Impact of ocean stratification on submarine melting of a major Greenland outlet glacier

Submarine melting is an important balance term for tidewater glaciers1,2 and recent observations point to a change in the submarine melt rate as a potential trigger for the widespread acceleration of outlet glaciers in Greenland3-5. Our understanding of the dynamics involved, and hence our ability to interpret past and predict future variability of the Greenland Ice Sheet, however, is severely impeded by the lack of measurements at the ice/ocean interface. To fill this gap, attempts to quantify the submarine melt rate and its variability have relied on a paradigm developed for tidewater glaciers terminating in fjords with shallow sills. In this case, the fjords’ waters are mostly homogeneous and the heat transport to the terminus, and hence the melt rate, is controlled by a single overturning cell in which glacially modified water upwells at the ice edge, driving an inflow at depth and a fresh outflow at the surface1. Greenland’s fjords, however, have deep sills which allow both cold, fresh Arctic and warm, salty Atlantic waters, circulating around Greenland, to reach the ice sheet margin3,6,7. Thus, Greenland’s glaciers flow into strongly stratified fjords and the generic tidewater glacier paradigm is not applicable. Here, using new summer data collected at the margins of Helheim Glacier, East Greenland, we show that melting is driven by both Atlantic and Arctic waters and that the circulation at the ice edge is organized in multiple, overturning cells that arise from their different properties. Multiple cells with different characteristics are also observed in winter, when glacial run off is at a minimum and there is little surface outflow. These results indicate that stratification in the fjord waters has a profound impact on the melting dynamics and suggest that the shape and stability of Greenland’s glaciers are strongly influenced by layering and variability in the Arctic and Atlantic waters. 

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Delayed Freshwater Export from a Greenland tidewater glacial fjord

Freshwater from the Greenland Ice Sheet is routed to the ocean through narrow fjords along the coastline where it impacts ecosystems both within the fjord and on the continental shelf, regional circulation, and potentially the global overturning circulation. However, the timing of freshwater export is sensitive to the residence time of waters within glacial fjords. Here, we present evidence of seasonal freshwater storage in a tidewater glacial fjord using hydrographic and velocity data collected over 10 days during the summers of 2012 and 2013 in Saqqarleq (SQ), a midsize fjord in West Greenland. The data revealed a rapid freshening trend of −0.05 ± 0.01 g/kg/day and −0.04 ± 0.01 g/kg/day, in 2012 and 2013, respectively, within the intermediate layer of the fjord (15–100 m) less than 2.5 km from the glacier terminus. The freshening trend is driven, in part, by the downward mixing of outflowing glacially−modified water near the surface and increasingly stratifies the fjord from the surface downwards over the summer melt season. We construct a box model which recreates the first−order dynamics of the fjord and describes freshwater storage as a balance between friction and density−driven exchange outside the fjord. The model can be used to diagnose the timescale for this balance to be reached, and for SQ we find a month lag between subglacial meltwater discharge and net freshwater export. These results indicate a fjord−induced delay in freshwater export to the ocean that should be represented in large−scale models seeking to understand the impact of Greenland freshwater on the regional climate system

Impact of icebergs on the seasonal submarine melt of Sermeq Kujalleq

The role of icebergs in narrow fjords hosting marine-terminating glaciers in Greenland is poorly understood, even though iceberg melt results in a substantial freshwater flux that can exceed the subglacial discharge. Furthermore, the melting of deep-keeled icebergs modifies the vertical stratification of the fjord and, as such, can impact ice–ocean exchanges at the glacier front. We model an idealised representation of the high-silled Ilulissat Icefjord in West Greenland with the MITgcm ocean circulation model, using the IceBerg package to study the effect of submarine iceberg melt on fjord water properties over a runoff season, and compare our results with available observations from 2014. We find the subglacial discharge plume to be the primary driver of the seasonality of circulation, glacier melt and iceberg melt. Furthermore, we find that melting of icebergs modifies the fjord in three main ways: first, icebergs cool and freshen the water column over their vertical extent; second, iceberg-melt-induced changes to fjord stratification cause the neutral buoyancy depth of the plume and the export of glacially modified waters to be deeper; third, icebergs modify the deep basin, below their vertical extent, by driving mixing of the glacially modified waters with the deep-basin waters and by modifying the incoming ambient waters. Through the combination of cooling and causing the subglacial-discharge-driven plume to equilibrate deeper, icebergs suppress glacier melting in the upper layer, resulting in undercutting of the glacier front. Finally, we postulate that the impact of submarine iceberg melt on the neutral buoyancy depth of the plume is a key mechanism linking the presence of an iceberg mélange with the glacier front, without needing to invoke mechanical effects.publishedVersio

Estimating ocean heat transports and submarine melt rates in Sermilik Fjord, Greenland, using lowered acoustic Doppler current profiler (LADCP) velocity profiles

Author Posting. © International Glaciological Society, 2012. This article is posted here by permission of International Glaciological Society for personal use, not for redistribution. The definitive version was published in Annals of Glaciology 53 (2012): 50-58, doi:10.3189/2012AoG60A050.Submarine melting at the ice-ocean interface is a significant term in the mass balance of marine-terminating outlet glaciers. However, obtaining direct measurements of the submarine melt rate, or the ocean heat transport towards the glacier that drives this melting, has been difficult due to the scarcity of observations, as well as the complexity of oceanic flows. Here we present a method that uses synoptic velocity and temperature profiles, but accounts for the dominant mode of velocity variability, to obtain representative heat transport estimates. We apply this method to the Sermilik Fjord-Helheim Glacier system in southeastern Greenland. Using lowered acoustic Doppler current profiler (LADCP) and hydrographic data collected in summer 2009, we find a mean heat transport towards the glacier of 29 × 109 W, implying a submarine melt rate at the glacier face of 650 m a-1. The resulting adjusted velocity profile is indicative of a multilayer residual circulation, where the meltwater mixture flows out of the fjord at the surface and at the stratification maximum.Funding for this work came from US National Science Foundation (NSF) Office of Polar Programs (OPP) grant 0909373 and the WHOI Arctic Research Initiative.2013-05-0

On the Relation between Thermohaline Anomalies and Water Mass Transformation in the Eastern Subpolar North Atlantic

Decadal thermohaline anomalies carried northward by the North Atlantic Current are an important source of predictability in the North Atlantic region. Here, we investigate whether these thermohaline anomalies influence surface-forced water mass transformation (SFWMT) in the eastern subpolar gyre using the reanalyses EN4.2.2 for the ocean and the ERA5 for the atmosphere. In addition, we follow the propagation of thermohaline anomalies along two paths: in the subpolar North Atlantic and the Norwegian Sea. We use observation-based datasets (HadISST, EN4.2.2, and Ishii) between 1947 and 2021 and apply complex empirical orthogonal functions. Our results show that when a warm anomaly enters the eastern subpolar gyre, more SFWMT occurs in light-density classes (27.0–27.2 kg m−3). In contrast, when a cold anomaly enters the eastern subpolar gyre, more SFWMT occurs in denser classes (27.4–27.5 kg m−3). Following the thermohaline anomalies in both paths, we find alternating warm–salty and cold–fresh subsurface anomalies, repeating throughout the 74-yr-long record with four warm–salty and cold–fresh periods after the 1950s. The cold–fresh anomaly periods happen simultaneously with the Great Salinity Anomaly events. Moreover, the propagation of thermohaline anomalies is faster in the subpolar North Atlantic (SPNA) than in the Norwegian Sea, especially for temperature anomalies. These findings might have implications for our understanding of the decadal variability of the lower limb of the Atlantic meridional overturning circulation and predictability in the North Atlantic region.publishedVersio

Increased Greenland melt triggered by large-scale, year-round cyclonic moisture intrusions

Surface melting is a major driver of Greenland's mass loss. Yet, the mechanisms that trigger melt are still insufficiently understood because seasonally based studies blend processes initiating melt with positive feedbacks. Here, we focus on the triggers of melt by examining the synoptic atmospheric conditions associated with 313 rapid melt increases, detected in a satellite-derived melt extent product, equally distributed throughout the year over the period 1979–2012. By combining reanalysis and weather station data, we show that melt is initiated by a cyclone-driven, southerly flow of warm, moist air, which gives rise to large-scale precipitation. A decomposition of the synoptic atmospheric variability over Greenland suggests that the identified, melt-triggering weather pattern accounts for ∼40 % of the net precipitation, but increases in the frequency, duration and areal extent of the initiated melting have shifted the line between mass gain and mass loss as more melt and rainwater run off or accumulate in the snowpack. Using a regional climate model, we estimate that the initiated melting more than doubled over the investigated period, amounting to ∼28 % of the overall surface melt and revealing that, despite the involved mass gain, year-round precipitation events are participating in the ice sheet's decline

Quantifying flow regimes in a Greenland glacial fjord using iceberg drifters

Large, deep-keeled icebergs are ubiquitous in Greenland's outlet glacial fjords. Here we use the movement of these icebergs to quantify flow variability in Sermilik Fjord, southeast Greenland, from the ice mélange through the fjord to the shelf. In the ice mélange, a proglacial mixture of sea ice and icebergs, we find that icebergs consistently track the glacier speed, with slightly faster speeds near terminus and episodic increases due to calving events. In the fjord, icebergs accurately capture synoptic circulation driven by both along-fjord and along-shelf winds. Recirculation and in-/out-fjord variations occur throughout the fjord more frequently than previously reported, suggesting that across-fjord velocity gradients cannot be ignored. Once on the shelf, icebergs move southeastward in the East Greenland Coastal Current, providing wintertime observations of this freshwater pathway.Funded by The National Science Foundation. Grant Numbers: OCE-1130008, ARC-0909274 and The University of Oregon

The outflow from Hudson Strait and its contribution to the Labrador Current

Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 55 (2008): 926-946, doi:10.1016/j.dsr.2008.03.012.Hudson Strait delivers a large amount of fresh water to the subpolar North Atlantic due to a large riverine input into the upstream Hudson Bay System and to the rerouting of Arctic Ocean waters. The fresh waters flowing out of Hudson Strait feed the Labrador Current, a current that has a significant impact on the climate and ecosystem of the entire northeastern seaboard. The lack of measurements from the strait have, until recently, made it difficult to determine the relative contribution of Hudson Strait to the properties and variability of the Labrador Current compared to other sources. This study describes the first year round observations of the outflow as obtained from a moored array deployed midstrait from August 2004 to 2005, and from a highresolution hydrographic section conducted in September of 2005. The outflow from Hudson Strait has the structure of a buoyant boundary current spread across the sloping topography of its southern edge. The variability in the flow is dominated by the extreme semidiurnal tides and by vigorous, mostly barotropic, fluctuations over several days. The fresh water export is seasonally concentrated between June and March with a peak in NovemberDecember, consistent with the seasonal riverine input and seaice melt. It is highly variable on weekly timescales due to synchronous salinity and velocity variations. The estimated volume and liquid fresh water transports during 20042005 are respectively of 11.2 Sv and 7888 (2829) mSv relative to a salinity of 34.8 (33). This implies that the Hudson Strait outflow accounts for approximately 15% of the volume and 50% of the fresh water transports of the Labrador Current. This larger than previously estimated contribution is partially due to the recycling, within the Hudson Bay System, of relatively fresh waters that flow into Hudson Strait, along its northern edge. It is speculated that the source of this inflow is the outflow from Davis Strait.Straneo acknowledges support from the Woods Hole Oceanographic Institution's Ocean and Climate Change Institute and the Comer Foundation, in particular, as well as support for NSF OCE0629411. Support to FJS from NSERC Research Grant and the Canadian Program on Energy Research and Development

On the connection between dense water formation, overturning, and poleward heat transport in a convective basin

Author Posting. © American Meteorological Society, 2006. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 36 (2006): 1822-1840, doi:10.1175/JPO2932.1.An isopycnal, two-layer, idealized model for a convective basin is proposed, consisting of a convecting, interior region and a surrounding boundary current (buoyancy and wind-driven). Parameterized eddy fluxes govern the exchange between the two. To balance the interior buoyancy loss, the boundary current becomes denser as it flows around the basin. Geostrophy imposes that this densification be accompanied by sinking in the boundary current and hence by an overturning circulation. The poleward heat transport, associated with convection in the basin, can thus be viewed as a result of both an overturning and a horizontal circulation. When adapted to the Labrador Sea, the model is able to reproduce the bulk features of the mean state, the seasonal cycle, and even the shutdown of convection from 1969 to 1972. According to the model, only 40% of the poleward heat (buoyancy) transport of the Labrador Sea is associated with the overturning circulation. An exact solution is presented for the linearized equations when changes in the boundary current are small. Numerical solutions are calculated for variations in the amount of convection and for changes in the remotely forced circulation around the basin. These results highlight how the overturning circulation is not simply related to the amount of dense water formed. A speeding up of the circulation around the basin due to wind forcing, for example, will decrease the intensity of the overturning circulation while the dense water formation remains unvaried. In general, it is shown that the fraction of poleward buoyancy (or heat) transport carried by the overturning circulation is not an intrinsic property of the basin but can vary as a result of a number of factors.This work was supported by NSF OCE 02-40978 and by the Climate Institute at the Woods Hole Oceanographic Institution (WHOI)

An Interdisciplinary Perspective on Greenland’s Changing Coastal Margins

Greenland’s coastal margins are influenced by the confluence of Arctic and Atlantic waters, sea ice, icebergs, and meltwater from the ice sheet. Hundreds of spectacular glacial fjords cut through the coastline and support thriving marine ecosystems and, in some places, adjacent Greenlandic communities. Rising air and ocean temperatures, as well as glacier and sea-ice retreat, are impacting the conditions that support these systems. Projecting how these regions and their communities will evolve requires understanding both the large-scale climate variability and the regional-scale web of physical, biological, and social interactions. Here, we describe pan-Greenland physical, biological, and social settings and show how they are shaped by the ocean, the atmosphere, and the ice sheet. Next, we focus on two communities, Qaanaaq in Northwest Greenland, exposed to Arctic variability, and Ammassalik in Southeast Greenland, exposed to Atlantic variability. We show that while their climates today are similar to those of the warm 1930s­–1940s, temperatures are projected to soon exceed those of the last 100 years at both locations. Existing biological records, including fisheries, provide some insight on ecosystem variability, but they are too short to discern robust patterns. To determine how these systems will evolve in the future requires an improved understanding of the linkages and external factors shaping the ecosystem and community response. This interdisciplinary study exemplifies a first step in a systems approach to investigating the evolution of Greenland’s coastal margins