Sea Project Research Articles

The use of NOSTRADAMUS, a numerical transport model, for simulating concentrations and distributions of chromium, cadmium and lead in the Southern North Sea

A numerical transport model, named NOSTRADAMUS, has been developed to simulate concentrations and distributions of a range of EU List I and II metals, including Cr, Cd and Pb in the water column of the southern North Sea. For Cd and Pb, the model takes advantage of data obtained during the observational phase of the Natural Environment Research Council (NERC) North Sea Project (NSP) during 1988–89 to drive the model and to test the results of model simulations. For Cr, there are no water column measurements from the NSP, and so data from other studies, including those from the more recent NERC LandOcean Interaction Study (LOIS), have been used to assess model results.

Development of a model to reproduce observed suspended sediment distributions in the southern North Sea using Principal Component Analysis and Multiple Linear Regression

As part of the NERC North Sea Project (1987–1992), concentrations of fine suspended particulate matter were determined at over 100 locations on 15 monthly survey cruises. Each of these monthly data sets were interpolated to provide continuous “synoptic” representations over the southern North Sea (south of 56°N). Here, statistical techniques are used in conjunction with numerical model simulations to interpret these data. Principal Component Analysis is performed on these monthly series of observations to locate statistically significant sources and sinks of suspended particulate matter. This analysis reveals the month-to-month variability of a primary source, accounting for around 80% of the total variation in the observations, and located in the vicinity of The Wash estuary and adjacent coast. A numerical dispersion model is then developed to simulate the erosion, settling and transport of fine suspended sediment from this source. The formulations and associated coefficients used to describe re-erosion and settling rates are then fine-tuned by comparison with observations. In the second part of this study, the sediment model developed is used to simulate dispersion from discrete sources. Thence a Multiple Linear Regression technique, as described by McManus and Prandle (1994) (in Marine Pollution Bulletin 28, 451–455), is used to fit these modelled dispersion patterns to the original observations to determine rates of sediment supply from these discrete sources. This technique reveals that (i) riverine sources are not statistically significant and (ii) the East Anglian sources are small in magnitude compared with the Dover Strait and northern North Sea sources. Further analysis explains this apparent contradiction with the first part of the study by illustrating how the longer effective flushing times of the two coastal sources amplifies their net contribution to the suspended particulate matter distribution in the southern North Sea. Estimates of mean annual supply from the statistically significant sources (in 10 6 t) are Dover Strait, 44.4, northern North Sea, 41.7, The Wash, 3.2, and Suffolk Coast, 0.7. Comparable estimates published in the 1993 North Sea Quality Status Report (North Sea Task Force, Olsen and Olsen, Denmark) are generally in reasonable agreement. Likewise, both the location and rates of supply from the sources determined in this study are in broad agreement with the earlier estimates of McCave (1987) (in Journal of the Geological Society 144, 149–152). Thus, the success achieved provides encouragement towards the goal of developing robust suspended sediment models of shelf seas.

Atmospheric and oceanic forcing of Weddell Sea ice motion

The data from sea ice buoys, which were deployed during the Winter Weddell Sea Project 1986, the Winter Weddell Gyre Studies 1989 and 1992, the Ice Station Weddell in 1992, the Antarctic Zone Flux Experiment in 1994, and several ship cruises in Austral summers, are uniformly reanalyzed by the same objective methods. Geostrophic winds are derived after matching of the buoy pressure data with the surface pressure fields of the European Centre for Medium Range Weather Forecasts. The ratio between ice drift and geostrophic wind speeds is reduced when winds and currents oppose each other, when the atmospheric surface layer is stably stratified, and when the ice is under pressure near coasts. Over the continental shelves, the spatial inhomogeneity of tidal and inertial motion effectively controls the variability of divergence for periods below 36 hours. Far from coasts, speed ratios, which presumably reflect internal stress variations in the ice cover, are independent of drift divergence on the spatial scale of 100 km. To study basin‐scale ice dynamics, all ice drift data are related to the geostrophic winds based on the complex linear model [Thorndike and Colony, 1982] for daily averaged data. The composite patterns of mean ice motion, geostrophic winds, and geostrophic surface currents document cyclonic basin‐wide circulations. Geostrophic ocean currents are generally small in the Weddell Sea. Significant features are the coastal current near the southeastern coasts and the bands of larger velocities of ≈6 cm s−1 following the northward and eastward orientation of the continental shelf breaks in the western and northwestern Weddell Sea. In the southwestern Weddell Sea the mean ice drift speed is reduced to less than 0.5% of the geostrophic wind speed and increases rather continuously to 1.5% in the northern, central, and eastern Weddell Sea. The linear model accounts for less than 50% of the total variance of drift speeds in the southwestern Weddell Sea and up to 80% in the northern and eastern Weddell Sea.

An analytical and numerical study of surface and bottom boundary layers with variable forcing and application to the North Sea

A study is made concerning the evolution of a tidally generated bottom layer and a wind-driven surface layer with a time-dependent surface stress in view of its application to thermal stratification problems in shelf seas. Three test cases are defined. In the first two a linear stratification is assumed initially while a stress is applied either at the bottom or at the surface. Analytical solutions can then be derived using simplifying assumptions. They show how the evolution depends on the forcing parameters, i.e. initial density gradient, rotation, forcing period, amplitude of the tidal current or magnitude of the wind stress, ellipticity and polarisation of the tidal or surface (stress) ellipse. It is shown that the analytical theory is in good agreement with results obtained using a numerical model incorporating a turbulence closure scheme. An important result is the occurrence of a resonance and an increased growth of the surface layer when the time scale of the surface stress is comparable with the rotation period. No such resonance has been found for the bottom layer forced by the semi-diurnal tide, In a third test case a simulation is made of the seasonal cycle at station CS in the North Sea, The results are compared with the temperature data from the North Sea Project, A series of numerical experiments is performed to verify parts of the theoretical analysis.

Inter-annual variability in the temperature of the North Sea

A numerical model is formulated to study inter-annual variability in the temperatures of the North Sea. Vertical diffusion is simulated using a simplified semi-analytical approach. Sensitivity tests examine the impact of changes in wind speed and air temperature, including specific anomalies observed during the 15-month survey period of the U.K. North Sea Project. In the North Sea, the sea surface temperature closely follows the air temperature, with a mean 1-2°C above the latter. Their seasonal amplitudes are closely similar in shallow water, but the sea surface amplitude is somewhat reduced in deeper water. Any increase in wind speed forces the sea surface temperature to converge even more closely towards the ambient air temperature. Beneath the surface, increasing depths both delay and attenuate surface variability—a process reinforced by thermal stratification that exists between March and October in deeper waters. Anomalies in observed sea surface temperatures can generally be directly related to concurrent air temperature anomalies with an indirect influence of anomalous wind conditions. However, air temperature anomalies are reduced in amplitude in their impact on depth-averaged sea temperatures because of the attenuating effect of water depth. Model simulations emphasise the essentially localised nature of the air-sea thermal balance in the central North Sea, with only a secondary effect of horizontal advection and dispersion. Any changes in mean temperature along the Atlantic boundary will have little influence on the more enclosed regions of the North Sea. Moreover, the effect of corresponding changes in the annual cycle will be reduced even further. Throughout this study, feedback effects of water temperature onto ambient air temperatures have not been considered. Future studies will need to incorporate this mechanism.

Determination of source concentrations of dissolved and particulate trace metals in the Southern North Sea

During the NERC North Sea Project, concentrations of dissolved and particulate Cd, Cu, Pb, Ni and Zn, together with suspended particulate matter (SPM), were determined at over 100 locations on each of four survey cruises. In an earlier study, McManus & Prandle (1994) described an inverse modelling technique to derive source concentrations. The inclusion of an SPM routine in this model (McManus & Prandle, submitted), and the use of partitioning coefficients ( K D ) means that this technique can now be used to determine source concentrations for particulate metals, as well as to refine the earlier dissolved estimates. These derived dissolved and particulate source concentrations are shown to be in fairly good agreement with values cited in the 1993 Quality Status Report (QSR; North Sea Task Force and International Council for the Exploration of the Sea, 1993). Moreover, the discrepancy in the earlier study between the 1987 QSR and model distributions for Pb (due to the neglect of scavenging by particulate material) is now resolved (International Conference on the Protection of the North Sea, 1987).

Arctic Ocean deep water masses in the western Iceland Sea

The Icelandic‐Danish contribution to the international Greenland Sea Project was devoted to a study of the distribution and transport of water masses in the Western Iceland Sea. Each year in September from 1987 to 1991, temperature and salinity were observed on a station net in the area off east Greenland from the Denmark Strait in the south to 72°30′N. The data reveal that the deep water column of the western Iceland Sea is occupied by a number of slightly different water masses, which all can be traced back to a formation in the Arctic Ocean. It is especially noticeable that in some years almost undiluted Eurasian Basin Deep Water can be observed as far south as the Denmark Strait.

Combining modelling and monitoring to determine fluxes of water, dissolved and particulate metals through the Dover Strait

Contaminant fluxes in a shelf sea system are determined from a series of interrelated studies involving monitoring, modelling and theoretical development. Year-long measurements of currents through the Dover Strait were made in 1990–1991 using both shore-based high frequency (HF) radar and a bottom-mounted acoustic Doppler current profiler (ADCP). These measurements were combined to determine both the residual component of tidal flow and the wind-forced residual flow resulting in an estimate of the net long term flow into the North Sea of 94,000 m 3 s −1—a value in close agreement with the most recent high resolution modelling of Salomon et al. (1993). The temporal variability in these radar and ADCP observations are compared with synoptic wind, tide gauge and numerical model data. The fluxes of the dissolved metals Cd, Cu, Ni, Pb and Zn through the Straits are then calculated using concentrations in the Strait derived from a study by McManus and Prandle (1994). The latter involved multiple regression of model simulations of dispersion (with the model flow through the Dover Strait corresponding to the above monitored value) against data from four surveys in the southern North Sea carried out in 1988–1989 as part of the North Sea Project. The mean concentrations determined from this inverse modelling technique depend directly on the net water flux through the Strait. Thus, since it is shown here that the results for the more conservative metals Cd, Cu, Ni and Zn agree closely with direct measurements by Statham et al. (1993), this lends further confidence to this new estimate of net flow derived from monitoring. The flux of suspended sediments is calculated using time and cross-sectionally averaged suspended sediment concentrations obtained during a cruise in June 1990 (Jones et al., 1993). The particulate metal fluxes are calculated by combining these suspended sediment concentrations with the dissolved metal concentrations and publised Kd (partitioning coefficient) values. These dissolved and particulate metal fluxes are also shown to be in reasonable agreement with the values derived by Statham et al. (1993). The net, particulate plus dissolved, flux of these metals through the Strait represents between one-sixth and one-third of the total from all other sources into the southern North Sea.

Salinity models of the southern North Sea

With the recent acquisition of a detailed set of salinity measurements in the southern North Sea as a result of the U.K. North Sea Project (1988–1989), it is now feasible to test a fine resolution model of the area thus superseding the bulk ☐ models used hitherto. A series of fine resolution, two-dimensional, advection-diffusion salinity models of the U.K. shelf and North Sea have been constructed. By comparison with the North Sea Project data these have inferred the most important influences on the salinity of the North Sea, in particular the importance of advection, of salinities on the open boundaries and of the evaporation/precipitation balance in the central North Sea. One of the models was also used in an attempt to explain the salinity anomaly seen during the North Sea Project. A mechanism for this is suggested.

Database applications supporting Community Research Projects in NERC marine sciences

Abstract Traditionally, oceanographic data management has been strongly cruise oriented. However, the organization of research into Community Research Projects, such as the North Sea Project and Biogeochemical Ocean Flux Study (BOFS), has produced a requirement for handling multidisciplinary data from a large number of research cruises as an integrated dataset. The code of practice for the project dictated that data were to be exchanged freely between project participants throughout the duration of the project, after which the data were to be brought into the public academic domain. The data management strategy therefore had to provide both a vehicle for data exchange within the project and a mechanism for subsequently making the data available to a wider community. These problems were solved by BODC through the adoption of two technologies. Relational database technology was used to integrate the data and, through access over the UK academic wide area network (JANET), provide the data exchange mechanism. Electronic publication of the datasets on CD-ROM provided the mechanism for expanding the dataset user base.

Comparisons between observations and model simulations of dissolved trace metal concentrations in the southern North Sea

As part of the NERC North Sea Project (1987–1992), dissolved concentrations of the metals Cd, Cu, Pb, Ni and Zn were determined at over 100 locations on each of four survey cruises. Model simulations are used to relate these observed distributions to their sources, namely: 1. inflows from the northern North Sea and the English Channel, 2. atmospheric and 3. riverine. A multiple linear regression technique is used to determine the dissolved concentration in each of the inflow sources. These derived values are shown to be in fairly good agreement with values cited in the 1987 Quality Status Report. The present study illustrates the relative contribution of each source in differing regions of the southern North Sea and also provides a quantitative assessment of the agreement between model and observed dissolved metal distributions.

Suppression of bottom water formation in the southeastern Weddell sea

The lack of bottom water formation in the southeastern Weddel Sea is investigated on the basis of CTD, current meter, and oxygen isotope data obtained in 1986 during the Winter Weddell Sea Project and in summer 1989 during the European Polastern Study. The principal underlying factor in suppressing the formation of bottom water is the narrow continental shelf in the region. This leads to two consequences not obtained in the western Weddel Sea: (1) the coastal polynya is able to extend out well over deep water and over th eswift-moving Antarctic Coastal Current, which acts to inhibit the acculmulation of salt released freezing in the polynya; and (2) the upper portions of Warm Deep Water come into close proximity with the glacial ice shelf floating above the continental shelf, thus providing heat for melting at the base of the ice shelf. Budgets for heat and salt derived from the winter data, along with measurements of δ 18O, indicate that this melting occurs at rates more than sufficient to compensate the combined effects of brine released by freezing in the polynya and the upward flux of salt from the Warm Deep Water. As a result, the Eastern Sshelf Water cannot acquire the salt concentrations needed fopr the formation of bottom water.

Evolution of the large‐scale temperature field in the Greenland Sea during 1988–89 from tomographic measurements

The Greenland Sea Ocean Acoustic Tomography Experiment was conducted during 1988–89, as one component of the international Greenland Sea Project, to study deep water formation and the response of the gyre to variations in wind stress and ice cover. Six acoustic transceivers moored in an array 200‐km across transmitted to one another at four hour intervals. Near the end of February, 1989, a sub‐surface temperature maximum at several hundred meters depth disappeared over a surprisingly large area of the central Greenland Sea. While the water column was modified to about 1000 m depth over much of the gyre, the surface remained colder than the deeper water, contrary to what might be expected from simple models of convective renewal.

Drag coefficients for winter Antarctic pack ice

This paper presents air‐ice and ice‐water drag coefficients referenced to 10‐m‐height winds for winter Antarctic pack ice based on measurements made from R/V Polarstern during the Winter Weddell Sea Project, 1986 (WWSP‐86), and from R/V Akademik Fedorov during the Winter Weddell Gyre Study, 1989 (WWGS‐89). The optimal values of the air‐ice drag coefficients, made from turbulent flux measurements, are C10 = (1.79 ± 0.06) × 10−3 for WWSP‐86 and (1.45 ± 0.09) × 10−3 for WWGS‐89. Neutral drag coefficient values are CN10 = 1.68 × 10−3 for WWSP‐86 and 1.44 × 10−3 for WWGS‐89. The slightly lower values for WWGS‐89 reflect a smaller surface roughness (z0) attributed to the thicker snow cover present in the 1989 study region (median mm for WWSP‐86 and 0.27 mm for WWGS‐89). These values are consistent with Arctic measurements for 80–100% concentration of sea ice and with those of Andreas et al. (this issue) for the Antarctic. A single (average) ice‐water drag coefficient for both WWSP‐86 and WWGS‐89, estimated from periods of ice drift thought to represent free‐drift conditions (air‐ice stress balanced by ice‐water drag and Coriolis force), is (1.13 ± 0.26) × 10−3, and the ice‐water turning angle . This drag value is significantly lower than Arctic values for thick multiyear ice, but it is similar to the values obtained by Langleben (1982) for first‐year Arctic ice. Consistent with previous findings for WWSP‐86, the free‐drift form of the momentum balance can be used to describe the observed WWGS‐89 ice drift observations by using an “effective” drag coefficient and turning angle that subsume the influence of ice‐ice interaction. For a typical Antarctic winter pack ice cover, it appears that the ice cover reduces the momentum flux from the atmosphere to the ocean by ∼33%.

The North Sea Project: an overview and the way forward

I am never content until I have constructed a (mechanical) model of the subject I am studying. If I succeed in making a model, I understand; otherwise I do not.

Dynamics of tidal mixing fronts in the North Sea

Twenty years since the discovery of tidal mixing fronts there are still few convincing observations of the velocity field associated with these structures. Simple models of shelf sea fronts predict strong along-front jets, weaker convergent circulations and instabilities. During the North Sea Project a series of studies of the Flamborough frontal system has used a new approach based upon novel combinations of modern instrumentation (HF radar, acoustic Doppler current profiler, Decca-Argos drifting buoys and towed undulating CTD) and have provided one of the first directly observed pictures of shelf sea frontal circulation. Observational confirmation of jetlike along-front flow has been found together with evidence of cross-frontal convergence. A new generation of eddy-resolving models will help to focus the next phase of frontal circulation studies in relation to questions concerning baroclinic instability and eddy generation.

Towards water quality models

Predictions of water quality involve the modelling both of physical processes, which underlie the transport and diffusion of all constituents, and of the sources, sinks, partitioning and interactive processes individual to those constituents. These processes are outlined, together with complementary modelling approaches: (i) development of sophisticated three-dimensional models to represent the physics, and sub-models of suspended sediment, microbiology and metal interactions for processes controlling nutrients, dissolved oxygen, phytoplankton, detritus and metals; (ii) a framework to link these component models; (iii) an accessible model with simpler physics for wide use in simulating constituent distributions, for comparison with measurements to infer sources, sinks and interactions. The North Sea Project measurements provide an input in process evaluation, and data to test the models.

The influence of horizontal circulation on the supply and distribution of tracers

A theoretical analysis of the mixing associated with (horizontal) advection and dispersion is used to show that the larger scale, longer-term distribution of conservative tracers in the southern North Sea is primarily determined by the residual circulation pattern. Model estimates of this circulation have been substantiated from new year-long high-frequency radar monitoring of flow through the Dover Strait. Model simulations of the saline balance and for the dissolved metals Cd, Cu, Pb, Zn and Ni are made for the North Sea Project (NSP) survey period, using river and atmospheric inputs specified from the 1987 Quality Status Report. Computed concentrations are in reasonable agreement with measured values (except for lead where losses by rapid adsorption were not included in the model). The flushing time, F T , of the southern North Sea (to 56° N) is approximately 240 days. For any conservative tracer and this value of F T , the observed seasonal amplitude in concentration corresponding to a seasonal inflow I cos wt is only 0.23 of the steady concentration corresponding to a continuous inflow I . By contrast with the above responses for ‘conservative’ tracers entering from rivers or adjacent seas, concentrations of certain ‘non-conservative’ tracers can be almost entirely determined by localized exchange processes. Thus the spatial pattern of seasonal variability in temperature and dissolved oxygen is shown to be closely correlated with the inverse of water depth. For such tracers, the influence of advection is small and generally confined to coastal or stratified waters. For shorter term processes, this influence of advection is further reduced. The transport of suspended sediments is highly dependent on particle size. While the transport of fast-settling sediments is complicated by periodic episodes of settlement and re-suspension, transport of slow-settling sediments (the main agent for adsorbed contaminants) approximates that of a dissolved tracer. Thus, to first order, simple models incorporating residual advection are adequate to relate inputs to concentrations (averaged over appropriate time and length scales) for reasonably conservative dissolved tracers. Likewise point-models are often adequate to study processes fundamentally concerned with vertical exchange rates particularly for shorter-term processes in well-mixed waters away from the coastal zone.

The distribution of aluminium in the Greenland Sea and its relationship to ventilation processes

During two cruises to the Greenland Sea in 1988 and 1989 a total of 690 on‐board aluminium (Al) determinations were performed on samples collected from profiles located along four transects in the Greenland Sea, as part of the Greenland Sea Project. Al concentrations range from extremely low values (0.4 nM) in the Polar waters of the East Greenland Current draining the Arctic Ocean, to 17.9 nM in the newly formed Norwegian Sea Deep Water (NSDW) at the periphery of the Greenland gyre. The Al content in the component water masses serves to constrain the make‐up of these water masses from the various mixing products of the region. The enrichment of Al in the NSDW both at its site of formation within the Greenland Sea (up to 17.9 nM) and in the Lofoten Basin of the Norwegian Sea (up to 40.3 nM) leads to the conclusion that there must be an additional, external, source of Al to these water masses. The apparent inability of water column or sedimentary diagenetic processes to provide this enrichment and the observation of enriched Al values associated with a high‐salinity shelf brine leads us to the conclusion that these brines may be responsible for the observed enrichments. The high concentration of Al in the Greenland Sea Deep Water compared with the water masses that produce it, suggests that Al may be acting as an inverse tracer of the processes that ventilate this basin by convective overturn, i.e. the Al concentration which increases with time, as a result of continuous brine or other external inputs, is reduced during convective events. It is also argued that this flushing of elevated Al water during convective events may have a similar counterpart in the Iceland Sea and that this mechanism could go some way toward solving the Al deficit that exists between the enriched North Atlantic Deep Water and its depleted precursor water masses observed in the intermediate waters of the Greenland and Iceland seas.

Topographic effects of the Maud Rise on the stratification and circulation of the Weddell Gyre

Hydrographic data indicate that due to the Maud Rise (65°S, 3°E) the upwelling of relatively warm and more saline Lower Circumpolar Deep Water (LCDW) in the Weddell Gyre and the entrainment of LCDW into the upper layer are enhanced, which increase the ocean heat loss and enrich the salinity of the upper layer. By these processes the water column is preconditioned for deep convection. An anomalous water column with a diameter of about 150–200 km, notable for its relatively cold deep water and its relatively saline upper layer, is situated above the crest of the Maud Rise. Water from its main pycnocline spreads laterally within the upper 500 m and compensates for the entrainment of LCDW into the upper layer.Current meter data from the Winter Weddell Sea Project 1986 show a marked temporal and spatial variability of the southward transport of LCDW in the lee of the Maud Rise. An anticyclone of LCDW, which had cut off from the main body of LCDW east of the Maud Rise, migrated southward along the western slope with a velocity of about 3 cm s−1. It was followed by a cyclone, which had probably formed at the front of the anomaly due to a strengthened flow, undergoing an enhanced meandering. The wavelength of the entire disturbance is estimated to be about 380 km, one-third of the circumference of the Maud Rise anomaly. If the disturbance repeats continuously, about two to three anticyclones detach per year.