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PCMDI > WCRP CMIP3 Model Output > Diagnostic Subprojects Printer Friendly Version
 
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The Madden-Julian Oscillation in General Circulation Models

PI: Kenneth R. Sperber
Institution: Lawrence Livermore National Laboratory / PCMDI
Abstract:
The Madden-Julian Oscillation (MJO) dominates the convective signal over the tropical Eastern Hemisphere, with teleconnections extending into the extratropics due to the large-scale convective extent and duration of the MJO convective anomalies. A methodology is utilized to analyze in a standardized fashion the MJO in general circulation models. This is attained by projecting 20-100 day bandpass filtered outgoing longwave radiation (OLR) from the models onto the two leading empirical orthogonal functions (EOFŐs) of observed OLR that characterize the propagation of MJO convection from the Indian Ocean to the central Pacific Ocean. The resulting principal component time series are then screened to isolate boreal winters during which they exhibit a lead-lag relationship consistent with observations. This PC subset is used for linear regression to determine the ability of the models to simulate the observed space-time variability of the MJO. Analysis of control integrations of the CMIP2+ models indicates that the vast majority of models underestimate the amplitude of the MJO convective anomalies by a factor of two or more, and the eastward propagation of convection is less coherent than observed, typically. For a given family of models, coupling to an ocean leads to better organization of the large-scale convection. The low-level moisture convergence mechanism for eastward propagation of the MJO is represented in limited cases, as is the vertical structure of moisture and divergence. The analysis of the CMIP2+ models provides a frame of reference for evaluating improvement in the simulation of the MJO in the IPCC control simulations.

It is envisioned that the IPCC control runs will first be analyzed to determine the models that give the most realistic representation of the MJO based on the afore-mentioned diagnostics. With this subset of models the climate change integrations will be analyzed to ascertain anthropogenically forced perturbations to this important mode of variability. Previous work by Slingo et al. (1999, QJRMS, 125, 583-609) has demonstrated an increased amplitude of observed MJO variability as the tropics warmed in the post-1976 regime transition compared to earlier decades. This suggests that additional warming due to anthropogenic climate change may strongly influence the MJO. During years when the MJO is active, the tendency is for below-normal rainfall over Austral-Asia, mainly due to reduced convection during the suppressed phase of the MJO. If the MJO becomes stronger in the future, this may have dire consequences for the agricultural based economies of the tropics.
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