Circulation and Modification of Warm Deep Water on the Central Amundsen Shelf

2014 ◽  
Vol 44 (5) ◽  
pp. 1493-1501 ◽  
Author(s):  
H. K. Ha ◽  
A. K. Wåhlin ◽  
T. W. Kim ◽  
S. H. Lee ◽  
J. H. Lee ◽  
...  
Keyword(s):  
Heat Transport ◽  
Deep Water ◽  
Shelf Area ◽  
Amundsen Sea ◽  
Glacial Ice ◽  
Ice Shelves ◽  
Oceanic Heat Transport ◽  
Eastern Side ◽  
Warm Deep Water ◽  
Western Side

Abstract The circulation pathways and subsurface cooling and freshening of warm deep water on the central Amundsen Sea shelf are deduced from hydrographic transects and four subsurface moorings. The Amundsen Sea continental shelf is intersected by the Dotson trough (DT), leading from the outer shelf to the deep basins on the inner shelf. During the measurement period, warm deep water was observed to flow southward on the eastern side of DT in approximate geostrophic balance. A northward outflow from the shelf was also observed along the bottom in the western side of DT. Estimates of the flow rate suggest that up to one-third of the inflowing warm deep water leaves the shelf area below the thermocline in this deep outflow. The deep current was 1.2°C colder and 0.3 psu fresher than the inflow, but still warm, salty, and dense compared to the overlying water mass. The temperature and salinity properties suggest that the cooling and freshening process is induced by subsurface melting of glacial ice, possibly from basal melting of Dotson and Getz ice shelves. New heat budgets are presented, with a southward oceanic heat transport of 3.3 TW on the eastern side of the DT, a northward oceanic heat transport of 0.5–1.6 TW on the western side, and an ocean-to-glacier heat flux of 0.9–2.53 TW, equivalent to melting glacial ice at the rate of 83–237 km3 yr−1. Recent satellite-based estimates of basal melt rates for the glaciers suggest comparable values for the Getz and Dotson ice shelves.

scholarly journals Some Implications of Ekman Layer Dynamics for Cross-Shelf Exchange in the Amundsen Sea

2012 ◽  
Vol 42 (9) ◽  
pp. 1461-1474 ◽  
Author(s):  
A. K. Wåhlin ◽  
R. D. Muench ◽  
L. Arneborg ◽  
G. Björk ◽  
H. K. Ha ◽  
...  
Keyword(s):  
Deep Water ◽  
Bottom Layer ◽  
Shelf Break ◽  
Ekman Transport ◽  
Shelf Area ◽  
Amundsen Sea ◽  
West Antarctic Ice Sheet ◽  
West Antarctic ◽  
Warm Deep Water ◽  
Slope Currents

Abstract The exchange of warm, salty seawater across the continental shelves off West Antarctica leads to subsurface glacial melting at the interface between the ocean and the West Antarctic Ice Sheet. One mechanism that contributes to the cross-shelf transport is Ekman transport induced by along-slope currents over the slope and shelf break. An investigation of this process is applied to the Amundsen Sea shelfbreak region, using recently acquired and historical field data to guide the analyses. Along-slope currents were observed at transects across the eastern and western reaches of the Amundsen slope. Currents in the east flowed eastward, and currents farther west flowed westward. Under the eastward-flowing currents, hydrographic isolines sloped upward paralleling the seabed. In this layer, declining buoyancy forces rather than friction were bringing the velocity to zero at the seabed. The basin water in the eastern part of the shelf was dominated by water originating from 800–1000-m depth off shelf, suggesting that transport of such water across the shelf frequently occurs. The authors show that arrested Ekman layers mechanism can supply deep water to the shelf break in the eastern section, where it has access to the shelf. Because no unmodified off-shelf water was found on the shelf in the western part, bottom layer Ekman transport does not appear a likely mechanism for delivery of warm deep water to the western shelf area. Warming of the warm bottom water was most pronounced on the western shelf, where the deep-water temperature increased by 0.6°C during the past decade.

scholarly journals Variability of Warm Deep Water Inflow in a Submarine Trough on the Amundsen Sea Shelf

2013 ◽  
Vol 43 (10) ◽  
pp. 2054-2070 ◽  
Author(s):  
A. K. Wåhlin ◽  
O. Kalén ◽  
L. Arneborg ◽  
G. Björk ◽  
G. K. Carvajal ◽  
...  
Keyword(s):  
Deep Water ◽  
Bottom Layer ◽  
Shelf Break ◽  
Ocean Currents ◽  
Local Wind ◽  
Amundsen Sea ◽  
Ice Shelves ◽  
Warm Deep Water ◽  
Buoyancy Forces ◽  
Lower Circumpolar Deep Water

Abstract The ice shelves in the Amundsen Sea are thinning rapidly, and the main reason for their decline appears to be warm ocean currents circulating below the ice shelves and melting these from below. Ocean currents transport warm dense water onto the shelf, channeled by bathymetric troughs leading to the deep inner basins. A hydrographic mooring equipped with an upward-looking ADCP has been placed in one of these troughs on the central Amundsen shelf. The two years (2010/11) of mooring data are here used to characterize the inflow of warm deep water to the deep shelf basins. During both years, the warm layer thickness and temperature peaked in austral fall. The along-trough velocity is dominated by strong fluctuations that do not vary in the vertical. These fluctuations are correlated with the local wind, with eastward wind over the shelf and shelf break giving flow toward the ice shelves. In addition, there is a persistent flow of dense lower Circumpolar Deep Water (CDW) toward the ice shelves in the bottom layer. This bottom-intensified flow appears to be driven by buoyancy forces rather than the shelfbreak wind. The years of 2010 and 2011 were characterized by a comparatively stationary Amundsen Sea low, and hence there were no strong eastward winds during winter that could drive an upwelling of warm water along the shelf break. Regardless of this, there was a persistent flow of lower CDW in the bottom layer during the two years. The average heat transport toward the ice shelves in the trough was estimated from the mooring data to be 0.95 TW.

Evaluation of four coupled climate models in the Amundsen Sea, Antarctica

2021 ◽  
Author(s):  
Kyriaki M. Lekakou ◽  
Ben G.M. Webber ◽  
Karen J. Heywood ◽  
David P. Stevens ◽  
Patrick Hyder
Keyword(s):  
Sea Ice ◽  
Continental Shelf ◽  
Deep Water ◽  
Climate Models ◽  
Warm Water ◽  
Low Resolution ◽  
Amundsen Sea ◽  
Ice Shelves ◽  
Medium Resolution ◽  
Warm Deep Water

<p>The Amundsen Sea glaciers, in West Antarctica, are among the world’s fastest discharges of ice into the ocean. The rapid thinning of these ice shelves can be largely explained by basal melting driven by the ocean. Relatively warm water reaches the continental shelf in the Amundsen Sea and deep bathymetric troughs facilitate warm deep water flow to the base of the ice shelves. However, time sparse observational data, and even poorly known bathymetry, contribute to the difficulty of quantifying the key ocean mechanisms, and their variability, that transport warm water onto the Amundsen Sea continental shelf and guide it southward into the ice shelf cavities. Nonetheless these processes should be represented in the coupled climate models, such as those used for CMIP6, which are being used to project future sea level rise.</p><p>Here we leverage recent observational campaigns and gains in process understanding to assess how well four of these models, UKESM1 and the HadGEM-GC3.1 family of models, represent the ocean processes forcing warm water onto the Amundsen Sea continental shelf. The three HadGEM models have the same external forcing but different horizontal resolutions, 1/12, ¼ and 1 degree. The 1 degree resolution UKESM1 is based on HadGEM3.1 but includes atmospheric chemistry, aerosols and marine biogeochemistry. A key finding is the medium resolution (1/4°) HadGEM-GC3.1 model’s inability to allow warm deep water intrusion onto the continental shelf, associated with a strong westward slope current that is not present in the other models. The medium resolution model represents well the annual cycle of sea ice in the Amundsen Sea, but overall has significantly less sea ice around Antarctica, compared with the other models and satellite observations. Despite its low resolution, UKESM1 represents well all the main ocean features, including the shelf-break undercurrent, warm deep water and realistic sea ice. It captures more significant interannual variability, in contrast to the low resolution HadGEM, for which the interannual variability is more suppressed. Of the four models considered here, the best performing models are the 1/12° HadGEM and UKESM1, followed by the low resolution HadGEM model, which reasonably represents warmer deep water on the continental shelf and a shallower mixed layer. The medium resolution HadGEM, despite its better resolution is less realistic than the two low resolution models.</p>

scholarly journals An oceanic heat transport pathway to the Amundsen Sea Embayment

2016 ◽  
Vol 121 (5) ◽  
pp. 3337-3349 ◽  
Author(s):  
Angelica R. Rodriguez ◽  
Matthew R. Mazloff ◽  
Sarah T. Gille
Keyword(s):  
Heat Transport ◽  
Transport Pathway ◽  
Amundsen Sea ◽  
Oceanic Heat Transport

Drivers of intermittent reduction in ocean heat transport into the Getz Ice Shelf cavity

2021 ◽  
Author(s):  
Nadine Steiger ◽  
Elin Darelius ◽  
Anna Wåhlin ◽  
Karen Assmann
Keyword(s):  
Heat Transport ◽  
Heat Content ◽  
Ocean Current ◽  
West Antarctica ◽  
Ice Shelf ◽  
Amundsen Sea ◽  
Ice Shelves ◽  
Coastal Polynya ◽  
The Right ◽  
The Antarctic

<p><span>Ice shelves in West Antarctica have been thinning during the last decades due to an increased supply of ocean heat that melts the ice from below. The Getz Ice Shelf in the western Amundsen Sea has experienced an inflow of warm water during 2016-2017, but intermittent events of reduced heat content occur during this period. The processes behind the variability of heat transport towards the Antarctic ice shelves on daily to decadal time scales are not well known. <br>Here, we present possible drivers and implications of these events of reduced heat content. We find that they are preceded by strong easterly winds that open up a coastal polynya and depress the cold Winter Water towards the ocean floor. Simultaneously, the ocean current flowing towards the ice shelf veers to the right and aligns with the ice shelf front rather than entering the ice shelf cavity. The heat transport into the ice shelf cavity is consequently reduced by 22% in winter 2016. These events do not occur during winter 2017, possibly due to stronger stratification and weaker winds.</span></p>

scholarly journals Oceanic heat transport onto the Amundsen Sea shelf through a submarine glacial trough

2007 ◽  
Vol 34 (2) ◽  
Author(s):  
Dziga P. Walker ◽  
Mark A. Brandon ◽  
Adrian Jenkins ◽  
John T. Allen ◽  
Julian A. Dowdeswell ◽  
...  
Keyword(s):  
Heat Transport ◽  
Amundsen Sea ◽  
Oceanic Heat Transport

scholarly journals Inflow of Warm Circumpolar Deep Water in the Central Amundsen Shelf*

2010 ◽  
Vol 40 (6) ◽  
pp. 1427-1434 ◽  
Author(s):  
A. K. Wåhlin ◽  
X. Yuan ◽  
G. Björk ◽  
C. Nohr
Keyword(s):  
Deep Water ◽  
Water Mass ◽  
Acoustic Doppler Current Profiler ◽  
Intermediate Water ◽  
Amundsen Sea ◽  
Ice Shelves ◽  
Circumpolar Deep Water ◽  
Ice Melt ◽  
West Antarctic ◽  
Glacier Ice

Abstract The thinning and acceleration of the West Antarctic Ice Sheet has been attributed to basal melting induced by intrusions of relatively warm salty water across the continental shelf. A hydrographic section including lowered acoustic Doppler current profiler measurements showing such an inflow in the channel leading to the Getz and Dotson Ice Shelves is presented here. The flow rate was 0.3–0.4 Sv (1 Sv ≡ 106 m3 s−1), and the subsurface heat loss was estimated to be 1.2–1.6 TW. Assuming that the inflow persists throughout the year, it corresponds to an ice melt of 110–130 km3 yr−1, which exceeds recent estimates of the net ice glacier ice volume loss in the Amundsen Sea. The results also show a 100–150-m-thick intermediate water mass consisting of Circumpolar Deep Water that has been modified (cooled and freshened) by subsurface melting of ice shelves and/or icebergs. This water mass has not previously been reported in the region, possibly because of the paucity of historical data.

scholarly journals A dynamically consistent analysis of circulation and transports in the southwestern Weddell Sea

1998 ◽  
Vol 16 (8) ◽  
pp. 1024-1038 ◽  
Author(s):  
M. Yaremchuk ◽  
D. Nechaev ◽  
J. Schroter ◽  
E. Fahrbach
Keyword(s):  
Surface Water ◽  
Heat Transport ◽  
Deep Water ◽  
General Circulation ◽  
Bottom Water ◽  
Weddell Sea ◽  
Global Ocean ◽  
Hydrographic Data ◽  
Poleward Heat Transport ◽  
Warm Deep Water

Abstract. An inverse model is applied for the analysis of hydrographic and current meter data collected on the repeat WOCE section SR4 in the Weddell Sea in 1989–1992. The section crosses the Weddell Sea cyclonic gyre from Kapp Norvegia to the northern end of the Antarctic Peninsula. The concepts of geostrophy, conservation of planetary vorticity and hydrostatics are combined with advective balances of active and passive properties to provide a dynamically consistent circulation pattern. Our variational assimilation scheme allows the calculation of three-dimensional velocities in the section plane. Current speeds are small except along the coasts where they reach up to 12 cm/s. We diagnose a gyre transport of 34 Sverdrup which is associated with a poleward heat transport of 28×1012 W corresponding to an average heat flux of 15 Wm–2 in the Weddell Sea south of the transect. This exceeds the estimated local flux on the transect of 2 Wm–2. As the transect is located mostly in the open ocean, we conclude that the shelf areas contribute significantly to the ocean-atmosphere exchange and are consequently key areas for the contribution of the Weddell Sea to global ocean ventilation. Conversion of water masses occuring south of the section transform 6.6±1.1 Sv of the inflowing warm deep water into approximately equal amounts of Weddell Sea deep water and Weddell Sea bottom water. The volume transport of surface water equals in the in- and outflow. This means that almost all newly formed surface water is involved in the deep and bottom water formation. Comparison with the results obtained by pure velocity interpolation combined with a hydrographic data subset indicates major differences in the derived salt transports and the water mass conversion of the surface water. The differences can be explained by deviations in the structure of the upper ocean currents to which shelf areas contribute significantly. Additionally a rigorous variance analysis is performed. When only hydrographic data are used for the inversion both the gyre transport and the poleward heat transport are substantially lower. They amount to less than 40% of our best estimate while the standard deviations of both quantities are 6.5 Sv and 37×1012 W, respectively. With the help of long-term current meter measurements these errors can be reduced to 2 Sv and 8×1012 W. Our result underlines the importance of velocity data or equivalent information that helps to estimate the absolute velocities.Key words. Oceanography: General (Arctic and antarctic oceanography) · Oceanography: Physical (General circulation; Hydrography)

scholarly journals An exploratory modelling study of perennial firn aquifers in the Antarctic Peninsula for the period 1979–2016

2021 ◽  
Vol 15 (2) ◽  
pp. 695-714
Author(s):  
J. Melchior van Wessem ◽  
Christian R. Steger ◽  
Nander Wever ◽  
Michiel R. van den Broeke
Keyword(s):  
Antarctic Peninsula ◽  
Climate Model ◽  
Liquid Water Content ◽  
Shelf Area ◽  
Accumulation Rates ◽  
Ice Shelf ◽  
Ice Shelves ◽  
Grid Points ◽  
Western Side ◽  
The Antarctic

Abstract. In this study, we focus on the model detection in the Antarctic Peninsula (AP) of so-called perennial firn aquifers (PFAs) that are widespread in Greenland and Svalbard and are formed when surface meltwater percolates into the firn pack in summer, which is then buried by snowfall and does not refreeze during the following winter. We use two snow models, the Institute for Marine and Atmospheric Research Utrecht Firn Densification Model (IMAU-FDM) and SNOWPACK, and force these (partly) with mass and energy fluxes from the Regional Atmospheric Climate MOdel (RACMO2.3p2) to construct a 1979–2016 climatology of AP firn density, temperature, and liquid water content. An evaluation using 75 snow temperature observations at 10 m depth and density profiles from 11 firn cores shows that output of both snow models is sufficiently realistic to warrant further analysis of firn characteristics. The models give comparable results: in 941 model grid points in either model, covering ∼28 000 km2, PFAs existed for at least 1 year in the simulated period, most notably in the western AP. At these locations, surface meltwater production typically exceeds 200 mmw.e.yr-1, with accumulation for most locations >1000mmw.e.yr-1. Most persistent and extensive are PFAs modelled on and around Wilkins Ice Shelf. Here, both meltwater production and accumulation rates are sufficiently high to sustain a PFA on 49 % of the ice shelf area in (up to) 100 % (depending on the model) of the years in the 1979–2016 period. Although this PFA presence is confirmed by recent observations, its extent in the models appears underestimated. Other notable PFA locations are Wordie Ice Shelf, an ice shelf that has almost completely disappeared in recent decades, and the relatively warm north-western side of mountain ranges in Palmer Land, where accumulation rates can be extremely high, and PFAs are formed frequently. PFAs are not necessarily more frequent in areas with the largest melt and accumulation rates, but they do grow larger and retain more meltwater, which could increase the likelihood of ice shelf hydrofracturing. We find that not only the magnitude of melt and accumulation is important but also the timing of precipitation events relative to melt events. Large accumulation events that occur in the months following an above-average summer melt event favour PFA formation in that year. Most PFAs are predicted near the grounding lines of the (former) Prince Gustav, Wilkins, and Wordie ice shelves. This highlights the need to further investigate how PFAs may impact ice shelf disintegration events through the process of hydrofracturing in a similar way as supraglacial lakes do.

Recent Changes in the Larsen-Weddell System

2020 ◽  
Author(s):  
Ted Scambos
Keyword(s):  
Deep Water ◽  
Shear Zones ◽  
Surface Melting ◽  
Mean Flow ◽  
Ice Shelf ◽  
Central Pacific ◽  
Ice Shelves ◽  
Weddell Gyre ◽  
Warm Deep Water ◽  
Westerly Winds

<p>A warming planet, and particularly the warming Pacific Ocean, has led to major changes in the Larsen-Weddell System. While somewhat less significant than those in the adjacent Amundsen-Bellingshausen Sea and its coastal ice, the changes are nonetheless dramatic indicators of a closely interconnected system, driven by increased westerly winds and their impact on surface melting and ice drift. The system very likely will see further major changes if warming continues through the 22<sup>nd</sup> Century.</p><p>A warming trend in the central Pacific over the past ~80 years has induced air circulation changes over the Southern Ocean and Antarctic Peninsula. A rise in the mean speed of westerly-northwesterly winds across the northern Peninsula led to more frequent foehn events, which in turn increased surface melting on the eastern Peninsula ice shelves, and were responsible for reduced sea ice cover and more frequent shore leads on the eastern edges of the ice shelves. This likely led to greater sub-ice-shelf circulation, possibly including solar-warmed surface water (in summer) and modified Weddell Deep Water (mWDW). Around 1986, structural evidence in the form of more disrupted shear zones and increased rifting suggests that the Larsen A and B ice shelves began to thin and weaken. At this progressed, a combination of increased surface ponding and reduced backstress on the iceshelves led to a series of catastrophic break-ups due to hydrofracture, in 1995 (Larsen A shelf) and 2002 (Larsen B).  More recently, thinning detected by altimetry on the northern Larsen C may have contributed to new fracturing and calving of a large iceberg there in 2016 (iceberg A-68), setting the ice shelf front significantly farther to the west than has previously been observed (since 1898).</p><p>Looking forward, if the trend in increased westerly winds and Southern Annular Mode index continues, it is anticipated (modelled) that the large clockwise Weddell Gyre will increase in mean flow speed, and that warm deep water entrained from the Antarctic Circumpolar Current will more frequently mix with the mid- to deep ocean layers in the Weddell Gyre. One outcome of this is likely to be advection of warm deep water into the Ronne Ice Shelf cavity, dramatically increasing the heat available for sub-ice-shelf melting there and potentially changing ice sheet flux from the outlet glaciers significantly.</p>

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