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Mesoscale Variability on The Eastern Brazilian Shelf through a Roms Nested Model L. F. Rezende a, b ; P. Silva a ; A. Peliz a ; J. Dubert a & M. Cirano c
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Mesoscale Variability on The Eastern Brazilian Shelf through a Roms Nested Model L. F. Rezende a, b ; P. Silva a; A. Peliz a ; J. Dubert a & M. Cirano c aUniversidade de Aveiro, Portugal b Universidade Estadual de Santa Cruz, Brazil c Universidade Federal da Bahia, Brazil Motivation: Model Results: A new model of the Eastern Brazilian Shelf has been developed to assess the ocean response on mesoscale features and structures. The main large scale dynamical features described on Stramma & England (1999) and Silveira et al. (2000) are present on the simulation. The annual mean superficial circulation captures the western influence of the South Equatorial Current; the formation of the Brazil Current south of 12°S; its intensification around 23°S; the formation and intensification of the well defined North Brazil Current, north of 12°S and the Equatorial Circulation, in the lower latitude section. A nesting capability has been integrated into ROMS to obtain higher resolution mesoscale solutions while preserving the large-scale circulation at affordable computational cost. It has been applied to a nested domain at 9 km resolution. In the southern part of the domain, centered from 16°S to20°S the topography is marked by two major banks, the Royal Charlotte bank and the Abrolhos bank. Between those, an extended and irregular shelf constraints and alters the flow direction and strength, inducing spatial variability in physical, chemical and biological features over the shelf. On the Southern Flanks of Abrolhos banks subsurface nutrient enrichment due to upwelling of nutrient-rich SACW has been reported on literature. The physical nature of this upwelling is not yet fully understood. The mesoscale grid simulations shows persistent eddies activities on the vicinity of the coast. These structures promote water stirring and eddies forced upwelling and so shall be capable to promote nutrient enrichment, to the oligotrophic shelf. The Eastern Brazilian Shelf is a narrow, oligotrophic and bathymetrically complex shelf, on the southwestern South Atlantic. Two main tasks motivates this work: 1. The evaluation of ocean – shelf interactions processes in promoting nutrient enrichment and shelf circulation variability. 2. Downscaling. The susceptability of a coastal bay system to ocean forcing. How does the large scale interact on the coastal dynamics. Large and the mesoscale Domains – South Atlantic Ocean, Eastern Coast of Brazil. Experiment Design: • Two model configurations were devised for the larger scale grid (1/4° resolution & 30 vertical levels) : • -Case 1 initiated and forced with climatological COADS (Levitus) data. • Case 2 initiated with SODA (The Simple Ocean Data Assimilation reanalysis of ocean climate variability); Forced with COADS data. interannual variability kept on monthly mean boundary conditions for 1990-2004. • The larger scale grid was nested on a mesoscale grid of 1/12° encompassing the mesoscale dynamics of the study area: Large Scale Grid: Left - Annual mean surface currents (cm.s-1) and stream function for the upper 120 meter, Tropical Water level ( the surface water mass on the South Atlantic). On the right, annual transport budget estimate on selected sections. Validation: Oceanic mesoscale variability may be estimated through global coverage altimeter data. The inter–comparison between model variability and altimetry derived mesoscale variability represents an efficient method to validate ocean models, of special interest when limited hydrographic and water currents measurements are available. Discussion: This work was focused on the impact of the large scale circulation on mesoscale processes through the implementation of a nested capability. The two model cases investigated (with COADS and SODA) revealed similar results for the nested sub-grid, but underestimated EKE on higher latitudes. Eddies activity has been noted and is being further investigated for seasonal patterns. Further developments will also focus on further sub-grid nesting downscale. Tools developed for this project will be adapted to such scales. Mean Zonal Eke : Mesoscale Grid: Left - Annual mean surface currents (cm.s-1) and stream function for the upper 120 meter. On the right, annual budget estimate on selected cross sections. On the southern part of this domain, on the Vicinity of Abrolhos Bank, intense upward flow ( revealed on transport budget) was associated to mesoscale eddies activities Ocean mesoscale variability was estimated on eddy kinetic energy (EKE), calculated from TOPEX/ Poseidon, ERS-1/-2 and Jason combined data for sea level anomaly (SLA), from AVISO database (Ducet et al. 2000). The altimeter data used covers over 12 years of data, from 14 October 1992 to 05 January 2005, and presents a 1/3° degree resolution. The model results covers five years of climatological model outputs of sea surface height (SSH) with snapshots every 10 days (run starts on year 10) and have a 1/4° spatial resolution. In order to compare the results, we have estimated the model eddy kinetic energy on similar procedure as altimetry anomalies. Zonal time averaged EKE was estimated for the domain, centered on latitudes range from 8° (±0.5°) S to 35° (±0.5°) S, for both data sets. The altimetry and model mean zonal time EKE results are inter-compared, for different model configurations, in order to evaluate the configuration that better represents the altimetry derived EKE. References: Ducet N, Le Tron PY, Reverdin G (2000) Global high-resolution mapping of ocean circulation from TOPEX/Poseidon and ERS_1 and _2. J Geophys Res 105(C8):19477–19498. Silveira, I. C. A., Schmidt, A. C. K., Campos, E. J. D., Godoi, S. S., Ikeda, Y., 2000. A Corrente do Brasil ao largo da costa leste brasileira. Revista Brasileira de Oceanografia 48 (2), 171-183. Stramma, L., England, M., 1999. On the water masses and mean circulation of the South Atlantic Ocean. Journal of Geophysical Research 104 (C9), 20,863- 20,883. Acknowledgments: Intense eddies activity revealed on the mesoscale dynamics, promotes upwelling of nutrient richer South Atlantic Central Waters to Coastal regions. On the left, current and temperature field. On the right, current and salinity field for surface (top), 50 meters and 100 meter depth. Lúcio Figueiredo Rezende is supported by the Programme Alβan, The European Union Programme of High Level Scholarship for Latin America, Scholarship no. E04D028784BR. Contact: Lucio@fis.ua.pt Mean Zonal EKE estimate for ROMS and altimetry data CASE 1 (Levitus) and CASE 2 (SODA).