Dissertation Abstract
A 3D high resolution coupled hydrodynamicÂ-biogeochemical model for the Western Mediterranean Sea. Interannual variability of primary and export production.
Bernardello, Raffaele 2010
Centre d'Estudis Avançats de Blanes- CSIC, Universitat Politecnica de Catalunya (UPC)- Consejo Superior de Investigaciones Cientificas (CSIC) (Spain), 216 pp.
Ocean carbon cycle has been object of increasing interest in the last few decades because of its prominent role in controlling global climate.
Carbon storage in the sea is a result of different processes involving physical, chemical and biological interactions. These interactions occur on a wide spectrum of spatial and time scales that field studies alone are often unable to encompass. Numerical modelling can help us to understand and quantify the carbon storage in the ocean providing a synoptic view of the oceanic circulation and its influence on tracers distribution.
This thesis analyses the dynamics that control the production and exportation of organic carbon, their interannual variability and their spatial heterogeneity in the Western Mediterranean Sea. The Mediterranean Sea is considered a model ocean because many of the processes that are fundamental to the general circulation of the world ocean also occur here. The western part is interested by deep water formation and by intense mesoscale activity. Different trophic conditions are found within small distance determining high spatial and temporal variability. Therefore, the results obtained in this study can be relevant also for other areas of the world ocean.
A hydrodynamic model based on the parallelized version (sbPOM) of the Princeton Ocean Model has been configured for the Western Mediterranean Sea with horizontal resolution of 1/20 degree and 52 sigma-layers in the vertical dimension. The model has been nested in one-way off-line into an Ocean General Circulation Model in order to specify the open lateral boundary conditions. The atmospheric forcing is based on the ERA-interim reanalysis data, provided by the European Centre for Medium-Range Weather Forecast (ECMWF). A biogeochemical model has been coupled to the hydrodynamic component in order to resolve non-conservative dynamics. The biogeochemical model uses nitrogen as currency and it has a partially described cycle of carbon associated to it. The coupled model has been run in hindcast mode for the 2001-2008 period and the outputs have been quantitatively and qualitatively validated with remote sensing data for sea surface temperature (SST), chlorophyll, Sea Level Anomaly (SLA) and Primary Production (PP).
The validation of results demonstrates that the model is able to reproduce with accuracy the interannual variability of spatial and seasonal patterns of SST, chlorophyll and PP. The simulated Mean Dynamic Topography (MDT) and the geostrophic circulation associated are in agreement with the principal patterns described in the area. The model estimates higher levels of Eddy Kinetic Energy (EKE) with respect to those obtained from remote sensing SLA maps. This is probably due to the high spatial resolution of the model that would be able to resolve mesoscale structures not captured by altimeter data.
Simulated PP presents a low mean interannual variability (1.5%; mean=143.8 g C/(sq. m per year)) while the Export Production (EP) is characterized by higher fluctuations (18.29%; mean=34.13 g C/(sq. m per year)). Model results suggest that in the northern areas, the variability of the Mixed Layer Depth is the main responsible for the interannual variability of PP and EP. On the other hand, in the southern portion of the area, mesoscale activity and frontal dynamics are responsible for maintaining high levels of PP and EP, reducing the interannual fluctuations linked to seasonal atmopheric dynamics.
Carbon storage in the sea is a result of different processes involving physical, chemical and biological interactions. These interactions occur on a wide spectrum of spatial and time scales that field studies alone are often unable to encompass. Numerical modelling can help us to understand and quantify the carbon storage in the ocean providing a synoptic view of the oceanic circulation and its influence on tracers distribution.
This thesis analyses the dynamics that control the production and exportation of organic carbon, their interannual variability and their spatial heterogeneity in the Western Mediterranean Sea. The Mediterranean Sea is considered a model ocean because many of the processes that are fundamental to the general circulation of the world ocean also occur here. The western part is interested by deep water formation and by intense mesoscale activity. Different trophic conditions are found within small distance determining high spatial and temporal variability. Therefore, the results obtained in this study can be relevant also for other areas of the world ocean.
A hydrodynamic model based on the parallelized version (sbPOM) of the Princeton Ocean Model has been configured for the Western Mediterranean Sea with horizontal resolution of 1/20 degree and 52 sigma-layers in the vertical dimension. The model has been nested in one-way off-line into an Ocean General Circulation Model in order to specify the open lateral boundary conditions. The atmospheric forcing is based on the ERA-interim reanalysis data, provided by the European Centre for Medium-Range Weather Forecast (ECMWF). A biogeochemical model has been coupled to the hydrodynamic component in order to resolve non-conservative dynamics. The biogeochemical model uses nitrogen as currency and it has a partially described cycle of carbon associated to it. The coupled model has been run in hindcast mode for the 2001-2008 period and the outputs have been quantitatively and qualitatively validated with remote sensing data for sea surface temperature (SST), chlorophyll, Sea Level Anomaly (SLA) and Primary Production (PP).
The validation of results demonstrates that the model is able to reproduce with accuracy the interannual variability of spatial and seasonal patterns of SST, chlorophyll and PP. The simulated Mean Dynamic Topography (MDT) and the geostrophic circulation associated are in agreement with the principal patterns described in the area. The model estimates higher levels of Eddy Kinetic Energy (EKE) with respect to those obtained from remote sensing SLA maps. This is probably due to the high spatial resolution of the model that would be able to resolve mesoscale structures not captured by altimeter data.
Simulated PP presents a low mean interannual variability (1.5%; mean=143.8 g C/(sq. m per year)) while the Export Production (EP) is characterized by higher fluctuations (18.29%; mean=34.13 g C/(sq. m per year)). Model results suggest that in the northern areas, the variability of the Mixed Layer Depth is the main responsible for the interannual variability of PP and EP. On the other hand, in the southern portion of the area, mesoscale activity and frontal dynamics are responsible for maintaining high levels of PP and EP, reducing the interannual fluctuations linked to seasonal atmopheric dynamics.
