Final Activity Report Summary - ECOGIB (Ecohydrodynamical data analysis and modelling studies of the Gibraltar Strait)
The overall objective of the project was to study the hydrodynamics of the Gibraltar Strait and its impact on the plankton ecosystem dynamics and the distribution of biochemical parameters using advanced numerical modelling and data assimilation techniques A two-way nested coupled (physical/biological) 3D modelling system was successfully implemented and applied to the climatological study of the hydrodynamics and the plankton ecosystem of the Gibraltar Strait and the Alboran Sea.
The main hydrodynamic features such as the Western Alboran Gyre (WAG) and the Eastern Alboran Gyre (EAG) were very well reproduced by the climatological model. Results showed that the Northwestern Alboran Sea is one of the most productive areas within the oligotrophic Mediterranean Sea. The current intensity of the Atlantic Jet increases progressively at the strait to obtain maximum values of about 1 m/s in the northeastern Mediterranean entrance (Spanish Coast). Maximum vertical (upwelling) velocities are also obtained at the same location. As water from the deep Atlantic Ocean enters the shallow strait, water layers are stretched upward. This results in an upward displacement of the nitracline towards the northeastern entrance of the strait increasing the nutrient transports into the Northwest Alboran Sea. Simulations are also performed in order to study the short-term variability of the ecohydrodynamics of the Gibraltar Strait in various real time variable atmospheric forcing cases.
Model results demonstrated the presence of two coastal upwelling regions associated with the prevailing easterly and westerly winds in the area of interest in agreement with observations. Model results indicate the periodic generation of an upwelling cell confined along the African coast of the Strait of Gibraltar induced during easterly wind bursts. The other upwelling region is a quasi-permanent coastal upwelling zone within the Northwest Alboran coastal area. The wind-induced coastal upwelling mechanism associated with the prevailing westerlies was shown to contribute but cannot solely explain the permanent pools of cold and nutrient rich surface waters. Analysis of model results shows that non-linear advection induces larger ageostrophic eastward transports in the upper layers following the direction of the Atlantic Jet alimenting the West Alboran Gyre while in the deeper layers water is moving northwestward due to increased positive (cyclonic) vorticity. In the northwestern coastal shelf area of the Alboran Basin a cyclonic circulation characterised by low advection is obtained. The nutrient-rich water transport originated from the strait along with the generation of cyclonic vorticity in the northwestern coastal area of the Alboran Sea results in accumulation of nutrients and permanent stimulation of primary production.
Finally the exploitation of the previously established 2-way nested modelling system by an operational forecasting application using remote sensing data assimilation was investigated. Remote sensing derived data of Sea Surface Temperature (SST), Sea Level Anomaly (SLA) and surface chlorophyll were assimilated in the model using different assimilation schemes. The SST assimilation consisted of correcting the surface heat flux by a relaxation of the numerical model surface-layer temperature towards the observed SST. The SLA and surface chlorophyll data were assimilated using a reduced order optimal interpolation scheme. Results showed that the Root Mean Square SST error was reduced more than 60% at the end of the assimilation experiment. The SST/SLA assimilation alone was shown to be responsible for a large part of the model surface chlorophyll correction demonstrating the assimilation impact of hydrodynamics on the evolution of the plankton ecosystem.
The main hydrodynamic features such as the Western Alboran Gyre (WAG) and the Eastern Alboran Gyre (EAG) were very well reproduced by the climatological model. Results showed that the Northwestern Alboran Sea is one of the most productive areas within the oligotrophic Mediterranean Sea. The current intensity of the Atlantic Jet increases progressively at the strait to obtain maximum values of about 1 m/s in the northeastern Mediterranean entrance (Spanish Coast). Maximum vertical (upwelling) velocities are also obtained at the same location. As water from the deep Atlantic Ocean enters the shallow strait, water layers are stretched upward. This results in an upward displacement of the nitracline towards the northeastern entrance of the strait increasing the nutrient transports into the Northwest Alboran Sea. Simulations are also performed in order to study the short-term variability of the ecohydrodynamics of the Gibraltar Strait in various real time variable atmospheric forcing cases.
Model results demonstrated the presence of two coastal upwelling regions associated with the prevailing easterly and westerly winds in the area of interest in agreement with observations. Model results indicate the periodic generation of an upwelling cell confined along the African coast of the Strait of Gibraltar induced during easterly wind bursts. The other upwelling region is a quasi-permanent coastal upwelling zone within the Northwest Alboran coastal area. The wind-induced coastal upwelling mechanism associated with the prevailing westerlies was shown to contribute but cannot solely explain the permanent pools of cold and nutrient rich surface waters. Analysis of model results shows that non-linear advection induces larger ageostrophic eastward transports in the upper layers following the direction of the Atlantic Jet alimenting the West Alboran Gyre while in the deeper layers water is moving northwestward due to increased positive (cyclonic) vorticity. In the northwestern coastal shelf area of the Alboran Basin a cyclonic circulation characterised by low advection is obtained. The nutrient-rich water transport originated from the strait along with the generation of cyclonic vorticity in the northwestern coastal area of the Alboran Sea results in accumulation of nutrients and permanent stimulation of primary production.
Finally the exploitation of the previously established 2-way nested modelling system by an operational forecasting application using remote sensing data assimilation was investigated. Remote sensing derived data of Sea Surface Temperature (SST), Sea Level Anomaly (SLA) and surface chlorophyll were assimilated in the model using different assimilation schemes. The SST assimilation consisted of correcting the surface heat flux by a relaxation of the numerical model surface-layer temperature towards the observed SST. The SLA and surface chlorophyll data were assimilated using a reduced order optimal interpolation scheme. Results showed that the Root Mean Square SST error was reduced more than 60% at the end of the assimilation experiment. The SST/SLA assimilation alone was shown to be responsible for a large part of the model surface chlorophyll correction demonstrating the assimilation impact of hydrodynamics on the evolution of the plankton ecosystem.