The MJO problem in GCMs: What are the missing physics?

Jiajin Lin

CIRES Climate Diagnostic Center  
This study evaluates the tropical intraseasonal variability, especially the fidelity of Madden-Julian Oscillation (MJO) simulations, in 14 climate models participating in the Inter-governmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4).  Eight years of daily precipitation from each model’s 20th century climate simulation are analyzed and compared with daily satellite retrieved precipitation. 
Space-time spectral analysis is used to obtain the variance and phase speed of dominant convectively coupled equatorial waves, including the MJO, Kelvin, equatorial Rossby (ER), mixed Rossby-gravity (MRG), and eastward inertio-gravity (EIG) and westward inertio-gravity (WIG) waves. The variance and propagation of the MJO, defined as the eastward wavenumbers 1-6, 30-70 day mode, are examined in detail.

Many of the models have signals of convectively coupled waves, with Kelvin and MRG-EIG waves especially prominent. However, the majority of the models with good signals show phase speeds that are too fast, and scale these disturbances to equivalent depths which are larger than the observed value. Interestingly, this scaling is consistent within a given model across modes, in that both the symmetric and antisymmetric modes scale similarly to a certain equivalent depth. Excessively deep equivalent depths suggest that these models may not have a large enough reduction in their “effective static stability” by diabatic heating.

Although the eastward MJO precipitation variance in most models is smaller than in observations, it does approach the observed value in several models. Moreover, the eastward MJO variance in many
models is significantly larger than its westward counterpart. However, the model variances in the MJO frequency band usually come from part of an over-reddened spectrum, and not from a pronounced spectral peak. The only model with a prominent spectral peak in MJO frequency band is CNRM-CM3. We did not find a systematic dependence of MJO variance on closure assumptions of the deep convection schemes, or on model resolution. Air-sea coupling significantly increases the MJO variance in the only model for which we have compared an uncoupled run with a coupled run.

It is found that equatorial precipitation in most of the models has stronger persistence than in observations, which tends to make their spectra too red. Factors affecting the persistence of precipitation, especially those associated with convective parameterization, are discussed. Several other physical processes that are important for the MJO, but are missing in current GCMs, will also be discussed.