Roger Proctor

A highly spatially resolved ecosystem model for the North West European Continental Shelf

Abstract This paper outlines an approach to complex spatio-temporal marine ecosystem modelling as applied to the North Western European Continental Shelf. The model presented here goes further than previous work, as we combine a higher resolution hydrodynamic model, the POL-3DB baroclinic model with the European Regional Seas Ecosystem Model. This combination of models includes many of the processes (benthic-pelagic coupling, dynamic zooplankton and nitrogen, phosphorous and silicate cycling) previous authors have identitied as missing from their models and partially responsible for the inadequacies of their simulations. Spatial distributions of key physical and ecological variables taken from the three dimensional high resolution hydrodynamic/ecological simulations are presented to illustrate how spatial and temporal variations in physical processes determine the onset of the spring bloom in the North Sea. A basic validation of these simulations is presented, which indicates the model reproduces many of the features of the seasonal cycles of nutrients and phytoplankton, but fails to simulate mesozooplankton biomass in a convincing manner. The reasons for this are discussed along with potential new research directions.

An initial assessment of the potential environmental impact of CO2 escape from marine carbon capture and storage systems

Abstract If carbon capture and storage is to be adopted as a CO2 mitigation strategy, it is important to understand the associated risks. The risk analysis consists of several elements such as leakage probability, assessing the strength of environmental perturbation, and quantifying the ecological, economic, and social impacts. Here, the environmental perturbation aspect is addressed by using a marine system model of the North West European Shelf seas to simulate the consequences of CO2 additions such as those that could arise from a failure of geological sequestration schemes. Little information exists to guide the choice of leak scenario and many assumptions are required; for consistency the assumptions err towards greater impact and what would be in likelihood extreme scenarios. The simulations indicate that only the largest leakage scenarios tested are capable of producing perturbations that are likely to have environmental consequences beyond the locality of a leak event. It is shown that, given the available evidence, the chemical perturbation of a sequestration leak, regionally integrated, is likely to be insignificant when compared with that from continued non-mitigated atmospheric CO2 emissions and the subsequent acidification of the marine system. The potential ecological impacts of a large environmental CO2 perturbation are reviewed, indicating that the biogeochemical functioning and biodiversity are sensitive. The key unknowns that must be addressed in future research are identified; namely, the fine scale dispersion of CO2 and the ability of ecological systems to recover from perturbation.

Modelling the global coastal ocean

Shelf and coastal seas are regions of exceptionally high biological productivity, high rates of biogeochemical cycling and immense socio-economic importance. They are, however, poorly represented by the present generation of Earth system models, both in terms of resolution and process representation. Hence, these models cannot be used to elucidate the role of the coastal ocean in global biogeochemical cycles and the effects global change (both direct anthropogenic and climatic) are having on them. Here, we present a system for simulating all the coastal regions around the world (the Global Coastal Ocean Modelling System) in a systematic and practical fashion. It is based on automatically generating multiple nested model domains, using the Proudman Oceanographic Laboratory Coastal Ocean Modelling System coupled to the European Regional Seas Ecosystem Model. Preliminary results from the system are presented. These demonstrate the viability of the concept, and we discuss the prospects for using the system to explore key areas of global change in shelf seas, such as their role in the carbon cycle and climate change effects on fisheries.

Thermohaline circulation of shallow tidal seas

The mechanisms controlling the temperature and salinity structure of shallow continental shelf seas have been understood for over thirty years, yet knowledge of what drives their large-scale circulation has remained relatively unknown. Here we describe a decade long programme of measurements, using satellite-tracked drifting buoys on the northwest European shelf, to draw attention to a striking picture of highly organised thermohaline circulation consisting of narrow, near surface, fast flowing jets. These are ubiquitous above sharp horizontal gradients in bottom temperatures and/or salinities. The circulation phenomena we describe are likely to be prevalent on all similar, wide, tidally energetic continental shelves including those off north-eastern China, Argentina and parts of the Arctic. The robust, repeatable observation of the key role of jets above bottom fronts results in a fundamental reassessment of how we view the dynamics of shelf seas.

Regional scale impacts of distinct CO2 additions in the North Sea

A marine system model applied to the North West European shelf seas is used to simulate the consequences of distinct CO(2) additions such as those that could arise from a failure of geological sequestration schemes. The choice of leak scenario is guided by only a small number of available observations and requires several assumptions; hence the simulations reported on are engineered to be worse case scenarios. The simulations indicate that only the most extreme scenarios are capable of producing perturbations that are likely to have environmental consequences beyond the locality of a leak event. Tidally driven mixing rather than air-sea exchange is identified as the primary mechanism for dispersal of added CO(2). We show that, given the available evidence, the environmental impact of a sequestration leak is likely to be insignificant when compared to the expected impact from continued non-mitigated atmospheric CO(2) emissions and the subsequent acidification of the marine system. We also conclude that more research, including both leak simulations and assessment of ecological impacts is necessary to fully understand the impact of CO(2) additions to the marine system.

The seasonal circulation and volume transport on the northwest European continental shelf: A fine-resolution model study

In this paper the circulation of the northwest European continental shelf is investigated using the first year-long density-evolving simulation at shelf wide scales and sub-Rossby Radius resolution (similar to 1.8 km). A series of numerical experiments are conducted to distinguish between the wind-, density-, and oceanic-driven components of the flow. These demonstrate that, while all components have a role throughout the year, the density- driven component is particularly important during the summer and autumn months. The time evolution of the density field makes a significant contribution to the seasonal variation of volume transport on shelf wide scales and is persistent in direction; whereas the wind- driven volume transport acts on much shorter timescales and is more variable in direction. The importance of the oceanic forcing is demonstrated, representing tidal residuals and large-scale oceanic sea level (pressure) variation; this forcing drives a substantial component of the circulation throughout the year. Twenty six satellite tracked drifters deployed in the summer of 2001 are used for a direct validation of the model currents. The model current speeds are found to be accurate to similar to 46% when averaged over similar to 40 d, but tend to be too slow. The summer volume fluxes are compared with estimates in the literature showing good agreement, although there is a suggestion that the North Sea inflows are overestimated. Comparisons with the coarser resolution model used for boundary conditions demonstrate the importance of fine-resolution to the details of the frontal currents with consequences for modeling exchange processes and biological activity in these regions

A Numerical Study of the Long- and Short-Term Temperature Variability and Thermal Circulation in the North Sea

A three-dimensional numerical study is presented of the seasonal, semimonthly, and tidal-inertial cycles of temperature and density-driven circulation within the North Sea. The simulations are conducted using realistic forcing data and are compared with the 1989 data of the North Sea Project. Sensitivity experiments are performed to test the physical and numerical impact of the heat flux parameterizations, turbulence scheme, and advective transport. Parameterizations of the surface fluxes with the Monin-Obukhov similarity theory provide a relaxation mechanism and can partially explain the previously obtained overestimate of the depth mean temperatures in summer. Temperature stratification and thermocline depth are reasonably predicted using a variant of the Mellor-Yamada turbulence closure with limiting conditions for turbulence variables. The results question the common view to adopt a tuned background scheme for internal wave mixing. Two mechanisms are discussed that describe the feedback of the turbulence scheme on the surface forcing and the baroclinic circulation, generated at the tidal mixing fronts. First, an increased vertical mixing increases the depth mean temperature in summer through the surface heat flux, with a restoring mechanism acting during autumn. Second, the magnitude and horizontal shear of the density flow are reduced in response to a higher mixing rate. Thermal and salinity fronts generate a seasonal circulation pattern in the North Sea. Their impact on the horizontal temperature distributions is found to be in good agreement with the observations. It is shown that, in the absence of strong wind forcing, both the vertical temperature distribution and the thermal circulation experience semimonthly variations in response to the spring-neap cycle in tidal mixing. At spring tides, the surface mixed layers are shallower, in agreement with observations at two mooring stations, and the baroclinic circulation intensifies, whereas the opposite occurs at neaps.

Forecast and hindcast simulations of the Braer oil spill

Abstract Model predictions of the movement of oil released during the Shetlands oil spill in January 1993 commenced immediately following the grounding of the Braer. The initial forecasts, made with a 5 nautical mile grid model, predicted oil along the west coast of the island but were too coarse to resolve the coastal bathymetry and circulation. This forced the development of a fine grid (924 m) hydrodynamic model of the region which was interfaced to surface wind forecasts and meshed to a shelf-wide storm surge model. Into this detailed model were merged particle tracking algorithms to simulate the movement and spreading of the oil. This model development required intensive efforts in the days following the spill. Some of the early predictions were in error, in part due to the complexity of the task and in part due to the lack of observational results to confirm the movement of the oil. However, hindcast simulations made in the months following the accident probably present a realistic account of the movement and spread of the oil. The hindcasts correctly simulate the presence of oil on the sea bed 100 km to the south of the spill site. This was an unexpected feature of the oil trajectory and its simulation has given additional insights into the dynamical processes that can influence the movement of oil.

The Role of Advection in Determining the Temperature Structure of the Irish Sea

Abstract The temperature structure of the Irish Sea is investigated using a 3-yr simulation with a high-resolution (∼1.8 km) three-dimensional baroclinic model (the Proudman Oceanographic Laboratory Coastal-Ocean Modelling System) and CTD and Advanced Very High Resolution Radiometer observations. This paper focuses on the extent to which (horizontal) advection determines the temperature structure. It is found that it has a significant effect on the depth-mean temperatures throughout the region and on the vertical profiles in seasonally stratified areas, such as the Celtic Sea and western Irish Sea. There is depth-mean advective heating during the summer in these stratified regions, whereas in well-mixed regions advection tends to reduce the amplitude of the seasonal cycle. Through an analysis of the terms in the temperature equation, the warming of the “cool pool” waters of the western Irish Sea can be attributed to the advection of partially well-mixed waters into the stratified region from the north. Th...

Advective controls on primary production in the stratified western Irish Sea: An eddy‐resolving model study

[1] The Proudman Oceanographic Laboratory Coastal Ocean Modelling System and the European Regional Seas Ecosystem Model are applied at eddy-resolving (∼1.8 km) scales to the stratified region of the western Irish Sea to investigate the effects of advective transport processes on the ecosystem. We find currents can transport nutrient-rich water into the otherwise nutrient-depleted surface layer of the stratified region, fueling intermittent production throughout the summer. The currents involved fall into three classes: large-scale wind and density-driven circulation, smaller-scale eddies, and tidally mediated dispersive phenomena; all appear to play a role in this area. A model experiment without ecosystem advection does not show the intermittent surface production; summer growth only occurs at the thermocline. This experiment gives a significantly lower total annual production of 110 ± 26 g C m -2 yr -1 , compared with 150 ± 40 g C m -2 yr -1 for the full model, which is in better agreement with observational estimates of 140 g C m-2 yr -1 . We calculate summer averages of the terms in the scalar transport equation, which show that advective transport of all nutrients dominates over vertical diffusion above the thermocline in most of the stratified region. The transport of nitrate, ammonia, and phosphate is significantly greater than the transport of silicate. This can be attributed to the lack of silicate recycling in the pelagic ecosystem. Only limited and anecdotal observational evidence exists to support these model results, which points to a need for observations of high spatial and temporal resolution to investigate these processes in conjunction with further model studies.