Study of the equatorial Atlantic Ocean mixing layer using a one-dimensional turbulence model

Abstracts

The General Ocean Turbulence Model (GOTM) is applied to the diagnostic turbulence field of the mixing layer (ML) over the equatorial region of the Atlantic Ocean. Two situations were investigated: rainy and dry seasons, defined, respectively, by the presence of the intertropical convergence zone and by its northward displacement. Simulations were carried out using data from a PIRATA buoy located on the equator at 23º W to compute surface turbulent fluxes and from the NASA/GEWEX Surface Radiation Budget Project to close the surface radiation balance. A data assimilation scheme was used as a surrogate for the physical effects not present in the one-dimensional model. In the rainy season, results show that the ML is shallower due to the weaker surface stress and stronger stable stratification; the maximum ML depth reached during this season is around 15 m, with an averaged diurnal variation of 7 m depth. In the dry season, the stronger surface stress and the enhanced surface heat balance components enable higher mechanical production of turbulent kinetic energy and, at night, the buoyancy acts also enhancing turbulence in the first meters of depth, characterizing a deeper ML, reaching around 60 m and presenting an average diurnal variation of 30 m.

Oceanic turbulence; General ocean turbulence model; Equatorial Atlantic Ocean; Turbulent kinetic energy

O modelo General Ocean Turbulence Model (GOTM) é aplicado para diagnosticar o campo de turbulência da camada de mistura oceânica (CM) na região equatorial do Oceano Atlântico. Foram investigadas as estações chuvosa e seca, definidas, respectivamente, pela presença da zona de convergência intertropical e pelo seu deslocamento para norte. Simulações foram realizadas usando dados da bóia PIRATA (0º, 23ºW) para o cálculo dos fluxos turbulentos de superfície e dados do Projeto NASA/GEWEX Surface Radiation Budget para "fechar" o balanço de radiação na superfície. Um esquema para assimilação de dados foi usado para considerar os mecanismos físicos não representados pelo modelo unidimensional. Para a estação chuvosa, os resultados mostraram uma CM rasa devido à menor tensão de cisalhamento na superfície e a estratificação estável da camada superior oceânica; a profundidade máxima alcançada é da ordem de 15 m com uma variação diurna média de 7 m de profundidade. Na segunda estação, a tensão de cisalhamento mais intensa e o aumento das trocas de calor em superfície geraram maior produção mecânica de energia cinética turbulenta e a noite o empuxo também favoreceu a formação de uma CM mais profunda, alcançando até 60 m, e com variação diurna de 30 m em média.

Turbulência oceânica; Modelo geral de turbulência oceânica; Oceano Atlântico equatorial; Energia cinética turbulenta

• BOLDING, K.; BURCHARD, H.; POHLMANN, T.; STIPS, A. Turbulent mixing in the Northern North Sea: a numerical model study. Cont. Shelf Res., v. 22, 2002. p. 2707-2724.
• BOURLÈS, B.; MCPAHADEN, M. J.; HERNANDES, F.; NOBRE, P.; CAMPOS, E.; YU, L.; PLANTON, S.; BUSALACCHI, A., MOURA, A. D.; SERVAIN, J.; TROTTE, J. The PIRATA Program. History, accomplishments, and future directions. Bull. Amer. Meteor. Soc, 2008. p. 1111-1125, DOI:10.
• BURCHARD, H.; BAUMERT, H. On the performance of a mixed-layer model based on the k-epsilon turbulence closure. J. Geophys. Res , v. 100, 1995. p. 8523-8540.
• BURCHARD, H.; BOLDING, K. Comparative analysis of four second-moment turbulence closure models for the oceanic mixed layer. J. Phys. Oceanogr, v. 31, 2001. p. 1943-1968.
• BURCHARD, H.; BOLDING, K.; VILLARREAL, M. GOTM – a general ocean turbulence model. Theory, applications and test cases. Tech.Rep. EUR 18745 EN, European Commission.
• CANUTO, V. M.; HOWARD, A.; CHENG, Y.; DUBOVIKOV, M. S. Ocean turbulence. Part I: One point closure model, momentum and heat vertical diffusivities. J. Phys. Oceanogr., v. 31 (6), 2001. p. 1413-1426.
• CARTON, J. A.; CAO X.; GIESE, B. S.; DA SILVA, A. M. Decadal and interannual SST variability in the tropical Atlantic. J. Phys. Oceanogr, v. 26, 1996. p. 1165-1175.
• CARTON, J. A.; ZHOU, Z. Annual cycle of sea surface temperature in the tropical Atlantic Ocean. J. Geophy. Res, v. 102, 1997. p. 27813-27824.
• CHANG, P.; SARAVANAN, L.; JI, L.; HEGERL, G.C. The effect of local sea surface temperatures on atmospheric circulation over the tropical Atlantic sector. J. Climate, v. 13, 2000. p. 2195-2216.
• DE BOYER MONTÉGUTE C.; MADEC, G.; FISHER, A. S.; LASAR, A.; IUDICONE, D. Mixed layer depth over the global ocean: an examination of profile data and a profile-based climatology. J. Geophys. Res, v. 109, 2004. DOI:10.
• FAIRALL, C. W.; BRADLEY, E. F.; HARE, J. E.; GRACHEV, A. A.; EDSON, J. B. Bulk Parameterization of Air-Sea Fluxes: Updates and Verification for the COARE Algorithm. J. of Climate, v. 16, 2003. p. 571-591.
• HASTENRATH, S. Climate Dynamics of the Tropics Kluwer Academic, 1991. 488p.
• JEFREY, C. D.; ROBINSON, I. S.; WOOLF, D. K.; DONLON, C. J. The response to phase-dependent wind stress and cloud fraction of the diurnal cycle of SST and air–sea CO2 exchange. Ocean Modelling v. 23, 2008. p. 33-48.
• LEVITUS, S.; BOYER, T. P. Temperature. World Ocean Atlas 1994 NOAA Atlas NESDIS 4. v. 4, 1994. 117p.
• MOISAN, J.; NIILER, P. The seasonal heat budget of the North Pacific: net heat fluxes and heat storage rates (1950-90). J. Phys. Oceanogr v. 28, 1998. p. 401-421.
• MONTEREY, G.; LEVITUS, S. Seasonal Variability of Mixed Layer Depth for the World Ocean NOAA Atlas NESDIS 14, Natl. Oceanic and Atmos. Admin., Silver Spring, Md. 5 p.
• PERES, J. R. Estudo do balanço de radiação sobre o oceano Atlântico Tropical na região do Arquipélago de São Pedro e São Paulo. Relatório Final de Iniciação Científica. Instituto de Astronomia, Geofísica e Ciências Atmosféricas. Universidade de São Paulo. 2008. 35 p. http://www.iag.usp.br/meteo/labmicro/publicacoes/relatorios_tecnicos/Jean_2008_Estudo_do_balanco_de_radiacao_sobre_o_oceano_Atlantico_tropical_na_regiao_do_ASPSP.pdf
• PHILANDER, S. G. El Niño, La Niña, and Southern Oscillation Academic Press, Londres, 289. 1990. 289 p.
• WEINGARTNER, T. J.; WEISBERG, R. H. On the annual cycle of equatorial upwelling in the central Atlantic Ocean. J. Phys. Oceanogr v. 21, 1991a. p. 68-82.
• WEINGARTNER, T. J.; WEISBERG, R. H. A description of the annual cycle in the sea surface temperature and upper ocean heat in the equatorial Atlantic. J. Phys. Oceanogr v. 21, 1991b. p. 82-96.
• WEISBERG, T. J.; TANG, T. Y. Further studies on the response of the equatorial thermocline in the Atlantic Ocean to the seasonal varying trade winds. J. Phys. Oceanogr v. 92, 1987. p. 3709-3727.
• YU, L.; XIANGZE, X.; WELLER, R. A. Role of net surface heat fluxes in seasonal variations of sea surface temperature in the tropical Atlantic Ocean. J. Climate v. 19, 2006. p. 6153-6169.
• YU, L.; XIANGZE, X.; WELLER, R. A. Multidecade Global Flux Datasets from the Objectively Analyzed Air-sea Fluxes (OAFlux) Project: Latent and Sensible Heat Fluxes, Ocean Evaporation, and Related Surface Meteorological Variables. Woods Hole Oceanographic Institution. OAFlux Project Technical Report, 2008. 64p.
• ZHANG, Y. C.; ROSSOW, W. B.; LACIS, A. A.; OINAS, V.; MISHCHENKO, M. I. Calculation of radiative fluxes from the surface to top of atmosphere based on ISCCP and other global data sets: refinements of the radiative transfer model and input data. J. Geophys. Res v. 109, D19105, 2004. DOI: 10.

Publication Dates

• Publication in this collection
  09 Sept 2010
• Date of issue
  June 2010

History

• Received
  02 Mar 2009
• Accepted
  02 Mar 2010
• Reviewed
  25 Nov 2009