Uncertainties of estimates of inertio-gravity energy in the atmosphere. Part I: Intercomparison of four analysis systems

On the contrary, the NCEP-NCAR reanalyses contain more IG motion, especially in the Southern Hemisphere extratropics. Each analysis contains more energy in the eastward IG motion than in its westward counterpart. The difference is about 2%-3% of the total wave energy and it is associated with the motions projected onto the Kelvin wave in the tropics.

The selected truncation parameters of the expansion (zonal, meridional, and vertical truncation) ensure that the projection provides the optimal fit to the input data on model levels. This approach is different from previous applications of the normal modes and under the linearity assumption it allows the application of the inverse projection to obtain details of circulation associated with a selected type of motion. The bulk of the IG motion is confined to the tropics.

For the successful reproduction of three-dimensional circulations by the normal modes it is important that the expansion includes a number of vertical modes.