Convenors: Mark Inall, Jo Hopkins, Mattias Green, Marie Porter

Transcription

1 SESSION 15: Ocean Shelf Exchage Convenors: Mark Inall, Jo Hopkins, Mattias Green, Marie Porter ORAL 1 Mesoscale eddies as shelf exchange mechanisms of Circumpolar Deep Water in the west Antarctic Peninsula Kathryn Gunn (Department of Earth Sciences, University of Cambridge, U.K.) Nicky White (Department of Earth Sciences, University of Cambridge, U.K.) Rob Larter (British Antarctic Survey, Cambridge, U.K.) Exchange of Circumpolar Deep Water across the west Antarctic Peninsula shelf break plays a significant role in ice shelf processes, which consequently affect ice sheet mass balance and global sea level. However, the physical mechanisms for this warm water circulation remain elusive. Here, we investigate shelf exchange using two-dimensional seismic imaging of the water column, attaining vertical and horizontal resolutions of ~10 m. 7 seismic profiles, acquired in February 2015 using the RRS James Clark Ross, are analysed. Seismic data were acquired using two Generator-Injector air guns fired every 10 s with a pressure of 2000 psi. Reflections were recorded on a 2.4 km streamer towed at 4.5 m depth with 192 receivers spaced every 12.5 m. Observed reflections are caused by changes of temperature (~ 80%) and salinity (~ 20%) hence delineating water masses of different physical properties. 10 XCTDs and XBTs plus a 38 khz echo-sounder profile were simultaneously acquired and used for calibration. Results show the upper boundary of the Winter Water layer, at m depth, as the brightest reflection across the entire survey, corresponding to temperatures -1 C. Curved, discontinuous reflections forming lens shapes, between m depth, are attributed to eddy-like intrusions of Upper Circumpolar Deep Water. Two warm core eddies, temperatures 1.5 C and length > 10 km, are imaged in Marguerite Trough. An eddy imaged in three dimensions, length 60 km and width 40 km, is calibrated with depressed temperature profiles and is believed to be recirculated intrusions. Between eddies, 2-15 km in length, are imaged in Belgica Trough. The seismic profiles indicate mesoscale eddies are a significant mechanism for warm water shelf exchange in the west Antarctic Peninsula and provide high resolution dimensional constraints.

2 ORAL 2 Title: Vigorous lateral export of the meltwater outflow from beneath an Antarctic ice shelf Alberto C. Naveira Garabato (National Oceanography Centre, Southampton - University of Southampton) Alexander Forryan (National Oceanography Centre, Southampton - University of Southampton), Pierre Dutrieux (Applied Physics Laboratory, University of Washington) Liam Brannigan (University of Stockholm) Louise C. Biddle (University of East Anglia) Karen J. Heywood (University of East Anglia) Adrian Jenkins (British Antarctic Survey) Yvonne L. Firing (National Oceanography Centre, Southampton) Satoshi Kimura (British Antarctic Survey) The processes regulating the physical exchanges across the front of the Pine Island Ice Shelf (one of the most rapidly melting Antarctic ice shelves) in the Amundsen Sea are investigated through the analysis of an extensive set of full-depth hydrographic, velocity and microstructure measurements, obtained as part of the ISTAR expedition in February 2014, and an idealised high-resolution numerical circulation model. The outflows of Glacially Modified Water (GMW), which contains elevated concentrations of meltwater from the ice shelf, are shown to be subject to very intense small-scale turbulent mixing within 5 kilometres of the ice front. The rates of turbulent kinetic energy dissipation and diapycnal mixing in the outflows are enhanced by up to four orders of magnitude relative to those in surrounding waters, which are characterised by turbulence levels typical of the open ocean. An investigation of the causes of the intensified turbulence reveals that it is sustained by submesoscale centrifugal instability of the GMW outflows. The instability drives a secondary circulation that induces rapid lateral mixing between GMW and pycnocline waters offshore, thereby preventing the GMW outflows from reaching the upper-ocean mixed layer and directly influencing surface climate in the region. The likely significance of this process for the fate of meltwater outflows from other Antarctic ice shelves will be discussed.

3 ORAL 3 Long-term variability of exchange between the Slope Current and the West Shetland Shelf B. Berx (Marine Scotland Science, Aberdeen, UK) J. Hopkins (National Oceanography Centre, Liverpool, UK) S. Jones (Scottish Association for Marine Science, Oban, UK) J. Huthnance (National Oceanography Centre, Liverpool, UK) M. Inall (Scottish Association for Marine Science, Oban, UK) Since 1994, mooring deployments on the West Shetland Shelf have monitored the strength of the North West European Slope Current in the Faroe-Shetland Channel (FSC), as part of a transport mooring array (TMA) observing the strength of Atlantic transport to the Nordic Seas. Analysis of the TMA observations to date has focused on quantifying the transport of water masses originating from the Atlantic Basin in general, and their exchange with the Nordic Seas. The FASTNEt (Fluxes Across Sloping Topography of the North East Atlantic) project aimed to study the ocean-shelf exchange, and in particular, the seasonal, inter-annual and regional variation along the North-West European continental shelf edge. As part of this project, the existing historic dataset has also been reexamined, and is the focus of the work presented here. We will present an overview of the data collected in the FSC since October 1994, and discuss the seasonal and inter-annual variability of along-slope and cross-slope transport estimates based on the moored profiling current meters. A coincident time series of satellite altimetry further provides spatial context to the pointmeasurements from current meters in the FSC. Between 2013 and 2015, several surface drifters (1m drogued) were also released to study exchange of the slope current with the coastal region around the Orkney and Shetland Islands. These drifter deployments also provide a Lagrangian context to the cross-shelf exchange in the region. As exploration for oil & gas continues to develop in the FSC, it will be key to continue elucidating the processes governing its circulation, and, in particular, the potential on-shelf transfer of hazardous substances.

4 ORAL 4 Shelf edge exchange and its influence on coastal oceanography Sam Jones (Scottish Association for Marine Science, Oban UK) Finlo Cottier (Scottish Association for Marine Science, Oban, UK) Mark Inall (Scottish Association for Marine Science, Oban, UK) The shallow waters west of Scotland feature strong variability in water properties on a variety of temporal scales. While the region is known to be subject to both coastal and oceanic influences, the causes of variability are poorly understood. The limited characterisation of changes in coastal waters impacts our ability to explain the behaviour of coastal ecosystems, and predict their resilience to future climate scenarios. This work uses historical data in conjunction with recent cruises and a coastal mooring to investigate the causes of variability in the waters west of Scotland. Two new inter-annual salinity time series on the European shelf are developed. The spatial variability in salinity in shallow waters is greatest during winter and increases by a factor of four between the shelf edge and the coastline. At the shelf edge, new observations of the along-slope current suggest that it is stronger but less stable during winter, leading to a greater availability of oceanic water on the outer Malin Shelf. However unlike other documented shelf regions, shelf edge processes do not directly influence Scottish coastal water properties. A baroclinic current originating in the Irish Sea is the main influence near the Scottish coast during quiescent periods, but wind forcing dominates shelf processes during most winters, with prevailing winds tending to drive oceanic water towards the coast. While salinity in the Sea of the Hebrides is moderately correlated to wind, coastal salinity is sensitive to both advective processes and freshwater runoff. On inter-annual time-scales, salinity on the Malin Shelf is higher when the North Atlantic Oscillation (NAO) is positive, whereas the northern Irish Sea is fresher during a positive NAO state. Salinity and flow pathways in Scottish coastal waters appear to be resilient both to changes in the Rockall Trough and a warming climate on decadal time-scales.

5 ORAL 5 Tidally-driven exchange at the shelf break Carl Spingys (University of Liverpool) Ric Williams (University of Liverpool) Mattias Green (University of Bangor) Jo Hopkins (National Oceanography Centre) Jonathan Sharples (University of Liverpool) Enhanced productivity in the shelf seas is dependent on a supply of nutrients from the open ocean, across the steep topography of the shelf break. The exchange at the shelf break is limited as the geostrophic flow is constrained to following isobaths, thus we need to explore processes that break the assumption of geostrophy. A new mechanism is proposed to provide an onshore transport onto continental slopes from baroclinic tides, where propagating tides provide a rectified transport from the correlation of their baroclinic velocity and layer thickness. This rectified transport is equivalent to the bolus transport considered in the open ocean and, for simple wave responses, the implied bolus velocity provides a significant contribution to the Stokes drift velocity. Analysis of 5 moorings in three different shelf seas reveal bolus transport contributions, which are often comparable to the expected contribution from the baroclinic M2 tide (with a mean error of typically 30%). The effect of the bolus transport is also estimated for other shelves drawing upon a global tidal dissipation model and estimates of climatological density. The estimates of bolus velocity reach the order of 1 cm s 1 in regions of strong internal tides. This bolus velocity might be important in systematically transferring heat and other tracers from the open ocean onto the shelves. This heat supply to the shelf reaches 1 to 10 W m 2 in localised regions, due to cooler water being returned when the shelf is cooler than the open ocean. This heat supply may be particularly important in ice-covered shelves, such as surrounding Greenland and the Antarctic Peninsula.

6 ORAL 6 Partially-standing internal tides in Whittard submarine canyon observed by an ocean glider Rob Hall, (Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia) Tahmeena Aslam (Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia and Centre for Environment Fisheries and Aquaculture Science) Veerle Huvenne (National Oceanography Centre, Southampton) An autonomous ocean glider is used to make the first direct measurements of internal tides within Whittard Canyon, a large, dendritic submarine canyon system that incises the Celtic Sea continental slope and a site of high benthic biodiversity. Observations of vertical isopycnal displacement at different sites fit a semi-analytical 2-D model of partially-standing internal tides - comprised of a major wave propagating up the canyon limbs and a minor wave reflected back offshore by supercritical topography near the canyon heads. The up-canyon internal tide energy flux in the primary study limb decreases from 9 to 4 kw m 1 over 28 km (a dissipation rate of W kg 1 ), comparable to elevated energy fluxes and internal tide driven mixing measured in other canyon systems. In the Whittard system, elevated mixing rates are inferred from collapsed temperature-salinity curves and eroded vertical oxygen concentrations gradients near the canyon heads. Glider observations are used to infer 1 C temperature and 15 μmol kg 1 oxygen concentration variability experienced by benthic organisms on the canyon walls due to the presence of internal tides. We hypothesise that this additional environmental stress may partly explain the high biodiversity observed within the canyon.

7 ORAL 7 A Future View of Ocean-Shelf Exchange and Shelf-Scale Circulation on the NW European Continental Shelf Jason Holt (NOC) Jeff Polton (NOC) John Huthnance (National Oceanography Centre, Liverpool, UK) James Harle (NOC) Sarah Wakelin (NOC) Enda O Dea (MetOffice) John Siddorn (MetOffice) The potential impacts of climate change on shelf seas is highly uncertain, with many vectors of atmospheric, oceanic and terrestrial change likely to play a role. Previous studies have highlighted the importance of change in oceanic properties (e.g. nutrients) as they are advected on-shelf. Here we demonstrate how substantial changes in circulation may also occur, with far- reaching consequences for on-shelf properties. We consider a 130 year simulation of the NEMO Atlantic Margin Model (AMM7) to This is driven by the atmospheric component of HADGEM2 (a CMIP5 Earth System Model under RCP 8.5) and a global ocean model (ORCA25, driven by the same atmosphere). Inflows into the North Sea through the Fair Isle channel and east of Shetland are found to substantially decrease, and the Shetland shelf current largely by-passes the North Sea. Moreover the inflow on the western flank of the Norwegian trench reverses. Together, this significantly reduces the cyclonic North Sea circulation and shifts the balance between oceanic and terrestrial influence in this region, seen by a decrease in salinity. Using perturbation experiments and simple geostrophic theory, we attribute the drop in North Sea inflow to a freshening of the North Sea and an increase in its sea surface height relative to the outer-shelf. This feedback process is initiated by a rapid drop in salinity in the northern boundary condition (between Iceland and Norway), causing a freshening of the Nordic seas and a rapid drop in ocean-shelf volume flux. This is sufficient to allow the density gradient between the North Sea and outer-shelf to switch sign. Shorter experiments with either no drop in boundary condition salinity or no river inflows (so a reduced oceanic inflow does not reduce the on-shelf density), both restore the North Sea circulation to close to its present day character.

8 POSTER 1 Interannual variability of ocean-shelf exchange along the North West European Shelf Harle, J. (NOC) Holt, J. (NOC) Inall, M. (Scottish Association for Marine Science, Oban, UK) Polton, J. (NOC) The biogeochemistry and ecosystems of the open-ocean and shelf seas are intimately connected. The Northwest European continental shelf receives a substantial fraction of its nutrients from the wider North Atlantic and exports carbon at depth, sequestering it from atmospheric exchange. We examine the physical processes influencing these ocean-shelf exchanges in a 1/12 degree basin-scale NEMO model with specific features relevant to shelf seas (e.g. tides and advanced vertical mixing schemes). The model is eddy resolving in the open-ocean, resolves barotropic scales on-shelf and has been run for the period We explore the connection between the shelf seas and open-ocean, and compare results with a 1/4 degree (eddy permitting) model that does not include shelf sea processes. Estimates of cross-shelf break fluxes and their interannual variability to the physical processes driving ocean-shelf exchange are presented. While the surface wind driven variability is similar in the two models, there are distinct differences between the 1/12 and 1/4 models at the shelf edge of the NW European Shelf. The 1/12 model is beginning to capture some of the off shelf transport at depth, exhibiting a reduction in off-shelf transport at depth after 1996 driven by a weakening in the northward slope current. We discuss possible implications these exchanges have for the nutrient and carbon fluxes between the shelf-seas and open-ocean.

9 POSTER 2 Automated typology of remotely-sensed shelf-sea fronts linked to stratification Peter I. Miller Remote Sensing Group, Plymouth Marine Laboratory Benjamin R. Loveday, Remote Sensing Group, Plymouth Marine Laboratory, Karen Guihou National Oceanographic Centre, Liverpool. Juliane Wihsgott School of Environmental Sciences, University of Liverpool. Stratification is of critical importance to the circulation, mixing and productivity of the ocean, and is expected to be modified by climate change. Hence it would be prudent to monitor the stratification of the global ocean, though this is currently only possible using in situ sampling, profiling buoys or underwater autonomous vehicles. Earth observation (EO) sensors cannot directly detect stratification, but can observe surface features related to the presence of stratification, for example shelf-sea fronts that separate tidally-mixed water from seasonally stratified water. Unfortunately, existing EO thermal front maps cannot distinguish tidal-mixing fronts from other types of front that have a different or no link with stratification, e.g. upwelling fronts, current or eddy boundary fronts, thermohaline fronts, or shelf-break fronts. This presentation describes efforts to create an automated typology of fronts by considering the integration of the thermal front map with bathymetric features, coincident EO data on ocean colour, and a model of tidal currents. Once the tidal mixing fronts are discriminated, a novel algorithm accumulates evidence for stratification from a sequence of classified front maps. In certain regions, this method can reveal the timing of the seasonal onset and breakdown of stratification. We also discuss further applications for the automated typology of ocean fronts in oceanography and studies of marine animal distribution.

10 POSTER 3 Observations of Antarctic Slope Current transport and dense water flow in the northwestern Weddell Sea Marina Azaneu (UEA), Karen Heywood (UEA) Bastien Queste (UEA) Andrew Thompson (Caltech) In early 2012 the GENTOO project deployed three Seagliders in the northwest Weddell Sea, acquiring high temporal and spatial resolution measurements east of the tip of the Antarctic Peninsula. The Antarctic Slope Front and its associated system of narrow currents influence both lateral and cross-slope transport of physical and biogeochemical properties. In the latter case, the ASF forms a physical and dynamical barrier to cross-shelf exchange, influencing local and global ocean dynamics as well as biogeochemical and ecological distributions. Here we explore the mechanisms that regulate cross-slope exchange processes and their impact on dense water export from the shelf. Dense bottom water spilling off the shelf is identified mostly over the slope, between the 500 and 1000 m isobaths (maximum depth of glider sampling). Analysing 11 cross-slope sections, we document the downstream evolution of the shelf break current on the eastern Antarctic Peninsula, around the Powell Basin, and west of the South Orkney Islands. Sections composited by isobath (equivalently f/h) reveal bottom-intensified flow and an intensification of variability at the continental shelf break. The along-slope transport of the slope current (upper 1000 m) system varies between 2-7 Sv, possibly related to high eddy activity offshore the slope region. Glider transects west of the South Orkney island indicate a northward flow, opposite to the previously assumed regime. These sections also show bottom intensified flows close to the slope, even in the absence of bottom water. The potential vorticity fields are used to identify potential instability mechanisms contributing to the cross-slope exchange of water mass properties.

11 POSTER 4 Fluxes of shelf-sediment derived trace metals to the open ocean: Seasonal variability and the influence of bathymetry Amber Annett (School of Geosciences, University of Edinburgh, UK. Now at: Rutgers University, New Jersey, USA) Walter Geibert (Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany) Alex Thomas (School of Geosciences, University of Edinburgh, UK) Antony Birchill (University of Plymouth, UK) Maeve Lohan (University of Plymouth, UK, and University of Southampton, UK) Peter Statham (University of Southampton, UK) Dagmara Rusiecka (University of Southampton, UK) Eric Achterberg (University of Southampton, UK, and GEOMAR Centre for Ocean Research, Kiel, Germany) Continental shelf sediments represent a source of macro- and micro-nutrients to the open ocean, in particular the essential trace metal iron (Fe). However, considerable uncertainties remain in quantifying the magnitude and variability of this source, including the seasonal variation in flux of Fe from sediments, and time scales of subsequent offshore advection. This study used naturally-occurring isotopes of the radioactive element radium (Ra) to investigate sedimentary flux. Like Fe, Ra accumulates in sediment pore waters and both can be supplied to the overlying water column by resuspension events and release of pore water. The four Ra isotopes decay at different known rates, from days to years, facilitating the use of Ra as a tracer of transport into the overlying water column and subsequent advection over multiple time and length scales. Our study focuses on activities of two short-lived Ra isotopes ( 224 Ra and 223 Ra, 3.7 and11.6 day halflives, respectively) and trace metal concentrations in spring and autumn across the Celtic Sea shelf break. Off-shelf transport fluxes were assessed from 2D sections. These include intermediate nepheloid layers (INLs) related to bathymetric features. Transport of shelf-derived Ra activity was modeled as either advective or diffusive transport, giving advective transport speeds of up to km d -1, and effective eddy diffusivity coefficients up to 1670 m s -2 at right angles to the shelf break. This highest transport was found for a shallow canyon INL in spring, with lower rates found the same INL in autumn, for deeper INLs, and above a spur. We further show that these INLs are enriched in Fe, manganese and cobalt, highlighting their importance for shelf-ocean fluxes. Calculated metal fluxes also vary between the two transect lines, with depth, and seasonally. We discuss this variation in off-shelf trace metal fluxes to explore the contribution of continental shelf sediments to the biogeochemical cycles and ocean budgets of trace metals.

12 POSTER 5 Cross-slope flow driven by deflection of the slope current onto the Malin Shelf Marie Porter (Scottish Association for Marine Science, Oban, UK) Mark Inall (Scottish Association for Marine Science, Oban, UK) Andrew Dale (Scottish Association for Marine Science, Oban, UK) Peter Miller (PML, Plymouth) Sam Jones (Scottish Association for Marine Science, Oban, UK) Between the Celtic Sea and the Malin Shelf the European slope is steep; largely preventing the exchange of water across it and allowing for the maintenance of a density driven, topographically trapped slope current. The Malin slope is less steep, thus a weaker topographic barrier to exchange. In July 2013 we released 30 GDP drifters (15 drogued at 15m and 15 drogued at 70m) on the Malin Slope, with the aim of seeding the slope current. Glider sections both up and down stream of this release point confirmed its presence at this time. However, after following the slope current for only 5 to 10 days the drifters crossed away from the slope and onto the shelf, following the 120 m contour. Of the 30 drifters released, 22 crossed the shelf break, subsequently remaining in a coherent on-shelf current, with an estimated transport of 0.53Sv. The drifters were advected eastward along the Irish coast, joining water from the Celtic Sea in the Irish Coastal Current. A glider transect across these drifter tracks shows that for the shallow drifters (drogued at 15m) the southern extent of the flow was bounded by a surface temperature front, clearly visible in satellite front maps. This front was crossed by the deeper drifters (drogued at 70m), suggesting that it weakens with depth. Furthermore, the deeper drifters track the top of a warm, saline feature centred below 70m between the 120m and 100m contours. Here we will show that while the presence of the slope current can be confirmed along the Malin Slope, there is a substantial leak of slope water onto the shelf. The drifters show that this water can reach as far as the Norwegian Shelf or the North Sea, providing a pathway of ocean water across the northern European Shelf.

13 POSTER 6 Ocean-shelf interaction and exchange John Huthnance (National Oceanography Centre, Liverpool) and FASTNEt team A brief review will be given of physical processes enabling ocean-shelf exchange of water and its contents. A recent study is the project FASTNEt ( Fluxes across sloping topography of the North East Atlantic ). This has made a variety of measurements in three contrasting sectors of shelf edge: the Celtic Sea south-west of Britain, the Malin-Hebrides shelf west of Scotland and the West Shetland shelf north of Scotland. Previous studies established the existence of flow along the continental slope in these areas, more persistently poleward in northern sectors. Modelling aims to diagnose and estimate the contribution of various processes to transports and to exchange along and across the slope. Estimates obtained so far will be presented; overall transport from drifters and moored current meters; effective diffusivity from drifter dispersion and salinity surveys; other estimates of velocity variance contributing to exchange. In addition to transport by the along-slope flow, possible process contributions which may be estimated include internal waves and their Stokes drift, tidal pumping, eddies and Ekman transports, in a wind-driven surface layer and in a bottom boundary layer. Overall estimates of exchange across the shelf edge here are large by global standards, several m 2 /s (Sverdrups per 1000 km). However, the large majority of this exchange is in tides and other motion of comparably short period, and is only effective for water properties or contents that evolve on a time-scale of a day or less.