Thomas Pohlmann

Predicting the thermocline in a circulation model of the North sea — part I: model description, calibration and verification

A model is presented that allows the prognostic simulation of the temperature in the North Sea. An approach proposed by Kochergin (1987 Three-dimensional coastal ocean models, American Geophysical Union, pp. 201–208). to determine the vertical eddy diffusion and eddy viscosity was applied for the surface and bottom mixed layer. At the sea surface observed temperatures are prescribed (Dirichlets boundary condition). They contain the effect of all components determining the heat flux in the surface layer. This includes, besides all advective and diffusive heat fluxes in the surface layer, the net-heat flux through the sea surface. By this means it is possible to circumvent the uncertainties in the determination of the heat flux through the atmosphere-sea interface. A calibration and verification of the presented model is carried out by comparing the model results firstly with a time series of actual temperature profiles and secondly with a climatological monthly mean of the bottom water temperature.

Currents and Transport in Water

The distribution of substances in the sea depends to a great extent on the characteristics of advective and diffusive transport. This chapter deals with the numerical simulation of the water movement in the North Sea. Both mean and extreme conditions are presented and discussed. The water movement is the basis for simulating the transport of (1) conservative and passive substance and (2) suspended particulate matter. Concerning the latter, settling, deposition and erosion are also considered.

Simulations of the north sea circulation, its variability, and its implementation as hydrodynamical forcing in ERSEM

Abstract The rationale is given of how the gross physical features of the circulation and the stratification of the North Sea have been aggregated for inclusion in the ecosystem box model ERSEM. As the ecosystem dynamics are to a large extent determined by small-scale physical events, the ecosystem model is forced with the circulation of a specific year rather than using the long-term mean circulation field. Especially the vertical exchange processes have been explicitly included, because the primary production strongly depends on them. Simulations with a general circulation model (GCM), forced by three-hourly meteorological fields, have been utilized to derive daily horizontal transport values driving ERSEM on boxes of sizes of a few 100 km. The daily vertical transports across a fixed 30-m interface provide the necessary short-term event character of the vertical exchange. For the years 1988 and 1989 the properties of the hydrodynamic flow fields are presented in terms of trajectories of the flow, thermocline depths, of water budgets, flushing times and diffusion rates. The results of the standard simulation with ERSEM show that the daily variability of the circulation, being smoothed by the box integration procedure, is transferred to the chemical and biological state variables to a very limited degree only.

Calculating the annual cycle of the vertical eddy viscosity in the North Sea with a three-dimensional baroclinic shelf sea circulation model

Abstract The vertical eddy viscosity ( A v ) is estimated using a three-dimensional baroclinic shelf sea model that treats the temperature as a prognostic quantity. A v is calculated by means of a turbulent closure approach proposed by Kochergin [(1987) Three-dimensional coastal ocean models , American Geophysical Union, pp. 201–208] which is closely related to a Mellor and Yamada [(1974) Journal of Atmospheric Science , 31 , pp. 1791–1806] level-2-model that has been used very successfully in a large number of applications. The annual cycle of the vertical eddy viscosity is discussed by looking at horizontal and vertical A v -distributions for the year 1988. These examples show that the vertical eddy viscosity is subject to a pronounced annual cycle which can be related to heating and cooling processes as well as to mixing induced by wind and bottom friction. A comparison of these results with A v -distributions calculated for the year 1987 additionally demonstrates a strong inter-annual variability.

Transport of conservative passive tracers in the North Sea: first results of a circulation and transport model

Abstract A simple Lagrangian transport model was applied to obtain estimates of transport routes of conservative passive tracers in the North Sea. A vertically integrated time-dependent flow field derived from the daily output of a three-dimensional baroclinic circulation model was used in the transport model. The simulation period covers the years from 1969 to 1982. The calculations were carried out in order to get estimates of both, the low-frequency flow in the North Sea and the resulting dispersion of matter within the sea. Point sources are assumed which release tracers continuously and with a constant rate. Hence the variability of the flow field can be visualized by the temporal and spatial fate of the released tracer ensemble. The model results are displayed in three different ways to provide a comprehensive description of the complex spacetime character of the system.

A Three Dimensional Circulation Model of the South China Sea

Abstract Up to now there is a great lack of observational data in the South China Sea. The best available information about the general hydrography of the region was the Naga Report compiled by Wyrtki already in 1961. The South China Sea is an equatorial region with a complex topography. It is a regime dominated by the monsoon and stratification is of enormous importance. A prognostic baroclinic circulation model was applied in order to increase our present knowledge and our understanding of this region. The gridsize of this 12- layer model is about 50 km in the horizontal. The layers have a thickness of 10 m to 3000 m, increasing with depth. Simulations were carried out for the winter- and for the summer monsoon, respectively. The calculation of temperature and salinity distributions which are consistent with the circulation provide insight into new features like deep-reaching up- and downwelling phenomena. A first validation of the model results was carried out in comparison with the observational data compiled by Klaus Wyrtki.

Nitrogen fluxes in the German Bight

Abstract From November 1989 to March 1992, the German Federal Maritime and Hydrographic Agency (BSH) carried out surveys of major physical and chemical parameters in the German Bight in connection with the TUVAS project (‘Transport, transfer and variability of contaminants and nutrients in the German Bight’). During roughly the same period, the interdisciplinary research project ‘Processes influencing contaminant fluxes in the North Sea (PRISMA)’ performed continuous measurements of atmospheric nutrient and contaminant deposition and numerical experiments with atmospheric, hydrodynamic and ecosystem models. Results from both projects, together with available information on river loads and data from the literature on sediment fluxes made it possible to construct nitrogen budgets for the German Bight during the period of the TUVAS surveys. Comparing the mean annual input fluxes, the major nitrogen flux into the water of the German Bight was from rivers (around 70%). The atmospheric contribution was around 30%.

A modelling study of SPM transport in the Bohai Sea

Abstract A 3-D SPM transport model is applied to the Bohai Sea in China. The model includes: (1) the movement of the SPM in the water body, (2) the movement of the sediment fine fraction in the sea bottom and (3) the deposition and erosion processes at the seabed. The model is of Lagrangian type, i.e., the tracer method is used. The influence of the SPM on the hydrodynamic condition is studied here because the Yellow River releases huge amount of SPM into the Bohai Sea. Thus, the usual model configuration is modified by taking into account the density change due to the SPM. The results show that the effect of the SPM can not be neglected near the river mouth area where the SPM concentration gradient is very high. With the SPM considered the SPM is disposed over a wider area. Based on the modified model a one-year simulation is performed for the year 1982. The results are compared with the CZCS satellite data at the same time. The results of the monthly averaged SPM concentrations in the Bohai Sea are compared between the seasons. They agree reasonably with observations. The exchange of SPM and water through the Bohai Strait is also studied. The results show that most of SPM from the Yellow River stays in the Bohai Sea.

Calculating the development of the thermal vertical stratification in the North Sea with a three-dimensional baroclinic circulation model

Abstract The vertical thermal stratification in the North Sea has been investigated with the help of a three-dimensional baroclinic circulation model. The main intention of the current investigation is to reproduce the actual conditions in the North Sea as realistically as possible. In order to obtain an appropriate representation of all the thermal parameters in space and time it is essential to use a model that treats temperature as a prognostic quantity. In particular, processes connected with the existence of the summer thermocline require attention. This is guaranteed by choosing adequate time- and space-dependent vertical eddy viscosity A v and diffusivity coefficients, respectively. Firstly, the month-to-month changes of the summer vertical stratification in the North Sea are discussed by presenting climatological monthly mean conditions (average over the period 1983–1992) of the following parameters: thermocline extension, depth, occurrence probability. These parameters are related to shear and buoyancy production of turbulent kinetic energy. It turned out that, in mid-summer, the location of the frontal zone is mainly controlled by tidal stirring and not by meteorological forcing. Secondly, in order to illustrate the pronounced inter-annual variability of these thermal parameters, actual daily mean situations are analyzed for the coldest and warmest summer, during this ten-year period. From these results, it is possible to conclude that compared with a warm summer, in a cold one a significantly higher shear production is responsible for the destruction or deepening of the thermocline, which in turn results in higher negative values of the buoyancy production term.

Annual balances of hexachlorocyclohexanes, polychlorinated biphenyls and triazines in the German Bight☆

Abstract During the PRISMA-Project concentrations of hexachlorocyclohexanes (α-HCH, γ-HCH), polychlorinated biphenyls (the ‘DIN-PCBs’: PCB 28, PCB 52, PCB 101, PCB 153, PCB 138, PCB 180), and triazines (atrazine, simazine, terbutylazine) were determined in the German Bight within the water column and in rain water. In addition, the HCHs, and PCBs were also analysed in sediment samples. The concentration values were combined with average lateral inflow and outflow rates of water masses into and out of the German Bight and with the respective inflow rates of the rivers Elbe, Weser and Ems. Thus an estimation of the annual balances of these compounds was accessible that allowed the following conclusions: a loss of 2 t yr−1 γ-HCH between input into and output out of the German Bight has to be stated, while more α-HCH is leaving this area at its northern boundary. This phenomenon can be explained by the different stability of the two HCH isomers. The mobility of PCBs has to be largely attributed to sediment transport, where during the summer period a larger amount is being transported into than out of the German Bight, while during spring the opposite observation was made. Furthermore, evidence is presented that stresses the importance of the riverine input of triazines to the German Bight. The role of their atmospheric input is not very pronounced within the coastal area, however, it has to be taken into consideration for the more distant North Sea regions. This holds, in particular, for the triazines, where atmospheric transport may become significant already in North Sea areas outside the plume of the river Elbe.