Connection of hydrological and hydrodynamic processes between the Gulf of Cádiz and Alborán Sea

La presente Tesis estudia dos procesos que son cruciales para comprender la respuesta de los productores primarios al forzamiento físico en el sistema golfo de Cádiz – estrecho de Gibraltar – mar de Alborán: (i) La dinámica de los aportes continentales de los ríos que descargan al golfo de Cádiz, que transportan, entre otros componentes, nutrientes que a su vez contribuyen de forma importante al crecimiento del fitoplancton en la franja costera del Golfo y estrecho de Gibraltar. (ii) Los procesos de mezcla vertical inducidos por las ondas internas generadas en el umbral de Camarinal, prestando especial atención a las mezclas verticales que se producen en el mar de Alborán y que pueden jugar un papel significativo en el suministro de nutrientes a la capa superior, ayudando por lo tanto al sostenimiento del fitoplancton en esta región. En cuanto a los aportes continentales del río Guadalquivir, la evolución de la pluma de turbidez hacia el estrecho de Gibraltar precisa que imperen condiciones de vientos con componente norte, noroeste u oeste. Por el contrario, el desarrollo de la pluma hacia el oeste precisa de vientos con componente este, sur o sureste. Por otra parte, como la mayor parte de las precipitaciones en la cuenca del Guadalquivir ocurren en coincidencia con vientos de componente oeste, los momentos de máxima descarga en la desembocadura del río ocurren bajo condiciones que favorecen el transporte por la franja costera de las sustancias inmersas en la pluma y otras propiedades asociadas a ella, hacia el estrecho de Gibraltar y mar de Alborán. En cuanto a los procesos de mezcla vertical inducidos por las ondas internas, se ha encontrado un mecanismo para la ocurrencia de mezcla vertical dentro de la picnoclina, en el que es esencial la implicación de modos baroclinos de orden superior (hasta el modo 9). La presencia de estos modos baroclinos de orden superior confieren una regularidad importante en la presencia de ondas internas en la región de Alborán, regularidad que se traspasa a los procesos de mezcla inducidos por estas ondas. Además, la pequeña escala vertical (unos 20 m) y horizontal (unos 100 m) de estos modos baroclinos requieren que los modelos numéricos, destinados a simular procesos de mezcla vertical en estas regiones, tengan una resolución espacial lo suficientemente alta para que permita resolver las longitudes de onda implicadas.The present thesis concerns the study of two processes that are crucial to understand the response of primary producers to physical forcing in the Gulf of Cadiz - Gibraltar Strait - Alborán Sea system: (i) The dynamics of the continental inputs of the rivers that discharge to the Gulf of Cadiz, which transport, among other components, nutrients that in turn contribute in a significant way to phytoplankton growth in the coastal margin of the Gulf and Strait of Gibraltar. (ii) The vertical mixing processes induced by internal waves generated at Camarinal Sill, paying particular attention to the vertical mixing that occurs in the Alborán Sea and which may play a significant role in the nutrient supply to the upper layer, thus helping to sustain phytoplankton growth in this region. Regarding the continental contributions of the Guadalquivir river, the evolution of the turbidity plume to the Strait of Gibraltar requires that conditions of winds with north, northwest or west component prevail. On the contrary, the development of the plume to the west needs winds with east, south or southeast component. On the other hand, since most of the rainfall in the Guadalquivir basin occurs in coincidence with winds of a westerly component, the moments of maximum discharge at the mouth of the river occur under conditions that favor the transport along the coastal margin of the substances immersed in the plume and other properties associated with it, the Straits of Gibraltar and Alborán Sea. Concerning the vertical mixing processes induced by the internal waves, a mechanism has been found for the occurrence of vertical mixing within the pycnocline, in which the implication of higher order baroclinic modes (up to mode 9) is essential. The presence of these higher order baroclinic modes confers an important regularity to the internal waves passages by the Alborán region, which is transferred to the mixing processes induced by these waves. In addition, the small vertical scale (about 20 m) and horizontal (about 100 m) of these baroclinic modes require that the numerical models intended to simulate vertical mixing processes in these regions, have a high enough spatial resolution to allow for solving the involved wavelengths

Hydrological Alteration Index as an Indicator of the Calibration Complexity of Water Quantity and Quality Modeling in the Context of Global Change

Modeling is a useful way to understand human and climate change impacts on the water resources of agricultural watersheds. Calibration and validation methodologies are crucial in forecasting assessments. This study explores the best calibration methodology depending on the level of hydrological alteration due to human-derived stressors. The Soil and Water Assessment Tool (SWAT) model is used to evaluate hydrology in South-West Europe in a context of intensive agriculture and water scarcity. The Index of Hydrological Alteration (IHA) is calculated using discharge observation data. A comparison of two SWAT calibration methodologies are done; a conventional calibration (CC) based on recorded in-stream water quality and quantity and an additional calibration (AC) adding crop managements practices. Even if the water quality and quantity trends are similar between CC and AC, water balance, irrigation and crop yields are different. In the context of rainfall decrease, water yield decreases in both CC and AC, while crop productions present opposite trends (+33% in CC and -31% in AC). Hydrological performance between CC and AC is correlated to IHA: When the level of IHA is under 80%, AC methodology is necessary. The combination of both calibrations appears essential to better constrain the model and to forecast the impact of climate change or anthropogenic influences on water resources

Baroclinic M2 circulation in Algeciras Bay and its implications for the water exchange with the Strait of Gibraltar: Observational and 3-D model results

The M2 tidal circulation in Algeciras Bay (Strait of Gibraltar) is analyzed using a 3-D, nonlinear, baroclinic, hydrodynamic model, in conjunction with observed data series. Results show the influence of the density stratification on the vertical structure of the M2 currents in Algeciras Bay, although its tidal dynamics shows major differences with respect to the Strait of Gibraltar. Whereas the M2 currents in the Strait present mainly barotropic behavior, the baroclinic effects prevail in Algeciras Bay. A notable finding is the presence of a tidal M2 counter-current system between the upper Atlantic and the lower Mediterranean water layers within the Bay, with amplitudes of up to 25 cm s−1. The interface between the two layers oscillates in antiphase relation with respect to the free-surface elevation, with amplitudes of almost 20 m. The presence of the submarine Algeciras Canyon was found to be determinant in the three-dimensional structure of tidal currents within the Bay, strengthening the baroclinic tidal regime of currents. This situation has quantitative consequences for the flow-exchange processes between Algeciras Bay and the outer Strait, with rates 20 times higher than those obtained when considering only the barotropic behavior, as well as inflow/outflow lateral recirculation volumes during half a tidal cycle that account for more than 20% of the net accumulated volume. This flow-exchange system was found to be affected by the nonlinear interaction processes between the first baroclinic period of resonance of Algeciras Bay and the M2 tide

Internal waves in the Strait of Gibraltar and their role in the vertical mixing processes within the Bay of Algeciras

This article presents some of the results of an oceanographic survey carried out in the Bay of Algeciras (Strait of Gibraltar) as part of a research project intended to assess the environmental quality of the Bay. One of the most interesting findings was the step-like patterns presented by density profiles within the Bay, which were indicative of notable vertical mixing activity there. The analysis of the observations indicates that those mixing processes may be explained by the interaction between the local internal tide dynamics and the large amplitude internal waves entering the Bay, which seem to originate from the Camarinal Sill region

The fate of Guadalquivir River discharges in the coastal strip of the Gulf of Cadiz. A study based on the linking of watershed catchment and hydrodynamic models.

A catchmentmodel for river basins and a hydrodynamicmodelwere combined in order to simulate the spreading of the turbidity plume produced by sediment discharges from the Guadalquivir River basin within the Gulf of Cádiz under different meteorological conditions. The current fields provided by the hydrodynamic model and a transport-diffusion scheme based on tracking virtual particles tracking released at the river mouth have enabled us to simulate turbidity plumes that show great similarity with the plumes observed in satellite images. The most relevant results of the study show that in the absence of winds, the plume tends to spread very slowly, gradually progressing northwards; this is because of the symmetry between the filling and draining flows at the mouth of the Guadalquivir and low intensity of the tidal currents beyond the mouth. In addition, the transport of the plume towards the Strait of Gibraltar requires wind conditions with a northerly, north-westerly or westerly component. Westwards transport, however, requires winds with an easterly, southerly, or south-easterly component. The periods of heaviest rainfall in the Guadalquivir River basin coincide with winds mainly from the west; therefore, the times ofmaximum discharge at the mouth of the river occurwhen there are wind conditions that favour the transport of the matter suspended in the plume, southwards along the coast, towards the Strait of Gibraltar and the Alboran Sea. Linking the watershed catchment and hydrodynamic models has proved its suitability to predict the evolution and reaching of the sediment plumes fromthe Guadalquivir River discharges and the experience encourages the use of that methodology to be applied in a future prediction systemfor the creation and evolution of those sediment plumes.We would like to thank the Regional Government of Andalusia ( P11-RNM-7722 project), the Spanish Government (TRUCO project RTI2018-100865-B-C22 ), the SUDOE INTERREG AGUAMOD and OCASO projects for supporting this work financially. We are also grateful to NASA for distributing the MODIS data used in this study, and to the AERONET project for the MODIS AQUA RGB True Color Images, especially those from the Malaga-San Jose and Huelva stations