John Eric Jones

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.

Sensitivity of Tidal Bed Stress Distributions, Near-Bed Currents, Overtides, and Tidal Residuals to Frictional Effects in the Eastern Irish Sea

Abstract A three-dimensional nonlinear high-resolution hydrodynamic model of the eastern Irish Sea is used to examine the influence of near-bed viscosity and bottom friction Upon the M2 tide and overrides in the region. Initially, bed stress in the model is related to the depth-mean current, the formulation used in a “classic” two-dimensional model. The resulting hybrid “two-dimensional/three-dimensional model” enables changes in current profile due to variations in eddy viscosity to he examined independently of those due to changes in bed stress. Also, results from the model are compared with those from a three-dimensional model with bed stress related to bottom current. Computed tidal elevations and surface currents are found to be fairly insensitive to the parameterization of bottom stress. However, tidal current profiles in the near-bed region and maximum bed stress are found to be sensitive to variations in near-bed viscosity and friction coefficient. Although reducing eddy viscosity in the near-bed ...

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.

Towards Water Quality Models [and Discussion]

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.

Influence of sea surface wind wave turbulence upon wind-induced circulation, tide–surge interaction and bed stress

A three-dimensional finite volume unstructured mesh model of the west coast of Britain, with high resolution in the coastal regions, is used to investigate the role of wind wave turbulence and wind and tide forced currents in producing maximum bed stress in the eastern Irish Sea. The spatial distribution of the maximum bed stress, which is important in sediment transport problems, is determined, together with how it is modified by the direction of wind forced currents, tide–surge interaction and a surface source of wind wave turbulence associated with wave breaking. Initial calculations show that to first order the distribution of maximum bed stress is determined by the tide. However, since maximum sediment transport occurs at times of episodic events, such as storm surges, their effects upon maximum bed stresses are examined for the case of strong northerly, southerly and westerly wind forcing. Calculations show that due to tide–surge interaction both the tidal distribution and the surge are modified by non-linear effects. Consequently, the magnitude and spatial distribution of maximum bed stress during major wind events depends upon wind direction. In addition calculations show that a surface source of turbulence due to wind wave breaking in shallow water can influence the maximum bed stress. In turn, this influences the wind forced flow and hence the movement of suspended sediment. Calculations of the spatial variability of maximum bed stress indicate the level of measurements required for model validation.