706 MONTHLY WEATHER REVIEW VOLUME 127 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 YI CHAO Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California (Manuscript received 9 April 1997, in ®nal form 13 May 1998) 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 pro®les 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 con®guration, 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 ¯ow and mixing, SST and the thermocline strati®cation in the eastern equatorial Paci®c 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 ¯ow 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 pro®les 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 in¯uence 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 de®ciencies in both GIM and MOM on the altimetric sea level data assimilation and transient tracer simulations are discussed. 1. Introduction OGCM development and improvement. The most wide- ly used OGCM is that developed at the Geophysical The past decade has witnessed increasing applications Fluid Dynamics Laboratory (Bryan 1969; Cox 1984; of ocean general circulation models (OGCMs) to the Pacanowski et al. 1991; Pacanowski 1995); this model study of the ocean's role in the climate over a wide is also known as the Modular Ocean Model (MOM). range of time- and spatial scales. With the increasing MOM is a level model since the prognostic variables demand, tremendous efforts have been devoted to in the model are carried at constant-depth levels. In a standard version of MOM, mixing of tracers occurs pre- dominantly along constant-depth levels, which is in- Corresponding author address: Dr. Dingming Hu, JISAO, Uni- consistent with our knowledge that tracer mixing occurs versity of Washington, Box 354235, Seattle, WA 98195. predominantly along isopycnal/neutral surfaces (Mont- E-mail: [email protected] gomery 1938; Iselin 1939; McDougall 1987a). Recent q 1999 American Meteorological Society Unauthenticated | Downloaded 09/26/21 10:49 PM UTC MAY 1999 HU AND CHAO 707 progress has been made in parameterizing isopycnal there is no data. Although a Kraus±Turner mixed-layer eddy mixing processes in level models (Redi 1982; Cox model can be introduced into MOM, it is beyond the 1987; Gent and McWilliams 1990; Gent et al. 1995); scope of this paper. Therefore, this study is not intended this has signi®cantly improved the ability of MOM to for a strict examination on the effect of the the different simulate the global ocean strati®cation and meridional vertical coordinate system on the simulation. In a model heat transport (England 1993; Hirst and Cai 1994; Dan- comparison, more differences in model formulation abasoglu et al. 1994). make it more dif®cult to understand the differences in Isopycnal mixing can be naturally incorporated in iso- model simulations. Nevertheless, understanding can still pycnic coordinate models. In these models, the prog- be gained on how each type of the models can be im- nostic variables are carried at layers of constant potential proved. Two of such examples are Chassignet et al. density (isopycnic layers), and thus both lateral mixing (1996) and Roberts et al. (1996). In the present study, and ¯ow occur strictly along isopycnic layers. Since the the major difference in model formulation between GIM early 1980s, many isopycnal ocean models have been developed, for example, the Miami Isopycnic Coordi- and MOM are vertical coordinates, a Kraus±Turner nate Ocean Model [MICOM hereafter, see Bleck et al. mixed layer, and an isopycnal depth diffusion. Effort is (1992) and Bleck and Chassignet (1994) for a complete made to investigate the effects of these differences on description], the isopycnal model developed by Ober- the model simulations. huber (1993), and other layer models (Schopf and This article is structured as follows. Section 2 de- Loughe 1995; Murtugudde et al. 1995; Gent and Cane scribes the isopycnal model. Section 3 describes the 1989; McCreary and Kundu 1988; Luther and O'Brien design of real-time simulations of the large-scale sea 1985). As more OGCMs are developed and improved, surface height and upper-ocean temperature for the pe- it has become apparent that the similarities and differ- riod of 1992±1993 with the GIM and MOM. Section 4 ences between these OGCMs need to be understood. describes several types of observations used for vali- However, a direct intercomparison between level and dation of the model intercomparison. Section 5 gives isopycnal ocean models is not as simple as it seems, the intercomparisons between the simulations with the because each OGCM is usually formulated with its own two global models and validations with the observa- unique con®guration, turbulence closure parameteriza- tions, and section 6 summarizes the results and conclu- tion, and primitive equation simpli®cation. This is par- sions of this study. ticularly true for existing isopycnal models. For in- stance, in the isopycnal models of Schopf and Loughe (1995) and Murtugudde et al. (1995), reduced-gravity approximation is assumed. In Oberhuber's model, ver- 2. Model description tical mixing is parameterized in a way analogous to that of the bulk mixed-layer parameterization, which differs a. The governing equations from the vertical eddy diffusivity parameterization ex- tensively used in OGCMs. In MICOM, the model is laid out on a Mercator projection, whereas most of the In the GIM used in this study, the interior ocean is OGCMs are con®gured on the earth's spherical surface. represented by a stack of layers of prescribed potential Note that for a given degree of horizontal resolution, density and the oceanic surface boundary layer is rep- the meridional grid size decreases poleward on a Mer- resented by a Kraus±Turner type of mixed-layer model cator projection and remains invariant in the spherical (Kraus and Turner 1967). The model governing±equa- coordinate. A fair model intercomparison has to be con- tions are the primitive conservation equations of mo- ducted on the same model mesh. mentum, mass, heat, and salt, with the Boussinesq and Such a direct intercomparison between isopycnal and hydrostatic approximations. Written in the generalized level models was achieved by Hu (1997), in which Hu's vertical coordinate s (Bleck 1978), the equations read global isopycnal model (GIM) outperformed MOM in simulating the climatology of the World Ocean. How- ever, Hu did not use a realistic surface mixed layer for ]vv2 e ]v 1 = 1 ( f 1 z)k 3 v 1 the sake of direct model comparison. In this study, a ]t s 2 ]z ]s Kraus±Turner mixed layer is introduced to the GIM. 12s Also the constant vertical mixing parameterization used ]s in Hu (1997) is replaced by a Richardson number±de- 1 52g=h 2 (= p 1 r= f) pendent mixing parameterization. Our object is to val- r sd s idate the ability of the GIM in simulating transient var- 0 iabilities of the world upper oceans. Observational data 1 ]z ]]v during 1992±93 are used for the validation. MOM sim- 1 =sMHs´ A = v 1 A MV, (1) ulation with the same con®guration and forcing is also ]z[]1212]s ]s ]z used to provide more consistent test in those areas where ]s Unauthenticated | Downloaded 09/26/21 10:49 PM UTC 708 MONTHLY WEATHER REVIEW VOLUME 127 ]]z ]z ]e c.