The GEOS-5 AOGCM
[ Latest Update: 2 February 2011 ]
Yury Vikhliaev, Max Suarez, Andrea Molod, Larry Takacs, Bin Zhao
The GEOS-5 AOGCM is designed to simulate climate variability on a wide range of time scales, from synoptic time scales to multi-century climate change, andhave been tested in coupled simulations and data assimilation mode.
The main components of the GEOS-5 AOGCM (Fig. 1) are the atmospheric model, the catchment land surface model, both developed by the GMAO (GEOS-5 AGCM, Rienecker et al. 2008), and MOM4, the ocean model developed by the Geophysical Fluid Dynamics Laboratory (Griffies et al. 2005). These two components exchange fluxes of momentum, heat and fresh water through a "skin layer" interface. The skin layer includes parameterization of the diurnal cycle and a sea ice model (LANL CICE, Hunke and Lipscomb 2008). All components are coupled together using the Earth System Modeling Framework (ESMF) interface. Here we describe the results from a single multi-decade simulation conducted in the latest tuning phase of the AOGCM.
The goal in having a multi-scale modeling system with unified physics is to be able to propagate improvements made to a physical process in one component into the other the components smoothly and efficiently.
The model system has also been coupled to a Satellite Data Simulation Unit that can compute satellite-consistent radiances or backscattering signals from simulated atmospheric profiles and condensates consistent with the unified microphysics within the multi-scale modeling system (Fig. 1).
Fig. 1 Components of the GEOS-5 AOGCM. Components in the black frame are used in a production of AGCM data assimilation system. Other components are tested in coupled and data assimilation modes.
2. Experiment Design
A 150-year validation experiment has been conducted to test the current performance of the GEOS-5 AOGCM. The resolution of the atmospheric component in this experiment is 2.5° x 2° latitude with 72 vertical levels up to 0.01hPa. The ocean resolution is 1° longitude and latitude, with meridional equatorial refinement to 1/3°, and 50 vertical levels. A coupled model configuration with a 1.5°x1° atmospheric grid, and 0.5° ocean grid is also being tested as the configuration for the GMAO's CMIP5 simulations. Initial conditions for the atmospheric component are taken from an uncoupled experiment forced by the observed sea surface temperature. The ocean is initialized with a steady state, with temperature and salinity from the Levitus and Boyer climatology. The atmospheric state in the coupled experiment is compared to the results from an uncoupled experiment and also to MERRA. This analysis allows us to understand which errors can be attributed to coupling and which errors result from deficiencies in the AGCM. The ocean state is validated using observations and the fields from the GMAO ocean data assimilation system.
The 150-year integration produced a stable, realistic mean climate and interannual climate variability. The key features of climate simulated by the model are summarized below.
The mean atmospheric state simulated by the AOGCM is similar to the atmospheric state simulated by the GEOS5 AGCM forced by the observed sea surface temperature (SST). Errors in the coupled simulation primarily have the same structure as errors in the uncoupled simulation, but usually with larger magnitude. These errors include: too strong surface wind stress in high latitudes; too strong cloud radiative forcing in low latitudes, and too weak cloud radiative forcing in high latitudes; errors in precipitation typical of most state-of-the-art climate models, e.g.,a strong double Inter-Tropical Convergence Zone (ITCZ).
On the ocean side, the upper ocean circulation and surface climate reach equilibrium in several decades, while the deep ocean circulation still exhibits a drift after 150 years of integration. SST is an important measure of the realism of the model climate, since it is used by the atmospheric component as a boundary condition. Figure 2 shows that the model SST simulation is quite realistic. The dominant errors shown in Figure 2 are typical for state-of-the-art climate models. For example, the warm SST bias near the eastern
boundaries of the oceans is due to deficiencies in simulating the orientation of the wind stress and the coastal
upwelling. SST errors in the regions of western boundary currents result from deficiencies in simulating the
strength of the western boundary currents and the separation location. These types of errors are typically reduced with the increased resolution.
Fig. 2 Sea surface temperature (top) from the GEOS-5 coupled integration, and the bias relative to the Reynolds climatology (bottom).
The leading mode of global SST variability is shown in Figure 3. This mode exhibits the prominent El Niño - Southern Oscillation pattern which varies on inter-annual time scales. Compared to observations, the model El Nino pattern is narrower and extends further into the western Pacific. In addition to the dominant signal in the equatorial Pacific, the model also captures the co-variability in the sub-tropical Pacific. Comparison between the model and observed time series of the leading SST mode shows that the model ENSO has reasonable time scales and irregularity, with a tendency to have slightly higher frequency than observed. Analysis of correlation between ENSO time series and 300mb geopotential height (not shown) demonstrates that the model simulates realistic tropical-extratropical ENSO teleconnections.
Fig. 3 The leading mode of global SST variability from the GEOS-5 AOGCM (left) and the Hadley center (right). Top panels show the leading empirical orthogonal functions of the global SST. Bottom panels show corresponding principal components.
Development of the GEOS-5 AOGCM is a significant advance in NASA's climate modeling. Current performance of the model is comparable to performance of the state-of-the art coupled climate models being used for the next IPCC assesment report. The model now routinely runs on the supercomputing clusters at the NASA Center for Climate Simulation (NCCS) and the NASA Advanced Supercomputing Division (NAS). Projects underway with the GEOS-5 AOGCM include weakly coupled ocean-atmosphere data assimilation, seasonal climate predictions and decadal climate prediction tests within the framework of Coupled Model Intercomparison Project Phase 5 (CMIP5). The decadal climate prediction experiments are being initialized using the weakly coupled atmosphere-ocean data assimilation based on MERRA. The results of these experiments will be distributed through the NCCS Earth Grid System node.