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Zhengxin Zhu

Abstract

A smoothly varying hybrid σθ coordinate, which changes from sigma at the bottom to nearly isentropic in the stratosphere, was implemented into a general circulation model. Multiyear simulations of the σθ coordinate GCM were successfully conducted. The results reveal that the tropical precipitation, water vapor transport, and large-scale circulation are significantly improved, compared to a control run using the σp coordinate. In boreal winter, the position and intensity of the South Pacific convergence zone is much better simulated in the position and intensity. In boreal summer, the Asian monsoonal rainfall increases substantially, and the precipitation maxima in the Bay of Bengal and west of India are more realistic. The changes in the moisture transport and convergence are found to be consistent with the precipitation changes. The tropical large-scale flow field is also improved in many aspects.

Possible reasons responsible for the improvements are analyzed and verified with sensitivity experiments. The improvements of water vapor transport and precipitation are more likely due to the better representation of large-scale dynamics by using the nearly isentropic coordinate in the upper levels, rather than due to more accurate representation of moisture advection process in the lower troposphere.

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Edwin K. Schneider and Zhengxin Zhu

Abstract

The annual cycle of sea surface temperature (SST) in the equatorial Pacific is compared for two simulations with a coupled atmosphere–ocean general circulation model. The simulations differ only in the optical properties of the ocean: sunlight penetrates below the topmost layer of the ocean model in one case but is completely absorbed in the top layer in the other. The simulation without the sunlight penetration produces an unrealistic annual cycle of SST with a strong semiannual component in the equatorial Pacific, whereas the simulation with sunlight penetration is more realistic. The change in the character of the annual cycle results from an increase in the effective heat capacity of the ocean associated with an increase in the depth of the mixed layer directly forced by the sunlight penetration. This produces a smaller amplitude of the annual cycle of SST at latitudes close to but off the equator. The zone of intense tropical convection then remains closer to the equator, leading to a reduced semiannual cycle of zonal wind stress at the equator. The reduction in the unrealistic semiannual wind stress forcing leads to a more realistic annual cycle in SST.

The simulation of the annual mean SST is also improved by the inclusion of the sunlight penetration, with a better simulation of the warm pool in the western equatorial Pacific and associated improvements in the atmospheric circulation. This improvement is also attributed to the increase in the mixed layer depth, which changes the ocean heat flux in the western equatorial Pacific by reducing the sensitivity of SST to upwelling.

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Zhengxin Zhu, John Thuburn, Brian J. Hoskins, and Peter H. Haynes

Abstract

A vertical discretization of the primitive equations in a general vertical coordinate is described that enables a primitive equation model to use terrain-following sigma levels near the ground and isentropic levels higher up, with a smooth transition region in between. Therefore, it combines many of the advantages of the computational efficiency of σ coordinates and the predictive and diagnostic potential of θ coordinates, and should be particularly useful for general circulation models to be used for studies of stratosphere-troposphere exchange and middle-atmosphere transport of trace gases. It is shown that the semi-implicit time scheme can be used in a straightforward manner with this discretization. A discussion is given of how to optimize the transition from sigma levels to isentropic levels so as to avoid model levels crossing each other. A numerical problem caused when very shallow, very strong inversions occur in the temperature field is countered by a form of vertical-scale selective dissipation. Baroclinic wave life cycles and full general circulation simulations have been successfully performed with a modified version of the European Centre for Medium-Range Weather Forecasts model.

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Ben P. Kirtman, J. Shukla, Bohua Huang, Zhengxin Zhu, and Edwin K. Schneider

Abstract

The Center for Ocean–Land–Atmosphere Studies anomaly coupled prediction system, using a sophisticated dynamical model of the tropical Pacific Ocean and the global atmosphere, is described. The resolution of the component models is moderate, with the atmospheric spectral model truncated at triangular total wavenumber 30 and 18 vertical levels. The ocean model is a Pacific Basin model with 0.5° latitude and 1.5° longitude resolution in the waveguide and 20 vertical levels. The performance of the uncoupled component models motivates the anomaly coupling strategy and has led to the development of a simple empirical technique for converting the 850-mb zonal wind into a zonal surface stress that is used in the prediction experiments described here. In developing ocean initial conditions, an iterative procedure that assimilates the zonal wind stress based on the simulated sea surface temperature anomaly error is applied. Based on a sample of 78 18-month hindcasts, the predictions have useful skill in the Nino-3 region for at least 12 months. The systematic error of the predictions is shown to be relatively small because the ocean initial conditions are in reasonable equilibrium with the ocean model. Finally, composites of the hindcast warm El Niño–Southern Oscillation (ENSO) events indicate that the model simulates the basic features of ENSO, but there are errors in the horizontal structure of the sea surface temperature anomaly that potentially limit the predictability of the model.

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Edwin K. Schneider, Bohua Huang, Zhengxin Zhu, David G. DeWitt, James L. Kinter III, Ben P. Kirtman, and J. Shukla

Abstract

A scheme for making seasonal to interannual predictions of El Niño–Southern Oscillation with a coupled atmosphere–ocean general circulation model that incorporates subsurface ocean measurements in the initial conditions is described. Anomaly initial conditions are used in order to reduce initial shock and climate drift. The ocean component of the prediction model has a nearly global domain, and the coupled model does not employ anomaly coupling or empirical statistical corrections.

Initial conditions for the ocean were obtained from a near-global ocean analysis produced by an ocean data assimilation system. The assimilation system uses a variationally formulated optimal interpolation method to analyze oceanic fields from temperature observations and a first-guess field provided by integrating a global ocean general circulation model. The period of the analysis was 1986 through 1992.

The anomaly initial conditions for the ocean were generated by adding the anomalies of the assimilated fields from the assimilation climatology to the coupled model climatology. A series of 28 1-yr hindcast experiments, four each year for the years 1986–1992, was carried out to test the scheme. The hindcasts show considerable skill in the equatorial Pacific.

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Edwin K. Schneider, Zhengxin Zhu, Benjamin S. Giese, Bohua Huang, Ben P. Kirtman, J. Shukla, and James A. Carton

Abstract

Results from multiyear integrations of a coupled ocean–atmosphere general circulation model are described. The atmospheric component is a rhomboidal 15, 18-level version of the Center for Ocean–Land–Atmosphere Studies atmospheric general circulation model. The oceanic component is the Geophysical Fluid Dynamics Laboratory ocean model with a horizontal domain extending from 70°S to 65°N. The ocean model uses 1.5° horizontal resolution, with meridional resolution increasing to 0.5° near the equator, and 20 vertical levels, most in the upper 300 m. No flux adjustments are employed.

An initial multiyear integration showed significant climate drift in the tropical Pacific sea surface temperatures. Several modifications were made in the coupled model to reduce these errors. Changes were made to the atmospheric model cloudiness parameterizations, increasing solar radiation at the surface in the western equatorial Pacific and decreasing it in the eastern Pacific, that improved the simulation of the time-mean sea surface temperature. Large errors in the wind direction near the western coast of South America resulted in large mean SST errors in that region. A procedure to reduce these errors by extrapolating wind stress values away from the coast to coastal points was devised and implemented.

Results from the last 17 years of a 62-yr simulation are described. The model produces a reasonably realistic annual cycle of equatorial Pacific sea surface temperature. However, the upper-ocean thermal structure has serious errors. Interannual variability for tropical Pacific sea surface temperatures, precipitation, and sea level pressure that resemble the observed El Niño–Southern Oscillation (ENSO) in structure and evolution is found. However, differences from observed behavior are also evident. The mechanism responsible for the interannual variability appears to be similar to the delayed oscillator mechanism that occurs in the real climate system.

The structure of precipitation, sea level pressure, and geopotential anomalies associated with the tropical Pacific sea surface temperature interannual variability are isolated and described. The coupled model is capable of producing structures that are similar to those observed.

It is concluded that atmosphere–ocean general circulation models are beginning to capture some of the observed characteristics of the climatology of the tropical Pacific and the interannual variability associated with the El Niño–Southern Oscillation. Remaining obstacles to realistic simulations appear to include ocean model errors in the eastern equatorial Pacific, errors associated with cloud–radiation interactions, and perhaps errors associated with inadequate meridional resolution in the atmospheric model equatorial Pacific.

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