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Seasonality of freshwater in the east Greenland current system from 2014 to 2016
Full text linkThe initial 2 years of Overturning in the Subpolar North Atlantic Program mooring data (2014–2016) provide the first glimpse into the seasonality of freshwater in the complete East Greenland Current system. Using a set of eight moorings southeast of Greenland at 60∘ N, we find two distinct, persistent velocity cores on the shelf and slope. These are the East Greenland Coastal Current, which carries cold, fresh water from the Arctic and Greenland along the shelf, and the East Greenland/Irminger Current over the slope, which is a combination of cold, fresh waters and warm, salty waters of Atlantic origin. Together, these currents carry 70% of the freshwater transport across the subpolar North Atlantic east of Greenland. The freshwater transport referenced to a salinity of 34.9 is approximately equipartitioned between the coastal current (East Greenland Coastal Current) and the fresh portion of the slope current (East Greenland Current), which carry 42 ± 6 and 32 ± 6 mSv, respectively. The coastal and slope current freshwater transports have staggered seasonality during the observed period, peaking in December and March, respectively, suggesting that summer surveys have underestimated freshwater transport in this region. We find that the continental slope is freshest in the winter, when surface cooling mixes freshwater off the shelf. This previously unmeasured freshwater over the slope is likely to enter the Labrador Sea downstream, where it can impact deep convection
Past to Future and Land to Sea: constraining global glacier models by observations and exploring ice-ocean interactions
No full textGlacier mass loss is an iconic process induced by anthropogenic climate change. It threatens human livelihood at coasts affected by the rising sea level and in glacierized hydrological basins where the glacial runoff is essential for water availability. Moreover, as glacier mass loss adds large amounts of freshwater to the oceans, it might alter ocean circulation in a way that affects marine ecosystems and the climate system. Only recently, satellite-data processing revealed mass changes on an individual glacier level (outside the large ice sheets), but only for the last two decades. Glacier mass change observations become increasingly sparse going back in time. Therefore, the glaciers’ past contribution to global mean sea level rise can only be reconstructed using numerical models. Since glacier mass change will continue during this century, it is vital to understand how this will affect global mean sea level, ocean circulation, and regional hydrology. Again, this is only possible using numerical models. Hence, it is essential to improve these models by incorporating previously neglected processes of glacier mass change into them, mainly in the form of parametrizations, and by constraining them using observations. Moreover, it is crucial to understand the uncertainties of results produced by numerical models, as they can never fully represent the natural world, which also hinges on the amount and quality of observational data. This work will tackle aspects of three issues in numerically modeling glacier mass changes: past glacier mass change reconstructions’ uncertainties, future mass change projections’ uncertainties, specifically regarding marine-terminating glaciers, and ice-ocean interactions in the northern hemisphere outside the Greenland ice sheet. All three issues are relevant in addressing the question of how glaciers respond to changes in their mass balance due to climatic changes and what consequences such changes have for the Earth system and, ultimately, human livelihood. It is found that the further outside the glaciological and meteorological observations’ spatial and temporal domain a numerical model is applied, the more uncertain reconstructed glacier mass changes become. Similarly, one primary source of uncertainty in future glacier mass change projections is the difference in climate models’ outputs of near-surface temperatures and precipitation. More accurately describing marine-terminating glacier dynamics and considering volume changes below sea level reduces estimates of future glacier contribution to global mean sea level rise systematically. However, significant uncertainties due to uncertainty about appropriate values for parameters involved in modeling (marine-terminating) glaciers’ dynamics are detected. Concerning ice-ocean interactions, it was found that including the freshwater input from glacier mass loss in the northern hemisphere (outside the Greenland ice sheet) in an ocean general circulation model significantly impacts the simulated high-latitude ocean circulation. Finally, a first estimate of the ice mass glaciers lose due to melting directly into the ocean was produced
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Understanding iceberg and glacier melt from ocean observations in Greenland fjords
Full text linkThe glacial fjords that connect the Greenland Ice Sheet to the North Atlantic control ocean heat transport toward the ice sheet and the downstream fate of glacier meltwater. This thesis builds on a growing body of research into Greenland fjord dynamics, focusing on aspects of glacier-fjord systems that are especially challenging to observe: sub-annual ocean variability beneath a floating ice tongue; iceberg meltwater properties and distribution; and the distribution and cycling of environmental mercury.Ice discharge to the ocean can be moderated by ice tongues, floating extensions of glaciers that buttress the upstream ice flow. In Chapter 3, an ice-tethered mooring record from beneath the 79 North Glacier ice tongue shows that ocean warming observed on the continental shelf is advected into the fjord and reaches the glacier grounding line within 6 months, indicating that basal melt of the ice tongue is sensitive to regional ocean variability. Icebergs calved from tidewater glaciers are a major component of fjord freshwater and heat budgets in fjords, but there are few observations to constrain iceberg melt models. In Chapter 4, meltwater plume intrusions are identified based on their temperature and salinity properties in two surveys of a large iceberg in Sermilik Fjord in southeast Greenland. The intrusions are distributed around the iceberg between 80-250 m depth and drive upwelling over vertical scales averaging 15-50 m, with the plume height primarily controlled by stratification. A standard melt plume model does not recreate the observed melt concentrations even with adjustments to the model coefficients, suggesting that more substantial modifications to the model physics are needed to accurately simulate iceberg melt and upwelling.In Chapter 5, results from a recent survey in Sermilik Fjord show that glacially modified waters are depleted in the toxic trace element mercury relative to regional ocean waters, indicating that glacier melt is not a significant source of environmental mercury in that system. We hypothesize that mercury is removed from the water column in the ice melange region near the glacier terminus through scavenging and settling of suspended sediments from iceberg melt and runoff
Heat and freshwater transport through the central Labrador Sea
Full text linkAuthor 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): 606-628, doi:10.1175/JPO2875.1.The seasonal and interannual variations in the export of Labrador Sea Water (LSW), and in the heat and freshwater transport through the central Labrador Sea, are examined for two different periods: from 1964 to 1974, using Ocean Weather Station Bravo data, and from 1996 to 2000, using data collected from profiling floats. A typical seasonal cycle involves a 300-m thickening of LSW (convection) followed by an equivalent thinning (restratification). Restratification is characterized by a drift of properties toward boundary current values that is indicative of a vigorous lateral exchange. The net result is a convergence of heat and salt, between 200 and 700 m, that balances the net surface heat loss to the atmosphere and partially offsets the surface freshwater accumulation due to surface, lateral exchange. Interannual variations in the export of LSW can be explained by taking into account changes in the central Labrador Sea–boundary current density gradient, which governs the lateral exchange. Interannual variations in how much heat is converged into the region, on the other hand, mostly reflect changes in the temperature of LSW. This only partly explains, however, the increased convergence of heat that occurs during the late 1990s. In years in which convection does not occur, restratification trends continue throughout the entire year, albeit at a reduced rate.This work was supported by NSF Grant OCE
02-40978, the John E. and Anne W. Sawyer Endowed
Fund, and the Grayce B. Kerr Fund
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Interannual variability of oceanographic conditions on the continental shelf outside of Sermilik Fjord
Full text linkFreshwater flux from the Greenland Ice Sheet has the capacity to affect regional and global circulation as well as contribute to sea level rise. Outlet glaciers are an important part of this process. They provide pathways for inland glaciers and ice sheets to melt and drain into the surrounding oceans. The Helheim Glacier, one of the largest outlet glaciers in East Greenland, is connected to the North Atlantic Ocean through the Sermilik Fjord. Warmer waters that penetrate the fjord can reach the glacier and contribute to melting, but the mechanism for this transport is not fully understood. In this thesis, we analyze mooring and summer conductivity, temperature, and depth (CTD) profiles from over a decade of research cruises. Our analysis focuses on the continental shelf adjacent to the fjord and broadens the current understanding of the variability of water masses entering and exiting the fjord. We found that the presence of the two main water masses, Polar Water (fresher and colder water) and Atlantic Water (higher salinity and warmer water), varied each year. The variation was most notable in the depth and extent of the two water masses, with Atlantic Water dominating the water column in some years and Polar Water in others. We also observed a consistent seasonal cycle for each year, with two minima of temperature and salinity, one concurrent maximum in temperature and salinity, and one isolated maximum of temperature. This behavior shows that despite the large daily variability in the region, there is still a consistent seasonal trend of temperature and salinity from one year to the next. The drivers of the seasonal cycle may be linked to the freshwater content over the shelf as well as the interannual variability observed in the water masses
On the connection between dense water formation, overturning, and poleward heat transport in a convective basin
Full text linkAuthor 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)
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Observations of ocean transport and mixing across scales in the Subpolar North Atlantic and Greenland
Full text linkOcean dynamics in the subpolar North Atlantic play an important role in the global climate system: Deep convection in the open ocean drives a large-scale overturning circulation that redistributes heat around the globe and cold currents circulating around Greenland protect marine-terminating glaciers from warmer water offshore that drives ice-melt and sea level rise. However, the drivers of natural variability within this complex system, and how the region will respond to anthropogenic forcing, are not fully understood. This thesis sheds light on two regions of the subpolar North Atlantic: the Irminger Sea and the Greenland continental shelf, where the mesoscale dynamics that deliver heat to these otherwise cold regions are studied.The Irminger Sea is a region of deep convection, contributing to the water mass transformation and large-scale circulation. In Chapter 1, observations from profiling Argo floats and repeat hydrography are used to track buoyancy changes in the Irminger interior and study the region’s recovery from wintertime convection. This work shows that the slow recovery of the Irminger interior after a year of particularly strong deep convection can be attributed to reduced transport of warm, buoyant eddies from the Irminger Current.On Greenland’s continental shelf, cold, fresh waters insulate the Greenland Ice Sheet. Here, cross-shelf exchange that delivers heat on-shelf is studied. In Chapter 2, observations from a two-week ship-based survey of Narsaq Trough are used to investigate how troughs (glacially carved depressions in the continental shelf) modify the local continental shelf environment. This snapshot suggests that the trough drives a subsurface exchange flow that produces mid-depth waters in the trough that are a mixture of on-shelf and off-shelf waters. Chapter 3 contextualises the findings from Chapter 2 with two decades of repeat observations over Narsaq Trough by voluntary observing ships. This extensive time-series shows a bathymetrically-steered exchange flow is a persistent feature at the trough, both above and below the shelf, driving cross-shelf exchange of heat and salt.Cumulatively, the studies described here show how eddies and bathymetrically-steered flows at the boundaries of the sub-polar North Atlantic basins supply heat to regions which are unreachable by large-scale ocean processes
Temperature, salinity, dissolved oxygen, turbidity, and fluorescence profiles from Sermilik Fjord and the Southeast Greenland shelf, collected in August 2023
No full textProfiles of temperature, salinity, and turbidity from Sermilik Fjord (Greenland) during August 2010
No full textTemperature, salinity, dissolved oxygen, and turbidity profiles from Sermilik Fjord, East Greenland, collected in July and August 2019
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