Numerical simulation of the barotropic tides in the Tunisian Shelf and the Strait of Sicily

We have investigated the barotropic tides in the Tunisian shelf and the Strait of Sicily using the Regional Ocean Modelling System (ROMS) with very high-resolution. Model performance was evaluated with respect to tide gauge, satellite data, and current meter measurements. The model fields faithfully reproduced the major feature of the barotropic tidal currents and agreed well with existing tidal elevation and phase observations. General features for the various semidiurnal constituents are nearly similar to each other with maximum amplitude in the Gulf of Gabes. The larger tidal currents occur over the continental shelves. In the Adventure Bank, the current is essentially of diurnal type whereas in the Gulf of Gabes it is of semidiurnal type.Tidal energy lost, which is primarily due to bottom stress dissipation, is predominantly in the Gulf of Gabes ( 61%), the Strait of Sicily, and the Strait of Messina. The forcing function for internal tides shows for both M₂ and K₁ constituents, significant spatial variability in the Strait of Sicily. This suggests that some internal tides will be generated in these regions and could thus explain the observed strong diurnal internal waves in the Adventure Bank.     

Residual circulation and sediment distribution in the Ria de Aveiro lagoon, Portugal

A two-dimensional hydrodynamic and sediment transport models have been developed and applied to the Ria de Aveiro lagoon, Portugal. The application consisted in the study of the residual circulation, the residence time as well as the sediment dynamics in the lagoon. High residence time for particles situated at the far end of the main channels points out that they are retained for a long time and tend to remain there, with high probability of being deposited into the bed. Low residence time for the central areas implies that the particles are flushed out more rapidly toward the lagoon mouth, revealing that the water and the particles exchange are very effective there. Although the residual circulation due to the rivers induces an overall transport toward the lagoon mouth, the residual circulations due to the winds induce particular circulation patterns (clockwise or counter-clockwise eddies) at some locations of the lagoon, depending on the wind direction. It was found that the suspended sediment concentrations along the main channels are induced by tidal asymmetries (resulting in areas of ebb and flood dominance), as well as by the wind stress and rivers runoff effects. They contribute, in general, to increase the sediment export toward the ocean, although the wind stress may induce currents and circulations in the opposite direction of the tidal currents. High turbid zones are observed for strong tidal currents, associated with tidal asymmetries, as well as with high rivers runoff. The erosion–deposition budget indicates a tendency for sediment accumulation at important areas of the lagoon, namely the shallow ones.     

A conceptual model of the strongly tidal Columbia River plume

The Columbia River plume is typical of large-scale, high discharge, mid-latitude plumes. In the absence of strong upwelling winds, freshwater from the river executes a rightward turn and forms an anticyclonic bulge before moving north along the Washington coast. In addition to the above dynamics, however, the river plume outflow is subject to large tides, which modify the structure of the plume in the region near the river mouth. Observations based on data acquired during a summer 2005 cruise indicate that the plume consists of four distinct water masses; source water at the lift-off point, and the tidal, re-circulating and far-field plumes. In contrast to most plume models that describe the discharge of low-salinity estuary water into ambient high-salinity coastal water, we describe the Columbia plume as the superposition of these four plume types.We focus primarily on a conceptual summary of the dynamics and mutual interaction of the tidal and re-circulating plumes. The new tidal plume flows over top of the re-circulating plume and is typically bounded by strong fronts. Soon after the end of ebb tide, it covers roughly 50–100% of the re-circulating plume surface area. The fronts may penetrate well below the re-circulating plume water and eventually spawn internal waves that mix the re-circulating plume further. The re-circulating plume persists throughout the tidal cycle and corresponds to a freshwater volume equivalent to 3–4 days of river discharge. Finally, the plume water masses are distinguished from one another in term of surface chlorophyll concentration, suggesting that the above classification may also describe different biological growth regimes. The low-salinity re-circulating plume serves as an extension of the estuary into the coastal ocean, or an “estuary at sea”, because residence times during periods of high river flow are greater than those in the estuary.