The structure and dynamics of tropical-midlatitude interactions

The structure and dynamics of tropical-midlatitude interactions are investigated using a global numerical forecast model (the Navy's NOGAPS model). The model responses to SST anomalies in the tropical Pacific Ocean are analyzed in terms of the model normal modes, with an emphasis on investigating the impact of anomalous tropical forcing on the dynamics of midlatitude flow. The dynamics of the problem are analyzed within the framework of the complete baroclinic forecast model, rather than in a greatly simplified version of the model.;A new diagnostic technique is developed whereby the normal modes are partitioned according to their latitudinal variances in order to obtain estimates of the tropical and extratropical contributions to the energy of the anomalous response. Estimates of the tropical and extratropical contributions to the anomalous response energy can thus be obtained.;The extent to which the structure and dynamics of the anomalous response in the NOGAPS model fit the conceptual framework derived from idealized modeling studies is investigated. It is found that modifications to this framework may be required when systems that include realistic vertical structure and baroclinic processes are considered. The analysis reveals that the extratropical response has a significant baroclinic component, in addition to the well known equivalent barotropic one. It is shown that the anomalous wave pattern in the extratropics is actually composed of two distinct types of horizontal wave motion superimposed on one another. The first type is associated with meridionally propagating large-scale waves that originate in the tropics and have equivalent barotropic structure. The second type is associated with zonally propagating waves that originate in midlatitudes, and whose structures are consistent with synoptic-scale baroclinic disturbances. Moreover, the synoptic-scale anomalies appear to originate in the regions where the meridionally propagating wave trains intersect the midlatitude westerlies, suggesting that the anomalous synoptic-scale development may be excited by the meridionally propagating waves. The synoptic-scale anomalies appear after only six to ten days of simulation, suggesting that anomalous tropical forcing may substantially influence midlatitude synoptic-scale flow at preferred locations on time scales less than two weeks.