Alejandro J. Souza

The modification of tidal ellipses by stratification in the Rhine ROFI

We report on recent observations of the vertical structure of density and velocity profiles, using a bottom mounted ADCP, in the Rhine ROFI system in the North Sea which confirms previous indications that the presence of stratification modifies the vertical structure of the tidal ellipse characteristics. During periods of stratification, the ellipses change from degener- ate to a more circular pattern, with the surface ellipse rotating clockwise and the bottom ellipse rotating anticlockwise. The surface to bottom ellipticity difference ae is found to be closely related to a bulk Richardson number which incorporates both the stratification and a measure of the tidal shear. An explanation of the observed dependency of elliptieity on the density structure is offered in terms of the different thickness of the frictional layers for clockwise and anticlockwise motion in a rotating system. The changes in polarization of the flow are large enough to introduce a significant cross-shore velocity component which enhances the vertical shear in the tidal flow and is responsible for the strong semi-diurnal variation of stratification observed in this ROFI system.

Semidiurnal switching of stratification in the region of freshwater influence of the Rhine

Observations in the Rhine region of freshwater influence (ROFI) system in the North Sea show evidence of large semidiurnal oscillations in stability, superimposed on a mean stratification, occurring throughout the stratified region at times of reduced mixing. The amplitude of this semidiurnal variation is of the same order as the mean stability and frequently results in conditions being mixed or nearly mixed once per tide. It is deduced that this semidiurnal variation results primarily from cross-shore tidal straining which interacts with the density gradient to induce stratification. This conceptual picture of the contributing processes has been tested in a one-dimensional point model forced by the observed slopes and the local density gradients. The model exhibits the same qualitative behavior as the observations, produces oscillations in stratification of the amplitude observed, and confirms the critical role of cross-shore tidal straining. The large cross-shore shear nder stratified conditions is identified with the changes in ellipse configuration which are observed between mixed and stratified conditions. The occurrence of semidiurnal variations in stability in the Rhine ROFI is thus inferred to be a consequence of the development of mean stability whenever the horizontal density gradients relax in conditions of low stirring.

A numerical study of the ex-ROFI of the Colorado River

The freshwater discharge of the Colorado River into the Gulf of California has been reduced to negligible quantities since the construction of the Hoover Dam in 1935. These radical anthropogenic changes in the hydrography of the Colorado River Delta had striking repercussions on both physical and biological processes. Using historical river discharge data, the changes in the flow dynamics and hydrographic patterns before and after the drastic freshwater reduction are studied numerically, using a three-dimensional nonlinear shelf model. The results are applied to assess the environmental impact of the reduction of river discharge on the area. Satellite imagery is also used to compare our results with observed fronts.

The Malin cascade in winter 1996

We report evidence for cascading of shelf-edge water down the continental slope northwest of Ireland at 55N in February 1996. The measured surface density contrast between shelf-edge and slope waters was 0.01 kg m-3 and the resulting dense plume was 20-50 m thick over the continental slope and penetrated to a depth of 500 m. Cascade waters were warmer but saltier than overlying slope waters and their shelf-edge origin was confirmed by relatively higher chlorophyll concentrations and a distinctive nutrient signature. Other cross-slope transects 100 km north and south of the cascade site showed dense water at the shelf edge (a situation assumed to be favorable for cascading) but no deep downslope penetration of shelf water. A one-and-a-half layer dynamical model indicates that the observed downslope penetration of the cascade can be accounted for by the measured excess density of shelf-edge waters even with a realistic level of entrainment of overlying slope waters.

A two-dimensional (x-z) model of tidal straining in the Rhine ROFI

Recent observations of the Rhine ROFI system in the North Sea suggest that the dynamics and structure are very variable, due to the fact that the system responds very rapidly to changes in the stirring conditions due to tides and winds. They also show very strong semi-diurnal variability, which is the result of the interaction between tidal straining and the cross-shore density gradients. This hypothesis is tested using a two-dimensional slice model. The model is successful in reproducing the qualitative behaviour of the observations, with the semi-diurnal oscillations in stratification of the right amplitude and at the same phase of the tidal cycle as the observations. The model also shows the effect of the stratification on generating the cross-shore tidal shear which then interacts with the main density gradients, hence the semi-diurnal variability in stratification. The model confirms previous hypotheses that suggest the importance of wind direction in enhancing or suppressing stratification.

Current structure in the Rhine region of freshwater influence

Observation with HF radar, ADCPs and conventional current meters in the Rhine ROFI system in the North Sea indicate that the dominant currents are due to tidal flow, with important contributions from winds and density-driven circulation. The tidal currents are dominated by the semidiumal components, principally the M2, with tidal ellipses parallel to the coast and a semimajor axis of the order of 1 m s ’ at the surface. The wind-driven flow varies between 1 and 3% of the wind speed and the rotation varies with depth in qualitative agreement with Ekman theory. After removal of the tidal and wind-driven component the residual t?~w within this region is generally parallel to the coast (northeastward) with average surface speeds of about 10 cm s-t. A convergence zone in the surface flow was observed at the outer edge of the coastal zone. The vertical distribution of residual velocity closely follows the Heaps (1972) profile for density-driven flow, with slight deviations probably due to the tidal rectification (-2 cm s-t).

Modelling SPM on the NW European shelf seas

Abstract The Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS) has been developed to tackle multidisciplinary studies in coastal/shelf environments. The central core is a sophisticated three-dimensional hydrodynamic model that provides realistic flow fields to interact with, and transport environmental parameters. The model uses realistic forcing with ocean currents and hydrography at the boundary, atmospheric forcing and tides. Suspended particulate matter (SPM) is transported within the water, like any other scalar in the model; it is advected using a high-order scheme and mixed vertically using diffusion coefficients from a second moment turbulence closure. In addition, the SPM transport is influenced by a number of processes unique to this problem, such as: the transfer between the lowest level of the model and the bed layer in which the resuspension and deposition takes place is dependent on the erosion rate, the particle settling velocity and the critical stresses for erosion and deposition. The particular application described in this paper is an annual simulation for the NW European shelf for a single sediment class. The simulation shows the expected seasonal variation in spatial patterns and a clear tidal signal in the SPM resuspension. Also evident is a fortnightly/monthly variability due to the spring-neaps and the M2-N2 cycles. There are also quarter-diurnal and semi-diurnal cycles in SPM, due to tidal resuspension and advection respectively. The above tidal suspension behaviour, as well as, the net transport direction are in agreement with historical observations of SPM.

Scattering signatures of suspended particles: an integrated system for combining digital holography and laser diffraction.

The use of laser diffraction is now common practice for the determination of an in situ particle size distribution in the marine environment. However, various imaging techniques have shown that particles vary greatly in shape, leading to uncertainty in the response of laser diffraction instruments when subjected to this diverse range of complex particles. Here we present a novel integrated system which combines both digital in-line holography and a LISST-100 type C, to simultaneously record in-focus images of artificial and natural particles with their small-angle forward scattering signature. The system will allow for further development of a reliable alternative to Mie Theory when using laser diffraction for the in situ measurement of complex suspended particles. A more detailed knowledge of the performance of laser diffraction when subjected to the wide variety of complex particles found in the marine environment will then be possible.

Tidal mixing modulation of sea surface temperature and diatom abundance in Southern California

In the Southern California Bight a clear seasonal cycle in temperature and diatom abundance is observed, with maximum temperatures in summer and maximum diatom abundance in spring, decreasing in summer. Within this seasonal cycle of temperature and diatom abundance, there is a weak fortnightly temperature variability. Here, we show that diatom abundance has lunar as well as seasonal variability, with the highest abundance corresponding to the coldest days within the lunar cycle. This suggests that at least part of the temperature and diatom abundance variability may be due to bottom tidal mixing. To explore the effect of tidal mixing and tidal straining on the control of the thermal structure of the water column in the Southern California region, a one-dimensional, hydrodynamic numerical model is used. The model is successful in explaining the seasonal cycle in temperature and partially explains the fortnightly and monthly variability in temperature. The observed temperature minimum shows a lag of about 2–3 days, when compared with model results. This lag is probably due to the fact that the model does not include the effect of internal waves, which will be an extra source of mixing and may have an advective effect that will modify the water-column structure and the diatom distribution.

An investigation of recent decadal-scale storm events in the eastern Irish Sea

The Proudman Oceanographic Laboratory Coastal Modelling System coupled to the WAve Model (POLCOMS-WAM) modeling system has been used to model combined tides, surges, waves, and wave-current interaction in the Irish Sea on a 1.85 km grid. A method for data analysis is presented to determine what factors and interactions contribute to extreme conditions in a region of interest. An 11 year hindcast (1996-2006) has been performed to investigate the meteorological conditions that cause extreme surge and/or wave conditions in Liverpool Bay. A one-way nested model approach was used. For waves, a 1 degrees North Atlantic WAM model forces the boundary of the Irish Sea model, driven by ERA-40 wind (similar to 1 degrees resolution every 6 h). To capture the external surge generated outside of the Irish Sea, the (1/9 degrees x 1/6 degrees) Proudman Operational surge model extending to the continental shelf edge was run for tide and surge and was forced by Met Office mesoscale winds (similar to 12 km resolution every hour). The data implied that the largest surges at Liverpool are generally driven by winds from the south to the west while the largest waves are forced by winds from the west to the northwest. The worst storm conditions in Liverpool Bay result under southwesterly wind conditions that veer to the west. The large tidal range in the region acts to enhance the impact of the surge through tide-surge interaction. Moreover, the highest water levels in Liverpool Bay are in response to southwesterly winds combined with high-water spring tide. Even though no significant surge occurs at this time, the flood threat is at its greatest