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Simulation of the Madden–Julian Oscillation in a Coupled General Circulation Model. Part II: The Role of the Basic State

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  • 1 NERC Centre for Global Atmospheric Modelling, Department of Meteorology, University of Reading, Reading, United Kingdom
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Abstract

In Part I of this study it was shown that air–sea coupling had a positive impact on some aspects of the simulation of the Madden–Julian oscillation (MJO) by a GCM. However, errors in the basic-state climate of that GCM appeared to be preventing the MJO-related convection from propagating into the west Pacific. In this paper, the actual impact of these errors will be addressed. An integration of a flux-adjusted version of the coupled model has been performed, which has reduced basic-state errors in the west Pacific. In this version of the coupled GCM the MJO does propagate into the west Pacific. The simulation of the MJO by a coupled model with the same atmospheric component but a different ocean GCM is also analyzed. This coupled GCM has similar systematic errors in low-level zonal wind and precipitation to the model studied in Part I, but with warmer SSTs. Results from this experiment, together with the other available evidence, suggest that it is the errors in the low-level zonal wind component in the west Pacific that prevent the MJO from propagating into this region in the coupled GCM rather than the errors in absolute values of SST.

Corresponding author address: Dr. Peter Inness, NERC Centre for Global Atmospheric Modelling, Department of Meteorology, University of Reading, P.O. Box 243, Earley Gate, Reading RG6 6BB, United Kingdom.Email: pete@met.rdg.ac.uk

Abstract

In Part I of this study it was shown that air–sea coupling had a positive impact on some aspects of the simulation of the Madden–Julian oscillation (MJO) by a GCM. However, errors in the basic-state climate of that GCM appeared to be preventing the MJO-related convection from propagating into the west Pacific. In this paper, the actual impact of these errors will be addressed. An integration of a flux-adjusted version of the coupled model has been performed, which has reduced basic-state errors in the west Pacific. In this version of the coupled GCM the MJO does propagate into the west Pacific. The simulation of the MJO by a coupled model with the same atmospheric component but a different ocean GCM is also analyzed. This coupled GCM has similar systematic errors in low-level zonal wind and precipitation to the model studied in Part I, but with warmer SSTs. Results from this experiment, together with the other available evidence, suggest that it is the errors in the low-level zonal wind component in the west Pacific that prevent the MJO from propagating into this region in the coupled GCM rather than the errors in absolute values of SST.

Corresponding author address: Dr. Peter Inness, NERC Centre for Global Atmospheric Modelling, Department of Meteorology, University of Reading, P.O. Box 243, Earley Gate, Reading RG6 6BB, United Kingdom.Email: pete@met.rdg.ac.uk

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