A Global Isopycnal OGCM: Validations Using Observed Upper-Ocean Variabilities during 1992–93

Dingming Hu Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington

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Yi Chao Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California

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Abstract

In this study, a global isopycnal ocean model (GIM) is described and used for a simulation of variabilities of the global upper ocean during 1992–93. The GIM simulations are compared and validated with both the available observations and simulations with the Geophysical Fluid Dynamics Laboratory Modular Ocean Model (MOM). The observations include sea surface height from TOPEX/Poseidon (T/P), sea surface temperature (SST) from weekly National Centers for Environmental Prediction analysis, and vertical temperature profiles from gridded expandable bathythermographs (XBTs) data. The major differences between the GIM and MOM used in this study are the vertical coordinates, a Kraus–Turner mixed layer, and a tracer-transport velocity associated with an isopycnal-depth diffusion. Otherwise, the two models are formulated in the same parameter space, model configuration, and boundary conditions. The effects of these differences in model formulation on the model simulations are investigated.

Due to the difference in the orientation of interior flow and mixing, SST and the thermocline stratification in the eastern equatorial Pacific in GIM are more sensitive to the wind-driven upwelling than they are in MOM. In GIM there is no effective means to transfer heat between the upwelling cold water and the surrounding warm water since subsurface flow and mixing predominantly occur along isopycnic layers. As a result, the SST tends to be cold and the front tends to be sharp compared with the observations in the wind-driven upwelling region. The sharp front could potentially cause numerical instability in GIM. Thus, a large isopycnal-depth diffusivity has to be used to maintain the model stability since the isopycnal-depth diffusion is the most effective way to reduce the steep slope of isopycnals and the strength of the front associated with the cold upwelling in GIM. But the large isopycnal-depth diffusion results in excessive smoothing in the meridional isotherm doming in the equatorial and tropical thermocline. The trade-off between the numerical instability and the excessive isopycnal smoothing points to the necessity of improvement in the isopycnal-depth diffusion.

Sea level variabilities during 1992–93 simulated with both GIM and MOM are in good agreement with T/P observations. However, MOM poorly simulates the vertical distribution of the seasonal temperature anomalies in the upper ocean (the baroclinic component of the sea level variability) during 1992–93. Due to the lack of a realistic surface mixed layer, the MOM-simulated temperature profiles have a sharp subsurface gradient, which is not evident in both the GIM simulation and the XBT observation. As a result, the region below the subsurface gradient is almost insulated from the influence of the seasonal temperature variation. The Kraus–Turner mixed layer used in GIM helps to improve the model-simulated seasonal variations of the upper-ocean temperature and the background sea level variability. Implications of deficiencies in both GIM and MOM on the altimetric sea level data assimilation and transient tracer simulations are discussed.

Corresponding author address: Dr. Dingming Hu, JISAO, University of Washington, Box 354235, Seattle, WA 98195.

Email: hu@atmos.washington.edu

Abstract

In this study, a global isopycnal ocean model (GIM) is described and used for a simulation of variabilities of the global upper ocean during 1992–93. The GIM simulations are compared and validated with both the available observations and simulations with the Geophysical Fluid Dynamics Laboratory Modular Ocean Model (MOM). The observations include sea surface height from TOPEX/Poseidon (T/P), sea surface temperature (SST) from weekly National Centers for Environmental Prediction analysis, and vertical temperature profiles from gridded expandable bathythermographs (XBTs) data. The major differences between the GIM and MOM used in this study are the vertical coordinates, a Kraus–Turner mixed layer, and a tracer-transport velocity associated with an isopycnal-depth diffusion. Otherwise, the two models are formulated in the same parameter space, model configuration, and boundary conditions. The effects of these differences in model formulation on the model simulations are investigated.

Due to the difference in the orientation of interior flow and mixing, SST and the thermocline stratification in the eastern equatorial Pacific in GIM are more sensitive to the wind-driven upwelling than they are in MOM. In GIM there is no effective means to transfer heat between the upwelling cold water and the surrounding warm water since subsurface flow and mixing predominantly occur along isopycnic layers. As a result, the SST tends to be cold and the front tends to be sharp compared with the observations in the wind-driven upwelling region. The sharp front could potentially cause numerical instability in GIM. Thus, a large isopycnal-depth diffusivity has to be used to maintain the model stability since the isopycnal-depth diffusion is the most effective way to reduce the steep slope of isopycnals and the strength of the front associated with the cold upwelling in GIM. But the large isopycnal-depth diffusion results in excessive smoothing in the meridional isotherm doming in the equatorial and tropical thermocline. The trade-off between the numerical instability and the excessive isopycnal smoothing points to the necessity of improvement in the isopycnal-depth diffusion.

Sea level variabilities during 1992–93 simulated with both GIM and MOM are in good agreement with T/P observations. However, MOM poorly simulates the vertical distribution of the seasonal temperature anomalies in the upper ocean (the baroclinic component of the sea level variability) during 1992–93. Due to the lack of a realistic surface mixed layer, the MOM-simulated temperature profiles have a sharp subsurface gradient, which is not evident in both the GIM simulation and the XBT observation. As a result, the region below the subsurface gradient is almost insulated from the influence of the seasonal temperature variation. The Kraus–Turner mixed layer used in GIM helps to improve the model-simulated seasonal variations of the upper-ocean temperature and the background sea level variability. Implications of deficiencies in both GIM and MOM on the altimetric sea level data assimilation and transient tracer simulations are discussed.

Corresponding author address: Dr. Dingming Hu, JISAO, University of Washington, Box 354235, Seattle, WA 98195.

Email: hu@atmos.washington.edu

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  • Arakawa, A., and V. R. Lamb, 1977: Computational design of the basic dynamical processes of the UCLA general circulation model. Methods Comput. Phys.,17, 174–265.

  • Bleck, R., 1978: Finite difference equations in generalized vertical coordinates. Part I: Total energy conservation. Contrib. Atmos. Phys.,51, 360–372.

  • ——, 1979: Finite difference equations in generalized vertical coordinates. Part II: Potential vorticity conservation. Contrib. Atmos. Phys.,52, 95–105.

  • ——, and E. Chassignet, 1994: Simulating the oceanic circulation with isopycnic-coordinate models. The Oceans: Physical–Chemical Dynamics and Human Impact, S. K. Majumdar and E. W. Miller, Eds., Pennsylvania Academy of Science, 17–39.

  • ——, H. P. Hanson, D. Hu, and E. B. Kraus, 1989: Mixed layer–thermocline interaction in a three-dimensional isopycnic coordinate model. J. Phys. Oceanogr.,19, 1417–1439.

  • ——, C. Rooth, D. Hu, and L. T. Smith, 1992: Salinity-driven thermocline transients in a wind- and thermohaline-forced isopycnic coordinate model of the North Atlantic. J. Phys. Oceanogr.,22, 1486–1505.

  • Bryan, K., 1969: A numerical method for the study of the circulation of the World Ocean. J. Comput. Phys.,4, 347–376.

  • Callahan, P., 1993: TOPEX/Poseidon project GDR users handbook. Internal Doc. JPL D-8944, Rev. A, Jet Propulsion Laboratory, Pasadena, CA, 84 pp. [Available from Jet Propulsion Laboratory, Pasadena, CA 91109.].

  • Chao, Y., and S. G. H. Philander, 1993: On the structure of the Southern Oscillation. J. Climate,6, 450–469.

  • ——, and L.-L. Fu, 1995: A comparison between the TOPEX/Poseidon data and a global ocean general circulation model during 1992–93. J. Geophys. Res.,100, 24 965–24 976.

  • ——, D. Halpern, and C. Perigaud, 1993: Sea surface height variability during 1986–1988 in the tropical Pacific Ocean. J. Geophys. Res.,98, 6947–6959.

  • Chassignet, E., L. T. Smith, R. Bleck, and F. Bryan, 1996: A model comparison: Numerical simulations of the north and equatorial Atlantic Oceanic circulation in depth and isopycnic coordinates. J. Phys. Oceanogr.,26, 1849–1867.

  • Cox, M. D., 1984: A primitive equation, 3-dimensional model of the ocean. Ocean Group Tech. Rep. 2, 143 pp. [Available from Geophysical Fluid Dynamics Laboratory/NOAA, Princeton, NJ 08542.].

  • ——, 1987: Isopycnal diffusion in a z-coordinate ocean model. Ocean Modeling,74, 1–5.

  • Danabasoglu, G., J. C. McWilliams, and P. R. Gent, 1994: The role of mesoscale tracer transport in the global ocean circulation. Science,264, 1123–1126.

  • Duffy, P. B., D. E. Eliason, A. J. Bourgeois, and C. C. Covey, 1995:Simulation of bomb radiocarbon in two global ocean general circulation models. J. Geophys. Res.,100, 22 545–22 563.

  • England, M. H., 1993: Representing the global-scale water masses in ocean general circulation models. J. Phys. Oceanogr.,23, 1523–1552.

  • Fu, L. L., E. J. Christensen, C. A. Yamarone, M. Lefebvre, Y. Menard, M. Dorrer, and P. Escudier, 1994: TOPEX/POSEIDON mission overview. J. Geophys. Res.,99, 24 369–24 381.

  • Gent, P. R., and M. A. Cane, 1989: A reduced gravity, primitive equation model of the upper equatorial ocean. J. Comput. Phys.,81, 444–480.

  • ——, and J. C. McWilliams, 1990: Isopycnal mixing in ocean circulation models. J. Phys. Oceanogr.,20, 150–155.

  • ——, J. Willebrand, T. J. McDougall, and J. C. McWilliams, 1995: Parameterizing eddy-induced tracer transports in ocean circulation models. J. Phys. Oceanogr.,25, 463–474.

  • Greatbatch, R. J., and K. G. Lamb, 1990: On parameterizing vertical mixing of momentum in non-eddy resolving ocean models. J. Phys. Oceanogr.,20, 1634–1637.

  • Hellerman, S., and M. Rosenstein, 1983: Normal monthly wind stress over the World Ocean with error estimates. J. Phys. Oceanogr.,13, 1093–1104.

  • Hirst, A. C., and W. Cai, 1994: Sensitivity of a World Ocean GCM to changes in subsurface mixing parameterization. J. Phys. Oceanogr.,24, 1256–1279.

  • Hu, D., 1991: Diapycnal mixing in a joint mixed-layer/isopycnic coordinate model of wind- and thermohaline-driven ocean general circulation. Ph.D. dissertation, University of Miami, 201 pp. [Available from University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149.].

  • ——, 1996a: The computation of diapycnal diffusive and advective scalar fluxes in multilayer isopycnic-coordinate ocean models. Mon. Wea. Rev.,124, 1834–1851.

  • ——, 1996b: On the sensitivity of thermocline depth and meridional heat transport to vertical diffusivity in OGCMs. J. Phys. Oceanogr.,26, 1480–1494.

  • ——, 1997: Global-scale water masses, meridional circulation, and heat transport simulated with a global isopycnal ocean model. J. Phys. Oceanogr.,27, 96–120.

  • Huang, R. X., 1993: Real fresh water flux as a natural boundary condition for the salinity balance and thermohaline circulation forced by evaporation and precipitation. J. Phys. Oceanogr,23, 2428–2446.

  • Iselin, C., 1939: The influence of vertical and lateral turbulence on the characteristics of waters at mid depths. Eos., Trans. Amer. Geophys. Union,20, 414–417.

  • Kraus, E. B., and J. S. Turner, 1967: A one-dimensional model of the seasonal thermocline. Part II: The general theory and its consequences. Tellus,19, 98–106.

  • Levitus, S., 1982: Climatological Atlas of the World Ocean. NOAA Professional Paper 13, 173 pp. [Available from U.S. Government Printing Office, Washington, DC 20402.].

  • Luther, M. E., and J. J. O’Brien, 1985: A model of the seasonal circulation in the Arabian Sea forced by observed winds. Progress in Oceanography, Vol. 14, Pergamon Press, 353–385.

  • McCreary, J. P., and P. K. Kundu, 1988: A numerical investigation of the Somali current during the southwest monsoon. J. Mar. Res.,46, 25–58.

  • McDougall, T. J., 1987a: Neutral surfaces. J. Phys. Oceanogr.,17, 1950–1965.

  • ——, 1987b: Thermobaricity, cabbeling, and water-mass conversion. J. Geophys. Res.,92, 5448–5464.

  • Mellor, G. L., and T. Ezer, 1991: A Gulf Stream model and an altimetry assimilation scheme. J. Geophys. Res.,96, 8779–8795.

  • Montgomery, R. B., 1938: Circulation in upper layers of southern North Atlantic deduced with use of isentropic analysis. Papers in Physical Oceanography and Meteorology, Vol. 6, Woods Hole Oceanographic Institute.

  • Murtugudde, R., M. Cane, and V. Prasad, 1995: A reduced-gravity, primitive equation, isopycnal ocean GCM: Formulation and simulations. Mon. Wea. Rev.,123, 2864–2887.

  • Oberhuber, J. M., 1993: Simulation of the Atlantic circulation with a coupled sea ice–mixed layer–isopycnal general circulation model. Part I: Model description. J. Phys. Oceanogr.,23, 808–829.

  • Pacanowski, R., 1995: MOM 2 documentation: User’s guide and reference manual. Ocean Group Tech. Rep. 3, Geophys. Fluid Dyn. Lab., Princeton, NJ, 232 pp. [Available from Geophysical Fluid Dynamics Laboratory/NOAA, Princeton, NJ 08542.].

  • ——, and S. G. H. Philander, 1981: Parameterization of vertical mixing in numerical models of tropical oceans. J. Phys. Oceanogr.,11, 1443–1451.

  • ——, K. Dixon, and A. Rosati, 1991: Modular ocean model user’s guide. Ocean Group Tech. Rep. 2, Geophys. Fluid Dyn. Lab., Princeton, NJ. [Available from Geophysical Fluid Dynamics Laboratory/NOAA, Princeton, NJ 08542.].

  • Redi, M. H., 1982: Oceanic isopycnal mixing by coordinate rotation. J. Phys. Oceanogr.,12, 1154–1158.

  • Reynolds, R. W., and T. M. Smith, 1995: A high-resolution global sea surface temperature climatology. J. Climate,8, 1571–1583.

  • Roberts, M. J., R. Marsh, A. L. New, and R. A. Wood, 1996: An intercomparison of a Bryan–Cox-type ocean model and an isopycnic ocean model. Part I: The subpolar gyre and high latitude processes. J. Phys. Oceanogr.,26, 1495–1527.

  • Schopf, P. S., and A. Loughe, 1995: A reduced-gravity isopycnal ocean model: Hindcasts of El Niño. Mon. Wea. Rev.,123, 2839–2863.

  • Wunsch, C., 1988: Electric modeling of the North Atlantic. Part II: Transient tracers and the ventilation of the Eastern Basin thermocline. Philos. Trans. Roy. Soc. London,A325, 201–236.

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