scholarly journals Deep Bottom Mixed Layer Drives Intrinsic Variability of the Antarctic Slope Front

2019 ◽  
Vol 49 (12) ◽  
pp. 3163-3177 ◽  
Author(s):  
Wilma Gertrud Charlotte Huneke ◽  
Andreas Klocker ◽  
Benjamin Keith Galton-Fenzi
Keyword(s):  
Continental Shelf ◽  
Mixed Layer ◽  
Process Model ◽  
Formation Rate ◽  
Ekman Transport ◽  
Intrinsic Variability ◽  
Subsequent Increase ◽  
The Stability ◽  
Basal Melting ◽  
The Antarctic

AbstractThe Antarctic Slope Front (ASF) is located along much of the Antarctic continental shelf break and helps to maintain a barrier to the movement of Circumpolar Deep Water (CDW) onto the continental shelf. The stability of the ASF has a major control on cross-shelf heat transport and ocean-driven basal melting of Antarctic ice shelves. Here, the ASF dynamics are investigated for continental shelves with weak dense shelf water (DSW) formation, which are thought to have a stable ASF, common for regions in East Antarctica. Using an ocean process model, this study demonstrates how offshore bottom Ekman transport of shelf waters leads to the development of a deep bottom mixed layer at the lower continental slope, and subsequently determines an intrinsic variability of the ASF. The ASF variability is characterized by instability events that affect the entire water column and occur every 5–10 years and last for approximately half a year. During these instability events, the cross-shelf density gradient weakens and CDW moves closer to the continent. Stronger winds increase the formation rate of the bottom mixed layer, which causes a subsequent increase of instability events. If the observed freshening trend of continental shelf waters leads to weaker DSW formation, more regions might be vulnerable for the ASF variability to develop in the future.

scholarly journals Acceleration and Overturning of the Antarctic Slope Current by Winds, Eddies, and Tides

2019 ◽  
Vol 49 (8) ◽  
pp. 2043-2074 ◽  
Author(s):  
Andrew L. Stewart ◽  
Andreas Klocker ◽  
Dimitris Menemenlis
Keyword(s):  
Sea Ice ◽  
Continental Shelf ◽  
Continental Slope ◽  
Mesoscale Eddy ◽  
Shelf Break ◽  
Ekman Transport ◽  
Global Ocean ◽  
Antarctic Slope Current ◽  
The Antarctic ◽  
Continental Shelf Break

AbstractAll exchanges between the open ocean and the Antarctic continental shelf must cross the Antarctic Slope Current (ASC). Previous studies indicate that these exchanges are strongly influenced by mesoscale and tidal variability, yet the mechanisms responsible for setting the ASC’s transport and structure have received relatively little attention. In this study the roles of winds, eddies, and tides in accelerating the ASC are investigated using a global ocean–sea ice simulation with very high resolution (1/48° grid spacing). It is found that the circulation along the continental slope is accelerated both by surface stresses, ultimately sourced from the easterly winds, and by mesoscale eddy vorticity fluxes. At the continental shelf break, the ASC exhibits a narrow (~30–50 km), swift (>0.2 m s−1) jet, consistent with in situ observations. In this jet the surface stress is substantially reduced, and may even vanish or be directed eastward, because the ocean surface speed matches or exceeds that of the sea ice. The shelfbreak jet is shown to be accelerated by tidal momentum advection, consistent with the phenomenon of tidal rectification. Consequently, the shoreward Ekman transport vanishes and thus the mean overturning circulation that steepens the Antarctic Slope Front (ASF) is primarily due to tidal acceleration. These findings imply that the circulation and mean overturning of the ASC are not only determined by near-Antarctic winds, but also depend crucially on sea ice cover, regionally-dependent mesoscale eddy activity over the continental slope, and the amplitude of tidal flows across the continental shelf break.

scholarly journals Eddy Generation and Jet Formation via Dense Water Outflows across the Antarctic Continental Slope

2016 ◽  
Vol 46 (12) ◽  
pp. 3729-3750 ◽  
Author(s):  
Andrew L. Stewart ◽  
Andrew F. Thompson
Keyword(s):  
Kinetic Energy ◽  
Continental Shelf ◽  
Continental Slope ◽  
Process Model ◽  
Local Maximum ◽  
Eddy Kinetic Energy ◽  
Eddy Generation ◽  
Slope Flow ◽  
Energy And Momentum ◽  
The Antarctic

AbstractAlong various stretches of the Antarctic margins, dense Antarctic Bottom Water (AABW) escapes its formation sites and descends the continental slope. This export necessarily raises the isopycnals associated with lighter density classes over the continental slope, resulting in density surfaces that connect the near-freezing waters of the continental shelf to the much warmer circumpolar deep water (CDW) at middepth offshore. In this article, an eddy-resolving process model is used to explore the possibility that AABW export enhances shoreward heat transport by creating a pathway for CDW to access the continental shelf without doing any work against buoyancy forces. In the absence of a net alongshore pressure gradient, the shoreward CDW transport is effected entirely by mesoscale and submesoscale eddy transfer. Eddies are generated partly by instabilities at the pycnocline, sourcing their energy from the alongshore wind stress, but primarily by instabilities at the CDW–AABW interface, sourcing their energy from buoyancy loss on the continental shelf. This combination of processes induces a vertical convergence of eddy kinetic energy and alongshore momentum into the middepth CDW layer, sustaining a local maximum in the eddy kinetic energy over the slope and balancing the Coriolis force associated with the shoreward CDW transport. The resulting slope turbulence self-organizes into a series of alternating along-slope jets with strongly asymmetrical contributions to the slope energy and momentum budgets. Cross-shore variations in the potential vorticity gradient cause the jets to drift continuously offshore, suggesting that fronts observed in regions of AABW down-slope flow may in fact be transient features.

The Origin and Fate of Subantarctic Mode Water in the Southern Ocean

2021 ◽  
Author(s):  
Zhi Li ◽  
Matthew H. England ◽  
Sjoerd Groeskamp ◽  
Ivana Cerovečki ◽  
Yiyong Luo
Keyword(s):  
Mixed Layer ◽  
Formation Rate ◽  
Ekman Transport ◽  
The South ◽  
Regional Variability ◽  
Mode Water ◽  
South Indian ◽  
Subantarctic Mode Water ◽  
Circumpolar Current ◽  
Mixed Layers

AbstractSubantarctic Mode Water (SAMW) forms in deep mixed layers just north of the Antarctic Circumpolar Current in winter, playing a fundamental role in the ocean uptake of heat and carbon. Using a gridded Argo product and the ERA-Interim reanalysis for years 2004-2018, the seasonal evolution of the SAMW volume is analyzed using both a kinematic estimate of the subduction rate and a thermodynamic estimate of the air-sea formation rate. The seasonal SAMW volume changes are separately estimated within the monthly mixed layer and in the interior below it. We find that the variability of SAMW volume is dominated by changes in SAMW volume in the mixed layer. The seasonal variability of SAMW volume in the mixed layer is governed by formation due to air-sea buoyancy fluxes (45%, lasting from July to August), entrainment (35%), and northward Ekman transport across the Subantarctic Front (10%). The interior SAMW formation is entirely controlled by exchanges between the mixed layer and the interior (i.e. instantaneous subduction), which occurs mainly during August-October. The annual mean subduction estimate from a Lagrangian approach shows strong regional variability with hotspots of large SAMW subduction. The SAMW subduction hotspots are consistent with the distribution and export pathways of SAMW over the central and eastern parts of the south Indian and Pacific Oceans. Hotspots in the south Indian Ocean produce strong subduction of 8 and 9 Sv for the light and southeast Indian SAMW, respectively, while SAMW subduction of 6 and 4 Sv occurs for the central and southeast Pacific SAMW, respectively.

scholarly journals On the evolution of an ice shelf melt channel at the base of Filchner Ice Shelf, from observations and viscoelastic modeling

2021 ◽  
Author(s):  
Angelika Humbert ◽  
Julia Christmann ◽  
Hugh F. J. Corr ◽  
Veit Helm ◽  
Lea-Sophie Höyns ◽  
...  
Keyword(s):  
Ice Shelf ◽  
Climate Conditions ◽  
Ice Shelves ◽  
Grounding Line ◽  
The Stability ◽  
Viscoelastic Modeling ◽  
Basal Melting ◽  
The Antarctic ◽  
Measurement Period

Abstract. Ice shelves play a key role in the stability of the Antarctic Ice Sheet due to their buttressing effect. A loss of buttressing as a result of increased basal melting or ice shelf disintegration will lead to increased ice discharge. Some ice shelves exhibit channels at the base that are not yet fully understood. In this study, we present in-situ melt rates of a channel which is up to 330 m high and located at the southern Filchner Ice Shelf. Maximum observed melt rates are 2.3 m a−1. Melt rates decline inside the channel along flow and turn into freezing 55 km downstream of the grounding line. While closer to the grounding line melt rates are higher within the channel than outside, this reverses further downstream. Comparing the evolution of this channel under present-day climate conditions over 250 years with its present geometry reveals a mismatch. This mismatch indicates melt rates two times higher were necessary over the past 250 years to form today's channel geometry. In contrast, forcing the model with present-day melt rates results in a closure of the channel, which contradicts observations. Time series of melt rate measurements show strong tidally-induced variability in vertical strain-rates. We found no evidence of seasonality, but discrete pulses of increased melting occurred throughout the measurement period. The type of melt channel in this study diminishes with distance from the grounding line and are hence not a destabilizing factor for ice shelves.

Problems of The Antarctic Treaty System (Political and Legal Aspects)

2021 ◽  
pp. 91
Author(s):  
Pavel Gudev
Keyword(s):  
Continental Shelf ◽  
Legal Aspects ◽  
National Interests ◽  
Antarctic Waters ◽  
Interstate Conflicts ◽  
National Jurisdiction ◽  
Antarctic Treaty ◽  
The Stability ◽  
The Antarctic ◽  
Antarctic Treaty System

The article considers the key factors that affect the stability of the Antarctic Treaty System (ATS), erode its basic norms and provisions, lead to strengthening of political and legal contradictions between countries and, in general, to a prospective strengthening of interstate conflicts. Among those factors are the attempts of some claimant states to form maritime zones of sovereignty, sovereign rights and jurisdiction in the Antarctic waters and the process of defining the outer limits of the continental shelf within the framework of the Commission on the Limits of the Continental Shelf initiated by them. The author shows how justifying their actions with references to the rights and powers granted to them under modern international maritime law, and above all the 1982 UN Convention on the Law of the Sea, leads to an imbalance of the entire System. The unresolved questions concerning the applicability of the concept of common heritage of mankind (CCH) to the Antarctic, the legality of the formation of maritime zones around the sub-Antarctic islands and restrictions on the exercise of national jurisdiction on the continent itself increase pressure on the sustainability of the established legal regime. It is in the interests of the Russian Federation not to allow the complete destruction of the established system of governance, and if it is inevitable, to be ready to pursue an active policy to defend its national interests.

scholarly journals Connecting Antarctic Cross-Slope Exchange with Southern Ocean Overturning

2013 ◽  
Vol 43 (7) ◽  
pp. 1453-1471 ◽  
Author(s):  
Andrew L. Stewart ◽  
Andrew F. Thompson
Keyword(s):  
Southern Ocean ◽  
Continental Shelf ◽  
Continental Slope ◽  
Process Model ◽  
Baroclinic Instability ◽  
Mesoscale Eddies ◽  
Deep Ocean ◽  
Overturning Circulation ◽  
Produce Water ◽  
The Antarctic

Abstract Previous idealized investigations of Southern Ocean overturning have omitted its connection with the Antarctic continental shelves, leaving the influence of shelf processes on Antarctic Bottom Water (AABW) export unconsidered. In particular, the contribution of mesoscale eddies to setting the stratification and overturning circulation in the Antarctic Circumpolar Current (ACC) is well established, yet their role in cross-shelf exchange of water masses remains unclear. This study proposes a residual-mean theory that elucidates the connection between Antarctic cross-shelf exchange and overturning in the ACC, and the contribution of mesoscale eddies to the export of AABW. The authors motivate and verify this theory using an eddy-resolving process model of a sector of the Southern Ocean. The strength and pattern of the simulated overturning circulation strongly resemble those of the real ocean and are closely captured by the residual-mean theory. Over the continental slope baroclinic instability is suppressed, and so transport by mesoscale eddies is reduced. This suppression of the eddy fluxes also gives rise to the steep “V”-shaped isopycnals that characterize the Antarctic Slope Front in AABW-forming regions of the continental shelf. Furthermore, to produce water on the continental shelf that is dense enough to sink to the deep ocean, the deep overturning cell must be at least comparable in strength to wind-driven mean overturning on the continental slope. This results in a strong sensitivity of the deep overturning strength to changes in the polar easterly winds.

Movements of southern elephant seals

1996 ◽  
Vol 74 (8) ◽  
pp. 1485-1496 ◽  
Author(s):  
B. J. McConnell ◽  
M. A. Fedak
Keyword(s):  
Southern Ocean ◽  
Continental Shelf ◽  
Antarctic Peninsula ◽  
Average Rate ◽  
Stationary Phases ◽  
Mirounga Leonina ◽  
Elephant Seals ◽  
South Georgia ◽  
Southern Elephant Seals ◽  
The Antarctic

Twelve southern elephant seals (Mirounga leonina) were tracked for an average of 119 days as they left their breeding or moulting beaches on the island of South Georgia between 1990 and 1994. Females travelled either eastward up to 3000 km away to the open Southern Ocean or to the continental shelf on or near the Antarctic Peninsula. Males either stayed close to South Georgia or used South Georgia as a base for shorter trips. The females all left South Georgia in a directed manner at an average rate of 79.4 km/day over at least the first 15 days. Thereafter travel was interrupted by bouts of slower travel or stationary phases. The latter were localized at sites on the continental shelf or along its edge. Three seals that were tracked over more than one season repeated their outward direction of travel and used some of the same sites in subsequent years. The magnitude of the movements makes most of the Southern Ocean potentially available to elephant seals.

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