface layer temperatures above 1 °C. Observations of temperature and salinity supported that the outlet glacier is a floating ice shelf with near-glacial subsurface temperatures at the freezing point. Melting from the surface layer significantly influenced the ice foot morphology of the glacier terminus. Hence, melting of the tidewater outlet glacier was found to be critically dependent on the retreat of sea ice adjacent to the terminus and the duration of open water.

Spring bloom dynamics in a subarctic fjord influenced by tidewater outlet glaciers (Godthåbsfjord, SW Greenland)

AbstractIn high‐latitude fjord ecosystems, the spring bloom accounts for a major part of the annual primary production and thus provides a crucial energy supply to the marine food web. However, the environmental factors that control the timing and intensity of these spring blooms remain uncertain. In 2013, we studied the spring bloom dynamics in Godthåbsfjord, a large fjord system adjacent to the Greenland Ice Sheet. Our surveys revealed that the spring bloom did not initiate in the inner stratified part of the fjord system but only started farther away from tidewater outlet glaciers. A combination of out‐fjord winds and coastal inflows drove an upwelling in the inner part of the fjord during spring (April–May), which supplied nutrient‐rich water to the surface layer. This surface water was subsequently transported out‐fjord, and due to this circulation regime, the biomass accumulation of phytoplankton was displaced away from the glaciers. In late May, the upwelling weakened and the dominant wind direction changed, thus reversing the direction of the surface water transport. Warmer water was now transported toward the inner fjord, and a bloom was observed close to the glacier terminus. Overall, our findings imply that the timing, intensity, and location of the spring blooms in Godthåbsfjord are controlled by a combination of upwelling strength and wind forcing. Together with sea ice cover, the hydrodynamic regime hence plays a crucial role in structuring food web dynamics of the fjord ecosystem.

A tidewater glacier landform assemblage in Belcher Inlet, Canadian Arctic

Belcher Glacier, a 35 km long tidewater outlet glacier of the 12 000 km2 ice cap on Devon Island (Dowdeswell et al. 2004), is one of the fastest-flowing glaciers in the Canadian Arctic (Van Wychen et al. 2014) (Fig. 1). Belcher Glacier and neighbouring Fitzroy Glacier to the southeast account for about 55% of the iceberg calving loss from the Devon Ice Cap (Van Wychen et al. 2014). The terminus of Belcher Glacier remained relatively stable between the 1960s (light blue dashed line in Fig. 1a) and 2000 (Landsat 7 satellite image in Fig. 1a). In contrast, the unnamed glacier immediately to the north retreated 2 km during this period (Fig. 1a). Belcher Glacier and the unnamed glacier retreated around 500 and 250 m, respectively, between 2000 and 2014 (dark blue dashed line in Fig. 1a). The bed topography of Belcher Glacier, which is around 250 m below sea level at the present-day glacier margin (Fig. 1c) and remains below sea level in the lower 11 km of the glacier, suggests that its terminus region may become unstable in the event of future retreat. Seafloor mapping of Belcher Inlet beyond the termini of Belcher Glacier and the unnamed glacier (Fig. 1a), together with sub-bottom profiling, provide information about the dynamic behaviour of tidewater glaciers. Fig. 1. ( a ) Swath bathymetry of Belcher Inlet, Arctic Canada (acquired in 2006), superimposed on 2000 Landsat 7 satellite imagery. Multibeam acquisition systems Kongsberg EM300 and Kongsberg EM3002S. Frequency 30 and 300 kHz, respectively. Grid-cell size 10 m. Light blue and dark blue dashed lines are terminus positions derived from 1960s and 2014 aerial photographs, respectively. ( b ) Multibeam-bathymetric image of the seafloor immediately in front of Belcher Glacier. ( c ) 2014 airborne radar profile of Belcher Glacier, showing the depth of the …

Modelling subglacial discharge and its influence on ocean heat transport in Arctic fjords

Tidewater outlet glaciers are directly connected to the ocean via ice walls or floating shelves. Melting and freezing of ice, runoff, englacial, and subglacial discharge of freshwater and ocean heat transport are therefore potential feedback processes between glacial ice flow and ocean circulation. Subglacial discharge occurs at the base of tidewater glacier outlets where out-flowing freshwater forms a convective buoyant plume ascending close to the glacier face and, due to entrainment, transports relatively warm and saline ambient bottom water up towards the surface. Plume dynamics, typically occurring at sub-grid scales in regional ocean models, therefore has to be parameterized in areas where ice-ocean interactions occur, as for example in Arctic fjords. Here, we develop and analyze a new simple boundary condition of subglacial discharge where entrainment-induced transport between the subsurface and surface layer is described. A sensitivity study showed that subglacial discharge increased ocean heat transport near the glacier whereas the impact from plume-entrainment became relatively small further from the glacier. Subglacial discharge was shown to have a significant influence on surface concentrations. The impact from subglacial discharge was demonstrated in a regional model of Godthabsfjord (64°N), located at the west coast of Greenland, where surface concentrations near the glacier were shown to be sensitive to subglacial discharge in accordance with observations.

Heat sources for glacial ice melt in a west Greenland tidewater outlet glacier fjord: The role of subglacial freshwater discharge

AbstractThe melting of tidewater outlet glaciers from the Greenland Ice Sheet contributes significantly to global sea level rise. Accelerated mass loss is related to melt processes in front of calving glaciers, yet the role of ocean heat transports is poorly understood. Here we present the first direct measurements from a subglacial plume in front of a calving tidewater outlet glacier. Surface salinity in the plume corresponded to a meltwater content of 7%, which is indicative of significant entrainment of warm bottom water and, according to plume model calculations, significant ice melt. Energy balance of the area near the glacier showed that ice melt was mainly due to ocean heat transport and that direct plume‐associated melt was only important in periods with high meltwater discharge rates of ~100 m3 s−1. Ocean mixing outside of the plume area was thus the primary heat source for melting glacier ice.

Glacial meltwater and primary production are drivers of strong CO<sub>2</sub> uptake in fjord and coastal waters adjacent to the Greenland Ice Sheet

Abstract. The Greenland Ice Sheet releases large amounts of freshwater, which strongly influences the physical and chemical properties of the adjacent fjord systems and continental shelves. Glacial meltwater input is predicted to strongly increase in the future, but the impact of meltwater on the carbonate dynamics of these productive coastal systems remains largely unquantified. Here we present seasonal observations of the carbonate system over the year 2013 in the surface waters of a west Greenland fjord (Godthåbsfjord) influenced by tidewater outlet glaciers. Our data reveal that the surface layer of the entire fjord and adjacent continental shelf are undersaturated in CO2 throughout the year. The average annual CO2 uptake within the fjord is estimated to be 65 g C m−2 yr−1, indicating that the fjord system is a strong sink for CO2. The largest CO2 uptake occurs in the inner fjord near to the Greenland Ice Sheet and high glacial meltwater input during the summer months correlates strongly with low pCO2 values. This strong CO2 uptake can be explained by the thermodynamic effect on the surface water pCO2 resulting from the mixing of fresh glacial meltwater and ambient saline fjord water, which results in a CO2 uptake of 1.8 mg C kg−1 of glacial ice melted. We estimated that 28% of the CO2 uptake can be attributed to the input of glacial meltwater, while the remaining part is due to high primary production. Our findings imply that glacial melt\\-water is an important driver for undersaturation in CO2 in fjord and coastal waters adjacent to large ice sheets.

Seasonal and interannual phytoplankton production in a sub-Arctic tidewater outlet glacier fjord, SW Greenland

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 524:27-38 (2015) - DOI: https://doi.org/10.3354/meps11174 Seasonal and interannual phytoplankton production in a sub-Arctic tidewater outlet glacier fjord, SW Greenland Thomas Juul-Pedersen1,*, Kristine E. Arendt1, John Mortensen1, Martin E. Blicher1, Dorte H. Søgaard1,3, Søren Rysgaard1,2,4 1Greenland Climate Research Centre, Greenland Institute of Natural Resources, PO Box 570, 3900 Nuuk, Greenland 2Centre for Earth Observation Science, CHR Faculty of Environment Earth and Resources, University of Manitoba, 499 Wallace Building, Winnipeg, MB R3T 2N2, Canada 3University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark 4Arctic Research Centre, Aarhus University, C. F. Møllers Allé 8, 8000 Aarhus C, Denmark *Corresponding author: thpe@natur.gl ABSTRACT: This study describes seasonal patterns and proposes likely drivers of an unusual phytoplankton primary production pattern in the outer-sill region of a tidewater outlet glacier-influenced fjord (Godthåbsfjord) in SW Greenland. It is based on monthly measurements of pelagic primary production and hydrographic conditions during a 7 yr period. Total annual primary production during 2005 to 2012 was between 84.6 and 139.1 g C m-2 yr-1. Two phytoplankton blooms of similar magnitude reoccur in the fjord every year. A ‘classical’ spring bloom of up to 1743 mg C m-2 d-1 occurred in late April/early May in a water column almost fully mixed due to tidal forces at the fjord sill. After the spring bloom, primary production decreased in June, after which a summer bloom of up to 1383 mg C m-2 d-1 built up. This bloom coincided with the development of a pycnocline caused by substantial runoff from the Greenland Ice Sheet every year during midsummer. This observation supports a hypothesis that fjord circulation modes and subglacial freshwater discharge, leading to upwelling of nutrient rich water, stimulate primary production in the fjord. Future changes in the timing or magnitude of meltwater runoff from the Greenland Ice Sheet are thus likely to affect phytoplankton dynamics in the fjord. KEY WORDS: Phytoplankton production · Seasonal patterns · Time series · Glacial meltwater · Sub-Arctic fjord · Greenland Full text in pdf format PreviousNextCite this article as: Juul-Pedersen T, Arendt KE, Mortensen J, Blicher ME, Søgaard DH, Rysgaard S (2015) Seasonal and interannual phytoplankton production in a sub-Arctic tidewater outlet glacier fjord, SW Greenland. Mar Ecol Prog Ser 524:27-38. https://doi.org/10.3354/meps11174 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 524. Online publication date: March 30, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

Seasonal variability of the circulation system in a west Greenland tidewater outlet glacier fjord, Godthåbsfjord (64°N)

Many tidewater outlet glacier fjords surround the coast of Greenland, and their dynamics and circulation are of great importance for understanding the heat transport toward glaciers from the ice sheet. Thus, fjord circulation is a critical aspect for assessing the threat of global sea level rise due to melting of the ice sheet. However, very few observational studies describe the seasonal dynamics of fjord circulation. Here we present the first continuous current measurements (April–November) from a deep mooring deployed in a west Greenland tidewater outlet glacier fjord. Four distinct circulation phases are identified during the period, and they are related to exchange processes with coastal waters, tidal mixing, and melt processes on the Greenland Ice Sheet. During early summer, warm intermediate water is transported toward the glacier at an average velocity of about 7 cm s−1. In late summer, the average velocity decreases to 3 cm s−1 during a period with significant subglacial freshwater discharges. During this period, a large variability in current velocities is also observed. The associated average heat transport in an intermediate-depth range corresponds to 568 GW in early summer and is reduced to 287 GW in late summer. These heat fluxes are at the higher end of previously reported fluxes. Our measurements show that the intermediate heat transport varies over time and during summer provides a major contribution to the heat budget and, thereby, potentially to glacial melt. We suggest that intermediate heat transport may play a similar important role in other fjords around Greenland.

Ice–ocean interaction and calving front morphology at two west Greenland tidewater outlet glaciers

Abstract. Warm, subtropical-originating Atlantic water (AW) has been identified as a primary driver of mass loss across the marine sectors of the Greenland Ice Sheet (GrIS), yet the specific processes by which this water mass interacts with and erodes the calving front of tidewater glaciers is frequently modelled and much speculated upon but remains largely unobserved. We present a suite of fjord salinity, temperature, turbidity versus depth casts along with glacial runoff estimation from Rink and Store glaciers, two major marine outlets draining the western sector of the GrIS during 2009 and 2010. We characterise the main water bodies present and interpret their interaction with their respective calving fronts. We identify two distinct processes of ice–ocean interaction which have distinct spatial and temporal footprints: (1) homogenous free convective melting which occurs across the calving front where AW is in direct contact with the ice mass, and (2) localised upwelling-driven melt by turbulent subglacial runoff mixing with fjord water which occurs at distinct injection points across the calving front. Throughout the study, AW at 2.8 ± 0.2 °C was consistently observed in contact with both glaciers below 450 m depth, yielding homogenous, free convective submarine melting up to ~200 m depth. Above this bottom layer, multiple interactions are identified, primarily controlled by the rate of subglacial fresh-water discharge which results in localised and discrete upwelling plumes. In the record melt year of 2010, the Store Glacier calving face was dominated by these runoff-driven plumes which led to a highly crenulated frontal geometry characterised by large embayments at the subglacial portals separated by headlands which are dominated by calving. Rink Glacier, which is significantly deeper than Store has a larger proportion of its submerged calving face exposed to AW, which results in a uniform, relatively flat overall frontal geometry.

Ice-dammed lake drainage cools and raises surface salinities in a tidewater outlet glacier fjord, west Greenland

The drainage of ice-dammed lakes in the form of outburst floods in Greenland is detected regularly by remote sensing, and these events are expected to occur more frequently in a warmer climate. However, their impact on ice sheet stability and neighboring water bodies is still unknown. In this interdisciplinary study, we investigate lake drainages from the Greenland Ice Sheet into a west Greenland fjord by analyzing simultaneous time series of satellite observations and direct hydrographic measurements of temperature and salinity in the fjord. Satellite images show that, in general, lake drainages have occurred quasiperiodically during the last decade. A particular sequence of drainage events was observed by satellite in 2009 and was analyzed together with the first direct hydrographic observations. Signs of ice-dammed lake drainages were observed by a downstream mooring located just below the intertidal zone. The release of freshwater occurred at the fjord subsurface at a tidewater outlet glacier. The downstream in-water sequence of property changes in relation to these drainage events was observed as an almost immediate decrease in surface layer temperature (~2°C) followed within a week by the arrival of a high-saline pulse (~ +5 units) with elevated salinity lasting for several days during the passage. During lake drainages, large amounts of relatively warm and saline intermediate water are brought to the near-surface layers by entrainment processes near the glacier front, and this influences the hydrography of the fjord but also impacts the ecosystem through upwelling of nutrient-rich intermediate water.

Oceanic heat delivery via Kangerdlugssuaq Fjord to the south‐east Greenland ice sheet

AbstractAcceleration of the Greenland Ice Sheet (GrIS) tidewater outlet glaciers has increased the ice sheet's contribution to global sea level rise over the last two decades. Coincident increases in atmospheric temperatures around Greenland explain some of the increased ice loss, but warm Atlantic‐origin water (AW) is increasingly recognized as contributing to the accelerating ice‐mass loss, particularly, via the outlet glaciers of south‐east (SE) Greenland. However, there remains a lack of understanding of the variability in heat content of the water masses found to the east of Greenland and how this heat is communicated to the outlet glaciers of the GrIS. Here a new analysis is presented of ocean/GrIS interaction in which the oceanic heat flux toward the ice sheet in Kangerdlugssuaq Fjord (0.26 TW) is an order‐of‐magnitude greater than that reported for the other major outlet glacier of SE Greenland (Helheim). Heat delivered by AW to the calving front of Kangerdlugssuaq is equivalent to ∼10 m d−1 melt (i.e., 30–60% of the ice flow speed), and thus is highly significant. During the observational campaign in September 2010 warm Polar Surface Water (PSWw) melted a substantial volume of ice within the fjord; equivalent to 25% of the volume melted by AW alone. Satellite‐derived sea surface temperatures show large interannual variability in PSWw over the 20 year period 1991–2011. Anomalously warm PSWw was observed within the fjord prior to the well‐documented major ice front retreats of May 2004 and November 2010.

High sensitivity of tidewater outlet glacier dynamics to shape

Abstract. Variability in tidewater outlet glacier behavior under similar external forcing has been attributed to differences in outlet shape (i.e., bed elevation and width), but this dependence has not been investigated in detail. Here we use a numerical ice flow model to show that the dynamics of tidewater outlet glaciers under external forcing are highly sensitive to width and bed topography. Our sensitivity tests indicate that for glaciers with similar discharge, the trunks of wider glaciers and those grounded over deeper basal depressions tend to be closer to flotation, so that less dynamically induced thinning results in rapid, unstable retreat following a perturbation. The lag time between the onset of the perturbation and unstable retreat varies with outlet shape, which may help explain intra-regional variability in tidewater outlet glacier behavior. Further, because the perturbation response is dependent on the thickness relative to flotation, varying the bed topography within the range of observational uncertainty can result in either stable or unstable retreat due to the same perturbation. Thus, extreme care must be taken when interpreting the future behavior of actual glacier systems using numerical ice flow models that are not accompanied by comprehensive sensitivity analyses.