Linear Equatorial Wave Mode Initialization in a Model of the Tropical Pacific Ocean: An Initialization Scheme for Tropical Ocean Models

Abstract

Data assimilation in models of the tropical oceans can generate spurious equatorial wave modes which are potentially harmful to the model background fields. The amplitudes of these spurious wave modes can often be large and, in general, depend upon the nature and size of the imbalances introduced into the model by the data assimilation process. Spurious equatorial Kelvin waves are likely to give rise to the most detrimental effects since they are dynamically very important in the real ocean, and am responsible for a variety of phenomena in the equatorial and coastal regions of the ocean. In this paper, an initialization scheme based on the technique of normal mode initialization (used for many years by meteorologists) is developed, which can be used to suppress spurious equatorial wave modes during data assimilation experiments. The initialization scheme maps information from observational data onto the linear planetary wave solutions of the discretized model equations of motion. In addition, equatorial Kelvin waves and Yanai waves driven by sea surface forcing in the model are retained. Using this method, the rate of growth of the assimilation errors between one assimilation time and the next can be reduced considerably. It is shown that the model need only he initialized in a narrow channel spanning the equator, and that the initialization method also works well in a fully nonlinear model.