Four-dimensional structures of the MJO and Kelvin wave in the CSU and CCM3 AGCMs

Simulation of tropical variability is important for several reasons. First, in order for a climate simulation to be deemed realistic, it should produce a reasonable basic state AND variability, since it is this day-to-day variability that is experienced by vegetation, humans, and the earth's surface. Second, because tropical weather is dominated by wave activity, a model that better simulates certain wave types may have better tropical forecast skill. Finally, successful simulation of such wave disturbances may provide insight into how such waves are excited and maintained.

In this paper, we use the method of Wheeler et al (1999) to construct composites of the Madden Julian Oscillation (MJO) and Kelvin wave as seen in the Colorado State University (CSU) and NCAR Community Climate Model (CCM, v3.6) Atmospheric General Circulation models (AGCMs). In constructing these composites, we consider the spectral characteristics of these wave types, the geographic distribution of their variance, and their four-dimensional structure, as seen in several fields.

For the CSU GCM, it is found that isolated differences in the basic state meridional winds in the eastern Pacific may dirupt the propagation of these wave types. Furthermore, at certain longitudes, extratropical influences affect the signatures of these wave types. In the CCM3, the addition of a convective triggering function based on the buoyancy of boundary layer parcels improves the spectral representation and propagation characteristics of the MJO.

The 4-dimensional composites of these wave types reveal deficiencies in their simulation that are not readily apparent in either the spectral plots, or in the geographic distributions of their variance. The fact that such deficiencies may be "hidden" in these analyses illustrates the need for detailed compositing of these structures when evaluating the realism of simulated wave types.