Atlas: A Spectral Coupled Ocean-Atmosphere Model by Ramesh C. Balgovind

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Computational Techniques and Applications Conference and Workshops - CTAC99
September 20-24, 1999
The Australian National University
Canberra, ACT, Australia

Organizers
Mike Osborne, Bob Gingold, Steve Roberts, David Harrar II, Thanh Tran, Bob Anderssen, Henry Gardner, Markus Hegland, Lutz Grosz

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A Spectral Coupled Ocean-Atmosphere Model
by
Ramesh C. Balgovind
HPCCC, Bureau of Meteorology, Melbourne, Victoria, Australia.
Coauthors: Carsten Frederiksen (BMRC, Bureau of Meteorology, Melbourne, Victoria, Australia), Jorgen Frederiksen (CSIRO, Atmospheric Research and Cooperative Research Centre for Southern Hemisphere Meteorology, Aspendale, Victoria, Australia)

We describe the formulation and implementation of a spectral coupled ocean-atmosphere model of intermediate complexity. The atmospheric domain is global and is coupled to a basin ocean which covers half the globe representing the Pacific ocean. The atmospheric part of the model, based on the primitive equations, is formulated in terms of vorticity, divergence and potential temperature. The ocean part of the model is formulated in terms of vorticity, divergence and temperature using the Boussinesq approximation. The atmospheric and oceanic components are coupled through surface heat fluxes and wind stresses. Both the atmospheric and oceanic components of the model are solved using spectral techniques in which the fields are expanded in terms of spherical harmonics. Symmetry constraints are applied to the coupling terms and to the ocean model variables.

Multi-decadal integrations of the model have been performed including realistic topography and seasonal variations in the atmospheric forcing and for resolutions ranging from low (rhomboidal wave number 15) to intermediate (rhomboidal wave number 31) to high (rhomboidal wave number 60). The model provides quite realistic atmospheric and oceanic circulations including their seasonal variations. In particular,El-Nino Southern Oscillations (ENSO) are produced with realistic amplitudes and frequency spectra even with resolutions as low as wave number 15. This contrasts with the poor ENSO amplitudes in most low resolution grid point ocean models. The reasons for the realistic ENSO fluctuations in the spectral model are shown to be related to the improved conservation properties and the fact that the spectral model is stable with realistic and small dissipation while commonly used grid point models require too large dissipation for stability.

Date received: July 18, 1999