Jörg-Olaf Wolff

The dynamical balance, transport and circulation of the Antarctic Circumpolar Current

The physical elements of the circulation of the Antarctic Circumpolar Current (ACC) are reviewed. A picture of the circulation is sketched by means of recent observations from the WOCE decade. We present and discuss the role of forcing functions (wind stress, surface buoyancy flux) in the dynamical balance of the flow and in the meridional circulation and study their relation to the ACC transport. The physics of form stress at tilted isopycnals and at the ocean bottom are elucidated as central mechanisms in the momentum balance. We explain the failure of the Sverdrup balance in the ACC circulation and highlight the role of geostrophic contours in the balance of vorticity. Emphasis is on the interrelation of the zonal momentum balance and the meridional circulation, the importance of diapycnal mixing and eddy processes. Finally, new model concepts are described: a model of the ACC transport dependence on wind stress and buoyancy flux, based on linear wave theory; and a model of the meridional overturning and the mean density structure of the Southern Ocean, based on zonally averaged dynamics and thermodynamics with eddy parametrization.

Assessing water renewal time scales for marine environments from three-dimensional modelling: A case study for Hervey Bay, Australia

We apply the three-dimensional COupled Hydrodynamical Ecological model for REgioNal Shelf seas (COHERENS) to compute water renewal time scales for Hervey Bay, a large coastal embayment situated off the central eastern coast of Australia. Water renewal time scales are not directly observable but are derived indirectly from computational studies. Improved knowledge of these time scales assists in evaluating the water quality of coastal environments and can be utilised in sustainable marine resource management. Results from simulations with climatological September forcing are presented and compared to cruise data reported by Ribbe (2006. A study into the export of saline water from Hervey Bay, Australia. Estuarine Coastal and Shelf Science 66, 550-558). A series of simulations using idealised forcing provides detailed insight into water renewal pathways and regional differences in renewal timescales. We find that more than 85% of the coastal embayments water is fully renewed within about 50-80 days. The eastern and western shallow coastal regions are ventilated more rapidly than the central, deeper part of the domain. The climatological simulation yields temperature and salinity patterns that are consistent with the observed situation and water renewal time scales in the range of those derived from idealised model studies. While the reported simulations involve many simplifications, the global assessment of the renewal time scale is in the range of a previous estimate derived for this coastal embayment from a simpler model and observational data.

Distribution and characteristics of marine habitats in a subpolar bay based on hydroacoustics and bed shear stress estimates - Potter Cove, King George Island, Antarctica

Marine habitats worldwide are increasingly pressurized by climate change, especially along the Antarctic Peninsula. Well-studied areas in front of rapidly retreating tidewater glaciers like Potter Cove are representative for similar coastal environments and, therefore, shed light on habitat formation and development on not only a local but also regional scale. The objective of this study was to provide insights into habitat distribution in Potter Cove, King George Island, Antarctica, and to evaluate the associated environmental processes. Furthermore, an assessment concerning the future development of the habitats is provided. To describe the seafloor habitats in Potter Cove, an acoustic seabed discrimination system (RoxAnn) was used in combination with underwater video images and sediment samples. Due to the absence of wave and current measurements in the study area, bed shear stress estimates served to delineate zones prone to sediment erosion. On the basis of the investigations, two habitat classes were identified in Potter Cove, namely soft-sediment and stone habitats that, besides influences from sediment supply and coastal morphology, are controlled by sediment erosion. A future expansion of the stone habitat is predicted if recent environmental change trends continue. Possible implications for the Potter Cove environment, and other coastal ecosystems under similar pressure, include changes in biomass and species composition.

East Frisian Wadden Sea hydrodynamics and wave effects in an unstructured-grid model

An unstructured-grid model (FVCOM) coupled to a surface wave model (FVCOM-SWAVE) with two different setups is used to investigate the hydrodynamic and wave energy conditions during a moderate wind and a storm situation in the southern North Sea. One setup covers the whole North Sea with moderately increased grid resolution at the coast, whereas the other is a very high-resolution Wadden Sea setup that is one-way coupled to the coarser North Sea model. The results of both model setups are validated, compared to each other and analysed with a focus on longshore currents and wave energy. The numerical results show that during storm conditions, strong wave-induced longshore currents occur in front of the East Frisian Wadden Sea islands with current speeds up to 1 m/s. The model setup with the higher resolution around the islands shows even stronger currents than the coarser setup. The wave-current interaction also influences the surface elevation by raising the water level in the tidal basins. The calculated wave energies show large differences between moderate wind and storm conditions with time-averaged values up to 200 kW/m.

Eddy Fluxes and Second-Order Moment Balances for Nonhomogeneous Quasigeostrophic Turbulence in Wind-Driven Zonal Flows*

Abstract The balances of momentum and second-order moments (potential enstrophy, energies, and potential vorticity flux) of wind-driven zonal flow, using a suite of numerical eddy resolving experiments in a two-layer channel, governed by quasigeostrophic dynamics, are investigated. The flow regime in these experiments does not satisfy the usual scaling of quasigeostrophic large-scale dynamics: relative vorticity is a significant contribution to the quasigeostrophic potential vorticity (QPV) in the deep layer and the lateral Reynolds stress divergence is comparable to the interfacial form stress in the top layer. The balances of second-order moments confirm that the eddy-induced fluxes of QPV and layer thickness are downgradient but significant contributions of triple moments occur. Existing parameterizations and scaling laws of the eddy fluxes of QPV and layer thickness are tested against data from the numerical experiments and it is shown that the usual downgradient forms of parameterization with diffusi...

Atmosphere–Ocean Momentum Exchange in General Circulation Models

A series of two-layer quasigeostrophic solutions for the ocean circulation driven by a steady wind in a channel with topography and in a flat bottom rectangular basin are presented in which the atmosphere and ocean are inertially coupled through the surface stress relation. The only other frictional processes are biharmonic lateral friction (under free-slip boundary conditions) and topographic form stress; there is no bottom friction involved. The results indicate that realistic momentum balances can be obtained on this physical basis. Two types of solutions are obtained, which are called (i) the I series in which the inertial coupling relation is applied directly in the earth reference frame with no current averaging and almost steady stream fields occur and (ii) the S series in which the inertial coupling relation is applied for long current averaging periods, of the order 100 days, rather than instantaneously. The solutions for the longer current averaging periods produce vigorous eddy fields, but their time-mean is very similar to the corresponding solution with no current averaging. Surface Stokes drift streamfields are also generated by the inertial coupling mechanism. Some implications of the results for general circulation modeling are discussed.

On the variability of the Charnock constant and the functional dependence of the drag coefficient on wind speed: Part II-Observations

An analytical expression for the 10 m drag law in terms of the 10 m wind speed at the maximum in the 10 m drag coefficient, and the Charnock constant is presented, which is based on the results obtained from a model of the air-sea interface derived in Bye et al. (2010). This drag law is almost independent of wave age and over the mid-range of wind speeds (5−17 ms−1) is very similar to the drag law based on observed data presented in Foreman and Emeis (2010). The linear fit of the observed data which incorporates a constant into the traditional definition of the drag coefficient is shown to arise to first-order as a consequence of the momentum exchange across the air-sea boundary layer brought about by wave generation and spray production which are explicitly represented in the theoretical model.

Numerical study on wave dynamics and wave-induced bed erosion characteristics in Potter Cove, Antarctica

Wave generation, propagation, and transformation from deep ocean over complex bathymetric terrains to coastal waters around Potter Cove (King George Island, South Shetland Islands, Antarctica) have been simulated for an austral summer month using the Simulating Waves Nearshore (SWAN) wave model. This study aims to examine and understand the wave patterns, energy fluxes, and dissipations in Potter Cove. Bed shear stress due to waves is also calculated to provide a general insight on the bed sediment erosion characteristics in Potter Cove.A nesting approach has been implemented from an oceanic scale to a high-resolution coastal scale around Potter Cove. The results of the simulations were compared with buoy observations obtained from the National Data Buoy Center, the WAVEWATCH III model results, and GlobWave altimeter data. The quality of the modelling results has been assessed using two statistical parameters, namely the Willmott’s index of agreement D and the bias index. Under various wave conditions, the significant wave heights at the inner cove were found to be about 40–50 % smaller than the ones near the mouth of Potter Cove. The wave power in Potter Cove is generally low. The spatial distributions of the wave-induced bed shear stress and active energy dissipation were found to be following the pattern of the bathymetry, and waves were identified as a potential major driving force for bed sediment erosion in Potter Cove, especially in shallow water regions. This study also gives some results on global ocean applications of SWAN.

Distribution and trajectories of floating and benthic marine macrolitter in the south-eastern North Sea

In coastal waters the identification of sources, trajectories and deposition sites of marine litter is often hampered by the complex oceanography of shallow shelf seas. We conducted a multi-annual survey on litter at the sea surface and on the seafloor in the south-eastern North Sea. Bottom trawling was identified as a major source of marine litter. Oceanographic modelling revealed that the distribution of floating litter in the North Sea is largely determined by the site of origin of floating objects whereas the trajectories are strongly influenced by wind drag. Methods adopted from species distribution modelling indicated that resuspension of benthic litter and near-bottom transport processes strongly influence the distribution of litter on the seafloor. Major sink regions for floating marine litter were identified at the west coast of Denmark and in the Skagerrak. Our results may support the development of strategies to reduce the pollution of the North Sea.

Fer receives the 2017 Georg Wüst Prize

Prof. Dr. Ilker Fer received the Georg Wüst Prize 2017 at the European Geophysical Union (EGU) General Assembly in Vienna, Austria, April 2017. This biannual prize is awarded for outstanding contributions to the general field of oceanography and is sponsored by the German Society for Marine Research and Ocean Dynamics. The following is a transcript of the citation by Profs. Koch and Wolff and the response by Prof. Fer.