Seasonal overturning of the Labrador Sea as observed by Argo floats

Author Posting. © American Meteorological Society, 2017. 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 47 (2017): 2531-2543, doi:10.1175/JPO-D-17-0051.1.Argo floats are used to investigate Labrador Sea overturning and its variability on seasonal time scales. This is the first application of Argo floats to estimate overturning in a deep-water formation region in the North Atlantic. Unlike hydrographic measurements, which are typically confined to the summer season, floats offer the advantage of collecting data in all seasons. Seasonal composite potential density and absolute geostrophic velocity sections across the mouth of the Labrador Sea assembled from float profiles and trajectories at 1000 m are used to calculate the horizontal and overturning circulations. The overturning exhibits a pronounced seasonal cycle; in depth space the overturning doubles throughout the course of the year, and in density space it triples. The largest overturning [1.2 Sv (1 Sv ≡ 106 m3 s−1) in depth space and 3.9 Sv in density space] occurs in spring and corresponds to the outflow of recently formed Labrador Sea Water. The overturning decreases through summer and reaches a minimum in winter (0.6 Sv in depth space and 1.2 Sv in density space). The robustness of the Argo seasonal overturning is supported by a comparison to an overturning estimate based on hydrographic data from the AR7W line.NSF OCE-1459474 supported this work.2018-04-1

Water exchange between the continental shelf and the cavity beneath Nioghalvfjerdsbræ (79 North Glacier)

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 7648–7654, doi:10.1002/2015GL064944.The mass loss at Nioghalvfjerdsbræ is primarily due to rapid submarine melting. Ocean data obtained from beneath the Nioghalvfjerdsbræ ice tongue show that melting is driven by the presence of warm (1°C) Atlantic Intermediate Water (AIW). A sill prevents AIW from entering the cavity from Dijmphna Sund, requiring that it flow into the cavity via bathymetric channels to the south at a pinned ice front. Comparison of water properties from the cavity, Dijmphna Sund, and the continental shelf support this conclusion. Overturning circulation rates inferred from observed melt rates and cavity stratification suggest an exchange flow between the cavity and the continental shelf of 38mSv, sufficient to flush cavity waters in under 1 year. These results place upper bounds on the timescales of external variability that can be transmitted to the glacier via the ice tongue cavity.NASA Grant Number: NNX13AK88G, NSF Grant Number: OCE-14340412016-03-2

Heat, salt, and freshwater budgets for a glacial fjord in Greenland

Author Posting. © American Meteorological Society, 2016. 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 46 (2016): 2735-2768, doi:10.1175/JPO-D-15-0134.1.In Greenland’s glacial fjords, heat and freshwater are exchanged between glaciers and the ocean. Submarine melting of glaciers has been implicated as a potential trigger for recent glacier acceleration, and observations of ocean heat transport are increasingly being used to infer the submarine melt rates. The complete heat, salt, and mass budgets that underlie such methods, however, have been largely neglected. Here, a new framework for exploring glacial fjord budgets is developed. Building on estuarine studies of salt budgets, the heat, salt, and mass transports through the fjord are decomposed, and new equations for calculating freshwater fluxes from submarine meltwater and runoff are presented. This method is applied to moored records from Sermilik Fjord, near the terminus of Helheim Glacier, to evaluate the dominant balances in the fjord budgets and to estimate freshwater fluxes. Throughout the year, two different regimes are found. In the nonsummer months, advective transports are balanced by changes in heat/salt storage within their ability to measure; freshwater fluxes cannot be inferred as a residual. In the summer, a mean exchange flow emerges, consisting of inflowing Atlantic water and outflowing glacially modified water. This exchange transports heat toward the glacier and is primarily balanced by changes in storage and latent heat for melting ice. The total freshwater flux increases over the summer, reaching 1200 ± 700 m3 s−1 of runoff and 1500 ± 500 m3 s−1 of submarine meltwater from glaciers and icebergs in August. The methods and results highlight important components of fjord budgets, particularly the storage and barotropic terms, that have been not been appropriately considered in previous estimates of submarine melting.The data collection and analysis was funded by NSF Grants ARC-0909373, OCE-113008, and OCE-1434041

Observations of water mass transformation and eddies in the Lofoten basin of the Nordic Seas

Author Posting. © American Meteorological Society, 2015. 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 45 (2015): 1735–1756, doi:10.1175/JPO-D-14-0238.1.The Lofoten basin of the Nordic Seas is recognized as a crucial component of the meridional overturning circulation in the North Atlantic because of the large horizontal extent of Atlantic Water and winter surface buoyancy loss. In this study, hydrographic and current measurements collected from a mooring deployed in the Lofoten basin from July 2010 to September 2012 are used to describe water mass transformation and the mesoscale eddy field. Winter mixed layer depths (MLDs) are observed to reach approximately 400 m, with larger MLDs and denser properties resulting from the colder 2010 winter. A heat budget of the upper water column requires lateral input, which balances the net annual heat loss of ~80 W m−2. The lateral flux is a result of mesoscale eddies, which dominate the velocity variability. Eddy velocities are enhanced in the upper 1000 m, with a barotropic component that reaches the bottom. Detailed examination of two eddies, from April and August 2012, highlights the variability of the eddy field and eddy properties. Temperature and salinity properties of the April eddy suggest that it originated from the slope current but was ventilated by surface fluxes. The properties within the eddy were similar to those of the mode water, indicating that convection within the eddies may make an important contribution to water mass transformation. A rough estimate of eddy flux per unit boundary current length suggests that fluxes in the Lofoten basin are larger than in the Labrador Sea because of the enhanced boundary current–interior density difference.The work was supported by NSF OCE 0850416.2015-12-0

Pathways for the export of Arctic change into the North Atlantic

The goal of the Pathways for the Export of Arctic Change into the North Atlantic project was to measure the exchange between the Hudson Bay System and the Labrador Sea, which occurs in the Hudson Strait. This exchange is of climactic relevance since a large amount of fresh water flows through the Hudson Strait into the Labrador Sea, where it can modulate the exchange of heat with the atmosphere. It is also of regional importance since the exchange influences the climate of Hudson Bay, which is home to a large indigenous population. The project consisted of deploying four subsurface moorings, over a one-year period, beginning August 2008 and ending September 2009. The moorings were positioned across the strait with Mooring A located on the south side and Moorings E, F, and G on the north side. The moorings were equipped with instruments to measure conductivity, temperature, pressure, ice draft and velocity.The National Science Foundation Grant Number OCE-0751554 provided funding for the project

A laboratory study of iceberg side melting in vertically sheared flows

Author Posting. © American Meteorological Society, 2018. 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 48 (2018): 1367-1373, doi:10.1175/JPO-D-17-0185.1.An earlier study indicates that the side melting of icebergs subject to vertically homogeneous horizontal velocities is controlled by two distinct regimes, which depend on the melt plume behavior and produce a nonlinear dependence of side melt rate on velocity. Here, we extend this study to consider ice blocks melting in a two-layer vertically sheared flow in a laboratory setting. It is found that the use of the vertically averaged flow speed in current melt parameterizations gives an underestimate of the submarine side melt rate, in part because of the nonlinearity of the dependence of the side melt rate on flow speed but also because vertical shear in the horizontal velocity profile fundamentally changes the flow splitting around the ice block and consequently the velocity felt by the ice surface. An observational record of 90 icebergs in a Greenland fjord suggests that this effect could produce an average underestimate of iceberg side melt rates of 21%.A. F. was supported by NA14OAR4320106 from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce. C. C. was supported by NSF OCE-1658079 and F. S. was supported by NSF OCE-1657601 and NSF PLR-1743693.2018-12-1

Using acoustic travel time to monitor the heat variability of glacial Fjords

Author Posting. © American Meteorological Society, 2021. 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 the Atmospheric and Oceanic Technology 38(9), (2021): 1535–1550, https://doi.org/10.1175/JTECH-D-20-0176.s1.Monitoring the heat content variability of glacial fjords is crucial to understanding the effects of oceanic forcing on marine-terminating glaciers. A pressure-sensor-equipped inverted echo sounder (PIES) was deployed midfjord in Sermilik Fjord in southeast Greenland from August 2011 to September 2012 alongside a moored array of instruments recording temperature, conductivity, and velocity. Historical hydrography is used to quantify the relationship between acoustic travel time and the vertically averaged heat content, and a new method is developed for filtering acoustic return echoes in an ice-influenced environment. We show that PIES measurements, combined with a knowledge of the fjord’s two-layer density structure, can be used to reconstruct the thickness and temperature of the inflowing water. Additionally, we find that fjord–shelf exchange events are identifiable in the travel time record implying the PIES can be used to monitor fjord circulation. Finally, we show that PIES data can be combined with moored temperature records to derive the heat content of the upper layer of the fjord where moored instruments are at great risk of being damaged by transiting icebergs.FS and MA acknowledge funding from the Kerr Family Foundation and the Grossman Family Foundation through the Woods Hole Oceanographic Institution. MA is supported by a grant from the National Science Foundation Office of Polar Programs (1332911). FS and RS acknowledge support from NSF OCE-1657601 and from the Heising-Simons Foundation

How fast is the Greenland ice sheet melting?

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Scambos, T., Straneo, F., & Tedesco, M. How fast is the Greenland ice sheet melting? Arctic Antarctic and Alpine Research, 53(1), (2021): 221–222, https://doi.org/10.1080/15230430.2021.1946241.THE ISSUE The Greenland Ice Sheet and the glacier-covered areas of Alaska and other Arctic lands are losing ice at an accelerating rate, contributing billions of tons of water to sea level rise. WHY IT MATTERS Ice loss from the ice sheets contributes directly to sea level rise. These losses are likely to increase rapidly as warming in the Arctic continues. Surface melt and runoff is now increasing more quickly than all other factors driving Greenland’s ice loss, although faster glacier outflow remains important. Increased ice loss from Alaska’s glaciers is also due mainly to surface melting. Given these trends, and the rapid warming in the Arctic (twice the global rate of warming), the Arctic is poised to lose ice even more rapidly and raise sea level. STATE OF KNOWLEDGE Since 2000, the net loss of ice from the Greenland Ice Sheet has increased five-fold, from 50 billion to about 250 billion tons per year1,2 (362 billion tons is equal to 1 mm in sea level rise). Ice losses in the Gulf of Alaska region have risen from about 40 to 70 billion tons per year3. These trends are confirmed by three independent satellite methods, using gravitational changes, elevation changes, and changes in the mass budget (the net difference between snowfall and the combination of glacier outflow and runoff)1. In total, the Arctic currently contributes approximately 350 billion tons (~1 mm) to sea level each year, primarily from Greenland, Alaska, and Arctic Canada. Recent measurements of the rate of sea level rise are 3.0 mm per year, with the additional rise coming from other glaciers and Antarctica (~0.4. mm) and expansion of the oceans due to warming (~1.7 mm)4. Slightly cooler summer seasons for Greenland in 2013 and 2014, and again in 2017 and 2018, temporarily reduced the rate of ice loss. Ocean temperatures cooled in some places along the western Greenland coast, slowing glacier outflow there5. However, strong melting in 2015, 2016 and 2019 again contributed large amounts of runoff to the ocean2. Because surface melt is closely tied to seasonal weather conditions, it is more variable than ice loss due to increased glacier outflow. Despite this variability, the overall warming trend of Arctic air and ocean has driven greatly increased melting and ice loss in Greenland and Alaska in the past two decades. As spring and summer temperatures have increased, net runoff of meltwater has grown dramatically (Figure 1). Ice loss due to faster glacier flow has remained stable overall and is unlikely to accelerate as rapidly as melting. Current increases in surface melt runoff rate are about twice that of ice loss due to increased ice flow speed1. As intense summer melt seasons like 2012, 2016, and 2019 become more common, further increases in melt runoff are inevitable.This work was supported by the Office of Polar Programs, National Science Foundation, and NSF’s Study of Environmental Arctic Change

Nonlinear response of iceberg side melting to ocean currents

Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 44 (2017): 5637–5644, doi:10.1002/2017GL073585.Icebergs calving into Greenlandic Fjords frequently experience strongly sheared flows over their draft, but the impact of this flow past the iceberg is not fully captured by existing parameterizations. We present a series of novel laboratory experiments to determine the dependence of submarine melting along iceberg sides on a background flow. We show, for the first time, that two distinct regimes of melting exist depending on the flow magnitude and consequent behavior of melt plumes (side-attached or side-detached), with correspondingly different meltwater spreading characteristics. When this velocity dependence is included in melt parameterizations, melt rates estimated for observed icebergs in the attached regime increase, consistent with observed iceberg submarine melt rates. We show that both attached and detached plume regimes are relevant to icebergs observed in a Greenland fjord. Further, depending on the regime, iceberg meltwater may either be confined to a surface layer or distributed over the iceberg draft.National Oceanic and Atmospheric Administration, U.S. Department of Commerce Grant Number: NA14OAR4320106; NSF Grant Numbers: OCE-1434041, OCE-1658079, PLR-1332911, OCE-14340412017-12-1

Export of strongly diluted Greenland meltwater from a major glacial fjord

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 4163-4170, doi:10.1029/2018GL077000.The Greenland Ice Sheet has been, and will continue, losing mass at an accelerating rate. The influence of this anomalous meltwater discharge on the regional and large‐scale ocean could be considerable but remains poorly understood. This uncertainty is in part a consequence of challenges in observing water mass transformation and meltwater spreading in coastal Greenland. Here we use tracer observations that enable unprecedented quantification of the export, mixing, and vertical distribution of meltwaters leaving one of Greenland's major glacial fjords. We find that the primarily subsurface meltwater input results in the upwelling of the deep fjord waters and an export of a meltwater/deepwater mixture that is 30 times larger than the initial meltwater release. Using these tracer data, the vertical structure of Greenland's summer meltwater export is defined for the first time showing that half the meltwater export occurs below 65 m.National Science Foundation Grant Number: OCE-15368562018-11-0