Weddell Sea Continental Slope Research Articles

Toward Parameterizing Eddy-Mediated Transport of Warm Deep Water across the Weddell Sea Continental Slope

Abstract The transport of warm deep water (WDW) onto the Weddell Sea continental shelf is associated with heat flux and strongly contributes to the melting of Antarctic ice shelves. The small radius of deformation at high latitudes makes it difficult to accurately represent the eddy-driven component of onshore WDW transport in coarse-resolution ocean models so that parameterization becomes necessary. The Gent and McWilliams/Redi (GM/Redi) scheme was designed to parameterize mesoscale eddies in the open ocean. Here, it is assessed to what extent the GM/Redi scheme can generate a realistic transport of WDW across the Weddell Sea continental slope. To this end, the eddy parameterization is applied to a coarse-resolution idealized model of the Weddell Sea continental shelf and slope, and its performance is evaluated against a high-resolution reference simulation. With the GM/Redi parameterization applied, the coarse model simulates a shoreward WDW transport with a heat transport that matches the high-resolution reference and both the hydrographic mean fields and the mean slopes of the isopycnals are improved. A successful application of the GM/Redi parameterization is only possible by reducing the GM diffusivity over the continental slope by an order of magnitude compared to the open ocean value to account for the eddy-suppressing effect of the topographic slope. When the influence of topography on the GM diffusivity is neglected, the coarse model with the parameterization either under- or overestimates the shoreward heat flux. These results motivate the incorporation of slope-aware eddy parameterizations into regional and global ocean models. Significance Statement Mesoscale eddies drive warm water across the continental slope and onto the continental shelf of the Weddell Sea, where it melts the adjacent Antarctic ice shelves. This process is not resolved in ocean models employing a coarse horizontal resolution akin to state-of-the-art climate models. This work addresses this issue by modifying and applying a well-established eddy parameterization to this specific case. The parameterization works particularly well when it accounts for the effect of sloping topography, over which eddy transports are weaker. We expect this modification also to be of benefit to regional and global models.

Coherent Seasonal Acceleration of the Weddell Sea Boundary Current System Driven by Upstream Winds

AbstractThe Weddell Sea is of global importance in the formation of dense bottom waters associated with sea ice formation and ocean‐ice sheet interaction occurring on the shelf areas. In this context, the Weddell Sea boundary current system (BCS) presents a major conduit for transporting relatively warm water to the Weddell Sea ice shelves and for exporting some modified form of Wedell Sea deep and bottom waters into the open ocean. This study investigates the downstream evolution of the structure and the seasonality of the BCS along the Weddell Sea continental slope, combining ocean data collected for the past two decades at three study locations. The interannual‐mean geostrophic flow, which follows planetary potential vorticity contours, shifts from being surface intensified to bottom intensified along stream. The shift occurs due to the densification of water masses and the decreasing surface stress that occurs westward, toward the Antarctic Peninsula. A coherent along‐slope seasonal acceleration of the barotropic flow exists, with maximum speed in austral autumn and minimum speed in austral summer. The barotropic flow significantly contributes to the seasonal variability in bottom velocity along the tip of the Antarctic Peninsula. Our analysis suggests that the winds on the eastern/northeastern side of the gyre determines the seasonal acceleration of the barotropic flow. In turn, they might control the export of Weddell Sea Bottom Water on seasonal time scales. The processes controlling the baroclinic seasonality of the flow need further investigation.

Antarctic Thermocline Dynamics along a Narrow Shelf with Easterly Winds

AbstractDetermining the role of Southern Ocean warm intermediate water for driving melting of the Antarctic ice sheet is a major challenge in assessing future sea level rise. Analysis of 2859 CTD profiles obtained between 1977 and 2016 by ships and instrumented seals at the Weddell Sea continental slope reveals a seasonal rise of the Antarctic Slope Front thermocline by more than 100 m during the summer. The signal at Kapp Norvegia (17°W) corresponds with a seasonal warming downstream at the Filchner Trough (40°W), indicating that a coherent evolution of the slope front along the shelf break regulates the onshore flow of warm deep water. Climatological cross sections of the slope front hydrography show that downwelling of Antarctic Surface Water forms a secondary front above the warm deep water interface during summer. Enhanced baroclinic growth rates at this front suggest that the wind-driven suppression of the thermocline is partially compensated by a shallower eddy overturning cell when surface water is present. A simple model of the Weddell Gyre boundary current reveals that wintertime densification of surface waters is crucial for maintaining the deep thermocline along the eastern Weddell Sea coast. The sensitivity of the warm inflow to the cross-frontal density gradient implies a positive feedback with ice shelf melting that may lead to an abrupt transition into a high melting state once warm water rises over the shelf break depth. Despite its regional focus, this study highlights the role of upper ocean buoyancy fluxes for controlling the thermocline depth along seasonally ice-covered narrow shelf regions with cyclonic along-slope winds.

Southern Weddell Sea shelf edge geomorphology: Implications for gully formation by the overflow of high‐salinity water

Submarine gullies are the most common morphological features observed on Antarctic continental slopes. The processes forming these gullies, however, remain poorly constrained. In some areas, gully heads incise the continental shelf edge, and one hypothesis proposed is erosion by overflow of cold, dense water masses formed on the continental shelf. We examined new multibeam echo sounder bathymetric data from the Weddell Sea continental slope, the region that has the highest rate of cold, dense water overflow in Antarctica. Ice Shelf Water (ISW) cascades downslope with an average transport rate of 1.6 Sverdrups (Sv) in the southern Weddell Sea. Our new data show that within this region, ISW overflow does not deeply incise the shelf edge. The absence of gullies extending deeply into the glacial sediments at the shelf edge implies that cold, high salinity water overflow is unlikely to have caused the extensive shelf edge erosion observed on other parts of the Antarctic continental margin. Instead, the gullies observed in the southern Weddell Sea are relatively small and their characteristics indicative of small‐scale slides, probably resulting from the rapid accumulation and subsequent failure of proglacial sediment during glacial maxima.

Early summer thermohaline characteristics and mixing in the western Weddell Sea

Abstract The Ice Station POLarstern (ISPOL) cruise revisited the western Weddell Sea in late 2004 and obtained a comprehensive set of conductivity–temperature–depth (CTD) data. This study describes the thermohaline structure and diapycnal mixing environment observed in 2004 and compares them with conditions observed more than a decade earlier. Hydrographic conditions on the central western Weddell Sea continental slope, off Larsen C Ice Shelf, in late winter/early spring of 2004/2005 can be described as a well-stratified environment with upper layers evidencing relict structures from intense winter near-surface vertical fluxes, an intermediate depth temperature maximum, and a cold near-bottom layer marked by patchy property distributions. A well-developed surface mixed layer, isolated from the underlying Warm Deep Water (WDW) by a pronounced pycnocline and characterized by lack of warming and by minimal sea-ice basal melting, supports the assumption that upper ocean winter conditions persisted during most of the ISPOL experiment. Much of the western Weddell Sea water column has remained essentially unchanged since 1992; however, significant differences were observed in two of the regional water masses. The first, Modified Weddell Deep Water (MWDW), comprises the permanent pycnocline and was less saline than a decade earlier, whereas Weddell Sea Bottom Water (WSBW) was horizontally patchier and colder. Near-bottom temperatures observed in 2004 were the coldest on record for the western Weddell Sea over the continental slope. Minimum temperatures were ∼0.4 and ∼0.3 °C colder than during 1992–1993, respectively. The 2004 near-bottom temperature/salinity characteristics revealed the presence of two different WSBW types, whereby a warm, fresh layer overlays a colder, saltier layer (both formed in the western Weddell Sea). The deeper layer may have formed locally as high salinity shelf water (HSSW) that flowed intermittently down the continental slope, which is consistent with the observed horizontal patchiness. The latter can be associated with the near-bottom variability found in Powell Basin with consequences for the deep water outflow from the Weddell Sea.

Acoustical characterization of sediments by Parasound and 3.5 kHz systems: Related sedimentary processes on the southeastern Weddell Sea continental slope, Antarctica

Subbottom profiling was performed during four Polarstern expeditions to the southeastern Weddell Sea continental slope, northwest of Lyddan Ice Rise, Antarctica. Analogue records from high resolution, seismic reflection systems (3.5 kHz and Parasound) were used to classify sediment echo types. A comparison between the two systems shows the advantage of the Parasound system. The smaller beam angle produces a higher spatial resolution, which reduces the diffraction hyperbolae considerably. The higher vertical resolution and deeper penetration provide more detailed information concerning seismic stratification. Using four basic echo categories (P = Prolonged, L = Layered D = Diffraction hyperbolae, W = Wedging subbottoms), ten discrete sediment echo patterns were distinguished. Their regional distribution and the depth of acoustic signal penetration into the sediment were mapped. SW-NE trending sediment ridges with channels running parallel on the southeastern side of the ridges were traced in the eastern part of the study area. The prolonged and highly reflective echo types of the channel bottoms indicate deposition of predominantly coarse-grained sediments affected by erosional processes. These channels are interpreted as drainage systems for bottom water currents derived from the shelf and steep upper slope. The flow direction is towards the northeast, counter to the Weddell Gyre. In contrast, the sediment ridges show up to 80 m subbottom penetration and an echo type of distinct, multi-layered reflectors running parallel to subparallel with the surface reflector. The sediments of the ridges are interpreted to be “levee”-sediments, overspilled from the channels and preferentially deposited on the northwestern flank due to the Coriolis force. Sedimentary units separated by wedge shaped reflectors are widespread on the gently inclined terrace in the middle part of the continental slope. These sediments are presumed to be produced by syn-sedimentary slumping and proximal debris flows having a northeastern direction of movement.

Mixing of the Weddell Sea continental slope

Abstract Antarctic Bottom Water is produced by a mixing of water masses which occurs on Antarctic continental slopes. This paper discusses the mechanisms involved, judging the success or failure of each mechanism by use of two observational criteria. These are (a) that the plume of dense water on the slope should reach the foot of the slope, and (b) that the mass flux should increase by 100 to 300%. Two- and three-dimensional plume models (the latter of which have been successful in describing the properties of the Norwegian and Mediterranean outflows) give results which are incompatible with observations. These plume models are steady , however. The possibility that time variability may be an important factor in the dynamics is examined by modelling the dense water as a thermal. However, this too gives results which do not agree with observations. It is shown that the dominant effect involves the equation of state at low temperatures. In particular, the increase of the thermal expansion coefficient α with depth must be included (nonlinearities, which can produce cabbeling on the small scale, are unimportant here). This effect produces a strange situation. At the top of the slope, the plume is colder—and therefore more dense—than its surroundings. As it falls down the slope it can become even more dense than its surroundings, despite the fact that its environment is stably stratified. The result of this is an effective source of internal energy which accelerates the plume down the slope. At greater depths, where the plume is nearly at the same temperature as its surroundings, this energy source disappears and normal plume dynamics resume. Thus, if the equation of state is included, results in agreement with observations are obtained.