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.

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.

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.

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 Variability of Circumpolar Deep Water transport onto the Amundsen Sea Continental shelf through a shelf break trough

2013 ◽  
Vol 118 (12) ◽  
pp. 6603-6620 ◽  
Author(s):  
K. M. Assmann ◽  
A. Jenkins ◽  
D. R. Shoosmith ◽  
D. P. Walker ◽  
S. S. Jacobs ◽  
...  
Keyword(s):  
Continental Shelf ◽  
Water Transport ◽  
Deep Water ◽  
Shelf Break ◽  
Amundsen Sea ◽  
Circumpolar Deep Water

scholarly journals Deep water circulation in the Hunter Channel (Southwest Atlantic) in a late Pleistocene and Holocene by benthonic foraminifera

2019 ◽  
Vol 59 (1) ◽  
pp. 133-142
Author(s):  
N. P. Lukashina
Keyword(s):  
Late Pleistocene ◽  
Deep Water ◽  
Sea Water ◽  
Bottom Layer ◽  
Water Circulation ◽  
Southwest Atlantic ◽  
East Part ◽  
The North ◽  
Lower Circumpolar Deep Water ◽  
Argentine Basin

Was reconstructed deep-sea water circulation near the Hunter Channel (Rio Grande Rise – South-West Atlantic) in a late Pleistocene and Holocene (MIS 4-MIS 1) by benthonic foraminifera. Was studied three cores of bottom sediment. Now moves the upper North Atlantic deep water (NADW) through the Hunter Channel from the North to the South. The lower NADW in the same direction came in MIS 2 and in MIS 4. There was the lower Circumpolar deep water (CPDW), NADW and Antarctic bottom water (AnBW) in MIS 3 periodically. CPDW prevail in a near bottom layer and in Holocene and in the late Pleistocene before the Hunter Channel sidewise the Argentine Basin. So in the Hunter Channel and on the way to it from south side for all studied period AnBW was almost not. Dissolution of carbonates during the Holocene happens in the deepest east part of the Hunter Channel. In Ice Ages processes of dissolution amplified and affected east part of the channel. Dissolution happen and happened not at the expense of AnBW, and at the expense of NADW which becomes there aggressive in relation to a calcium carbonate.

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 The Impact of Overturning and Horizontal Circulation in Pine Island Trough on Ice Shelf Melt in the Eastern Amundsen Sea

2019 ◽  
Vol 49 (1) ◽  
pp. 63-83 ◽  
Author(s):  
Benjamin G. M. Webber ◽  
Karen J. Heywood ◽  
David P. Stevens ◽  
Karen M. Assmann
Keyword(s):  
Continental Shelf ◽  
Ocean Circulation ◽  
Circulation Model ◽  
Shelf Break ◽  
Ekman Pumping ◽  
Overturning Circulation ◽  
Amundsen Sea ◽  
Ice Shelves ◽  
Basal Melting ◽  
The Impact

AbstractThe ice shelves around the Amundsen Sea are rapidly melting as a result of the circulation of relatively warm ocean water into their cavities. However, little is known about the processes that determine the variability of this circulation. Here we use an ocean circulation model to diagnose the relative importance of horizontal and vertical (overturning) circulation within Pine Island Trough, leading to Pine Island and Thwaites ice shelves. We show that melt rates and southward Circumpolar Deep Water (CDW) transports covary over large parts of the continental shelf at interannual to decadal time scales. The dominant external forcing mechanism for this variability is Ekman pumping and suction on the continental shelf and at the shelf break, in agreement with previous studies. At the continental shelf break, the southward transport of CDW and heat is predominantly barotropic. Farther south within Pine Island Trough, northward and southward barotropic heat transports largely cancel, and the majority of the net southward temperature transport is facilitated by baroclinic and overturning circulations. The overturning circulation is related to water mass transformation and buoyancy gain on the shelf that is primarily facilitated by freshwater input from basal melting.

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>

scholarly journals Wind-Driven Processes Controlling Oceanic Heat Delivery to the Amundsen Sea, Antarctica

2019 ◽  
Vol 49 (11) ◽  
pp. 2829-2849 ◽  
Author(s):  
Tiago S. Dotto ◽  
Alberto C. Naveira Garabato ◽  
Sheldon Bacon ◽  
Paul R. Holland ◽  
Satoshi Kimura ◽  
...  
Keyword(s):  
Continental Shelf ◽  
Water Mass ◽  
Vertical Displacement ◽  
Temperature Variability ◽  
Shelf Break ◽  
Ekman Pumping ◽  
Amundsen Sea ◽  
Ice Shelves ◽  
Property Changes ◽  
Basal Melting

AbstractVariability in the heat delivery by Circumpolar Deep Water (CDW) is responsible for modulating the basal melting of the Amundsen Sea ice shelves. However, the mechanisms controlling the CDW inflow to the region’s continental shelf remain little understood. Here, a high-resolution regional model is used to assess the processes governing heat delivery to the Amundsen Sea. The key mechanisms are identified by decomposing CDW temperature variability into two components associated with 1) changes in the depth of isopycnals [heave (HVE)], and 2) changes in the temperature of isopycnals [water mass property changes (WMP)]. In the Dotson–Getz trough, CDW temperature variability is primarily associated with WMP. The deeper thermocline and shallower shelf break hinder CDW access to that trough, and CDW inflow is regulated by the uplift of isopycnals at the shelf break—which is itself controlled by wind-driven variations in the speed of an undercurrent flowing eastward along the continental slope. In contrast, CDW temperature variability in the Pine Island–Thwaites trough is mainly linked to HVE. The shallower thermocline and deeper shelf break there permit CDW to persistently access the continental shelf. CDW temperature in the area responds to wind-driven modulation of the water mass on-shelf volume by changes in the rate of inflow across the shelf break and in Ekman pumping-induced vertical displacement of isopycnals within the shelf. The western and eastern Amundsen Sea thus represent distinct regimes, in which wind forcing governs CDW-mediated heat delivery via different dynamics.

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