Long-term transport processes in coastal seas with time scales from weeks to seasons time scale are controlled by residual circulation. In the Bohai Sea, an ultrashallow shelf sea of China, tidal ...residual is almost the dominant factor to the circulation due to slight stratification and weak wind in summer. In order to establish an adequate hydrodynamic base to the ecosystem dynamics of the Bohai Sea, the differences of tide-induced Lagrangian and Eulerian mean circulation are discussed and calculated in this contribution. The Stokes drift is analyzed theoretically. According to Longuet-Higgins Deep-Sea Res. 16 (1969) 431, the Lagrangian flow is the sum of the Eulerian flow and the Stokes drift that is induced by the mean kinetic energy and coastal nonlinear interaction. Stokes drift is large in the coastal sea and in areas where the vorticity and/or divergence are large. Vertical mass transports by Stokes drift are also the result of nonlinear interaction of current, water level and topography. Hamburg Shelf Ocean Model (HAMSOM) is applied in the Bohai Sea to simulate the tides and tidal currents. The tide-induced Lagrangian mean circulation and the Eulerian one are calculated at the same time. In the area where the Stokes drift is in the same direction as the Eulerian residual, the Lagrangian one is stronger than the Eulerian one. Where they are pointing in opposite directions, the Lagrangian one is small, like in the southwest of the Bohai Bay, Liaodong Bay and Bohai Strait. The Lagrangian residual current flows into the Bohai Bay along its southern bank causing deposition of Huanghe River sediments. This is in agreement with observations.
A particle tracking model was used to investigate the annual spring invasion of the North Sea by Calanus finmarchicus copepodites which overwinter in deep water off the Scottish continental shelf. ...Flow fields generated by a hydrodynamic model (HDM) were used to simulate the advection of zero drag particles representing the copepods. Particles were released simultaneously from a regular lattice of start positions at a given depth (D1), and ascended at a fixed rate (dD/dt) until they reached a final depth (D2) in the surface layers. The proportion of particles reaching target areas in the northern North Sea was relatively insensitive to a moderate degree of variation (±20%) around chosen default values of the vertical migration parameters (D1, D2 and dD/dt), derived from field data. The inclusion of horizontal diffusion velocities and diel vertical migration in surface layers did not significantly affect the results. Sensitivity to wind direction was investigated by applying flow fields from HDM runs with different wind forcing scenarios. For the default vertical migration parameters, only north‐westerly winds resulted in particles entering the North Sea from release locations north of the Iceland–Scotland Ridge, where dense aggregations of overwintering copepods were encountered during field surveys. The particle tracking model predicted that the major routes for the spring Calanus invasion into the North Sea were the East of Shetland Atlantic Inflow and the Norwegian Trench Atlantic Inflow, which agreed with seasonal trends observed in Continuous Plankton Recorder data. Overall, despite its relative simplicity, particle tracking was confirmed as a robust tool to explore the causal mechanisms behind the annual invasion of the North Sea by C. finmarchicus emerging from diapause in the deep waters off the Scottish continental shelf.
The circulation and the hydrography of the Bohai Sea are simulated with the Hamburg Shelf Ocean Model (HAMSOM). The model is three-dimensional, prognostic baroclinic and has a resolution of 5 min in ...latitude and longitude and 10 layers in the vertical. It is initialised and forced with the five main tidal constituents, temperature and salinity distributions taken from the Levitus database, monthly mean river run-off values and European Centre for Medium Range Weather Forecast (ECMWF) re-analysed data of air pressure, wind stress and of those parameters relevant for the calculation of heat fluxes. The simulation period covers 14 years from 1980 to 1993 due to the availability of the time-dependent ECMWF forcing.
The results are analysed by means of time series and EOFs focussing on the interpretation of fluctuations with periods above the tidal cycle. Furthermore, tracer simulations are carried out and turnover times are calculated in order to evaluate the importance of these fluctuations on the renewal and transport of water masses in the Bohai Sea.
One of the major outcomes of the investigation is the overall dominance of the annual cycle in all hydrographic parameters and the importance of stochastic weather fluctuations on the transport of water masses in the Bohai Sea.
Advection of Calanus finmarchicus in the eastern North Atlantic was analysed using a particle‐tracking model based on the Hamburg Shelf Ocean Model (HAMSOM). Quasi‐static seasonal mean flowfields ...were simulated, archived and interpolated to represent a climatological‐mean annual cycle. Particles had a simple prescribed depth profile comprising deep overwintering, spring ascent, a shallow‐water phase followed by descent to overwintering depth. Export routes for C. finmarchicus from the model area were identified to the south of Greenland and to the north of the Lofoten Basin. Self‐sustaining overwintering areas were identified by observing how closely particles returned to their origins after one calendar year. Several such areas were found, notably in the Norway and Lofoten Basins, and in the Færoe–Shetland Channel. The particle tracking was run for up to 10 years to demonstrate persistence of these cycles. Known features of the winter and summer distributions of C. finmarchicus were reproduced by the model. The success of the HAMSOM in simulating both the shallow and deep circulation of the eastern North Atlantic and Norwegian Sea was critical to the identification of these spatio‐temporal cycles of C. finmarchicus.
Biogeochemical constituents, especially the different phases of nitrogen, were investigated in the central and southern Bohai Sea, China, during fall 1998 and spring 1999 by repeatedly sampling a ...regular station grid.
Apart from at the area off the Huanghe River, dissolved organic nitrogen dominated the nitrogen fraction, indicating the seasonally maturated stage of the ecosystem.
Inputs of dissolved inorganic nitrogen from the Yellow Sea (by the “Yellow Sea Warm Current”) as well as by the Huanghe River fuelled primary production and led to high primary production at the entrance of the Bohai Sea during both seasons and in the shallow areas, especially at the Huanghe River mouth, during spring. Beside the nitrogen import, resuspension and remineralisation processes of nitrogen contributed to the ongoing phytoplankton blooms and, thus, to the conversion of nitrogen.
In spite of the high variability indicated by strong changes within a fortnight, seasonal differences could be detected, such as specific contribution of phytoplankton to suspended matter. Correlations of particulate nitrogen with total suspended matter as well as changes in concentrations of the different nitrogen species as well as in primary production rates helped in distinguishing between conversion and transport/diffusion processes.
Overall, the Bohai Sea was revealed as a huge “bioreactor” for the conversion of nitrogen, and—due to permanent nitrogen input and extensive water–sediment interaction processes—as a possible important sink on a global scale for nitrogen via denitrification processes.
As part of the interdisciplinary investigation of Calanus finmarchicus migrations between oceanic and shelf areas off north‐west Europe (the ICOS project), a three‐dimensional, prognostic baroclinic ...circulation model system, consisting of a coarse and a nested fine scale model, was constructed. The aim was to simulate the flow fields around Iceland, the Faroe Islands and the north‐west European continental shelf and slope, including the northern North Sea. The coarse scale model covered the northern North Atlantic and provided the far field flow to the finer scale model. The latter was initialized and forced with climatological seasonal (winter and spring) means of temperature, salinity, river run‐off, and other parameters necessary for the calculation of heat fluxes. Seasonal average simulations of flow and hydrographic conditions were performed, together with simulations of specific scenarios of uniform wind forcing from different directions. A feature common to all the results was a topographically guided northwards flow along the continental shelf edge, with a southwards counter‐flow in the vicinity of the Faroe–Shetland Channel. The extent and strength of the shelf edge current, of its counter flow, and of the re‐circulation cell in the northern North Sea, were dependent on the prevailing wind direction. The modelling results were consistent with the accepted circulation patterns in the area, and a comparison with field measurements in the Faroe–Shetland Channel showed that the main characteristics of the flow were well replicated, although the estimated strength of the slope current was somewhat less than in the field observations.
In high-latitude oceans with seasonal ice cover, the ice and the low-salinity mixed layer form an interacting barrier for the heat flux from the ocean to the atmosphere. The presence of a less dense ...surface layer allows ice to form, and the ice cover reduces the heat loss to the atmosphere. The ice formation weakens the stability at the base of the mixed layer, leading to stronger entrainment and larger heat flux from below. This heat transport retards, and perhaps stops, the growth of the ice cover. As much heat is then entrained from below as is lost to the atmosphere. This heat loss further reduces the stability, and unless a net ice melt occurs, the mixed layer convects. Two possibilities exist: (1) A net ice melt, sufficient to retain the stability, will always occur and convection will not take place until all ice is removed. The deep convection will then be thermal, deepening the mixed layer. (2) The ice remains until the stability at the base of the mixed layer disappears. The mixed layer then convects, through haline convection, into the deep ocean. Warm water rises towards the surface and the ice starts to melt, and a new mixed layer is reformed. The present work discusses the interactions between ice cover and entrainment during winter, when heat loss to the atmosphere is present. One crucial hypothesis is introduced: “When ice is present and the ocean loses sensible heat to the atmosphere and to ice melt, the buoyancy input at the sea surface due to ice melt is at a minimum”. Using a one-dimensional energy-balance model, applied to the artificial situation, where ice melts directly on warmer water, it is found that this corresponds to a constant fraction of the heat loss going to ice melt. It is postulated that this partitioning holds for the ice cover and the mixed layer in the high-latitude ocean. When a constant fraction of heat goes to ice melt, at least one deep convection event occurs, before the ice cover can be removed by heat entrained from below. After one or several convection events the ice normally disappears and a deep-reaching thermal convection is established. Conditions appropriate for the Weddell Sea and the Greenland Sea are examined and compared with field observations. With realistic initial conditions no convection occurs in the warm regime of the Weddell Sea. A balance between entrained heat and atmospheric heat loss is established and the ice cover remains throughout the winter. At Maud Rise convection may occur, but late in winter and normally no polynya can form before the summer ice melt. In the central Greenland Sea the mixed layer generally convects early in winter and the ice is removed by melting from below as early as February or March. This is in agreement with existing observations.
As part of the interdisciplinary investigation of Calanus finmarchicus migrations between oceanic and shelf areas off north-west Europe (the ICOS project), a three-dimensional, prognostic baroclinic ...circulation model system, consisting of a coarse and a nested fine scale model, was constructed. The aim was to simulate the flow fields around Iceland, the Faroe Islands and the north-west European continental shelf and slope, including the northern North Sea. The coarse scale model covered the northern North Atlantic and provided the far field flow to the finer scale model. The latter was initialized and forced with climatological seasonal (winter and spring) means of temperature, salinity, river run-off, and other parameters necessary for the calculation of heat fluxes. Seasonal average simulations of flow and hydrographic conditions were performed, together with simulations of specific scenarios of uniform wind forcing from different directions. A feature common to all the results was a topographically guided northwards flow along the continental shelf edge, with a southwards counter-flow in the vicinity of the Faroe-Shetland Channel. The extent and strength of the shelf edge current, of its counter flow, and of the re-circulation cell in the northern North Sea, were dependent on the prevailing wind direction. The modelling results were consistent with the accepted circulation patterns in the area, and a comparison with field measurements in the Faroe-Shetland Channel showed that the main characteristics of the flow were well replicated, although the estimated strength of the slope current was somewhat less than in the field observations.
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