Nutrient transport from an artificial upwelling of deep sea water

The transport of nutrient-rich, deep sea water from an artificial upwelling pipe has been simulated. A numerical model has been built within a commercial Computational Fluid Dynamics (CFD) package. The model considers the flow of the deep sea water after it is ejected from the pipe outlet in a negatively buoyant plume (densimetric Froude number = −2.6), within a stably stratified ocean environment subject to strong ocean current cross flow. Two cross-flow profiles were tested with momentum flux ratios equal to 0.92 and 3.7. The standard k-ε turbulence model has been employed and a range of turbulent Schmidt and Prandtl numbers tested. In all cases the results show that the rapid diffusion of heat and salinity at the pipe outlet causes the plume to attain neutral buoyancy very rapidly, preventing strong fountain-like behavior. At the higher momentum flux ratio fountain-like behavior is more pronounced close to the pipe outlet. The strong cross-current makes horizontal advection the dominant transport process downstream. The nutrient plume trajectory remains largely within one relatively thin stratified layer, making any ocean cross-flow profile less important. Very little unsteady behavior was observed. The results show that the nutrient is reduced to less than 2% of its inlet concentration 10 meters downstream of the inlet and this result is largely independent of turbulent Prandtl or Schmidt number. Initial results would suggest that if such an artificial upwelling were to be viable for an ocean farming project, a large number of upwelling pipes would be necessary. Further work will have to determine the minimum nutrient concentration required to sustain a viable phytoplankton population and the required spacing between upwelling pipes.     

The vertical distribution and diurnal migration of copepods at different stations in the Irish Sea

The vertical distribution of plankton and hydrographic factors was studied at five stations within the Irish Sea, differing in depth, strength of tidal currents and location with respect to non-tidal currents. Neither the plankton nor the hydrographic factors showed vertical homogeneity at any station, even when there were strong tidal currents. The day depth of bothPseudocalanus elongatus andMicrocalanus pusillus varied considerably at different stations and at different seasons at the same station.P. elongatus showed diurnal migrations on some but not all occasions. No clear diurnal migration was shown byM. pusillus. Some of the vertical distributions of both species, both by day and over 24 h, could be correlated with physical or chemical factors. These included stratification involving differences in temperature or salinity, but in some cases only differences in nutrient concentration were detected. Although layers of seawater of similar density are unlikely to maintain their identity for very long periods they are probably more common than is generally realized and sufficiently persistent to have important effects on the distribution of plankton. The determination of nutrient concentrations provides a convenient and effective means of detecting such layers.     

Solar and anthropogenic imprints on Lake Masoko (southern Tanzania) during the last 500 years

The Masoko crater-lake in southern Tanzania provides a continuous record of environmental changes covering the last 500 years. Multi-proxy studies were performed on a 52 cm sediment core retrieved from the deepest part of the lake. Magnetic, organic carbon, geochemical proxies and pollen assemblages indicate a dry climate during the ‘Little Ice Age’ (AD 1550–1850), confirming that the LIA in eastern Africa resulted in marked and synchronous hydrological changes. However, the direction of response varies between different African lakes (low versus high lake-levels), indicating strong regional contrasts that prevent the clear identification of climate trends over eastern Africa at this time. Inferred changes in Masoko lake-levels closely resemble the record of solar activity cycles, indicating a possible control of solar activity on the climate in this area. This observation supports previous results from East African lakes, and extends this relationship southward. Finally, anthropogenic impact is observed in the Masoko sediments during the last 60 years, suggesting that human disturbance significantly affected this remote basin during colonial and post-colonial times.     

Millennial timescale carbon cycle and climate change in an efficient Earth system model

A new Earth system model, GENIE-1, is presented which comprises a 3-D frictional geostrophic ocean, phosphate-restoring marine biogeochemistry, dynamic and thermodynamic sea-ice, land surface physics and carbon cycling, and a seasonal 2-D energy-moisture balance atmosphere. Three sets of model climate parameters are used to explore the robustness of the results and for traceability to earlier work. The model versions have climate sensitivity of 2.8–3.3°C and predict atmospheric CO₂ close to present observations. Six idealized total fossil fuel CO₂ emissions scenarios are used to explore a range of 1,100–15,000 GtC total emissions and the effect of rate of emissions. Atmospheric CO₂ approaches equilibrium in year 3000 at 420–5,660 ppmv, giving 1.5–12.5°C global warming. The ocean is a robust carbon sink of up to 6.5 GtC year⁻¹. Under ‘business as usual’, the land becomes a carbon source around year 2100 which peaks at up to 2.5 GtC year⁻¹. Soil carbon is lost globally, boreal vegetation generally increases, whilst under extreme forcing, dieback of some tropical and sub-tropical vegetation occurs. Average ocean surface pH drops by up to 1.15 units. A Greenland ice sheet melt threshold of 2.6°C local warming is only briefly exceeded if total emissions are limited to 1,100 GtC, whilst 15,000 GtC emissions cause complete Greenland melt by year 3000, contributing 7 m to sea level rise. Total sea-level rise, including thermal expansion, is 0.4–10 m in year 3000 and ongoing. The Atlantic meridional overturning circulation shuts down in two out of three model versions, but only under extreme emissions including exotic fossil fuel resources.     

Northern Hemisphere atmospheric blocking as simulated by 15 atmospheric general circulation models in the period 1979–1988

 As a part of the Atmospheric Model Intercomparison Project (AMIP), the behaviour of 15 general circulation models has been analysed in order to diagnose and compare the ability of the different models in simulating Northern Hemisphere midlatitude atmospheric blocking. In accordance with the established AMIP procedure, the 10-year model integrations were performed using prescribed, time-evolving monthly mean observed SSTs spanning the period January 1979–December 1988. Atmospheric observational data (ECMWF analyses) over the same period have been also used to verify the models results. The models involved in this comparison represent a wide spectrum of model complexity, with different horizontal and vertical resolution, numerical techniques and physical parametrizations, and exhibit large differences in blocking behaviour. Nevertheless, a few common features can be found, such as the general tendency to underestimate both blocking frequency and the average duration of blocks. The problem of the possible relationship between model blocking and model systematic errors has also been assessed, although without resorting to ad-hoc numerical experimentation it is impossible to relate with certainty particular model deficiencies in representing blocking to precise parts of the model formulation. Received: 16 July 1997/Accepted: 20 October 1997     

Potential field modelling of the Baltica–Avalonia (Thor–Tornquist) suture beneath the southern North Sea

Magnetic anomaly maps of the Trans-European Suture Zone (TESZ) highlight the contrast between the highly magnetic crust of Baltica and the less magnetic terranes to the SW of the suture. Although the TESZ is imaged on gravity maps, anomalies related to postcollisional rifting and reactivated rift structures tend to dominate.

Seismic and potential field data have been used to construct 2 -D crustal models along three profiles crossing the Baltica–Avalonia suture in the southern North Sea (SNS). The first of these models lies along a transect assembled from reflection line GECO SNST 83-07 and refraction profile EUGENO-S 2; the other two models are coincident with MONA LISA profiles 1 and 2. Additional structural information and density information for the cover sequence is available from released wells, while magnetic susceptibility values are compatible with values measured from borehole core samples.

Magnetic anomalies related to the suture are interpreted as due to magnetic Baltican basement of the Ringkøbing-Fyn High dipping SW beneath nonmagnetic Avalonian basement underlying the western part of the SNS. Low-amplitude, long-wavelength magnetic anomalies occurring outboard of the suture are interpreted as due to a mid-crustal magnetic body, possibly a buried magmatic complex. This might represent the ‘missing’ arc related to inferred southward subduction of the Tornquist Sea, or an exotic element emplaced during the collision between Avalonia and Baltica. The present model supports an imbricated structure within Baltica as indicated by the latest reprocessing of the MONA LISA seismic data.