The representation of the Madden-Julian oscillation (MJO) has proven to be a challenging task for general circulation models (GCMs). Slingo et al. (1996) analyzed the MJO in 15 GCMs and reported that, as seen in upper level wind fields, the models generally simulated the MJO with shorter periods and weaker amplitudes than is seen in ECMWF analyses. Furthermore, nearly all had more power at higher frequencies than the analyses. In this paper, we examine the
structural and propagation characteristics of the MJO in a recent version of the CSU GCM. In addition to the fundamental analyses of Slingo et al. (1995), we present a detailed examination of the zonal wavenumber, spatial structure, and propagation characteristics of the MJO.
For this study, we have chosen the difference between the zonal wind at 200 mb and the zonal wind at 850 mb (U200-U850) as an index of MJO activity. This index is then subjected to a 20-100 day bandpass filter. Lag-longitude correlation matricies of this index indicate that the MJO in the model tends to be a zonal wavenumber 2-3 disturbance, rather than wavenumber 1 as is seen in the observations. While eastward propagation is the dominant cause of local variability at any given longitude, there are noticeably more standing and westward propagating waves in the western Pacific warm pool reigon than are seen in the observations.
Our current efforts are directed at determing whether the eastward propagating part of the MJO index indeed represents the structure of the MJO. This question will be answered by compositing various fields (i.e., precipitation, heating, OLR, etc.) for just the eastward moving waves (via use of a 2D FFT filtering process). We are also particularly interested in the standing and westard moving parts of the index, as this behavior leads to questions such as "Is the model MJO a wave-CISK type disturbance or simply a response to a standing, oscillatory heat source?" Finally, a model run with a coupled,
interactive ocean layer model is currently underway way to examine the impact of the ocean on the model MJO.
REFERENCES
J. M. Slingo, K. R, Sperber, J. S. Boyle, J.-P. Ceron, M. Dix, B. Dugas, W. Ebisuzaki, J. Fyfe, D. Gergory, J.-F. Gueremy, J. Hack, A. Harzallah, P. Inness, A. Kitoh, W. K.-M. Lau, B. McAvaney, R. Madden, A. Matthews, T. N. Palmer, C.-K Park, D. Randall, and N. Renno, 1996: Intraseasonal oscillations in 15 atmospheric general circulation models: results from an AMIP diagnostic subproject. Climate Dynamics, 12: 325-357