A Normal-Mode Analysis of Rapid Teleconnections in a Numerical Weather Prediction Model. Part I: Global Aspects

Abstract

Global-scale interactions between the tropics and extratropics are investigated using a version of tile U.S. Navy's global operational numerical weather prediction model. The primary goals of this study are 1) to demonstrate the importance of atmospheric teleconnections for medium-range numerical weather prediction and 2) to analyze the evolution and dynamic structure of the response in a sophisticated numerical forecast model. The model normal modes are used as the principal diagnostic tool for analyzing the response to sea surface temperature anomalies in the tropical Pacific. By monitoring the energy growth in the dominant horizontal and vertical modes and comparing these with conventional difference-field diagnostics, it is shown that the character of the long-term response is well established within one to two weeks after the heating anomaly is introduced. The growth rates and structures of these modes provide insights into the dynamic processes that control the model response. In the tropics, enhanced convection is clearly the dominant forcing mechanism for these modes. In the extratropics, a more complicated picture arises in which both meridionally propagating energy and in situ instabilities in the ambient flow appear to be important mechanisms for producing the observed wave patterns. The results clearly demonstrate that tropical forcing can have a significant global impact on time scales relevant to medium-range numerical weather prediction. In Part II of this study, the normal-mode diagnostic approach is extended by developing a technique for partitioning the modes according to their latitudinal variances in order to examine the tropical and extratropical responses in further detail. It is shown that the modes are a powerful and flexible tool for diagnosing the behavior of a complicated model.