Subramanian, A. C., M. Jochum, A. J. Miller, R. Murtugudde, R. B. Neale and D. E. Waliser, 2011:
The Madden Julian Oscillation in CCSM4
Journal of Climate, 24, 6261-6282.
We assess the ability of the Community Climate System Model-4 (CCSM-4) to represent
the Madden Julian Oscillation, the dominant mode of intraseasonal variability in the tropical
atmosphere. We use the US CLIVAR MJO Working Group prescribed diagnostic tests to
evaluate the model's mean state, variance and wavenumber-frequency characteristics in a
20-year simulation of the intraseasonal variability in zonal winds at 850 hPa (U850) and
200 hPa (U200) and Outgoing Longwave Radiation (OLR). Unlike its predecessor, CCSM4
reproduces a number of aspects of MJO behavior more realistically.
CCSM4 produces coherent, broadbanded and energetic patterns in eastward propagating
intraseasonal zonal winds and OLR in the tropical Indian and Pacific Oceans that are
generally consistent with MJO characteristics. Strong peaks occur in power spectra and
coherence spectra with periods between 20-100 days and zonal wavenumbers between 1 and
3. Model MJO's, however, tend to be more broadbanded in frequency than in observations.
Broadscale patterns, as revealed in combined EOFs of U850, U200 and OLR, are remarkably
consistent with observations and indicate that large-scale convergence-convection coupling
occurs in the simulated MJO.
Relations between MJO in the model and its concurrence with other climate states are
also explored. MJO activity (defined as the percentage of time the MJO index exceeds
1.5) is enhanced during El Nino events compared to La Nina events both in the model
and observations. MJO activity is increased during periods of anomalously strong negative
meridional wind shear in the Asian Monsoon region, and also during strong negative Indian
Ocean Zonal Mode states, in both the model and observations.