Anna Wåhlin

The Mediterranean outflow splitting—a comparison between theoretical models and CANIGO data

Abstract The Mediterranean outflow in the Atlantic is vertically subdivided into two main cores that are identified by temperature and salinity maxima. Hydrological data collected west of the Strait of Gibraltar and off the south and south-west coasts of Portugal in September 1997 have been used in two different models to shed some light on the mechanisms that cause a splitting of the Mediterranean outflow. These models are (i) a steady, one-dimensional streamtube model, and (ii) a local, time-dependent model that includes cross-stream variations. The streamtube model, applied at the northern, respectively, the southern part of the outflow, appears capable of reproducing the observed density difference between the two cores. The vertical separation, however, was underestimated. The results from the local model suggest that a bump in the bottom topography may be a determining factor for the splitting tendency of the flow

Topographic steering of dense currents with application to submarine canyons

The influence of a submarine canyon on a dense bottom current flowing along the continental shelf is considered. It is shown that the front of the current moves forward with the Nof velocity, establishing a stationary plume parallel to the depth contours behind it. The stationary flow has a frictional transport directed downhill, perpendicular to the depth contours. This transport converges inside topographic depressions. Consequently, the heavy water accumulates in such areas and the current may change direction and flow inside the depression rather than along the depth contours. Submarine canyons can influence the along-shelf flow in this manner. A canyon that intersects the continental slope has the ability to channel dense water down to the deep sea. The rate at which water is drained from the along-shelf flow is determined from the canyons geometry and the magnitude of bottom friction. If the along-shore transport upstream of the canyon exceeds the capacity of the canyon, the plume divides into two branches. A portion of the flow is then steered down into the canyon while the rest continues on the downstream side of it. The distribution of dense water inside the canyon is obtained analytically for a simplified description of bottom friction, as well as numerically for more complex friction parameterizations. The results are remarkably similar. This indicates that the downward flow is controlled by the topography and the magnitude of bottom friction rather than the choice of how friction is parameterized.

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

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 transportwarm densewater ontothe shelf,channeledby bathymetric troughs leadingto 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.

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

AbstractThe 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 f...

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

Theexchangeofwarm,saltyseawateracrossthecontinentalshelvesoffWestAntarcticaleadstosubsurface glacial melting at the interface between the ocean and the West Antarctic Ice Sheet. One mechanism that contributesto the cross-shelf transportis Ekmantransportinduced by along-slope currentsoverthe slope and shelf break. An investigation of this process is applied to the Amundsen Sea shelfbreak region, using recently acquiredandhistoricalfielddatatoguidetheanalyses.Along-slopecurrentswereobservedattransectsacross 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.Thebasin water in the easternpart ofthe shelfwas dominatedby wateroriginating 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 0.68C during the past decade.

Limitations of the streamtube model

Abstract A common feature of one-dimensional streamtube models is that the pressure gradient includes only the effect of the density difference between the plume and the ambient water on a sloping bottom. Variations of the streamtube height and density along the flow path are thus neglected. In the present paper a streamtube model is used to highlight the assumptions underlying the simplification and to obtain the limitations this prerequisite presents. The results indicate that the parameter range for which the simplification is applicable is largest when the initial outflow is parallel to the isobaths. More specific limits are given for three major outflows — the Mediterranean, the Faroe Bank Channel and the Denmark Strait outflow.

Delivering 21st century Antarctic and Southern Ocean science

Abstract The Antarctic Roadmap Challenges (ARC) project identified critical requirements to deliver high priority Antarctic research in the 21st century. The ARC project addressed the challenges of enabling technologies, facilitating access, providing logistics and infrastructure, and capitalizing on international co-operation. Technological requirements include: i) innovative automated in situ observing systems, sensors and interoperable platforms (including power demands), ii) realistic and holistic numerical models, iii) enhanced remote sensing and sensors, iv) expanded sample collection and retrieval technologies, and v) greater cyber-infrastructure to process ‘big data’ collection, transmission and analyses while promoting data accessibility. These technologies must be widely available, performance and reliability must be improved and technologies used elsewhere must be applied to the Antarctic. Considerable Antarctic research is field-based, making access to vital geographical targets essential. Future research will require continent- and ocean-wide environmentally responsible access to coastal and interior Antarctica and the Southern Ocean. Year-round access is indispensable. The cost of future Antarctic science is great but there are opportunities for all to participate commensurate with national resources, expertise and interests. The scope of future Antarctic research will necessitate enhanced and inventive interdisciplinary and international collaborations. The full promise of Antarctic science will only be realized if nations act together.

Pathways of Meltwater Export from Petermann Glacier, Greenland

AbstractIntrusions of Atlantic Water cause basal melting of Greenland’s marine-terminating glaciers and ice shelves, such as that of Petermann Glacier, in northwest Greenland. The fate of the resulting glacial meltwater is largely unknown. It is investigated here, using hydrographic observations collected during a research cruise in Petermann Fjord and adjacent Nares Strait onboard icebreaker (I/B) Oden in August 2015. A three end-member mixing method provides the concentration of Petermann ice shelf meltwater. Meltwater from Petermann is found in all of the casts in adjacent Nares Strait, with the highest concentration along the Greenland coast in the direction of Kelvin wave phase propagation. The meltwater from Petermann mostly flows out on the northeast side of the fjord as a baroclinic boundary current, with the depth of maximum meltwater concentrations approximately 150 m and shoaling along its pathway. At the outer sill, which separates the fjord from the ambient ocean, approximately 0.3 mSv (1 Sv ...

Subinertial Oscillations on the Amundsen Sea Shelf, Antarctica

AbstractMooring data from the western flank of Dotson trough, Amundsen Sea shelf region, show the presence of barotropic oscillations with a period of 40–80 h. The oscillations are visible in velocity, temperature, salinity, and pressure and are comparable to tides in magnitude. The period of the oscillations corresponds to topographic Rossby waves of low group velocity and a wavelength of about 40 km, that is, the half-width of the channel. It is suggested that these resonant topographic Rossby waves cause the observed peak in the wave spectra. The observations show that sparse CTD data from this region should be treated with caution and need to be complemented with moorings or yo-yo stations in order to give a representative picture for the hydrography.

Correlation between Synthetic Aperture Radar Surface Winds and Deep Water Velocity in the Amundsen Sea, Antarctica

The recent observed thinning of the glacier ice shelves in the Amundsen Sea (Antarctica) has been attributed to warm deep currents, possibly induced by along-coast winds in the vicinity of the glacial ice sheet. Here, high resolution maps of wind fields derived from Synthetic Aperture Radar (SAR) data have been studied and correlated with subsurface measurements of the deep water velocities in the Amundsen Sea area. Focus is on periods with low ice coverage in 2010 and 2011. In 2010, which had comparatively low ice coverage, the results indicate a more rapid response to wind forcing in the deep currents than in 2011. The SAR wind speed maps have better spatial resolution than available reanalysis data, and higher maximum correlation was obtained with SAR data than with reanalysis data despite the lower temporal resolution. The maximum correlation was R = 0.71, in a direction that is consistent with wind-driven Ekman theory. This is significantly larger than in previous studies. The larger correlation could be due to the better spatial resolution or the restriction to months with minimum ice coverage. The results indicate that SAR is a useful complement to infer the subsurface variability of the ocean circulation in remote areas in polar oceans.