Modal view of atmospheric circulation
MODES applies three-dimensional linear wave theory for the decomposition of global circulation in terms of normal-mode functions (NMFs). NMFs used by MODES are eigensolutions of the linearized primitive equations in the terrain-following sigma coordinates and were derived by Kasahara and Puri (1981, Mon. Wea. Rev). The horizontal structure functions are the Hough harmonics while the vertical structure equation is solved numerically using realistic stratification profiles of the troposphere+stratosphere. The top boundary condition is applied near the zero pressure level. For details, see https://gmd.copernicus.org/articles/8/1169/2015/
MODES outputs quantify spatial and temporal variability associated with the two main circulation regimes, the Rossby wave (or balanced) regime and the inertia-gravity wave (or unbalanced) regime. The approach is most useful in the tropics where the two special NMF solutions, the Kelvin wave and the mixed Rossby-gravity wave, account for a significant part of tropical variability.
Recently published book describes theory and applications of normal-mode functions in weather and climate dynamics and numerical weather prediction research: https://www.springer.com/gp/book/9783030609627
MODES pages provide real-time and archive results of modal decomposition of the operational deterministic ECMWF 10-day forecasts. Selected outputs of modal analysis of reanalysis and interesting datasets are also here.