On Normal Mode Aided Modelling: Application to Asian Monsoon

Summary A new concept of utilizing generalized normal mode initialization (NMI) for modelling is investigated. This NMI-balancing adapts real data to model solutions. For eigenmodes of typical (climatological) structure and a simple 1- or more-layer shallow water model, anomalous flow of special episodes or events can be identified with mode groups giving distinctive signals. If these signals are part of the model solution or of the complement of the data to the model solution and how this is changed by introducing additional physical mechanisms to the model, gives hints for improvement. In applying this to multi-scale interactions of Asian monsoon, in particular a seventeen-years background knowledge and the anomalous flow of years 1991, 1977, 1978 we have a challenging test frame. Particular topics are: (1) Mei Yu rains in China, the representation of particular aspects of them by certain mode groups, the degree of independence of the corresponding signals on other modes, and the role of dissipation; (2) detection of El Ni?o-Southern Oscillation (ENSO) in atmospheric (reanalysis) data minus the NMI-balanced model solution; (3) Madden-Julian Oscillations (MJO) and the introduction of diabatic processes, and finally (4) aspects of dynamic stability. From among the results which are new or we simply have become aware of we pull out that atmospheric processes generally can be represented by modes or mode groups neither straight forward nor uniquely. Introduction of some dissipation schemes to the model was not helpful but rather destroyed the realistic structure of atmospheric dynamics. A parameterization of diabatic processes passes signals present in the data but not in the NMI-balanced model solution through to such being part of the latter, in case of MJO but not yet in case of ENSO. Ocean dynamics proves to be indispensable. Nevertheless there are imprints of essential atmospheric processes hidden on certain (e.g., most unstable) modes and partially recovered by NMI-balancing. Received July 19, 1999 Revised December 29, 1999