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Volume and Heat Transports in the World Oceans from an Ocean General Circulation Model

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Volume and Heat Transports in the World Oceans from an Ocean General Circulation Model “main” — 2009/11/10 — 21:05 — page 181 — #1 Revista Brasileira de Geof´ısica (2009) 27(2): 181-194 © 2009 Sociedade Brasileira de Geof´ısica ISSN 0102-261X www.scielo.br/rbg VOLUME AND HEAT TRANSPORTS IN THE WORLD OCEANS FROM AN OCEAN GENERAL CIRCULATION MODEL Luiz Paulo de Freitas Assad1, Audalio Rebelo Torres Junior2, Wilton Zumpichiatti Arruda3, Affonso da Silveira Mascarenhas Junior4 and Luiz Landau5 Recebido em 29 setembro, 2008 / Aceito em 9 junho, 2009 Received on September 29, 2008 / Accepted on June 9, 2009 ABSTRACT. Monitoring the volume and heat transports around the world oceans is of fundamental importance in the study of the climate system, its variability, and possible changes. The application of an Oceanic General Circulation Model for climatic studies needs that its dynamic and thermodynamic fields are in equilibrium. The time spent by the model to reach this equilibrium is called spin-up time. This work presents some results obtained from the application of the Modular Ocean Model version 4.0 initialized with temperature, salinity, velocity and sea surface height data already in equilibrium from an ocean data assimilation experiment conducted by Geophysical Fluid Dynamics Laboratory (GFDL). The use of this dataset as initial condition aimed to diminish the spin-up time of the oceanic model. The model was integrated for seven years. Volume and heat transports in different sections around the world oceans showed to be in good agreement with the literature. The results showed a well defined seasonal cycle starting at the second integration year, also some important dynamic and thermodynamic aspects of the Global Ocean Circulation, as the great conveyor belt, are well reproduced. The obtained results could constitute an important data source to be used as initial and boundary conditions in regional ocean model experiments as well as for long integration runs in order to study oceanic climate variability. Keywords: ocean climate, computational modeling, volume transport, heat transport. RESUMO. O monitoramento dos transportes de volume e calor nos oceanos e´ de fundamental importanciaˆ para o estudo do sistema climatico,´ sua variabilidade e poss´ıveis mudanc¸as. A aplicac¸ao˜ de Modelos de Circulac¸ao˜ Global dos Oceanos em estudos climaticos´ necessita que os dados dinamicosˆ e termodinamicosˆ gerados pelos mesmos se encontrem em equil´ıbrio. Esse trabalho apresenta alguns resultados obtidos a partir da aplicac¸ao˜ do Modular Ocean Model , em sua versao˜ 4.0, inicializado com campos de temperatura, salinidade, velocidade e elevac¸ao˜ da superf´ıcie livre em equil´ıbrio, oriundos do experimento de assimilac¸ao˜ de dados oceanicosˆ conduzido pelo Geophysical Fluid Dynamics Laboratory (GFDL). O uso desse conjunto de dados objetivou a diminuic¸ao˜ do tempo de “spin-up” do modelo oceanicoˆ utilizado. O modelo foi integrado por sete anos, ao fim dos quais foram estimados transportes de volume e calor em diferentes sec¸oes˜ ao redor do globo. Tais estimativas demonstraram boa concordanciaˆ com valores estimados em trabalhos anteriores. Os resultados demonstraram a ocorrenciaˆ de um ciclo sazonal oceanicoˆ bem definido a partir do segundo ano de integrac¸ao˜ do modelo, assim como importantes aspectos da circulac¸ao˜ oceanicaˆ global como o “conveyor belt”, os giros subtropicais oceanicosˆ e o escoamento associado a` Corrente Circumpolar Antartica.´ O experimento realizado demonstrou a importanciaˆ desse modelo como fonte de dados a ser utilizado como condic¸oes˜ iniciais e de contorno em modelos regionais de circulac¸ao˜ oceanica,ˆ assim como para longas integrac¸oes˜ a serem aplicadas em estudos de variabilidade e mudanc¸as climaticas.´ Palavras-chave: clima oceanico,ˆ modelagem computacional, transporte de volume, transporte de calor. 1Universidade Federal do Rio de Janeiro, COPPE, Centro de Tecnologia, Bloco I, sala 214, Laboratorio´ de Metodos´ Computacionais em Engenharia, Av. Athos da Silveira Ramos, 149, Cidade Universitaria,´ Ilha do Fundao,˜ 21941-909 Rio de Janeiro, RJ, Brazil. Phone: +55 (21) 2562-8419 – E-mail: [email protected] 2Universidade Federal do Rio de Janeiro, CCMN, Instituto de Geociencias,ˆ Departamento de Meteorologia, Laboratorio´ de Modelagem de Processos Marinhos e Atmosfericos,´ Av. Athos da Silveira Ramos, s/n, Cidade Universitaria,´ Ilha do Fundao,˜ 21949-900 Rio de Janeiro, RJ, Brazil. Phone: +55 (21) 2562-9470 ext. 21 – E-mail: [email protected] 3Universidade Federal do Rio de Janeiro, CCMN, Instituto de Matematica,´ Av. Athos da Silveira Ramos, 149, Bl. C, CT, Cidade Universitaria,´ Ilha do Fundao,˜ 21949-900 Rio de Janeiro, RJ, Brazil. Phone: +55 (21) 2562-7508 ext. 222 – E-mail: [email protected] 4Centro Internacional para la Investigacion´ del Fenomeno´ El Nino˜ (CIIFEN), Escobedo #1204 y 9 de Octubre, P.O. Box 09014237, Guayaquil, Ecuador. Phone: +59 (34) 251-4770; Fax: +59 (34) 251-4771 – E-mail: [email protected] 5Universidade Federal do Rio de Janeiro, COPPE, Centro de Tecnologia, Bloco I, sala 214, Laboratorio´ de Metodos´ Computacionais em Engenharia, Av. Athos da Silveira Ramos, 149, Cidade Universitaria,´ Ilha do Fundao,˜ 21941-909 Rio de Janeiro, RJ, Brazil. Phone: +55 (21) 2562-8415 – E-mail: [email protected] “main” — 2009/11/10 — 21:05 — page 182 — #2 182 VOLUME AND HEAT OCEAN TRANSPORTS INTRODUCTION the transport for the Antarctic Circumpolar Current (ACC) and Agulhas Current (AC). This work presents the application of this Monitoring the volume and heat transports around the world kind of strategy to initialize a coarse resolution OGCM and also oceans is of fundamental importance to understand the cli- some results from 7 years of model integration (Assad, 2006). mate system, its variability and possible changes. In the last A brief description of the model and some characteristics of years, most of the climate studies focused on the following the applied methodology are given in the next section where also methods: global ocean numerical modeling (Maltrud & McClean, it will be described the grid setup, model parameterizations, for- 2003; Stammer et al., 2003) and hydrographic inverse methods cing, initial and boundary conditions applied in the simulation. (Schmitz, 1996; Ganachaud & Wunsch, 2000). The ocean volume and heat transports annual mean values and According to Peixoto & Oort (1992) the “climate” state could time series analysis of some dynamic and thermodynamic prog- be understood as a set of average quantities that characterizes nostic transports are presented and discussed. the structure and the behavior of the atmosphere, hydrosphere and cryosphere for a particular period. The application of an Ocean Global Circulation Model (OGCM) to make climatic fore- METHODOLOGY casts requires the model to be in dynamic and thermodynamic Grid setup equilibrium. In this work we use the Modular Ocean Model 4.0 (MOM 4.0) To start the integration of an OGCM it is necessary to spe- developed by GFDL under the administration of National Ocea- cify initial conditions. The initial fields are basically represented nic and Atmospheric Administration (NOAA). This z-coordinate by the following variables: temperature, salinity, velocity field and model has been largely adopted by the scientific community for sea surface height. In general, models are started from a state global climatic research investigations. It is mathematically for- of rest with climatological distribution of temperature and sali- mulated with a Navier-Stokes set of equations under the Bous- nity fields imposed. On this manner the model physics will gene- sinesq and hydrostatic approximations. The system of continu- rate dynamical and thermodynamical fields in equilibrium state. ous equations are basically completed by an equation of state The time needed by the model to reach this equilibrium state is of sea-water, the continuity equation for an incompressible fluid, called spin-up time and could range from days for the upper layers and conservation equations for temperature and salinity. A com- to hundred of years for the deep layers of the ocean. Long spin-up plete description of this model can be found in Pacanowsky times are related with the low velocity propagation of some inter- & Griffies (1999). nal ocean modes. Semtner & Chervin (1992) use a high resolu- The prognostic variables are space distributed following the tion OGCM to study the general circulation of the oceans. With staggered Arakawa B-grid system. The MOM 4.0 code also al- horizontal resolution of 0.5 0.5 and 20 levels in the verti- lows the construction of spherical global grids with numeri- ◦× ◦ cal, the model took 32.5 years to reach its dynamic equilibrium cal poles not necessarily coincident with the geographical ones. starting from a state of rest with imposed climatological tempe- This method minimizes problems related with the convergence rature and salinity fields from Levitus (1982). A way to initialize of meridians over the ocean that happens mainly on North Pole. an OGCM, that dramatically reduces the spin-up time, is the use The grid constructed on this work uses the tripolar grid method of all initial fields already in thermodynamic and dynamic equi- by Murray (1996). It adopts normal spherical coordinates south librium and not only the mass structure. These data can be ob- of 65◦ N while North of this latitude two numerical poles situated tained from another numerical experiment that has already rea- on land regions of Asia and North America are used. ched a “climate” state. Using temperature and salinity distribu- The longitudinal resolution is 1◦ worldwide while the latitu- tions from Navy’s Modular Ocean Data Assimilation System 1/8◦ dinal resolution increases from 1◦ to 1/3◦ within the 10◦N-10◦S climatological product (Fox et al., 2002 apud Maltrud & McClean, equatorial band. On the vertical, 50 levels are implemented. In 2003), Maltrud & McClean (2003) initialized a 1/10◦ resolution order to accommodate a higher resolution near the ocean surface, Parallel Ocean Program general circulation model. The model the first 22 levels are located in the top 220 m depth.
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