D.G. Bowers

Models of stratification and frontal movement in shelf seas

Abstract The availability of turbulent kinetic energy from tidal flow in the shelf sea areas of the oceans is considered. This energy source, together with wind mixing and buoyancy input by surface heating, is incorporated in a constant efficiency model of the vertical mixing in the shelf seas. The relative importance of wind and tidal stirring is assessed on the basis of observed potential energy distributions. The results of satellite infrared (i.r.) imagery are used to describe the variability of frontal positions. After the removal of tidal advection, there is a residual r.m.s. displacement of ∼7 km that cannot be explained by springs-neaps adjustment or seasonal changes. The minimal springs-neaps movement observed (∼4 km) leads to a modified energy model in which a feedback component reduces the efficiency of mixing as stratification becomes established. Predictions by the variable efficiency model of the annual cycle of the potential energy V are compared with observations at locations experiencing widely different levels of tidal mixing.

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 jet-like 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.

Residual and tidal flow at a tidal mixing front in the North Sea

Abstract A ship-mounted acoustic Doppler current profiler (ADCP) has been used to study the circulation in a frontal region of the North Sea, at the boundary between mixed and stratified water. The ship steamed backwards and forwards across a front on two sections for at least two tidal cycles to enable tidal and residual flows to be resolved. The residual flows parallel to the front are generally larger (by a factor of ∼3) than those perpendicular to it and exhibit significant structure. On a section near the Yorkshire coast, observations at neaps show that the flow is to the southeast with two regions where the flow is up to ∼6 cm s−1 separated by a zone of low flow. This structure is in qualitative agreement with the geostrophic flow based on the density field. At a section near the Dogger Bank there is evidence of a frontal jet, with currents reaching 15 cm s−1. However, the jet is not present all the time, and there are indications that it is stronger at springs and weaker at neaps. The cross-frontal residual current also is better defined at spring tides, with convergence at the surface and divergence near the bottom. The ADCP has enabled details of the tidal current profile to be elucidated. There are clear indications that the profile is influenced by the density structure. The tidal currents near the bottom lead those near the surface on both sides of the front, but on the stratified side this phase change is concentrated in a narrow band at the thermocline depth. On the mixed side, the phase change is spread more evenly over the water column. The tidal current vector rotates clockwise in the top 20 m and then anticlockwise in the rest of the water column, an effect known to be caused by bottom friction. A more subtle feature is the observed clockwise veering of the semi-major axis with depth from the surface.

An algorithm for the retrieval of suspended sediment concentrations in the Irish Sea from SeaWiFS ocean colour satellite imagery

A study was conducted in the Irish Sea with the aim of deriving an algorithm for the retrieval of suspended sediment concentrations from ocean colour imagery obtained from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). In situ observations of the diffuse attenuation coefficient, Kd , and irradiance reflectance, R, were obtained at wavelengths coincident with the SeaWiFS visible wavebands using a Profiling Reflectance Radiometer (PRR600, Biospherical Instruments Inc., San Diego). Results showed that surface reflectance at 665 nm (R665 ), rather than variations in the intrinsic colour of the ocean (using colour ratios), was the most widely applicable method of obtaining suspended sediment concentrations from ocean colour imagery in this region. The derived algorithm enabled the estimation of mineral suspended sediment (MSS) concentrations from ocean colour in the Irish Sea, accurate to within 1 mg l−1 (see equation below). Furthermore, the application of this algorithm to a SeaWiFS image of the Irish Sea accurately reproduced known regions of high turbidity with realistic MSS concentrations. MSS=0.0441R 2 665 + 1.1392R 665 + 1.7459 (R 2=0.9105, n=124, RMS error=0.907) Specific absorption and scattering coefficients were derived for all optically active in-water constituents, namely yellow substance (YS), mineral suspended sediments (MSS) and phytoplankton pigments (C). An optical model based on the empirically derived absorption and scattering coefficients reproduced the observed relationship between MSS and R 665. Model results highlighted the relative insensitivity of reflectance at 665 nm to variations in the concentrations of other in-water constituents, suggesting that the algorithm may be applicable to the Irish Sea throughout the year.

Absorption spectra of inorganic particles in the Irish Sea and their relevance to remote sensing of chlorophyll

Abstract Mineral suspended solids contribute significantly to the total suspended sediment load in shelf seas and estuaries. Even in moderate concentrations, mineral suspended solids affect algorithms for retrieving chlorophyll concentrations from remotely-sensed colour data. Brown and Simpson (1990) showed empirically that the slope of a log-log graph of chlorophyll against the blue-green reflectance ratio increased as the concentration of mineral suspended solids increased. In an attempt to explain this, we have measured the absorption spectrum of over 100 samples of mineral suspended solids collected from the Menai Strait in the Irish Sea. The absorption spectra of the mineral suspended solids particles on filters consistently show an exponential decrease with increasing wavelength across the visible spectrum. The absorption coefficient at a single wavelength is proportional to the concentration of mineral suspended solids. This spectrum has been incorporated into a simple model of the blue-green ratio...

Physical behaviour of a large, negative or inverse estuary

Abstract The observational behaviour of a large, negative or inverse estuary is discussed in the context of theoretical, and laboratory models of gravitationally driven circulations. The characteristics of Spencer Gulf, South Australia, in which evaporation exceeds precipitation all year round, and the spring-neap tidal cycle is greatly exaggerated, make it particularly instructive with regard to the role of turbulence in controlling the strength of the baroclinic circulation, and hence the exchange between Gulf and shelf waters. Seasonal influences, involving the reversal of the temperature gradient across the mouth, are shown to have a profound impact on this Gulf-shelf exchange. The development of strong temperature and salinity fronts which are compensatory with respect to density, removes almost all baroclinic forcing in the region and effectively blocks communication across the Gulf entrance during summer. It is apparent that the equilibrium of such systems relies on the sensitive interplay of influences primarily related to the nature of the change from estuary to shelf regime across the entrance, and the variability of turbulence in the region.

Mean position of tidal fronts in European-shelf seas

Abstract Different theoretical approaches to the stirring mechanism lead to the proposition that tidal fronts should lie parallel to contours ofh/u n , where the exponentn varies from3/4 to 3 according to the theory chosen. The geographical contours of the different parameters are almost parallel, which makes it difficult to distinguish between the theories by comparison with observation. In this paper, we have taken as large a data set as possible from European-shelf seas to try to determine the most appropriate mixing theory. A plot in theh-u plane of the mean surface-to-bottom temperature difference (δT) confirms the functional dependence on these variables but does not strongly favour any simple formh/u n over the full range of parameters. The results do, however, exhibit a satisfactory fit to the combined wind-tide stirring model ( Simpson et al. , 1978 , Journal of Geophysical Research , 83 , 4607–4614; Simpson and Bowers , 1984 , Annales Geophysicae , 2 , 411–416). For depths in excess of 50 m tidal stirring dominates and there is reasonable conformity ton = 3. With decreasing water depth, wind mixing becomes increasingly important and significantly inhibits persistent stratification for depths

Detecting the Zambezi River plume using observed optical properties.

In April 1998 a research cruise was carried out in the Mozambique Channel taking detailed optical, water quality and salinity measurements; the Zambezi plume waters were apparent through strong colour fronts with green waters in the plume and clear blue waters offshore. A good (r2 = 0.76) negative empirical relationship was found between the salinity (S) and yellow substance, represented by the absorption of filtered samples at 440 nm (g440). An empirical relationship was found between the salinity and a reflectance ratio: S(1/2 Zsd) = 37.45-7.07(R555/R490). An optical model in which light absorption is dominated by yellow substance and light scattering by inorganic particles confirms this empirical relationship. This has been applied to a SeaWIFS image to map salinity on the Sofala Bank, demonstrating the potential to remotely determine the distribution of the Zambezi River plume. It is thought that any significant level of chlorophyll would reduce the effectiveness of the above algorithm.

The relationship between concentrations of suspended particulate material and tidal processes in the Irish Sea

Abstract The time-dependence of suspended particulate material (SPM) and its control by tidal currents was investigated in the Irish Sea from April to October in 1987. Recording transmissometers and current meters were deployed in mixed water (h = 80m) in an area where strong reflectance in visible band imagery indicated high SPM concentrations. A seasonal signal was observed in the data with a reduction in beam attenuation (c) from late May to September. During spring and autumn a semi-diurnal variation in beam attenuation was interpreted as due to the advection of a west-east gradient in SPM. There was also evidence of local resuspension sincec was positively correlated with current speed. Both processes contributed to generally higher values ofc being attained during spring tides than at neap tides. There was no evidence of the influence of windstirring on SPM concentrations even when windspeeds exceeded 12.5 m s −1 . A model to simulate the tidal control of SPM concentrations by resuspension and tidal excursion showed good agreement with the data during spring tides. It was less successful for the neap periods when the concentrations of SPM remained low.

Seasonal changes in colour ratios and optically active contituents in the optical Case-2 waters of the Menai Strait, North Wales

A time series was conducted in the Menai Strait from March to December 1996 combining optical measurements by a colour sensor with measurements of light-absorbing constituents in the water. In order to improve estimations of pigment concentration in Case-2 waters, an optical model was used to synthesize the spectrum of sea-leaving radiance from the absorption properties of pure sea water, yellow substances, mineral suspended solids and phytoplankton. The original model was tested and improved by using a different set of parameters. Multiple regression was used to empirically relate colour ratios to pigments and total suspended solids.